Final Report Summary - MAMA (Unlocking research potential for multifunctional advanced materials and nanoscale phenomena)
Executive Summary:
MAMA project was founded on two scientific concepts key for materials physics: i) functional materials (mainly transition-metal oxides [TMOs]), ii) complex physical phenomena (e.g. superconductivity, magnetism, ferroelectricity, etc.). TMOs represent a large family of compounds where the complex interplay between lattice (strain, phonons) and electronic (charge, orbital, spin) degrees of freedom leads to unique states of matter which can exhibit gigantic response upon small applied perturbations. TMOs is therefore a fertile platform for investigating and exploiting a wide range of intricate phenomena such as high temperature superconductivity, ferroelectricity, magnetism, colossal magneto resistance, thermo electricity, half-metallic and multiferroic behaviour, two-dimensional electron liquids, anomalous metal-to-Mott transitions, quantum topological insulator or superconductors, etc. Furthermore, TMOs can be stacked in multilayer heterostructures that, on one hand, allow to combine in a single device the functionalities of their individual layers, and, on the other hand, can generate an even wider range of emergent novel properties, that can be surprisingly different from those of the individual building blocks. All such elements are at the core for the generation of innovative paths and solutions in the field of functionalities and advanced materials.
The MAMA project was successful in increasing the research capacity building within CNR-SPIN in the area of multifunctional materials (based on TMOs and hybrids) and the related nanoscale phenomena by: 1) recruiting highly qualified personnel, 2) acquiring powerful instrumentation for fabricating, characterizing, and modelling the materials targeted by our research, 3) by mobilizing and supporting know-how exchange between CNR SPIN researchers and European centres leader in our field. The European partnerships played a crucial role in the MAMA project. We involved 11 prestigious groups covering all areas of synthesis, analysis and modelling of complex materials: “Correlated Electron Systems and Oxide Physics” at the University of St. Andrews, “Magnetic and Superconducting Materials” at Leiden Institute of Physics, “Advanced Inorganic Materials” at the University of Twente, “New phenomena and Novel Functionalities in Artificial Oxide materials” and “Quantum electronics” at the DPMC Geneve, “Photonic Engineering” at the Risø National Laboratory Roskilde , “Nonlinear Magneto-optics” at the ETH Zurich University, “Nanoscience and Nanotechnology” at the IMDEA Madrid, “Microtechnology and Nanoscience” at the University of Chalmers, “Magnetism and Superconductivity” at the University of Warwick, “Theoretical Solid State Physics” at the Leibniz Institute of Dresden, “Realistic Theory of Strongly Correlates Systems” at the Institute for Advanced Simulation of Jülich.
In general, the actions developed within the MAMA consortium aimed at set up the foundations to overcoming the issue of sustainability in terms of new critical mass, of the growth of the research potential as well as of the need of maintaining international collaborations in order to effectively respond to rapid scientific changes. Concerning human potential, the recruitment actions have been successful in terms of group building and stability. This is demonstrated by the positive result of having four of the eight recruited researchers as permanent staff within the CNR-SPIN. Also, the level of international partnerships has been successfully strengthened and consolidated thanks to the intense program of know-how exchange. As result of such activities, The successful activities in this direction within the MAMA project allowed to place CNR-SPIN as a leader in coordinating a COST Action on synthesis, analysis and modelling of functional oxide materials which involves all the MAMA partners as well as virtually all European groups active in the field with the aim of strengthen the research within the European Research Area (ERA). In addition, CNR-SPIN groups actively participated and received project funding both in National and European competitive calls. The activity of developing innovative ideas for project involvement has been much intense within the MAMA project benefiting of the wide range of partnerships and expertise. This represents an essential asset to sustain and stimulate the successful participation of CNR-SPIN to the ERA.
Project Context and Objectives:
Multifunctional materials typically perform specific functions other than possessing a load bearing capacity. In this context, transition metal oxides (TMOs) are prototypes for multifunctional behaviour due to the wide variety of physical properties that they exhibit, including unconventional superconductivity, piezo- and ferro-electricity, colossal magneto resistance, multiferroicity and a number of exotic magnetic, charge and orbital orderings. Furthermore, they can be employed to develop oxide interface having properties at the nanometer scale that are qualitatively different from their single building blocks, allowing to engineer novel functionalities by benefiting, for instance, of the controlled growth of epitaxial heterostructures. The integration of layers of functional organic materials, often showing similarities with oxides in their transport mechanisms, represents an ultimate and even more ambitious challenge. Such features are believed to open the route to the fabrication of device prototypes where multiple functionalities of TMOs and functional organic layers are nano-integrated on the same chip. The range of application sectors is correspondingly large especially regarding the area of information and communications technology,as well as the material solutions forenergy generation, storage and transport.
Within the MAMA project the CNR-SPIN pointed to the highest level of competitiveness about issues of i) materials fabrication, by addressing the growth of very high quality samples in the different forms of epitaxial thin films and single crystals, also integrated together in complex heterostructures and ii) advanced material characterizations, both based on matter-light interaction (by resorting either to advanced laser applications or to synchrotron sources), on scanning probe techniques and on electromagnetic transport, as well as about iii) theoretical modelling and advanced computation to analyze and get insight into different physical properties of innovative materials. In order to tackle these issues, MAMA aimed at exploiting the available partnerships’ expertise and experimental endowment, complemented by the new resources.
The MAMA project aimed at reinforcing the research potential by mobilizing and supporting the human and material resources with excellent centres operating in the field of the synthesis, analysis, and modelling of physical properties for multi-functional materials (WP2), by recruiting new personnel (WP3), by increasing SPIN visibility and socioeconomic impact through targeted dissemination actions (WP5) and through the organization of three workshops and one conference (WP4) and through the acquisition of powerful tools for fabricating and characterizing the materials at the core of the research targets (WP6). Such requirement has been planned in the view of a synergic growth of the Institute’s research potential both in the experimental and theoretical areas. Indeed, only a reinforcement that involves all the CNR-SPIN research components will permit an harmonious achievement of the scientific objectives.
The MAMA activities refer to three main objectives inherent to 1) fabrication, 2) characterization and 3) computation of multifunctional advanced materials.
1) Fabrication: to reinforce research potential about the synthesis of multi-functional materials
With specific targets such as:
• Quality controlled samples
• Improve capacities for tailoring phenomena in innovative multi-functional materials. Develop specific expertise for a multi-scale or integrated approaches for the fabrication of functional materials.
Synthesis of multi-functional materials concerns different techniques and expertise for materials in the shape of bulk single crystals, thin-films and nano-particles. To obtain single crystals having high quality in the sense of defects and impurity level control, image furnaces are required to perform the so called floating-zone or the travelling-solvent floating-zone method. Such kind of equipment permits the realization of large samples whose dimensions can be of the order of few cm3 with a very low degree of contamination by spurious elements. The objective targeted under the MAMA project was to increase the competences to execute the processes for sample growth both for the general steps and for those which can be more dependent on the specific materials uponinvestigation. Hence, the exchange of know-how and experience on the basics of the techniques and the procedures for fabricating and manipulating the samples was of fundamental importance.
Concerning the nanostructures, the control in the growth of oxide thin films is not only essential for implementing most practical applications, but also allows to disclose new phenomena, as often reported in the case of epitaxial ultrathin films and of nanoparticle films, showing electronic and magnetic properties that are qualitatively different from the bulk material. An even wider field are interfaces in oxide epitaxial heterostructures, that can show properties that are qualitatively different from the single “building blocks”. In order to push the control in the growth of epitaxial thin films and heterostructures to the level of atomic engineering, an unprecedented capability in monitoring the growthis needed . Such aim was attained both by implementing advanced characterization techniques in the deposition system, for real-time monitoring of the process, and by analysing the as grown samples (possibly also during intermediate stages of growth) with a fast ultra-high-vacuum transfer to suitable analysis chambers. This concept has led to the design and to the implementation of the “Modular facility for Oxide Deposition and Analysis” (MODA) at CNR-SPIN.
2) Characterization: to enhance research capacities for analysis of multi-functional materials with specific targets as:
• Reinforcing the space, time, momentum and energy resolved approach for the analysis of innovative materials
• Enhance probing capacities at the mesoscopic and nanoscopicscale
The main target is to strengthen the capacity to perform different types of analyses on solid state materials: morphological, structural and compositional studies, electro-magnetic investigation, as well as spectroscopic, optical , etc.
While in the case of single crystals only ex-situ analysis can be considered, in the case of thin film samples characterization can take place either during growth (RHEED, fast photography of the plume), or within the same ultra-high-vacuum system (XPS, UPS, SPA-LEED and STM/AFM), or ex-situ (transport measurements at variable temperature and frequency in applied field, measurements performed within the scanning electron microscope, optical spectroscopies, X-ray diffraction and other characterizations, also performed within the partner’s laboratories). The first two kinds of analyses are performed within the MODA facility, where the fabrication and characterization aspect are closely intermixed. This is a crucial aspect for the multi-scale control of the synthesized materials.
A specific objective concerns the increase of research capacity inperforming micro- and nano-scale electrical characterization of materials and devices. The acquisition of a Scanning Electron Microscope (SEM) with ultra-high resolution imaging and microscopic probes significantly improved the research capacity for surface layer transport measurement, interconnect probing, and characterization of inhomogeneous or microstructured multi-functional advanced materials: nano-wires and tubes, ultra-thin films and mono-layer, electronic surface states, device interconnects, single and poly-crystals, organic materials and so on.
