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LIGHT ELEMENT MOLECULAR SUPERCONDUCTIVITY: AN INTERDISCIPLINARY APPROACH

Final Report Summary - LEMSUPER (LIGHT ELEMENT MOLECULAR SUPERCONDUCTIVITY: AN INTERDISCIPLINARY APPROACH)

Executive Summary:
Title: To understand existing and design new light element molecular superconducting systems with high transition temperatures

Turning an insulator into a superconductor is a challenge that has occupied scientific research for decades - superconductors have no electrical resistance and carry electricity without losing energy, so finding new superconductors with enhanced performance is of paramount importance for energy applications. Most superconductors have simple structures built from atoms, but superconductors made from molecules arranged in regular solid structures also exist. Here we have unveiled new molecular superconductors by resorting to chemistry (doping) and by employing physical means (application of ultrahigh pressures, generation of ultrahigh charge carrier densities by electrical fields). The success of our synergic approach (synthesis/measurement/theory) is illustrated by our results on the unconventional fulleride superconductors with the highest working temperature (at 38 K) for any molecular system where we showed that the electronic structure of the molecular building block directly controls the formation of the state of matter (an anomalous metal), from which both the superconductor and the parent antiferromagnetic Mott insulator emerge, providing this connection between molecule and property. As chemists can create new molecules, this opens the way to designing-in features conferring high-temperature superconductivity to classes of superconductor that are not restricted by the elements of the Periodic Table.

Project Context and Objectives:
Title: To understand existing and design new light element molecular superconducting systems with high transition temperatures

The objective of the project was to develop methodology to design light element (B, C, O) cluster/molecule-based superconducting systems with high transition temperatures. This was achieved by integrating chemical synthesis of new materials with physical control of electron density and delocalisation underpinned by theoretical understanding benchmarked against evidence from advanced spectroscopies and electronic property measurements in the unique multidisciplinary training environment provided by ten world-leading groups from the EU and Japan. The step change in the fundamental understanding of the novel competing electronic ground states from which superconductivity emerged was achieved by focussing on light element materials in which a fine balance exists between electron correlations and electron-phonon coupling. Light elements are cheap, abundant, non-toxic and environmentally benign and thus ideal candidates for sustainable energy-saving superconductor technologies without the need to use toxic and/or rare elements. The discovery of light element molecular superconductors with figures-of-merit needed for applications is a grand challenge requiring the fundamental research proposed here to identify proof-of-concept materials and scientific understanding.

In order to achieve the understanding and the experience necessary to design light element molecular superconducting systems, the synergic capabilities of the project team were integrated into four Core Technology Platforms to enable the synthesis-driven study of basic mechanisms of superconductivity. The Synthesis and Characterisation Platform addressed the synthetic targets of new structural and chemical classes of light element solids with enhanced functionalities defined as high superconducting transition temperatures and new competing electronic ground states (e.g. cooperative magnetism) for s/p-electron systems. The Control Platform addressed the systematic control of the competition between insulating and superconducting behaviour of these systems via the application of external stimuli such as ultrahigh pressures and very strong electric fields. The Measurement Platform addressed the identification of the new electronic ground states and their evolution with changes in external parameters. The Theory and Modelling Platform addressed the understanding of the electronic properties by modelling and evaluation of the key structural and chemical components of light element molecular superconductors and, through working-level engagement with the other platforms new high Tc materials.

Project Results:
CHALLENGES AND ISSUES, STATE OF THE ART

An important issue in superconductivity research has been, even after its electron-phonon BCS mechanism has been understood, the inability to design superconducting materials – namely to assemble appropriate building units to target the synthesis of superconductors with desired performance. Instead major breakthroughs and new families of superconductors, including the high-Tc cuprates, MgB2, and the Fe pnictides have emerged by serendipity. The dominance of electron-phonon mechanisms was seriously questioned by the high-Tc cuprates – because these superconductors arise by doping of strongly correlated insulators, the Mott insulators, the strong electron-electron repulsion and the resulting strong correlations play a dominant role in determining the properties. Despite theoretical proposals that attribute high-Tc to strong correlations rather than to phonons, the theory of superconductivity in these materials has remained highly controversial.

