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Engineering Exotic Phenomena at Oxide Interfaces

Final Report Summary - OXIDES (Engineering exotic phenomena at oxide interfaces)

Executive Summary:

Emergent phenomena at oxide interfaces constitute nowadays a major research topic of both fundamental and technological interests. Just as the engineering of physical properties at semiconductor interfaces was the crucial step in Si-based electronics, a next great advance might rely on the multiple novel functionalities of oxide interfaces. The objectives of OXIDES were (i) to develop advanced theoretical and simulation techniques to model the most relevant types of oxide interfaces and (ii) to use them to design new layered oxide materials with unique experimentally-confirmed properties.

Developing simulation methods for problems of technological interest typically requires addressing several levels of a multi-scale ladder, starting from an atomistic description of the constitutive functional material. When dealing with nanomaterials, whose functional properties are monitored by quantum effects directly controlled by their nanostructure, this starting level must necessarily be a truly quantum-mechanical atomistic description of the active materials. Within OXIDES, a special emphasis was put on the identification of advanced DFT methods providing improved description of complex OXIDES at the first-principles level and, then, on the development of second-principles approaches allowing to go to larger length and time scales. At the first-principles level, huge expertise has been accumulated regarding the modelling of important complex OXIDES and the recent pSIC approach and B1-WC hybrid functional were demonstrated to be valuable and tractable alternatives to usual LDA/GGA DFT methods in order to avoid pathological situations and achieve improved description of the electronic properties of different kinds of oxide interfaces, including self-consistent atomic relaxations. At the second-principles level, a radically new type of atomistic model potentials was proposed and developed. This approach is general and has a number of unique characteristics that significantly improve over the state-of-the-art in order to perform large-scale atomistic simulations and access the properties of nano-materials at operation conditions. Tools were also developed to access the thermoelecric properties of two-dimensional electron gas at oxide interfaces. A simple Landau-type formalism was proposed to describe ferroelectric capacitors with imperfect electrodes in finite bias and the first ever calculation of a ferroelectric tunnel junction in finite bias has been achieved at the first-principles level.

In close collaborations with the experimentalists, various artificial superlattices have then been proposed, which present unique characteristics for applications in electronics and spintronics: PbTiO3/SrTiO3 superlattices exhibiting unusual dielectric properties, BiFeO3/LaFeO3 superlattices that appear as promising candidates to achieve electric switching of the magnetisation at room temperature, LaNiO3/LaMnO3 superlattices in which interfacial Mn-Ni interaction induces a magnetic order leading to the occurrence of exchange-bias or SrTiO3/SrRuO3 superlattices developing a highly-confined half-metal spin-polarised 2-dimensional electron gas (2DEG). A lot of effort were also devoted to the characterisation of the 2DEG at the popular LaAlO3/SrTiO3 interface including the improved understanding of electron confinement, the study of superconducting and magnetic properties, the report of very large capacitance enhancement and negative compressibility, or the effects of Rashba spin-orbit interaction. Through joint experimental-theoretical works, we also provided strong convincing arguments demonstrating the intrinsic origin of the 2DEG, clarifying an intense debate and envisioning the possible exploitation of this 2DEG in electronics.

While the atomic-scale engineering of exotic properties at oxide interfaces might seem of purely academic interest, OXIDES also investigated what could be the real concrete impact of oxide interfaces in competitive technological applications. At the end of the project we can claim the identification of at least one immediate realisation. The integration of an oxide superlattice in a prototype electronic device was achieved and demonstrated some unique characteristics opening the door to specific applications not accessible with the usual technology. This is a concrete achievement deserving now the elaboration of functional demonstrators and action plans. The writing of a patent is on the way.

Project Context and Objectives:

The OXIDES project was targeting the engineering of exotic phenomena at oxide interfaces. It was a theory-driven project, also relying on strong and continuous interactions with experimentalists. The main objectives were (i) the development of advanced theoretical and simulation techniques to model the most relevant types of oxide interfaces and (ii) the use of these tools to design, in close collaboration with experimentalists a new generation of layered materials with unique experimentally-confirmed properties.