3)Computation: to enhance capacities for computational analyses and modelling of multi-functional advanced materials
With specific targets as:
• Need of improving computational multi-scale approaches to get insight into different physical properties of innovative materials
• Enhance capacities for ab-initio modeling of oxides and of heterostructures
• Improve research potential towards phenomenological and microscopic modelling of oxides and of heterostructures
The state of the art concerning the properties modelling of correlated systems and of heterostructures covers many topics and theoretical approaches in the area of quantum many-body systems as well as a high expertise for investigating the emerging complex electronic quantum structure. The present objective consists in acquiring competences about different theoretical and computational approaches for describing electronic and magnetic as well as spectroscopic properties of multi-functional materials benefiting from the international cross sharing information. This activity referred to the acquisition of competence to allow to perform realistic calculations of the full quantum structure. This implies the achievement of a know-how of the state-of-the-art of ab-initio methods based on density functional theory and other approaches that can suitably include the electron correlations or the coupling between electrons and vibrational modes.
The fabrication of multi-functional advanced materials is generally followed by the most powerful spectroscopic tools to study the resulting quantum structure. These tools include low energy electron diffraction, scanning tunnelling microscopy and spectroscopy, core level X-ray photoemission, Raman, optical, and angle resolved photoemission spectroscopy. The combination of these methods gives direct experimental access to the surface chemistry, crystallographic structure, and the electronic structure. The major objective in this framework was to enlarge the research potentiality for facing complex problems with integrated schemes of theoretical computations and through the synergy of them to provide a constructive feedback on the spectroscopic characterizations by isolating the basic aspects needed for the analysis.
Project Results:
The main S&T results related to the MAMA activities refer to different thematic areas of functional materials, based on transition metal oxides and hybrids, with special emphasis on the potential applications derived by the exploitation of the large variety of exhibited complex phenomena.
In particular, we can identify three areas of scientific impact of the achieved results with marks to specific topics:
1) Superconducting materials and hybrids, emergent quantum-topological states, quantum topological computation and new material solutionsbased on superconductorsfor information data storage and manipulation.
2) Electron correlated materials and heterostructures/interfaces based on transition metal oxides, novel phases of matter driven by spin-orbit coupling, the synthesis of oxide materials for novel electronics.
3) Opto-electronic materials, light-matter interaction for designing novel nanostructures and functionalities, fundamental excitations and time-dependent control of collective phenomena in electron correlated materials.
Hereafter we highlight few of the main scientific results related to the three mentioned topics.
Topic 1)
The main results in this area concern the study of superconducting heterostructures interfaced to ferromagnet, superconducting edge states with anomalous magnetic properties, and topological superconducting and/or insulating systems. These results have been obtained with the aim of developing, inthe long-term, novel materials for spintronics and topological quantum computation.
The basic concept of spintronics materials is that, unlike conventional electronic devices, which transmit information via the charge carried by an electron, theycan exploit the “spin”, i.e. intrinsic angular momentum of the electron.
Spintronics involves manipulating the spin degree of freedom to perform logic operations in devices. There is, however, a shortcoming: any such device requires a large spin current to operate, which in itself requires the use of a large electrical charge. Since the spin currents are dissipative, a large fraction of the input energy is wasted as heat. Superconductors are materials which, when cooled below a certain temperature, can carry a current without losing energy, thus provide one potential solution to this. If these materials could be inserted in spin-based devices, an energy-efficient source for the spin currents could be provided. In this direction, the solutions analysed within MAMA are made of heterostructures with superconductors (with spin-singlet or spin-triplet pairs) interfaced to magnetic systems (e.g. ferromagnets, helimagnet, antiferromagnet, etc.).
The system with spin-triplet superconductors are particularly attractive because the charge supercurrents would intrinsically carry a spin current too. In this framework, a novel form of interaction between spin and orbital degreesof freedom has been found in a spin-triplet superconductor (TSC)- ferromagnet (FM) heterostructure. In the TSC-FM system the orientation of the FM moment relative to the TSC vector order parameter is a crucial variable that controls the physical behavior. In addition to the pair breaking, spin-flip reflection processes at the interface with the FM scatter the triplet Cooper pairs between the spin up and down condensates, setting up an effective Josephson-like coupling between them. The pair-breaking and spin Josephson coupling both make significant contributions to the free energy of a TSC-FM junction through the proximity effect, interface electronic reconstruction, and the variation of the TSC gap. Although these contributions depend upon the direction of the FM's exchange field, the two effects do not necessarily act constructively. For a single-component p-wave TSC, we find that the variation of the gap controls the orientation of the FM's moment via the change in condensation energy. The stable configuration is either parallel or perpendicular to the TSC vector order parameter, depending on the alignment of the TSC gap with respect to the interface, thus evidencing a unique form of spin-orbital coupling. Otherwise, the competing orbital components of the chiral state generate a non-unique behavior and a first-order transition from the perpendicular to the parallel configuration as the FM exchange field is increased. These results highlight the potential of manipulating spin supercurrent by exploting the orbital character of the superconducting pairing [Phys. Rev. Lett. 111, 097003 ( 2013)].
Concerning the issue of superconducting edge states with anomalous magnetic properties, the recognition that the surface states in correlated materials reflect the nature of the interactionsand orders in the bulk is the key aspect for the understanding, and potential control, of these electronic states. Gapless modes at the boundary ofmaterials whose bulk is gapped are especially interestingsince the surface states are robust and may be topologicallyprotected; i.e. their existence relies on the global symmetries of the bulk state and does not depend on the details ofthe surface scattering and other sample-dependent parameters. The bulk gap may be due to the band structure, or,in a metal, may arise from electron-electron interaction, as in superconductors. Simple band insulators or conventional superconductors do not support robust low-energystates at the boundary. In this context, one of the main results is that the interference between singlet and triplet pairs can generate magnetic states at their boundary. Such magnetic effects can occur if triplet and singlet pairing get mixed and have a non-uniform spatial profile or by suitably controlling their phase difference. As a result, the bound states can be spin-polarized, leading to a finite surface magnetization, and still, spin current can flow along the interface while surface charge currents exhibit anomalous dependence on the magnetization [Phys. Rev. Lett. 110, 267002 (2013)].
Quantum topological computation relies on the possibility of having materials platform that can exhibit non-Abelian modes. The emergence of particles that exhibit a richer exchange statistics than the constituent electrons and ions in a material is among the most remarkable manifestation of “more is different.”Such particles fall into two broad categories: Abelian and non-Abelian. Interchanging Abelian anyons alters the system’s wave function by aphasethat is intermediate between the case of thebosons and the fermions. Non-Abelian anyons are more interesting because the exchange rotates the system quantum stateamong a degenerate set of locally indistinguishable ground states produced by the anyons. Non-Abelian anyons are still extremely interesting modes because they are fundamental ingredients to fault-tolerant topological quantum computation. The qubits are embedded in the system’s ground states and, due to the non-Abelian statistics, manipulated through anyon exchanges. The non locality with which the information is stored and processed intrinsically leads to immunity against decoherence stemming from local environmental perturbations. The challenge is then to identifying suitable platforms for non-Abelian excitations.
In this framework, one dimensional spin-triplet superconductors can have topological superconducting phase whose domain walls in the superconductor bind Majorana zero modes and realize confined Isinganyons with non-Abelian statistics. The challenge here is to find such nontrivialone-dimensional (1D) and two dimensional (2D) superconductors and to testproposals forengineering these phases in heterostructures fashioned from ingredients such as topological insulators, semiconductors, and spin-singlet superconductors.
One of the main finding in this context concerns the synthesis and the study of materials that can have spin-triplet superconductivity as well as the identification of hybrid platform where non-abelian states can occur. The system Sr2RuO4 is a good candidate for spin-triplet superconductivity. This material and the related heterostructures have been successfully synthesized. Relevant results about the property of such oxide superconductor are on the surface and bulk electronic structure via angle resolved photoemission study. Circularly polarised light has been used to disentangle the signals from the bulk and surface layers, thus opening the possibility of investigating many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting ofthe surface band. The outcome has underlined the role of spin-orbit coupling and the possible connection to the nature of the topological matter [New Journal of Physics 14, 063039 (2012)]. The hallmark of Sr2RuO4superconductivity is the presenceof time-reversal symmetry breaking (TRSB), a phenomenon most likely arising from the chiralp-wave structure of theCooper pairs. The possibility of synthesizing eutectic (i.e. mixed) systems, representing naturally occurring nanoscopic interfaces, has opened newroutes for testing the symmetry of the pairing wave functionin the proximity of normal or magnetic systems, as well as in searching for novel quantum configurations that could emerge at the interface between the embedded phases. The successful synthesis of eutectic oxides of the form Sr2RuO4- Sr3Ru2O7 led to the discovery of a superconducting state with time-reversal symmetry breaking due to the occurring at 2.5 K (i.e. well above1.5 K, the critical temperature of the Sr2RuO4 single phase)[Phys. Rev. B 85, 134527 (2012)].