This underscores the necessity for the development of new superconducting materials platforms, which was the main challenge of the project. Fresh impetus came from recent work, which revealed that our fundamental understanding of superconductivity can be significantly enhanced by studying light element materials with s/p-based outer electrons in which a fine balance existed between electron-phonon coupling and electron correlation energies.[1] Cs3C60 is a Mott-Hubbard insulator at ambient pressure but superconductivity at 38 K emerges under pressure solely by changing the extent of overlap between the outer wavefunctions of the constituent molecules. Tc also scales universally in a dome-like relationship with proximity to the Mott metal-insulator transition,[2] a hallmark of the effect of electron correlations characteristic of high-temperature superconductors. A new avenue into superconductivity research was also presented by the discovery that alkali-metal doping of the aromatic hydrocarbon picene, C22H14 led to a molecular superconductor (Tc = 18 K) with a very simple chemical constitution and in which the organic component contains only the light elements, carbon and hydrogen.[3] These materials advances, combined with the opportunities offered by physical control of electron density and delocalisation as achieved by the application of physical pressure [4] and charge carrier accumulation by electric fields [5] to access both high Tc and unusual electronic states, together produced an opportunity to make a transformative advance in the state-of-the-art of light element molecular superconductors.

SCIENTIFIC AND TECHNICAL APPROACH

The methodology utilised the complementary skills of the partners to address the basic mechanisms of superconductivity in light element systems. As a synthesis-driven study, the project relied foremost on innovative synthetic (solid/gas, soft chemistry) routes to create the new electronic states while mechanistic insight arose from the measurement (structural, magnetic, spectroscopic, thermodynamic, transport) and understanding (theory) of the resulting materials. Identification of the accessible ground states required physical tuning by pressure and electric field doping beyond that initially achievable chemically. The working approach involved a continuous search for new materials coupled with detailed structural and electronic characterisation at both ambient and high P using a panoply of in-house and facility-based state-of-the-art experimental techniques and in constant dialogue with theory.

ACHIEVED RESULTS

The project succeeded in taking the understanding of molecular high-Tc superconductivity, the metal-insulator transition in strongly correlated systems, and electric-field-induced superconductivity to an unprecedentedly advanced stage.

(1) Fulleride superconductivity – beyond BCS

Understanding how electron-electron interactions are controlled near the M-I crossover region is of generic significance for all strongly correlated systems and, in this regard, the study of hyperexpanded Cs3C60 in which fulleride frontier orbital degeneracy and geometrical frustration may be pressure-tuned [2] to access insulating, metallic and superconducting states is of paramount importance. Contrary to long-held beliefs, we showed that fullerides are simple members of the unconventional superconductivity family, i.e. the pairing interaction is something other than simply the conventional BCS electron-phonon interaction. We demonstrated this by showing that in Cs3C60, superconductivity emerges upon applied pressure out of an antiferromagnetic Mott insulating state and displays an unconventional behaviour –a superconductivity dome– explicable by the prominent role of strong electron correlations. The project team now identified the purely molecular aspect of the C603- superconductivity story by demonstrating that the parent insulating state involves Jahn-Teller distortion of the anion, driven by coupling of the localized t1u3 electrons to intramolecular phonons, thereby identifying the controlling role of the molecular electronic structure. [6] This on-molecule distortion is dynamic and creates an S = ½ ground state which produces the magnetism and from which superconductivity emerges.