OXIDES essentially considered the family of ABO3 perovskite compounds that, while sharing the same simple cubic reference structure, can exhibit a wide range of properties from insulators to metals and develop various types of orders ferroelectric, anti-ferrodistortive, ferromagnetic, orbital orders. While interest in this class of compounds is not new, breakthroughs in the synthesis of complex OXIDES have recently brought the field to an entirely new level, allowing artificial oxide nanostructures to be realised with atomic-level precision. Not only can high-quality ultrathin oxide films now be grown directly on silicon but, more generally, various functional OXIDES such as ferroelectrics, high-temperature superconductors and magnets can be combined at the nanoscale, thus offering tremendous new possibilities for creating artificial multifunctional materials and devices. It appeared recently that building artificial multilayers and superlattices of perovskite OXIDES gives not only the possibility to combine the intrinsic properties of the parent compounds but also sometimes to realise the engineering of radically new properties, fully relying on interfacial effects. Creating interfaces between different compounds indeed offers unique opportunities to further tune and couple their different degrees of freedom (charge, spin, lattice, orbital) through various interfacial effects like charge transfer, electrostatic coupling, symmetry breaking, strain engineering, frustration, in order to induce totally new phenomena. Exploring and exploiting these interfacial effects were the central challenges of OXIDES.

Engineering new properties in artificial heterostructures by taking advantage of interfacial phenomena requires both perfect control of the interfaces and making their role dominant within the material. For that purpose, OXIDES focused on epitaxial layered heterostructures (multilayers and superlattices) with ultrathin layer thicknesses (typically in the 0.4 - 10 nm range). Among the numerous possibilities of combining perovskite OXIDES, three specific types of interfaces have been selected, each of them motivated by a simple fundamental physical concept, targeting a specific technological application and requiring specific theoretical developments. OXIDES so explicitly considered: (i) insulating interfaces between insulating OXIDES, where novel couplings between structural instabilities can lead to unusual phenomena; (ii) conducting interfaces between insulating OXIDES, where an interfacial 2-dimensional electron gas (2DEG) might exhibit large thermoelectric power; and (iii) interfaces between metallic and insulating OXIDES, for a deeper understanding of screening.

The working strategy underlying the OXIDES project consists of a feedback loop combining the use of different modelling tools in a multiscale spirit and constant interactions with experimentalists. Modelling is often considered as the initial stage of a two independent steps process in which theoretical predictions are first made to be later confirmed experimentally. Alternatively, theoretical and experimental studies can also be combined at the same time in order to take advantage of the complementarity of the informations accessible at both levels in order to get a complete picture into a virtuous circle. Although OXIDES followed both avenues, one of the strength of the consortium was the close connection between partners who often successfully worked as a joint team following the second approach. At the end of the project significant realisations can be claimed. They include methodological developments made available to the scientific community, significant improvements in the understanding of previously known systems and the identification of new promising interfaces. Many efforts have also been devoted to the clarification of what could be the concrete impact of oxide interfaces in technological applications. The integration of an oxide superlattice in a prototype electronic device has been achieved. It demonstrated some unique characteristics opening the door to specific applications not accessible with the usual technology; the writing of a patent is on the way.

Project Results:

In line with its original objectives, OXIDES has realised the development of new advanced theoretical and simulation techniques and contributed to the identification and characterisation of highly-promising oxide layered structures. It has also explicitly investigated the possibilities to include such layered materials in concrete devices.

Development of theoretical and simulation techniques.

At the first-principles level, modelling in OXIDES relied on advanced atomic-scale approaches based on Density Functional Theory (DFT) within which the study of highly-correlated electron systems and/or specific interfaces (like metal/insulator interfaces) requires going beyond the usual approximations (LDA,GGA) and remains challenging. One of the original aspects of the project was to consider recently proposed pSIC [A. Filippetti et al., Eur. Phys. J. B 71, 139 (2009)] and B1-WC hybrid functional [D. Bilc et al., Phys. Rev. B 77, 165107 (2008)] techniques for the study of oxide interfaces, as a convincing alternative to usual LDA+U corrections.

Regarding the 2DEG appearing at polar oxide interfaces, both pSIC and B1-WC approaches revealed particularly adapted to achieve improved description of the system, to clarify the origin of the 2DEG [C. Cancellieri et al. Phys. Rev. Lett. 107, 056120 (2011); M.L. Reinle-Schmitt et al. Nat. Commun. 3, 932 (2012)] and to characterise its properties [P. Delugas et al., Phys. Rev. Lett. 106, 166807 (2011)]. A significant methodological advance concerned the development of a variational pSIC, giving access to forces and self-consistent structural relaxations within this framework [A. Filippetti et al. Phys. Rev. B 84, 195127 (2011)].