The search for topological nontrivial quantum modes has been also pursued with spin-singlet high temperature superconductors with d-wave symmetry such as those of the family of the cuprates, e.g. LaSrCuO, BaSrCuO, etc.. A major finding is that aπ−Josephson Junction, inserted in a topologically non trivial model ring, sustains a Majoranabound state, which is robust with respect to localand non local perturbations. Therealistic structure could be based on high temperature superconductors andtricrystal structure, similar to theone used to spot the d-wave order parameter. The presence of the Majorana bound state changesthe ground state of the topologically non trivial ring in a measurable way, with respect to that of aconventional one. This is an important issue for detecting the topological mode [Phys. Rev. B 88, 184512 (2013)].
Topic 2)
Concerning the correlated heterostructures and interfaces based on transition metal oxides the main results refer to the synthesis, analysis, manipulation and modelling of different oxide systems, with special focus on those of the type LaAlO-SrTiO.
These artificial thin-film oxide structures are among the most investigated systems in condensed matter especially becausethe atomic-scale engineering of such interfaces holds a great promise: theability to manipulate electrons by locally modifying orbitals shape and occupation, yieldingnew quantum states that can be exploited in novel electronic devices. If interfaces of semiconductors, which are rather featureless materials with a non magnetic rigid lattice, can nonetheless lead to fascinating physics, such as the quantum Hall effect, then the oxides have a huge potential. A variety of exotic low dimensional electronic systems can be achieved at the oxide interfaces, exploiting spin, charge, and orbital interactions as well as lattice vibrations.
This is the motivation for intense investigation of the interface between band insulators, e.g. LaAlO and SrTiO, which hosts a high mobility two -dimensional electron gas (2DEG)in spite ofthe large band gap of its bulk constituents, and exhibits intriguing transport, magnetic and superconducting properties.
One of the main questions addressed is about the mechanism for the generation of the 2DEG at the interface. Can defect-generated electrons, due to disorder or other extrinsic sources,explain all the observed properties of LaAlO-SrTiO? By employing electron energy loss spectroscopy(EELS) it has been possible to present evidence that nearly perfect LaAlO-SrTiO interfaces with negligible cation intermixing can be fabricated by pulsed laser deposition (PLD). In these samples the interfacial chargedensity decays over~3 unit cells (u.c.) within theSrTiO bulk and is not generated by oxygen vacancies concentrated at the interface. Samples with LaAlO thickness of 4-5 u.c. show an interfacial charge density less than 0.5electronper areal unitcelland a residual polarization of the LaAlO thin film. The direction and spatial dependence ofthe polarization allowed to conclude that the positive charge left behind by the electronsmigrated to the interfaceis located towards the surface of the LaAlO film.These are much relevant findings that rule out donor defects in SrTiO as the main source ofinterfacial conductivity and provide an unprecedented experimental evidence that in atomically abrupt LaAlO-SrTiO interfaces the2DEG is generated by an electronic reconstruction mechanism [Advanced Materials 24, 3952 (2012)].
The connection between the change of the orbital occupation and the metallic-insulating properties of the 2DEG as a function of the LaAlO thickness represents an important issue for the exploitation of the functionalities of such artificial structures. In this framework, the main finding is that an orbital reconstruction occurs also in insulating 2 u.c. LaAlO-SrTiO heterostructure before the appearance of the metallic state. Interestingly partial or complete inversion of the 3d-t2g energy levels in insulating samples is correlated to partial or complete coverage of the SrTiO surface by an ordered polar layer. A rumpling of the TiO and SrO atomic planes leads to a band splitting already for 2 u.c. thick heterostructures, showing that structural distortions appear in SrTiO before the insulator to metal transition. These results provide evidence that the orbital reconstruction may be due to the combination of interface symmetry breaking and of the transfer of localized electrons to the interface before the metallic state sets in[Advanced Materials 25, 2333 (2013)].
Another fundamental issue is given by the possible coexistence of superconductivity and ferromagnetism at the LaAlO-SrTiO interface. Different scenarios have been considered to address the existence of magnetic states at the interface. For instance, point-defects or oxygen vacancies may be responsible for the emergence of ferromagnetic clusters, otherwise electron-electron correlations may lead to novel quantum state in the presence of inversion symmetry breaking field at the LaAlO-SrTiO interface. Typical magnetic probes, e.g. torque magnetometry, scanning SQUID microscopy, cannot provide direct evidence that magnetism is indeed an intrinsic phenomenon related to Ti moments at the interface. By means of x-ray absorption spectroscopy (XAS) across the transition metals L-edges it has been possible to probe directly the magnetic and orbital properties of Ti at the interface. XAS performed with circularly or linearly polarized photons can detect the magnetic moments and the 3d energy splitting. The results performed on two types of artificial oxide structure, LaAlO-SrTiO and BiMnO-SrTiO allowed to demonstrate that magnetism at the interface is driven by the exchange interaction between interface states and localized moments due to Mn or Ti [Phys. Rev. Lett. 111, 087204 (2013)].
Concerning the synthesis of artificial oxide heterostructures, the main results concern the successful achievement of new fabrication routes. For instance, layer-by-layer growth of conductive interfaces has been only carried out at oxygen pressure of 10-6-10-2 mbar. Post deposition treatments have been suggested as a doable mean to decrease the amount of oxygen vacancies. However, such a post-growth process imposes some constraints in view of deposition of more complex heterostructures and multilayers. Opening the route to a higher pressure regime, where the interfaces are conducting and the growth is two-dimensional (2D), represents an important step forward to achieve novel artificial structures with peculiar interfacial electronic and magnetic phenomena. The successful control of the plume propagation allowed to grow conducting LaGaO-SrTiO interfaces at an oxygen pressure of 10-1 mbar. These results allowed to demonstrate that the sample properties critically depend on the target-to-substrate distance. The analysis of the ablation plume propagation into the background gas confirms also the direct influence of the ablation plume features on the interface properties. In particular, conductive interfaces are achieved when the substrate is located at a position where the plume is marked by a higher content of excited and oxidized species whose maximum kinetic energy is still of the order of 1 eV. In this situation the interfacial conducting properties are not dominated by oxygen defects contribution. Overcoming the constraint of low oxygen pressure growth allowed to get deeper understanding in the role and control of the various mechanism for the interface conductivity. This result opened also interesting perspectives in the understanding of the driving mechanisms for the electron gas formation at the polar-non polar interfaces, and more generally, of the growth processes of interfaces where the intrinsic electronic reconstruction has to be disentangled from the extrinsic growth effects [Applied Phys. Lett. 101, 031602 (2012)].
The advancement in materials deposited via pulsed laser deposition, homoepitaxial SrTiO3 thin films have led to the synthesis in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles. To have full access to the deposition parameters, the kinetic energy of the oncoming species is determined by exploiting plume diagnostics techniques and the resulting nucleation and growth processes are analysed by reflection high-energy electron diffraction and atomic force microscopy. The main result is that, although the kinetic energy of the oncoming species varies to a great extent, the diffusion process is mostly influenced by the stoichiometry. Under stoichiometric conditions, obtained only in a limited window of process parameters, the adatoms on the surface have the highest diffusivity, thus promoting a step-flow growth mode. Under nonstoichiometric conditions, both Sr- and Ti-rich, the diffusivity is strongly reduced. This results in a transition from a two-dimensional to a three-dimensional growth under Sr-rich conditions. Conversely, in the Ti-rich case, obtained at high laser fluence, the two-dimensional growth sustains until the end of the growth process. Such study allowed to attribute this effect to the high island density available at high laser fluence which facilitates the diffusion of adatoms to step edges despite of their reduced diffusion length [Applied Surface Science 258, 9116 (2012); Applied Phys. Lett. 103, 031607 (2013)].
Topic 3)
Concerning the topics of opto-electronic materials, light-matter interaction for analysing complex phenomena and designing novel nanostructures and functionalities, fundamental excitations and time-dependent control of collective orders in electron correlated materials, there are various results to be underlined.
Firstly, artificial heterostructures of the type LaAlO-SrTiO, LaGaO-SrTiO and NdGaO-SrTiO have been shown to exhibit similar transport upon light response, with a persistent photoconductivity showing a universal dynamics whose intensity depends on the room temperature sheet resistance of the sample rather than on the nature of the polar layer. The time dynamics indicates long time scales and a dramatic dependence of the photo response on the wavelength. Such effect in not only different quantitatively but also qualitatively, since the functional form versus time changes when the photon energy exceeds the SrTiO gap threshold. This is attributed to the emergence of different photo induced excited states with different decay dynamics under irradiation by above-gap and below-gap photons. This result brings also an interesting connection between oxide interfaces and photovoltaic junctions. Still the persistent perturbation induced by light allowed to suggest that photon-based spectroscopies might probe samples which are significantly away from their ground state. The persistent photoconductivity has been demonstrated to be related to the presence of strong interface electric fields and that the origin of photo-carriers may be linked to that of the electrons contributing to the 2DEG before illumination [Advanced Optical Materials 1, 834 (2013)].