However, although the similarities with the electronic phase diagrams of high-Tc superconductors such as the cuprates are striking, there existed no information on the metallic state out of which both the insulator and superconductor emerge. The relation of this parent state to both the pairing mechanism and the proximate antiferromagnetic insulator is at the heart of any attempt to understand high-temperature unconventional superconductivity and has been feverishly pursued for decades in the cuprates. The high symmetry and structural simplicity of the fullerides were here advantageous in pursuing a direct connection from (molecular) building unit to extended structure properties. The critical synthetic advance achieved here was the identification of a new family of fullerides (RbxCs3-xC60) that allowed us to traverse the Mott insulator-metal transition at ambient pressure, thus enabling the deployment of the panoply of techniques available to the project (diffraction, magnetization, IR & NMR spectroscopy, specific heat and Hc2 measurements) to probe the electronic and crystal structure changes associated with this transition. [7] We found an anomalous correlated metallic phase –the Jahn-Teller metal– which dominates the phase diagram at temperatures well above Tc and is the normal state at the maximum in Tc, where the superconducting pairing crosses over from conventional weak-coupling to unconventional strong coupling. The Jahn-Teller metal is a dynamic, microscopically heterogeneous coexistence of itinerant metallic electrons with localized electrons, which produce Jahn-Teller on-molecule distortions. This electronic state is the parent of an unconventional strongly-coupled superconductor and fades away into a conventional Fermi liquid metal and weak-coupling BCS superconductor when the molecular signatures disappear with lattice contraction. The optimal Tc in the fullerides –highest for any molecular material– associated with a strongly coupled (or extremely stable) Cooper pair and thus with an enhanced upper critical field to 80 T is found at the boundary between unconventional and conventional behaviour, where the balance between molecular (Jahn-Teller distortion) and extended lattice (itinerant electrons) features of the electronic structure is optimized.

(2) Aromatic superconductivity – a new frontier

The discovery of superconductivity in metal-doped phenacenes [3] opened a new direction building on the Cs3C60 correlated superconductors. The electronically active pi-electrons are carried by graphene-like (“armchair” edge) fragments. Significant progress towards the synthesis of new materials with enhanced properties and clarification of their electronic properties has been achieved by the partners. The experimental situation has been unclear regarding critical points of essential detail which have been preventing fundamental understanding and impeding progress –namely, the compositions and structures of the superconductors are unknown.

Progress was made in two fronts: (i) fine tuning of the synthetic methods employing both high temperature annealing and low-temperature reactions in liquid ammonia. This resulted in enhanced shielding fractions to 10-20% in metal-picene superconductors and the report of new K-[6]phenacene (Tc= 5-7 K) and -[7]phenacene (Tc =11 K) superconductors, [8] and (ii) physical characterization of Kxpicene led to the confirmation of the zero-resistance state complementing the results of magnetization measurements. [9] In addition, photoelectron spectroscopy provided evidence of the development of metallic states in Kxpicene thin films deposited on graphite substrates that displayed a sharp Fermi edge. [10] Finally, high pressure magnetic measurements of Kxpicene revealed a positive pressure coefficient of Tc, which was understood by considering the change in the Fermi surface topology upon pressurization. [11]

A common thread characterising the synthetic attempts to afford alkali salts of phenacenes has been the use of direct solid state reactions between the organic components and the metals at elevated temperatures. However, the isolated materials are poorly crystalline and invariably multiphase. Therefore, devising new synthetic routes to afford isolation and urgently-needed structural and electronic characterisation of highly-crystalline phase-pure materials in a reproducible fashion is of paramount importance and was carefully addressed in the project. As a result we developed facile soft-chemistry single-step solution-based routes to prepare crystalline solvent-free binary salts of phenanthrene radical anions with variable oxidation state under mild conditions. [12] Crystal structure determination revealed severe disruption of the herringbone packing configuration of the phenanthrene units that accompanies intercalation and is driven by bonding optimisation between alkali intercalants and phenanthrene benzene rings. No superconductivity is observed for materials with composition Cs(C14H10) (1) and Cs2(C14H10) (2), which contain the radical mono- and di-anion, respectively. The magnetic properties of (1) are consistent with those of a uniform S = ½ linear Ising chain, indicative of significant electronic contact between open-shell (C14H10)- ions mediated by co-ordinated Cs+ ions. In contrast, (C14H10)2- is closed-shell and diamagnetic. At the same time, the high-temperature preparative route was modified to replace the alkali metal reductants by their hydride derivatives. This new strategy has immediately led to the isolation and structural characterization of the dipotassium salts of both picene and anthracene. [13] In common with the phenanthride salts above, both these materials are diamagnetic. The new synthetic chemistry developed in the project is sufficiently flexible to open the way for the isolation of new families of alkali phenacides and acides required for the exploration of the electronic states at previously inaccessible oxidation states.