Regarding metal/ferroelectric interfaces, it was demonstrated that many DFT results reported in the literature are wrong and a practical approach was developed in order to identify systematically pathological situations often appearing within usual LDA/GGA calculations [M. Stengel et al. Phys. Rev. B 83, 235112 (2011)]. Going further, it was shown that B1-WC hybrid functional is a valuable alternative: it is a convincing approach to bypass such problems and achieve reasonable estimate of Schottky barriers at metal/ferroelectric interfaces [D. Bilc et al., unpublished]. A lot of efforts were also devoted to the computation of complex electronic band structures and to the understanding of their link with metal-induced gap states [P. Agualdo-Puente, PhD Thesis (2011)].

Independently of that, the finite-D method [M. Stengel et al. Nat. Phys. 5, 304 (2009)] was shown to be the proper tool to rationalise the concept of (hybrid) improper ferroelectricity [M. Stengel et al. Phys. Rev. B 86, 094112 (2012)]. Let us emphasise that OXIDES partners often pushed the limit of first-principles calculations to the simulation of very large systems of hundreds of atoms like in the study of polydomain structures of ferroelectric superlattices [P. Aguado-Puente et al. Phys. Rev. B 85, 184105 (2012); P. Zubko et al. Nano Letters 12, 2846 (2012)]. Also, a lot of insight has been accumulated regarding the accuracy of first-principles simulations in perovskites; it appeared that even for simple ABO3 compounds, all functionals do not quantify properly the competition between ferroelectric and anti-ferrodistortive instabilities [J. Wojdel et al., unpublished].

At the second-principles level, modelling in OXIDES included various developments. Regarding electronic properties, a tight-binding hamiltonian for electrons was proposed to determine the equilibrium distribution of the conduction charge in 2DEG [M. Stengel, Phys. Rev. Lett. 106, 136803 (2011); M. Verissimo-Alves et al. Phys. Rev. Lett. 108, 107003 (2012)]. A method, including a temperature-dependent relaxation time for the description of the transport properties of 2DEG within the Bloch-Boltzmann approach, was implemented in the BoltzTrap code [A.Filippetti unpublished]. Regarding lattice dynamical properties OXIDES developed a totally new effective potentials approach, that properly includes all ionic degrees of freedom, for the study of structural phase transitions with temperature [J. Wojdel et al., arXiv:1204.3394]. Regarding ferroelectric tunnel (FTJ) junctions, a significant step forward was also performed through the first ever ab initio simulation of a realistic FTJ under finite bias, combining DFT and non-equilibrium Green's function formalism [D. Bilc et al. ACS Nano 6, 1473 (2012)]; in this context, the limitations of the Brinkman-Dynes-Rowel model for the determination of electronic transport properties were also discussed.

At the Devonshire-Ginzburg-Landau and effective potentials levels, the main advance relies in the clarification of the appropriate expression to be used for the modelling of ferroelectric thin films and model capacitors with incomplete screening in zero and finite bias [C. Lichtensteiger, et al. in Oxide Ultrathin Films: Science and Technology ISBN: 978-3-527-33016-4 (Wiley, 2011), pp. 265-309.]. The multiscale combination of Landau-type models making use of this expression with first-principles simulations appeared very useful to speed up the calculations of FTJ in finite bias [D. Bilc et al. ACS Nano 6, 1473 (2012)] and was also applied to the study of YMnO3 thin films and capacitors [A. Prikockyt, PhD thesis 2012].

Design of a new generation of layered oxide materials

A lot of efforts were also devoted in OXIDES to the characterisation of different kinds of oxide interface. At the level of insulating interfaces between insulating OXIDES, the initial model system under investigation was PbTiO3/SrTiO3 superlattices that can develop a trilinear coupling between lattice modes yielding an unusual improper ferroelectric behaviour [E. Bousquet et al. Nature 452, 732 (2008)]. Intensive characterisation of such superlattices was made both at the theoretical [P. Aguado-Puente et al. Phys. Rev. Lett. 107, 217601 (2011); P. Aguado-Puente et al. Phys. Rev. B 85, 184105 (2012)] and experimental levels [P. Zubko et al. Phys. Rev. Lett. 104, 187601 (2010); A. Torres-Pardo et al. Phys. Rev. B 84, 220102 (2011); P. Zubko et al. Nano Letters 12, 2846 (2012)]. It rapidly appeared that those systems are much more complicated than initially thought, obliging us to propose a new type of effective potentials to study their properties at finite temperature. This new method was first applied to bulk PbTiO3, revealing a strong competition between ferroelectricity and oxygen rotations [J. Wojdel et al., arXiv:1204.3394]. It was further applied to bulk SrTiO3 and extended to thick PbTiO3/SrTiO3 superlattices. Beyond the study of PbTiO3/SrTiO3 superlattices, many efforts were also devoted to the search of new potentially interesting systems. In this context, it was for instance proposed that a trilinear coupling of lattice modes, similar to that in PbTiO3/SrTiO3, might be one of the most promising route to achieve electric switching of the polarisation at room temperature in multiferroic superlattices [Ph. Ghosez & J.-M. Triscone, Nat. Mat. 10, 269 (2011)]. We so intensively investigated to coupling between electric and magnetic orders in BiFeO3/LaFeO3 systems and we propose that such a superlattice might be a promising candidate to realise that effect [Z. Zanolli et al., unpublished]. Experimentalists are growing such systems and will try to confirm this prediction. Another independent example is the discovery of unprecedented interface effects in LaNiO3/LaMnO3 superlattices. In such systems, the interfacial Mn-Ni interaction induces a magnetic order within the nickelate layers, which leads to the occurrence of exchange-bias. Our experimental-theoretical work on LaNiO3/LaMnO3 [M. Gibert et al., Nat. Mat. 11, 195 (2012)] constitutes one of 2012's highlights of OXIDES. Finally let us notice that many efforts have also been devoted to the understanding of ferroelectric switching and domain wall motions in thin films [H. Ba et al., J. Phys. Cond. Mat. 23, 142201 (2011); P. Paruch et al. Phys. Rev. B 85, 214115 (2012)].