Concerning the exploitation of the interaction between light and matter for designing functional materials, the analysis of a polymer film illuminated by light has been considered for inducing reorientation of some of the constituent molecules, as well as for the mass migration. In particular, the light-induced molecular displacement has a high potential for having writing and erasing phenomena on the sample surface. For instance, it is known in literature that the advantage of using azo-polymers in the place of sacrificial photoresists in the fabrication of silicon micro- and nano-structures arrays has been successfully demonstrated. Usually, the light-induced mass-transport appears to occur preferentially in the direction of the electric field, while there is no prediction for a sensitivity to the wavefront structure of the writing beam. The use of optical vortex beams allowed to demonstrate that the patterns of mass-displacement can depend on the structure of the wavefront. Indeed, spiral reliefs are found to be sensitive to the vortex topological charge and in particular to the wavefront handedness, thus showing that the polymer is responding not only to the intensity distribution of the light field in the focus but also to its phase. This action has been demonstrated to be due to an unusual interference between longitudinal and transverse optical field components, made possible by the symmetry breaking taking place at the sample surface. This result is particularly relevant in view of the possible application in micro- and nano-litography, allowing the design of complex patterns by fully exploiting the light wavefront as an additional control parameter [Nature Communications 3, 989 (2012)].
The fundamental excitations induced by the interaction between X-ray and matter, within processes of inelastic resonant scattering, have been modelled and analysed to unveil the nature of complex quantum states. This is the case of spin liquids. They are unconventional states of matter which typically emerge in low-dimensional systems when the long-range order is hindered by thermal fluctuations or for special lattice connectivity that frustrates the spins interaction.
While the absence of an ordered pattern makes elusive the detection of spin liquids, the probing of the excitations spectrum can unveil their fundamental character. For a three-dimensional spin liquid on hyperkagomè, i.e. in the family of iridate oxides the Na4Ir3O8compound, the observed features largely deviate from those expected in one dimensional Heisenberg system. The analysis of the spinon Fermi surface and the magnetic response of three-dimensional spin liquid states when the time reversal symmetry can be broken due to the nontrivial phase shifts acquired by the spinons moving along loops within the unit cell allowed to demonstrate that the spinon Fermi surface has a topological transition as a function of the generated flux intensity. The main result is that such changes affect the spin dynamical structure factor in such a way that the gapless regions of the magnetic response occurring at specific symmetry points in the Brillouin zone depend on the intensity of the symmetry breaking flux. The spectrum is strongly linked to the character of the quantum spin liquid state with dominant peaks for a time reversal invariant configuration which get dispersing when nonzero flux phases are included. These results are relevant fingerprints which can be exploited to unveil the elusive nature of spin liquids by probing their magnetic excitations [Phys. Rev. B 88, 144422].
Finally, the use of optical second harmonic generation (SHG) spectroscopy has been successfully applied for shedding light on the nature of interfaces and atomic termination of functional materials, with special emphasis to the physics of LaAlO-SrTiO type of heterostructures. The SHG signal requires a breaking of the inversion symmetry and thus is an ideal tool for investigating the polar asymmetry developed at an interface. Compared to standard optical techniques, an advantage of SHG is that its probing depth is not a priori fixed, but depends on the space extension of the polar asymmetry at the specific interface. Therefore it only senses the interfacial property of interest here with background-free capability. SHG spectra from LaAlO-SrTiO interfaces showed unprecedented sensitivity on the interfacial termination. Whereas for the TiO2-terminated surface the intensity and spectral distribution of the SHG signal indicated the presence of charge injection at the interface, in the case of SrO termination a spectrally selective and very efficient suppression of SHG yield reveals that no injection of either mobile or localized charge occurs (with the possible exception of the first atomic plane). Hence, the lack of conduction in SrO-terminated LAO/STO interfaces cannot be ascribed to a disorder-induced charge localization effect [Phys. Rev. B 88, 035405 (2013)].
Potential Impact:
Concerning the potential impact and the main dissemination activities and exploitation of results, the activities of the MAMA Project succeeded in strengthening the human potential, the interaction with all the main stakeholders, the synergy with the science education and the regional productive sector.
Human potential
Eight researchers were recruited specifically for this project (of which four were female researchers and four male researchers).
All researchers contributed their expertise to unlock SPIN research potentials. Moreover, we were able to contribute in building their career in science, as well as preventing the brain drain and build a critical mass to develop research in innovative materials in the Campania Region. As shown in the table below, four researchers obtained a permanent position in SPIN within the lifetime of the project and four are hired with research fellowships and are associated to SPIN: Procolo Lucignano [CNR-SPIN: Permanent position since Oct 2011], Rosalba Fittipaldi [CNR-SPIN: Permanent position since Oct 2012], Paola Gentile [CNR-SPIN: Permanent position since Dec 2013], Antonio Ambrosio [CNR-SPIN: Permanent position since Dec 2013], Fabio Chiarella [CNR-SPIN: Temporary researcher recruited under the SMARTAGS project], Emiliano Di Gennaro [University of Naples: 5 years Tenure Track, associated to CNR-SPIN], Filomena Forte [University of Salerno: CNR-SPIN researcher, to be post-doc, associated to CNR-SPIN], Gabriella De Luca [University of Naples: Post-doc, associated to CNR-SPIN].
Interaction with stakeholders
The interaction with the main stakeholders (SMEs, Scientific community, Regional and National authorities, EU, etc.) has been developed in order to highlight the achievements in functional materials, concerning synthesis, analysis, and modelling) towards the areas of Information and communication technology (ICT) and Energy. In particular, the main issues faced within the ICT framework relates to spintronics, sensors, oxides based logic memory elements, and materials for topological quantum computation. Within the Energy thematic area, the focus has been on oxides for energy harvesting, themoelectrics, superconducting devices for low power electronics, and novel materials for photovoltaic.
Synergies with Science Education
These activities aimed at increasing awareness among stakeholders and general public.
The dissemination activities targeted to the general public consisted in measures towards high school students within the framework of the "Lauree scientifiche" project (Scientific university education). The activity took place in Fall-Winter 2012-2013 and included a plan of visits to the MAMA Project labs to allow high school students to experience hands-on activities concerning the synthesis and characterization of functional materials in the form of single crystals. Such project was conducted in tight collaboration with high school as well as university professors with the aim of informing students of the Salerno Area willing to begin a university curriculum in physics. A similar plan of visits was performed at the MAMA Project labs in Naples during the Fall-Winter 2012-2013 with demonstration of the facilities for design of thin films heterostructures based on oxides materials and on the application of different advanced probing techniques to control the quality of the materials and the optical properties.
Synergies with the Regional Productive Sector
Within the framework of the MAMA project, SPIN established interactions with industries and SMEs of the Campania Region.
Active interactions
- Commercial activities for high-quality control of materials for production through non damaging advanced microscopy and spectroscopy
Main opportunities
- Identification of potential industrial partners (e.g. European Microfusion Aerospace)
- SMEs involved in networking activities
- New industrial partners (within the SMARTAGS project)
The main technological paths identified by the Campania Region are: Aerospace [aeronautics, maintenance and transformation, space and vectors, propulsion], Energy [smart and micro grid, energy saving and environmental sustainability, renewable energy, biochemical, management and valorization of water resources], Logistics and Transports [automotive, railways, logistics, technological systems for mobility], energy efficient buildings, sustainable access, conservation techniques, sustainable materials, urban and local security, monitoring], new Materials and nanotechnologies [advanced materials, transports, energy saving, ICT, human health], Life sciences [bio-nanotechnologies, nutrogenomoics, biomedical engineering, food environmental and economic efficiency, quality and intergrity of the supply chain, diagnostics, new drugs, e-health]. The scientific and technological challenges in the field of nanosciences, nanotechonologies, materials and new production technologies, are strongly related to the reinforcement of the activities covered by the MAMA Project, namely those linked with the growth, the analysis, and the control, in the sense of physical properties, of novel functional materials of high quality.
Nevertheless, it is widely accepted that there is a low level of innovation demand in the entrepreneurial system of the Regions of Convergence. The system of production in the Region is greatly made up of SME that develop their actions in the sectors considered as traditional and in general not related to the high-tech. Moreover, in this area, there is a limited number of enterprises, which are a branch of trans-national big companies, but whose R&D activities are mainly performed in their headquarters. In addition, in the Campania Region the relationship between the enterprises and the research centres are not well established, and the transfer of know-how is almost absent.
A crucial aspect identified by the MAMA project is related to the capability of transferring the competition towards the best parts of the market playing more on the quality and the performance of the products than on the costs. In doing that, we can have an impact at Regional level by favoring, at the same time, the differentiation of the activities and in turn the support for the creation of new enterprises closely related to the thematic areas of the scientific research. Innovation in the field of material science have strong influences on transversal sectors of the economy and thus can represent a valid tool to sustain the quick transformations that follow the market demand.
Activities to Promote Awareness on European Funding Opportunities
The dissemination activity devoted to increase awareness on European funding opportunities for research in the new framework programme Horizon 2020 consisted in a one-day open initiative that took place on February 25, 2013 entitled: Horizon 2020 Regional dimension of research within the future funding programme and cohesion policy. The presentation was held by Andrew Bianco, Project Officer, from the DG Research. The event took place at Salerno University Campus and was attended by over 100 researchers and administrative staff, mainly form SPIN units of Naples and Salerno and from the University of Salerno.
Use and dissemination
Articles published in peer reviewed journals: Overall 70 articles published since September 2010. There are also
Presentations to conferences: During the lifetime of the project, researchers, supported by MAMA, participated at:
• 32 Conferences as invited speaker
• 2 Conferences for abstract oral presentations
• 3 Networking events
Researchers were invited to give talks at various Institutions in Europe and in USA.