(3) Other superconductors and related materials

A variety of other superconducting systems has been investigated in the project.

(i) Ion-gated superconductivity. Electric double layer transistors (EDLT) using organic electrolytes were employed to search for new superconductors. These devices enable the accumulation of a high density of carriers (~10^14 cm-2), high enough to induce superconductivity. They can also induce a variety of functionalities, including ferromagnetism, metal-insulator transitions, and chiral light emitting transistors, opening up a new interdisciplinary field, termed “iontronics”. Successful demonstration of EDLT devices has been made on several carbon-based materials, picene single crystals, polythiophenes [14] and carbon nanotubes. [15] Gate-induced superconductivity was observed in MoS2 and the Tc-carrier density phase diagram was established. [16] A new series of superconductors (MoSe2, MoTe2, WS2) was also unveiled. [17] In WS2, superconductivity with Tc = 8 K was discovered through in-situ monitoring of the resistance upon potassium intercalation. Another achievement of gate-induced superconductivity work is the confirmation of the two-dimensional nature of superconductivity by means of the anisotropic upper critical field, fluctuation dominated superconducting transition, and the vortex phase diagram. The most interesting result has been that the zero resistance in the ground state is destroyed by application of only a small magnetic field. This is due to quantum creep phenomena, where the flux cannot be pinned because of the strong 2D fluctuations. [18]

(ii) The non-centrosymmetric superconductor, La2C3. Isolation of the La2C3 (Tc = 11 K) and Y2C3 (18 K) superconductors (comprising C24- dimer units) with a non-centrosymmetric cubic structure raised the question of mixing of triplet character in the Cooper pairs. 139La-NMR spectroscopy showed that the coherence peak in the temperature dependence of the NMR relaxation rate, T1-1 at Tc is absent. This provided evidence for either exotic pairing in this system or the vortex response, quite a unique situation for a cubic superconductor. [19]

(iii) Co-intercalation of alkali metals and ammonia molecules. This soft chemistry reaction protocol has been frequently used in the synthesis of molecular superconductors –here we extended its use to other van der Waals bonded host materials such as the layered superconductor, FeSe (Tc~ 9 K) and discovered a complex structural and electronic phase diagram in (NH3)yAxFeSe upon pressurization to 40 GPa. Tc(P) shows an asymmetric double-dome-like dependence with an enhanced maximum Tc of 49 K at high pressures. [20]

(iv) Theoretical investigation of light element superconductors. A key development in the theoretical description of light element superconductors has been the proposal of the existence of a supersolid phase (co-existence of superconductivity and other electronic orders such as charge ordering) with strong electron-phonon interactions and strong correlations. [21]

DISCUSSION

The objectives of the project were divided into two classes, scientific and integration & training. The scientific objectives comprised: (a) unveiling new light element molecule-based superconductors, (b) identifying new electronic ground states in light element systems leading to superconductivity, and (c) developing new correlation-driven mechanisms for superconductivity in light element systems. All three scientific objectives were fully met as detailed in C.4 and key high-impact breakthroughs were achieved (new fulleride and phenacene-based superconductors unveiled, new superconductors created by electric field charge accumulation at unprecedentedly high density levels, new electronic states such the Jahn-Teller metal and 2D superconducting state were identified, the new field of iontronics was established, and new theoretical models for strongly correlated systems in the presence of significant electron-phonon interactions were developed). The potential for further scientific exploitation is not only in the field of superconductivity research but also by all involved in the synthesis, properties and device development of molecular materials for carbon-based electronics and technological applications. Notwithstanding the significant progress already made in the project, a bottleneck still remains to be completely overcome: namely, to achieve full structural and electronic characterisation of the phenacene-based superconductors. We believe that our breakthrough to access the precursor states in phase-pure highly-crystalline form offers the promise to overcome the remaining bottleneck in the immediate future.