At the level of conductive interfaces between insulating OXIDES, most of the efforts have been concentrated on 2DEG at the prototypical SrTiO3/LaAlO3 interface. Exploiting this 2DEG in various electronic applications is envisioned [D.G. Schlom and J. Mannhart, Nat. Mat. 10, 168 (2011); J. Mannhart and W. Haensch, Nature 487, 436-437 (2012); B. Keimer et al. Nat. Mat. 11, 751 (2012)] but would be hampered in case the origin of the 2DEG is extrinsic. Through a set of joint experimental-theoretical works, we have provided strong convincing arguments in favour of an intrinsic origin of the 2DEG [C. Cancellieri et al. Phys. Rev. Lett. 107, 056120 (2011); M.L. Reinle-Schmitt et al. Nat. Commun. 3, 932 (2012)]. In parallel, OXIDES partners were also particularly active in the characterisation of the 2DEG at the SrTiO3/LaAlO3 interface. This includes, at the theoretical level, improved understanding of the electron confinement at the interface [M. Stengel, Phys. Rev. Lett. 106, 136803 (2011); P. Delugas et al., Phys. Rev. Lett. 106, 166807 (2011)] or, at the experimental level, the study of its superconducting and magnetic properties [L. Li et al., Nature Physics, 7 762 (2011), S. Gariglio et al. Physics, 4, 59 (2011); N. Pavlenko et al., Phys. Rev. B 85, 020407 (2012)]; the report of very large capacitance enhancement and negative compressibility [L. Li et al. Science, 332, 825 (2011)], or the effects of Rashba spin-orbit interaction [A. D. Caviglia et al. Phys. Rev. Lett. 104, 126803 (2010); A. Fete et al. arXiv:1203.5239]. A specific attention was also devoted to the characterisation of the potentially interesting thermoelectric properties of the 2DEG. At that level, it was clearly shown experimentally [I. Pallecchi, et al. Phys. Rev. B 81, 085414 (2010)] and confirmed theoretically [A. Filippetti et al., unpublished], that contrary to the expectations, there is no enhancement of the thermoelectric properties compared to bulk doped SrTiO3.

At the level of interfaces between metallic and insulating OXIDES, we have demonstrated at the theoretical level that, contrary to the common belief, even symmetric ferroelectric tunnel junctions can exhibit a large tunnelling electroresistance effects [D. Bilc et al. ACS Nano 6, 1473 (2012)]. Besides, we have identified a new mechanism for the formation of a 2DEG at metal/insulator interfaces, based on the electronegativitity of the atoms at the interface: we have so predicted the possibility to create a spin-polarised half-metal extremely-confined 2DEG in SrTiO3/SrRuO3 superlattices [M. Verissimo-Alves et al. Phys. Rev. Lett. 108, 107003 (2012)] and also studied its thermoelectric properties [P. Garcia­a-Fernandez, et al. Phys. Rev. B 86, 085305 (2012)]. Finally a lot of theoretical and experimental knowledge has been acquired regarding the understanding of screening at metal/ferroelectric interfaces like those appearing in ferroelectric capacitors [C. Lichtensteiger, et al. in Oxide Ultrathin Films: Science and Technology ISBN: 978-3-527-33016-4 (Wiley, 2011), pp. 265-309.]. The first-principles modelling of Pt/LaAlO3/SrTiO3 capacitors under an external bias potential has been achieved [C. Cazorla and M. Stengel, Phys. Rev. B 85, 075426 (2012)] as well as the study of electrochemical ferroelectric switching [N. Bristowe et al. Phys. Rev. B 85, 024106 (2012)].