List of Websites:
Project website: http://mama.spin.cnr.it/
Institute website: http://www.spin.cnr.it/
MAMA project was founded on two scientific concepts key for materials physics: i) functional materials (mainly transition-metal oxides [TMOs]), ii) complex physical phenomena (e.g. superconductivity, magnetism, ferroelectricity, etc.). TMOs represent a large family of compounds where the complex interplay between lattice (strain, phonons) and electronic (charge, orbital, spin) degrees of freedom leads to unique states of matter which can exhibit gigantic response upon small applied perturbations. TMOs is therefore a fertile platform for investigating and exploiting a wide range of intricate phenomena such as high temperature superconductivity, ferroelectricity, magnetism, colossal magneto resistance, thermo electricity, half-metallic and multiferroic behaviour, two-dimensional electron liquids, anomalous metal-to-Mott transitions, quantum topological insulator or superconductors, etc. Furthermore, TMOs can be stacked in multilayer heterostructures that, on one hand, allow to combine in a single device the functionalities of their individual layers, and, on the other hand, can generate an even wider range of emergent novel properties, that can be surprisingly different from those of the individual building blocks. All such elements are at the core for the generation of innovative paths and solutions in the field of functionalities and advanced materials.
The MAMA project was successful in increasing the research capacity building within CNR-SPIN in the area of multifunctional materials (based on TMOs and hybrids) and the related nanoscale phenomena by: 1) recruiting highly qualified personnel, 2) acquiring powerful instrumentation for fabricating, characterizing, and modelling the materials targeted by our research, 3) by mobilizing and supporting know-how exchange between CNR SPIN researchers and European centres leader in our field. The European partnerships played a crucial role in the MAMA project. We involved 11 prestigious groups covering all areas of synthesis, analysis and modelling of complex materials: “Correlated Electron Systems and Oxide Physics” at the University of St. Andrews, “Magnetic and Superconducting Materials” at Leiden Institute of Physics, “Advanced Inorganic Materials” at the University of Twente, “New phenomena and Novel Functionalities in Artificial Oxide materials” and “Quantum electronics” at the DPMC Geneve, “Photonic Engineering” at the Risø National Laboratory Roskilde , “Nonlinear Magneto-optics” at the ETH Zurich University, “Nanoscience and Nanotechnology” at the IMDEA Madrid, “Microtechnology and Nanoscience” at the University of Chalmers, “Magnetism and Superconductivity” at the University of Warwick, “Theoretical Solid State Physics” at the Leibniz Institute of Dresden, “Realistic Theory of Strongly Correlates Systems” at the Institute for Advanced Simulation of Jülich.
In general, the actions developed within the MAMA consortium aimed at set up the foundations to overcoming the issue of sustainability in terms of new critical mass, of the growth of the research potential as well as of the need of maintaining international collaborations in order to effectively respond to rapid scientific changes. Concerning human potential, the recruitment actions have been successful in terms of group building and stability. This is demonstrated by the positive result of having four of the eight recruited researchers as permanent staff within the CNR-SPIN. Also, the level of international partnerships has been successfully strengthened and consolidated thanks to the intense program of know-how exchange. As result of such activities, The successful activities in this direction within the MAMA project allowed to place CNR-SPIN as a leader in coordinating a COST Action on synthesis, analysis and modelling of functional oxide materials which involves all the MAMA partners as well as virtually all European groups active in the field with the aim of strengthen the research within the European Research Area (ERA). In addition, CNR-SPIN groups actively participated and received project funding both in National and European competitive calls. The activity of developing innovative ideas for project involvement has been much intense within the MAMA project benefiting of the wide range of partnerships and expertise. This represents an essential asset to sustain and stimulate the successful participation of CNR-SPIN to the ERA.
Project Context and Objectives:
Multifunctional materials typically perform specific functions other than possessing a load bearing capacity. In this context, transition metal oxides (TMOs) are prototypes for multifunctional behaviour due to the wide variety of physical properties that they exhibit, including unconventional superconductivity, piezo- and ferro-electricity, colossal magneto resistance, multiferroicity and a number of exotic magnetic, charge and orbital orderings. Furthermore, they can be employed to develop oxide interface having properties at the nanometer scale that are qualitatively different from their single building blocks, allowing to engineer novel functionalities by benefiting, for instance, of the controlled growth of epitaxial heterostructures. The integration of layers of functional organic materials, often showing similarities with oxides in their transport mechanisms, represents an ultimate and even more ambitious challenge. Such features are believed to open the route to the fabrication of device prototypes where multiple functionalities of TMOs and functional organic layers are nano-integrated on the same chip. The range of application sectors is correspondingly large especially regarding the area of information and communications technology,as well as the material solutions forenergy generation, storage and transport.
Within the MAMA project the CNR-SPIN pointed to the highest level of competitiveness about issues of i) materials fabrication, by addressing the growth of very high quality samples in the different forms of epitaxial thin films and single crystals, also integrated together in complex heterostructures and ii) advanced material characterizations, both based on matter-light interaction (by resorting either to advanced laser applications or to synchrotron sources), on scanning probe techniques and on electromagnetic transport, as well as about iii) theoretical modelling and advanced computation to analyze and get insight into different physical properties of innovative materials. In order to tackle these issues, MAMA aimed at exploiting the available partnerships’ expertise and experimental endowment, complemented by the new resources.
The MAMA project aimed at reinforcing the research potential by mobilizing and supporting the human and material resources with excellent centres operating in the field of the synthesis, analysis, and modelling of physical properties for multi-functional materials (WP2), by recruiting new personnel (WP3), by increasing SPIN visibility and socioeconomic impact through targeted dissemination actions (WP5) and through the organization of three workshops and one conference (WP4) and through the acquisition of powerful tools for fabricating and characterizing the materials at the core of the research targets (WP6). Such requirement has been planned in the view of a synergic growth of the Institute’s research potential both in the experimental and theoretical areas. Indeed, only a reinforcement that involves all the CNR-SPIN research components will permit an harmonious achievement of the scientific objectives.
The MAMA activities refer to three main objectives inherent to 1) fabrication, 2) characterization and 3) computation of multifunctional advanced materials.
1) Fabrication: to reinforce research potential about the synthesis of multi-functional materials
With specific targets such as:
• Quality controlled samples
• Improve capacities for tailoring phenomena in innovative multi-functional materials. Develop specific expertise for a multi-scale or integrated approaches for the fabrication of functional materials.
Synthesis of multi-functional materials concerns different techniques and expertise for materials in the shape of bulk single crystals, thin-films and nano-particles. To obtain single crystals having high quality in the sense of defects and impurity level control, image furnaces are required to perform the so called floating-zone or the travelling-solvent floating-zone method. Such kind of equipment permits the realization of large samples whose dimensions can be of the order of few cm3 with a very low degree of contamination by spurious elements. The objective targeted under the MAMA project was to increase the competences to execute the processes for sample growth both for the general steps and for those which can be more dependent on the specific materials uponinvestigation. Hence, the exchange of know-how and experience on the basics of the techniques and the procedures for fabricating and manipulating the samples was of fundamental importance.
Concerning the nanostructures, the control in the growth of oxide thin films is not only essential for implementing most practical applications, but also allows to disclose new phenomena, as often reported in the case of epitaxial ultrathin films and of nanoparticle films, showing electronic and magnetic properties that are qualitatively different from the bulk material. An even wider field are interfaces in oxide epitaxial heterostructures, that can show properties that are qualitatively different from the single “building blocks”. In order to push the control in the growth of epitaxial thin films and heterostructures to the level of atomic engineering, an unprecedented capability in monitoring the growthis needed . Such aim was attained both by implementing advanced characterization techniques in the deposition system, for real-time monitoring of the process, and by analysing the as grown samples (possibly also during intermediate stages of growth) with a fast ultra-high-vacuum transfer to suitable analysis chambers. This concept has led to the design and to the implementation of the “Modular facility for Oxide Deposition and Analysis” (MODA) at CNR-SPIN.
2) Characterization: to enhance research capacities for analysis of multi-functional materials with specific targets as:
• Reinforcing the space, time, momentum and energy resolved approach for the analysis of innovative materials
• Enhance probing capacities at the mesoscopic and nanoscopicscale
The main target is to strengthen the capacity to perform different types of analyses on solid state materials: morphological, structural and compositional studies, electro-magnetic investigation, as well as spectroscopic, optical , etc.
While in the case of single crystals only ex-situ analysis can be considered, in the case of thin film samples characterization can take place either during growth (RHEED, fast photography of the plume), or within the same ultra-high-vacuum system (XPS, UPS, SPA-LEED and STM/AFM), or ex-situ (transport measurements at variable temperature and frequency in applied field, measurements performed within the scanning electron microscope, optical spectroscopies, X-ray diffraction and other characterizations, also performed within the partner’s laboratories). The first two kinds of analyses are performed within the MODA facility, where the fabrication and characterization aspect are closely intermixed. This is a crucial aspect for the multi-scale control of the synthesized materials.
A specific objective concerns the increase of research capacity inperforming micro- and nano-scale electrical characterization of materials and devices. The acquisition of a Scanning Electron Microscope (SEM) with ultra-high resolution imaging and microscopic probes significantly improved the research capacity for surface layer transport measurement, interconnect probing, and characterization of inhomogeneous or microstructured multi-functional advanced materials: nano-wires and tubes, ultra-thin films and mono-layer, electronic surface states, device interconnects, single and poly-crystals, organic materials and so on.