The integration & training objective was to establish durable integration between partner group activities via structured exchange and research activity coordination of young researchers. This was also fully met through the interdisciplinary, intersectorial and international nature of the consortium and the close durable collaboration that was developed by the partners.

CONCLUSIONS

The combined synthesis, measurement, control and theoretical effort in the project made a transformative advance in the state-of-the-art of light element molecular superconductors by extending the realm of superconducting materials through the discovery of new systems, the establishment of iontronics, a new field of property control, and the understanding of unconventional superconducting pairing mechanisms. The output of the project is thus fundamental knowledge of benefit to the scientific communities working in superconductivity, molecular electronics, functional molecular solids, correlated electron systems and the "soft" low temperature chemistry of solids.

REFERENCES
[1] M. Capone et al., Rev. Mod. Phys. 81, 943 (2009).
[2] A. Y. Ganin et al., Nature Mater. 7, 367 (2008); Y. Takabayashi et al., Science 323, 1585 (2009); A. Y. Ganin et al., Nature 466, 221 (2010).
[3] R. Mitsuhashi et al., Nature 464, 76 (2010).
[4] K. Shimizu et al., Nature 393, 767 (1998).
[5] K. Ueno et al., Nature Mater. 7, 855 (2008); J. T. Ye et al., Nature Mater. 9, 125 (2010).
[6] G. Klupp et al., Nature Commun. 3, 912 (2012).
[7] R. H. Zadik et al., Science Advances 1, e1500059 (2015).
[8] N. Huyen et al., unpublished.
[9] K. Teranishi et al., Phys. Rev. B 87, 060505(R) (2013).
[10] H. Okazaki et al., Phys. Rev. B 88, 245414 (2013).
[11] H. Aoki, J. Superconduct. Novel Magnet. 25, 1243 (2012).
[12] A. Stefancic et al., unpublished.
[13] F. D. Romero et al., unpublished.
[14] W. Shi et al., Adv. Funct. Mater. 24, 2005 (2014).
[15] H. Shimotani et al., Adv. Funct. Mater. 24, 3305 (2014).
[16] J. T. Ye et al., Science 338, 1193 (2012).
[17] W. Shi et al., unpublished.
[18] Y. Saito et al., unpublished.
[19] A. Potocnik et al., Phys. Rev. B 90, 104507 (2014).
[20] M. Izumi et al., Sci. Rep., 5, 9477 (2015).
[21] Y. Murakami et al., Phys. Rev. Lett. 113, 266404 (2014).


Potential Impact:
Impact

(1) The creation of a more robust European – Japanese research cooperation. The project as a whole relied on complementary skills between EU and Japanese partners, whereupon technical capability (ultra-high pressure, FET construction) in the Japanese partners was matched by materials (new hosts accessed by high-throughput discovery) and measurement (structure refinement, instrument construction) capability in Europe. Such synergies were utilized in allowing chemists, physicists and materials scientists to work together on the design and synthesis of novel molecular superconducting materials. The project also specifically addressed the training and exchange aspects of the programme where leading scientists from both the EU and Japan work together, transfer knowhow, exchange scientific personnel and early stage researchers between the participating laboratories and undertake common key experimental investigations in the settings of international facilities. Perhaps the greatest contribution towards the creation of a more robust European – Japanese research cooperation came from the ethos of the project itself where the research activity progressed within a closely-knit consortium without any consideration of the national origin of the participating teams.

Joint research activities will definitely continue after the end of the coordinated project as partners in groups of two or more members have already established durable collaborations. Funding is currently actively sought or has already been applied for through applications to bilateral funding calls.