Toward concrete applications

In parallel to fundamental investigations, the identification of realistic industrial applications for new oxide-based interfaces has been a challenging task of OXIDES. It was successfully carried out through the two-step process of (i) studying the properties of the interfaces, carefully measuring their physical properties in an application-oriented perspective and (ii) at the same time scanning the industrial environment. At the end of the project, we can claim the identification of at least one immediate application. The integration of an oxide superlattice in a prototype electronic device has been achieved. This is a success deserving the elaboration of functional demonstrators and action plans to give continuity to the applied research work initiated within OXIDES.

Potential Impact:

OXIDES has produced 64 peer-reviewed publications in high-impact journals (7 in Nature journals, 2 in Science, 13 in Physical Review Letters), amongst which 18 were joint works between two or more partners of the consortium, and performed more than 200 dissemination actions (presentations, press releases, interviews, public lectures).

Concrete impacts of OXIDES can be expected at different levels.

At the first-principles level, significant progresses have been realised regarding the accurate modelling of complex OXIDES within density functional theory (DFT). A huge expertise has been accumulated regarding the accuracy of first-principles methods in cubic perovskites, revealing that, even for simple ABO3 compounds, many usual functionals do not quantify properly the competition between ferroelectric and antiferrodistortive structural instabilities. It was highlighted that the pSIC and hybrid functional B1-WC approaches constitute valuable alternatives to LDA+U for the study of two-dimensional electron gas (2DEG) at oxide interfaces and a variational pSIC was developed opening the door to self-consistent relaxations within this framework. It was demonstrated that many DFT results on metal/ferroelectric interfaces reported in the literature are wrong and a practical approach was developed in order to identify systematically pathological situations; going further, it was shown that B1-WC hybrid functional is a practical way to bypass this problem. All these advances were published in international journals and are accessible to the whole community working on complex OXIDES; they contributed to improve the predictive power of DFT simulations in complex OXIDES and constitute a solid basis extremely useful to address the physics of more complex systems in the future.

At the second-principles level, OXIDES realised also important and useful developments. Regarding electronic properties, a tight-binding model for electrons was developed to determine the equilibrium distribution of the conduction charge in 2DEG. A method, including a temperature-dependent relaxation time for the description of the transport properties of 2DEG within the Bloch-Boltzmann approach, was implemented in the BoltzTrap code. Regarding lattice dynamical properties at finite temperatures, a totally new effective potentials approach was proposed and implemented. At the Landau theory and effective potentials levels, the appropriate expression to be used for the modelling of ferroelectric thin films and model capacitors with incomplete screening in zero and finite bias was derived, clarifying a long-standing debate. Some of these advances are still at their infancy. Nevertheless they initiate original ways to go beyond previous capabilities and pave the way to multiscale modelling of OXIDES and other functional materials. Some of these developments are already at the origin of new collabortive projects that will continue implementing original OXIDES ideas in order to attack the modelling of more and more complex multifunctional materials at larger length- and time-scales.

At the material design level, OXIDES was again particularly innovative in proposing new systems exhibiting exotic properties like LaNiO3/LaMnO3, BiFeO3/LaFeO3 or SrTiO3/SrRuO3 superlattices. OXIDES members also significantly contributed to promote the potential use of the 2DEG at LaAlO3/SrTiO3 interface in electronic devices by (i) reporting strong convincing arguments in favour of an intrinsic origin of the 2DEG and (ii) exploring and clarifying its intriguing properties: superconductivity, magnetism, large capacitance enhancement and negative compressibility, Rashba spin-orbit coupling... OXIDES results and ideas were disseminated through various high-impact journals, including different News and Views in Nature and Science high-impact journals. This objectively attests of the quality and concrete impact of OXIDES research within the scientific community working on oxide materials.

OXIDES paid finally a particular attention to the identification of realistic industrial applications for new oxide-based interfaces. At the end of the project, we can claim the identification of at least one immediate application. The integration of an oxide superlattice in a prototype electronic device has been achieved. This is a concrete achievement deserving the elaboration of functional demonstrators and action plans to give continuity to the applied research work initiated within OXIDES. The writing of a patent is on the way.

Project website: http://www.OXIDES.ulg.ac.be