3)Computation: to enhance capacities for computational analyses and modelling of multi-functional advanced materials
With specific targets as:
• Need of improving computational multi-scale approaches to get insight into different physical properties of innovative materials
• Enhance capacities for ab-initio modeling of oxides and of heterostructures
• Improve research potential towards phenomenological and microscopic modelling of oxides and of heterostructures
The state of the art concerning the properties modelling of correlated systems and of heterostructures covers many topics and theoretical approaches in the area of quantum many-body systems as well as a high expertise for investigating the emerging complex electronic quantum structure. The present objective consists in acquiring competences about different theoretical and computational approaches for describing electronic and magnetic as well as spectroscopic properties of multi-functional materials benefiting from the international cross sharing information. This activity referred to the acquisition of competence to allow to perform realistic calculations of the full quantum structure. This implies the achievement of a know-how of the state-of-the-art of ab-initio methods based on density functional theory and other approaches that can suitably include the electron correlations or the coupling between electrons and vibrational modes.
The fabrication of multi-functional advanced materials is generally followed by the most powerful spectroscopic tools to study the resulting quantum structure. These tools include low energy electron diffraction, scanning tunnelling microscopy and spectroscopy, core level X-ray photoemission, Raman, optical, and angle resolved photoemission spectroscopy. The combination of these methods gives direct experimental access to the surface chemistry, crystallographic structure, and the electronic structure. The major objective in this framework was to enlarge the research potentiality for facing complex problems with integrated schemes of theoretical computations and through the synergy of them to provide a constructive feedback on the spectroscopic characterizations by isolating the basic aspects needed for the analysis.
Project Results:
The main S&T results related to the MAMA activities refer to different thematic areas of functional materials, based on transition metal oxides and hybrids, with special emphasis on the potential applications derived by the exploitation of the large variety of exhibited complex phenomena.
In particular, we can identify three areas of scientific impact of the achieved results with marks to specific topics:
1) Superconducting materials and hybrids, emergent quantum-topological states, quantum topological computation and new material solutionsbased on superconductorsfor information data storage and manipulation.
2) Electron correlated materials and heterostructures/interfaces based on transition metal oxides, novel phases of matter driven by spin-orbit coupling, the synthesis of oxide materials for novel electronics.
3) Opto-electronic materials, light-matter interaction for designing novel nanostructures and functionalities, fundamental excitations and time-dependent control of collective phenomena in electron correlated materials.
Hereafter we highlight few of the main scientific results related to the three mentioned topics.
Topic 1)
The main results in this area concern the study of superconducting heterostructures interfaced to ferromagnet, superconducting edge states with anomalous magnetic properties, and topological superconducting and/or insulating systems. These results have been obtained with the aim of developing, inthe long-term, novel materials for spintronics and topological quantum computation.
The basic concept of spintronics materials is that, unlike conventional electronic devices, which transmit information via the charge carried by an electron, theycan exploit the “spin”, i.e. intrinsic angular momentum of the electron.
Spintronics involves manipulating the spin degree of freedom to perform logic operations in devices. There is, however, a shortcoming: any such device requires a large spin current to operate, which in itself requires the use of a large electrical charge. Since the spin currents are dissipative, a large fraction of the input energy is wasted as heat. Superconductors are materials which, when cooled below a certain temperature, can carry a current without losing energy, thus provide one potential solution to this. If these materials could be inserted in spin-based devices, an energy-efficient source for the spin currents could be provided. In this direction, the solutions analysed within MAMA are made of heterostructures with superconductors (with spin-singlet or spin-triplet pairs) interfaced to magnetic systems (e.g. ferromagnets, helimagnet, antiferromagnet, etc.).
The system with spin-triplet superconductors are particularly attractive because the charge supercurrents would intrinsically carry a spin current too. In this framework, a novel form of interaction between spin and orbital degreesof freedom has been found in a spin-triplet superconductor (TSC)- ferromagnet (FM) heterostructure. In the TSC-FM system the orientation of the FM moment relative to the TSC vector order parameter is a crucial variable that controls the physical behavior. In addition to the pair breaking, spin-flip reflection processes at the interface with the FM scatter the triplet Cooper pairs between the spin up and down condensates, setting up an effective Josephson-like coupling between them. The pair-breaking and spin Josephson coupling both make significant contributions to the free energy of a TSC-FM junction through the proximity effect, interface electronic reconstruction, and the variation of the TSC gap. Although these contributions depend upon the direction of the FM's exchange field, the two effects do not necessarily act constructively. For a single-component p-wave TSC, we find that the variation of the gap controls the orientation of the FM's moment via the change in condensation energy. The stable configuration is either parallel or perpendicular to the TSC vector order parameter, depending on the alignment of the TSC gap with respect to the interface, thus evidencing a unique form of spin-orbital coupling. Otherwise, the competing orbital components of the chiral state generate a non-unique behavior and a first-order transition from the perpendicular to the parallel configuration as the FM exchange field is increased. These results highlight the potential of manipulating spin supercurrent by exploting the orbital character of the superconducting pairing [Phys. Rev. Lett. 111, 097003 ( 2013)].
Concerning the issue of superconducting edge states with anomalous magnetic properties, the recognition that the surface states in correlated materials reflect the nature of the interactionsand orders in the bulk is the key aspect for the understanding, and potential control, of these electronic states. Gapless modes at the boundary ofmaterials whose bulk is gapped are especially interestingsince the surface states are robust and may be topologicallyprotected; i.e. their existence relies on the global symmetries of the bulk state and does not depend on the details ofthe surface scattering and other sample-dependent parameters. The bulk gap may be due to the band structure, or,in a metal, may arise from electron-electron interaction, as in superconductors. Simple band insulators or conventional superconductors do not support robust low-energystates at the boundary. In this context, one of the main results is that the interference between singlet and triplet pairs can generate magnetic states at their boundary. Such magnetic effects can occur if triplet and singlet pairing get mixed and have a non-uniform spatial profile or by suitably controlling their phase difference. As a result, the bound states can be spin-polarized, leading to a finite surface magnetization, and still, spin current can flow along the interface while surface charge currents exhibit anomalous dependence on the magnetization [Phys. Rev. Lett. 110, 267002 (2013)].
Quantum topological computation relies on the possibility of having materials platform that can exhibit non-Abelian modes. The emergence of particles that exhibit a richer exchange statistics than the constituent electrons and ions in a material is among the most remarkable manifestation of “more is different.”Such particles fall into two broad categories: Abelian and non-Abelian. Interchanging Abelian anyons alters the system’s wave function by aphasethat is intermediate between the case of thebosons and the fermions. Non-Abelian anyons are more interesting because the exchange rotates the system quantum stateamong a degenerate set of locally indistinguishable ground states produced by the anyons. Non-Abelian anyons are still extremely interesting modes because they are fundamental ingredients to fault-tolerant topological quantum computation. The qubits are embedded in the system’s ground states and, due to the non-Abelian statistics, manipulated through anyon exchanges. The non locality with which the information is stored and processed intrinsically leads to immunity against decoherence stemming from local environmental perturbations. The challenge is then to identifying suitable platforms for non-Abelian excitations.
In this framework, one dimensional spin-triplet superconductors can have topological superconducting phase whose domain walls in the superconductor bind Majorana zero modes and realize confined Isinganyons with non-Abelian statistics. The challenge here is to find such nontrivialone-dimensional (1D) and two dimensional (2D) superconductors and to testproposals forengineering these phases in heterostructures fashioned from ingredients such as topological insulators, semiconductors, and spin-singlet superconductors.
One of the main finding in this context concerns the synthesis and the study of materials that can have spin-triplet superconductivity as well as the identification of hybrid platform where non-abelian states can occur. The system Sr2RuO4 is a good candidate for spin-triplet superconductivity. This material and the related heterostructures have been successfully synthesized. Relevant results about the property of such oxide superconductor are on the surface and bulk electronic structure via angle resolved photoemission study. Circularly polarised light has been used to disentangle the signals from the bulk and surface layers, thus opening the possibility of investigating many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting ofthe surface band. The outcome has underlined the role of spin-orbit coupling and the possible connection to the nature of the topological matter [New Journal of Physics 14, 063039 (2012)]. The hallmark of Sr2RuO4superconductivity is the presenceof time-reversal symmetry breaking (TRSB), a phenomenon most likely arising from the chiralp-wave structure of theCooper pairs. The possibility of synthesizing eutectic (i.e. mixed) systems, representing naturally occurring nanoscopic interfaces, has opened newroutes for testing the symmetry of the pairing wave functionin the proximity of normal or magnetic systems, as well as in searching for novel quantum configurations that could emerge at the interface between the embedded phases. The successful synthesis of eutectic oxides of the form Sr2RuO4- Sr3Ru2O7 led to the discovery of a superconducting state with time-reversal symmetry breaking due to the occurring at 2.5 K (i.e. well above1.5 K, the critical temperature of the Sr2RuO4 single phase)[Phys. Rev. B 85, 134527 (2012)].