(2) The improved understanding of superconductivity in general and of novel superconducting materials in particular.

Step-change advances in superconductivity are associated with the discovery of new classes of superconducting materials, and LEMSUPER achieved this. The project was also ideally suited to provide mechanistic understanding of the formation of superconducting and competing insulating states. The field of high-temperature superconductivity remains incompletely understood despite extensive research ever since the discovery of superconductivity in the cuprates. The prime reason for this lies with the fact that the currently available high-temperature superconductors are rather complex systems with charge inhomogenites, structural disorder, symmetry changes upon doping and presence of strong electronic correlations. Therefore, key design questions regarding the synthesis of new classes of high-temperature superconductors, such as the selection of composition and structure, are currently difficult to answer because we are unable to link unambiguously their properties to the characteristics of the atomic building blocks of the materials. In the project, we demonstrated that the electronic structure of the molecular building blocks of selected unconventional superconductors such as the fullerides directly controls the formation of the state of matter (normal Fermi liquid vs anomalous metal), from which both the superconductor and the parent insulator emerge, providing this connection between molecule and property. The high symmetry and structural simplicity of the fullerides were advantageous in pursuing a direct connection from (molecular) building unit to extended structure properties.

(3) The organisation of successful joint research, activities, publications, and contributions to scientific events.

The dissemination activities of the coordinated project directly addressed these key objectives leading to high-visibility joint EU-Japan publications in peer-reviewed journals. As analysis and discussion of the plethora of collaborative experimental and theoretical results are finalised, a further stream of high-impact co-authored publications will follow. The consortium also generated new ideas and catalysed collaborations in the field of superconductivity in light-element molecular solids through its additional dissemination and training activities, notably through the organisation of two specialised International Workshops.

(4) A more intensive exchange and training of researchers.

The close collaboration between the partners resulted in an intensive programme of research exchanges and all-round training in superconductivity including that at major international facilities in Europe and Japan) which benefited the new generation of EU and Japanese young researchers involved in the consortium who became aware of the cross-disciplinary challenges in the field. In addition, a series of paedagogical lecture presentations with worked examples were delivered at the project meetings both by permanent scientists of the partner teams and invited guest lecturers from outside the team, including Advisory Board members, selected to deliver key training inaccessible within LEMSUPER. These early career scientists were aided to develop the network of international contacts which will help them in their scientific careers in the longer term. This is of tremendous importance for providing both the EU and Japan with the high quality workforce needed, both in academia and industry, to ensure that they remain competitive globally in a key scientific area such as superconductivity that has important ramifications for priority technological and societal issues such as energy and the environment.

(5) An improved performance of industrial products in the longer term.

The coordinated project was by its very nature of fundamental scientific importance rather than addressing directly issues related to industrial products. Nonetheless, the key contribution of LEMSUPER was to address intrinsically sustainable light element systems and turn the focus away from scarce element and toxic components. As the project produced results at the international forefront of these requirements, it has significant long-term benefits to society (towards environmentally benign energy infrastructure, reduced energy usage, use of sustainably sourced carbon-based components and corresponding reduction in the use of scarce metals such as neodymium or yttrium) and industry (new materials and associated products). In the short term, it was revealed that the critical current of power cables constructed using the light-element superconductor, MgB2 can be dramatically improved by mixing-in carbon, whose source was the coronene molecular system. This technological achievement clearly demonstrated the significant potential of light element superconductors with simple chemical composition not only in fundamental science but also in industrial applications.

DISSEMINATION

The entire consortium (including the contributions of all EU and Japanese partners) has produced to date 116 published articles (including 2 in Science, 1 in Science Advances, 7 in Nature Materials/Physics/Communications/Nanotechnology, 7 in Scientific Reports, 2 in PNAS, and 9 in Phys. Rev. Lett.) and 143 invited international conference presentations, including 8 plenary and 5 keynote lectures.”

List of Websites:

www.lemsuper.eu