The search for topological nontrivial quantum modes has been also pursued with spin-singlet high temperature superconductors with d-wave symmetry such as those of the family of the cuprates, e.g. LaSrCuO, BaSrCuO, etc.. A major finding is that aπ−Josephson Junction, inserted in a topologically non trivial model ring, sustains a Majoranabound state, which is robust with respect to localand non local perturbations. Therealistic structure could be based on high temperature superconductors andtricrystal structure, similar to theone used to spot the d-wave order parameter. The presence of the Majorana bound state changesthe ground state of the topologically non trivial ring in a measurable way, with respect to that of aconventional one. This is an important issue for detecting the topological mode [Phys. Rev. B 88, 184512 (2013)].
Topic 2)
Concerning the correlated heterostructures and interfaces based on transition metal oxides the main results refer to the synthesis, analysis, manipulation and modelling of different oxide systems, with special focus on those of the type LaAlO-SrTiO.
These artificial thin-film oxide structures are among the most investigated systems in condensed matter especially becausethe atomic-scale engineering of such interfaces holds a great promise: theability to manipulate electrons by locally modifying orbitals shape and occupation, yieldingnew quantum states that can be exploited in novel electronic devices. If interfaces of semiconductors, which are rather featureless materials with a non magnetic rigid lattice, can nonetheless lead to fascinating physics, such as the quantum Hall effect, then the oxides have a huge potential. A variety of exotic low dimensional electronic systems can be achieved at the oxide interfaces, exploiting spin, charge, and orbital interactions as well as lattice vibrations.
This is the motivation for intense investigation of the interface between band insulators, e.g. LaAlO and SrTiO, which hosts a high mobility two -dimensional electron gas (2DEG)in spite ofthe large band gap of its bulk constituents, and exhibits intriguing transport, magnetic and superconducting properties.
One of the main questions addressed is about the mechanism for the generation of the 2DEG at the interface. Can defect-generated electrons, due to disorder or other extrinsic sources,explain all the observed properties of LaAlO-SrTiO? By employing electron energy loss spectroscopy(EELS) it has been possible to present evidence that nearly perfect LaAlO-SrTiO interfaces with negligible cation intermixing can be fabricated by pulsed laser deposition (PLD). In these samples the interfacial chargedensity decays over~3 unit cells (u.c.) within theSrTiO bulk and is not generated by oxygen vacancies concentrated at the interface. Samples with LaAlO thickness of 4-5 u.c. show an interfacial charge density less than 0.5electronper areal unitcelland a residual polarization of the LaAlO thin film. The direction and spatial dependence ofthe polarization allowed to conclude that the positive charge left behind by the electronsmigrated to the interfaceis located towards the surface of the LaAlO film.These are much relevant findings that rule out donor defects in SrTiO as the main source ofinterfacial conductivity and provide an unprecedented experimental evidence that in atomically abrupt LaAlO-SrTiO interfaces the2DEG is generated by an electronic reconstruction mechanism [Advanced Materials 24, 3952 (2012)].
The connection between the change of the orbital occupation and the metallic-insulating properties of the 2DEG as a function of the LaAlO thickness represents an important issue for the exploitation of the functionalities of such artificial structures. In this framework, the main finding is that an orbital reconstruction occurs also in insulating 2 u.c. LaAlO-SrTiO heterostructure before the appearance of the metallic state. Interestingly partial or complete inversion of the 3d-t2g energy levels in insulating samples is correlated to partial or complete coverage of the SrTiO surface by an ordered polar layer. A rumpling of the TiO and SrO atomic planes leads to a band splitting already for 2 u.c. thick heterostructures, showing that structural distortions appear in SrTiO before the insulator to metal transition. These results provide evidence that the orbital reconstruction may be due to the combination of interface symmetry breaking and of the transfer of localized electrons to the interface before the metallic state sets in[Advanced Materials 25, 2333 (2013)].
Another fundamental issue is given by the possible coexistence of superconductivity and ferromagnetism at the LaAlO-SrTiO interface. Different scenarios have been considered to address the existence of magnetic states at the interface. For instance, point-defects or oxygen vacancies may be responsible for the emergence of ferromagnetic clusters, otherwise electron-electron correlations may lead to novel quantum state in the presence of inversion symmetry breaking field at the LaAlO-SrTiO interface. Typical magnetic probes, e.g. torque magnetometry, scanning SQUID microscopy, cannot provide direct evidence that magnetism is indeed an intrinsic phenomenon related to Ti moments at the interface. By means of x-ray absorption spectroscopy (XAS) across the transition metals L-edges it has been possible to probe directly the magnetic and orbital properties of Ti at the interface. XAS performed with circularly or linearly polarized photons can detect the magnetic moments and the 3d energy splitting. The results performed on two types of artificial oxide structure, LaAlO-SrTiO and BiMnO-SrTiO allowed to demonstrate that magnetism at the interface is driven by the exchange interaction between interface states and localized moments due to Mn or Ti [Phys. Rev. Lett. 111, 087204 (2013)].
Concerning the synthesis of artificial oxide heterostructures, the main results concern the successful achievement of new fabrication routes. For instance, layer-by-layer growth of conductive interfaces has been only carried out at oxygen pressure of 10-6-10-2 mbar. Post deposition treatments have been suggested as a doable mean to decrease the amount of oxygen vacancies. However, such a post-growth process imposes some constraints in view of deposition of more complex heterostructures and multilayers. Opening the route to a higher pressure regime, where the interfaces are conducting and the growth is two-dimensional (2D), represents an important step forward to achieve novel artificial structures with peculiar interfacial electronic and magnetic phenomena. The successful control of the plume propagation allowed to grow conducting LaGaO-SrTiO interfaces at an oxygen pressure of 10-1 mbar. These results allowed to demonstrate that the sample properties critically depend on the target-to-substrate distance. The analysis of the ablation plume propagation into the background gas confirms also the direct influence of the ablation plume features on the interface properties. In particular, conductive interfaces are achieved when the substrate is located at a position where the plume is marked by a higher content of excited and oxidized species whose maximum kinetic energy is still of the order of 1 eV. In this situation the interfacial conducting properties are not dominated by oxygen defects contribution. Overcoming the constraint of low oxygen pressure growth allowed to get deeper understanding in the role and control of the various mechanism for the interface conductivity. This result opened also interesting perspectives in the understanding of the driving mechanisms for the electron gas formation at the polar-non polar interfaces, and more generally, of the growth processes of interfaces where the intrinsic electronic reconstruction has to be disentangled from the extrinsic growth effects [Applied Phys. Lett. 101, 031602 (2012)].
The advancement in materials deposited via pulsed laser deposition, homoepitaxial SrTiO3 thin films have led to the synthesis in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles. To have full access to the deposition parameters, the kinetic energy of the oncoming species is determined by exploiting plume diagnostics techniques and the resulting nucleation and growth processes are analysed by reflection high-energy electron diffraction and atomic force microscopy. The main result is that, although the kinetic energy of the oncoming species varies to a great extent, the diffusion process is mostly influenced by the stoichiometry. Under stoichiometric conditions, obtained only in a limited window of process parameters, the adatoms on the surface have the highest diffusivity, thus promoting a step-flow growth mode. Under nonstoichiometric conditions, both Sr- and Ti-rich, the diffusivity is strongly reduced. This results in a transition from a two-dimensional to a three-dimensional growth under Sr-rich conditions. Conversely, in the Ti-rich case, obtained at high laser fluence, the two-dimensional growth sustains until the end of the growth process. Such study allowed to attribute this effect to the high island density available at high laser fluence which facilitates the diffusion of adatoms to step edges despite of their reduced diffusion length [Applied Surface Science 258, 9116 (2012); Applied Phys. Lett. 103, 031607 (2013)].
Topic 3)
Concerning the topics of opto-electronic materials, light-matter interaction for analysing complex phenomena and designing novel nanostructures and functionalities, fundamental excitations and time-dependent control of collective orders in electron correlated materials, there are various results to be underlined.
Firstly, artificial heterostructures of the type LaAlO-SrTiO, LaGaO-SrTiO and NdGaO-SrTiO have been shown to exhibit similar transport upon light response, with a persistent photoconductivity showing a universal dynamics whose intensity depends on the room temperature sheet resistance of the sample rather than on the nature of the polar layer. The time dynamics indicates long time scales and a dramatic dependence of the photo response on the wavelength. Such effect in not only different quantitatively but also qualitatively, since the functional form versus time changes when the photon energy exceeds the SrTiO gap threshold. This is attributed to the emergence of different photo induced excited states with different decay dynamics under irradiation by above-gap and below-gap photons. This result brings also an interesting connection between oxide interfaces and photovoltaic junctions. Still the persistent perturbation induced by light allowed to suggest that photon-based spectroscopies might probe samples which are significantly away from their ground state. The persistent photoconductivity has been demonstrated to be related to the presence of strong interface electric fields and that the origin of photo-carriers may be linked to that of the electrons contributing to the 2DEG before illumination [Advanced Optical Materials 1, 834 (2013)].
Concerning the exploitation of the interaction between light and matter for designing functional materials, the analysis of a polymer film illuminated by light has been considered for inducing reorientation of some of the constituent molecules, as well as for the mass migration. In particular, the light-induced molecular displacement has a high potential for having writing and erasing phenomena on the sample surface. For instance, it is known in literature that the advantage of using azo-polymers in the place of sacrificial photoresists in the fabrication of silicon micro- and nano-structures arrays has been successfully demonstrated. Usually, the light-induced mass-transport appears to occur preferentially in the direction of the electric field, while there is no prediction for a sensitivity to the wavefront structure of the writing beam. The use of optical vortex beams allowed to demonstrate that the patterns of mass-displacement can depend on the structure of the wavefront. Indeed, spiral reliefs are found to be sensitive to the vortex topological charge and in particular to the wavefront handedness, thus showing that the polymer is responding not only to the intensity distribution of the light field in the focus but also to its phase. This action has been demonstrated to be due to an unusual interference between longitudinal and transverse optical field components, made possible by the symmetry breaking taking place at the sample surface. This result is particularly relevant in view of the possible application in micro- and nano-litography, allowing the design of complex patterns by fully exploiting the light wavefront as an additional control parameter [Nature Communications 3, 989 (2012)].
The fundamental excitations induced by the interaction between X-ray and matter, within processes of inelastic resonant scattering, have been modelled and analysed to unveil the nature of complex quantum states. This is the case of spin liquids. They are unconventional states of matter which typically emerge in low-dimensional systems when the long-range order is hindered by thermal fluctuations or for special lattice connectivity that frustrates the spins interaction.
While the absence of an ordered pattern makes elusive the detection of spin liquids, the probing of the excitations spectrum can unveil their fundamental character. For a three-dimensional spin liquid on hyperkagomè, i.e. in the family of iridate oxides the Na4Ir3O8compound, the observed features largely deviate from those expected in one dimensional Heisenberg system. The analysis of the spinon Fermi surface and the magnetic response of three-dimensional spin liquid states when the time reversal symmetry can be broken due to the nontrivial phase shifts acquired by the spinons moving along loops within the unit cell allowed to demonstrate that the spinon Fermi surface has a topological transition as a function of the generated flux intensity. The main result is that such changes affect the spin dynamical structure factor in such a way that the gapless regions of the magnetic response occurring at specific symmetry points in the Brillouin zone depend on the intensity of the symmetry breaking flux. The spectrum is strongly linked to the character of the quantum spin liquid state with dominant peaks for a time reversal invariant configuration which get dispersing when nonzero flux phases are included. These results are relevant fingerprints which can be exploited to unveil the elusive nature of spin liquids by probing their magnetic excitations [Phys. Rev. B 88, 144422].
Finally, the use of optical second harmonic generation (SHG) spectroscopy has been successfully applied for shedding light on the nature of interfaces and atomic termination of functional materials, with special emphasis to the physics of LaAlO-SrTiO type of heterostructures. The SHG signal requires a breaking of the inversion symmetry and thus is an ideal tool for investigating the polar asymmetry developed at an interface. Compared to standard optical techniques, an advantage of SHG is that its probing depth is not a priori fixed, but depends on the space extension of the polar asymmetry at the specific interface. Therefore it only senses the interfacial property of interest here with background-free capability. SHG spectra from LaAlO-SrTiO interfaces showed unprecedented sensitivity on the interfacial termination. Whereas for the TiO2-terminated surface the intensity and spectral distribution of the SHG signal indicated the presence of charge injection at the interface, in the case of SrO termination a spectrally selective and very efficient suppression of SHG yield reveals that no injection of either mobile or localized charge occurs (with the possible exception of the first atomic plane). Hence, the lack of conduction in SrO-terminated LAO/STO interfaces cannot be ascribed to a disorder-induced charge localization effect [Phys. Rev. B 88, 035405 (2013)].
Potential Impact:
Concerning the potential impact and the main dissemination activities and exploitation of results, the activities of the MAMA Project succeeded in strengthening the human potential, the interaction with all the main stakeholders, the synergy with the science education and the regional productive sector.
Human potential
Eight researchers were recruited specifically for this project (of which four were female researchers and four male researchers).
All researchers contributed their expertise to unlock SPIN research potentials. Moreover, we were able to contribute in building their career in science, as well as preventing the brain drain and build a critical mass to develop research in innovative materials in the Campania Region. As shown in the table below, four researchers obtained a permanent position in SPIN within the lifetime of the project and four are hired with research fellowships and are associated to SPIN: Procolo Lucignano [CNR-SPIN: Permanent position since Oct 2011], Rosalba Fittipaldi [CNR-SPIN: Permanent position since Oct 2012], Paola Gentile [CNR-SPIN: Permanent position since Dec 2013], Antonio Ambrosio [CNR-SPIN: Permanent position since Dec 2013], Fabio Chiarella [CNR-SPIN: Temporary researcher recruited under the SMARTAGS project], Emiliano Di Gennaro [University of Naples: 5 years Tenure Track, associated to CNR-SPIN], Filomena Forte [University of Salerno: CNR-SPIN researcher, to be post-doc, associated to CNR-SPIN], Gabriella De Luca [University of Naples: Post-doc, associated to CNR-SPIN].
Interaction with stakeholders
The interaction with the main stakeholders (SMEs, Scientific community, Regional and National authorities, EU, etc.) has been developed in order to highlight the achievements in functional materials, concerning synthesis, analysis, and modelling) towards the areas of Information and communication technology (ICT) and Energy. In particular, the main issues faced within the ICT framework relates to spintronics, sensors, oxides based logic memory elements, and materials for topological quantum computation. Within the Energy thematic area, the focus has been on oxides for energy harvesting, themoelectrics, superconducting devices for low power electronics, and novel materials for photovoltaic.
Synergies with Science Education
These activities aimed at increasing awareness among stakeholders and general public.
The dissemination activities targeted to the general public consisted in measures towards high school students within the framework of the "Lauree scientifiche" project (Scientific university education). The activity took place in Fall-Winter 2012-2013 and included a plan of visits to the MAMA Project labs to allow high school students to experience hands-on activities concerning the synthesis and characterization of functional materials in the form of single crystals. Such project was conducted in tight collaboration with high school as well as university professors with the aim of informing students of the Salerno Area willing to begin a university curriculum in physics. A similar plan of visits was performed at the MAMA Project labs in Naples during the Fall-Winter 2012-2013 with demonstration of the facilities for design of thin films heterostructures based on oxides materials and on the application of different advanced probing techniques to control the quality of the materials and the optical properties.
Synergies with the Regional Productive Sector
Within the framework of the MAMA project, SPIN established interactions with industries and SMEs of the Campania Region.
Active interactions
- Commercial activities for high-quality control of materials for production through non damaging advanced microscopy and spectroscopy
Main opportunities
- Identification of potential industrial partners (e.g. European Microfusion Aerospace)
- SMEs involved in networking activities
- New industrial partners (within the SMARTAGS project)
The main technological paths identified by the Campania Region are: Aerospace [aeronautics, maintenance and transformation, space and vectors, propulsion], Energy [smart and micro grid, energy saving and environmental sustainability, renewable energy, biochemical, management and valorization of water resources], Logistics and Transports [automotive, railways, logistics, technological systems for mobility], energy efficient buildings, sustainable access, conservation techniques, sustainable materials, urban and local security, monitoring], new Materials and nanotechnologies [advanced materials, transports, energy saving, ICT, human health], Life sciences [bio-nanotechnologies, nutrogenomoics, biomedical engineering, food environmental and economic efficiency, quality and intergrity of the supply chain, diagnostics, new drugs, e-health]. The scientific and technological challenges in the field of nanosciences, nanotechonologies, materials and new production technologies, are strongly related to the reinforcement of the activities covered by the MAMA Project, namely those linked with the growth, the analysis, and the control, in the sense of physical properties, of novel functional materials of high quality.
Nevertheless, it is widely accepted that there is a low level of innovation demand in the entrepreneurial system of the Regions of Convergence. The system of production in the Region is greatly made up of SME that develop their actions in the sectors considered as traditional and in general not related to the high-tech. Moreover, in this area, there is a limited number of enterprises, which are a branch of trans-national big companies, but whose R&D activities are mainly performed in their headquarters. In addition, in the Campania Region the relationship between the enterprises and the research centres are not well established, and the transfer of know-how is almost absent.
A crucial aspect identified by the MAMA project is related to the capability of transferring the competition towards the best parts of the market playing more on the quality and the performance of the products than on the costs. In doing that, we can have an impact at Regional level by favoring, at the same time, the differentiation of the activities and in turn the support for the creation of new enterprises closely related to the thematic areas of the scientific research. Innovation in the field of material science have strong influences on transversal sectors of the economy and thus can represent a valid tool to sustain the quick transformations that follow the market demand.
Activities to Promote Awareness on European Funding Opportunities
The dissemination activity devoted to increase awareness on European funding opportunities for research in the new framework programme Horizon 2020 consisted in a one-day open initiative that took place on February 25, 2013 entitled: Horizon 2020 Regional dimension of research within the future funding programme and cohesion policy. The presentation was held by Andrew Bianco, Project Officer, from the DG Research. The event took place at Salerno University Campus and was attended by over 100 researchers and administrative staff, mainly form SPIN units of Naples and Salerno and from the University of Salerno.
Use and dissemination
Articles published in peer reviewed journals: Overall 70 articles published since September 2010. There are also
Presentations to conferences: During the lifetime of the project, researchers, supported by MAMA, participated at:
• 32 Conferences as invited speaker
• 2 Conferences for abstract oral presentations
• 3 Networking events
Researchers were invited to give talks at various Institutions in Europe and in USA.
List of Websites:
Project website: http://mama.spin.cnr.it/
Institute website: http://www.spin.cnr.it/
