Emergent phenomena in novel correlated materials

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

An important phenomenon in Nature is that of organization of many objects interacting together, which results into new entities with properties that are much more than the sum of the parts . For example the ability to think is a property of the brain as a whole and is the result of interactions that involves numerous neurons exchanging information in an organized way and is not a property of a single individual neuron. Similarly, in many technologically-important materials electrons also show a certain degree of order in that they correlate their motion with one another to avoid the strong repulsion that arise when they are brought close together. Such correlation effects can lead to surprising emergent material properties, which often can not be predicted in advance, such as superconductivity, where current flows with no resistance due to the fact that electrons travel in pairs in a very robust way. This proposal is to explore superconductivity and other novel form of electronic order stabilized by strong correlations in complex materials that are often not found in Nature but are artificially synthesized with the purpose to achieve certain material functionality. In 2008 the discovery of superconductivity a large class of materials based on Iron stimulated a revolution in condensed matter physics. This was most unexpected as usually Iron has strong ferromagnetic properties (attracting metals) that would normally destroy a superconducting state by breaking the special pairing between electrons. The large number of structural combinations in which iron-based superconductivity is found has raised the hope that the periodic table still holds the key to the discovery of new materials with extremely high superconducting temperatures which one day will revolutionize our way of living. In my first project I propose to take on the challenge of exploring deep into the nature of structural configurations, predicting electronic behaviors and testing experimentally novel superconductors. My second project aims to explore how electrons organize themselves in the presence of frustrated magnetic interactions. Imagine a restaurant with a number of triangular tables and a large number of male and female guests; if one tries to arrange guests such that everybody sits next to a person of the opposite sex, it cannot be realized even for one single table and many equally-unsatisfactory arrangements exist. The same kind of decision has to be made by magnetic spins which can point up or down on a triangular lattice and they cannot decide, so become frustrated. How electrons organize themselves and how they travel in such circumstances remains a mystery. Another amazing unexplored behaviour is that in which electrons are able to flow freely on the surface of a material but not inside it, giving rise to an insulator with a surface that conducts electricity. In this kind of topological insulator, as also in certain frustrated systems, conventional laws of physics do not apply as particles could be found in a superposition of several states at the same time, property that could be important for use in future quantum computers.For this research I use and plan to develop the most advanced tools for probing electron correlations in micron-size single crystalline materials using the highest magnetic fields in the world (a million times larger than earth's magnetic field), low temperatures near absolute zero and extreme high pressures to tune interactions and probe new electronic phases of matter.

Planned Impact

While future scientific endeavor is difficult to predict in advance, the discovery of unexpected physics displayed by strongly correlated systems with markedly improved multifunctional properties compared with widely used semiconductors will one day have a dramatic effect on our lives. Emergent phenomena, in which the correlated behavior of many particles leads to collective new properties, are of great significance across a broad range of areas in science. However, the objective is to tailor a material (starting with its chemical composition, constituent phases) in order to obtain a desired set of properties suitable for a given application. Today technological advances are based on semiconductor physics due to their high degree of precise processing. However, if the same can be achieved for strongly correlated materials which have multifunctional and unique properties, such as superconductivity and magnetism. not found in semiconductors, would open up remarkable technological possibilities. Superconductivity is one of the most exciting phenomenon and has the potential to change significantly our life with applications in energy storage and transport (reducing dramatically the energy cost as the current flows without resistance), in medical investigations (as the case of MRI machines) or high speed trains (like the bullet train in Japan). To answer the feasibility of room temperature superconductivity requires that we solve the mechanism of high temperature superconductivity and this research will aims to contribute significantly towards that effort. Discovery of a new phase of matter is an excitement that any scientist is drawn into exploration. As understading the nature of simple metals has helped to manipulate and process semiconductors in a very precise way, the predicted novel states of matter in geometrically frustarted systems with mobile electrons or the newly discovered toplogical insulators provide avenues for the manipulationg and realization of fault-tolerant quantum computing which one day will increase the ability to simulate and predict the behaviour of systems made of infinite number of contituents. The results of this research will made available to other scientists and to the wider public though websites, newsletters and science reports aimed at a cross-disciplinary audience and also significant effort will be made to ensure that the excitment and the wonder of scientific exploration is passed on to future generations of scientists through training of students and also engagement with schools.

Publications

10 25 50
 
Description Our research explores the unexpected emergent phenomena present in novel quantum materials mediated by strong electronic correlations. We have explored the fundamental electronic behaviour of newly discovered quantum materials using state-of-the art experimental techniques, including ultra-high magnetic fields, synchrotron facilites and first-principle band structure calculations. We have created precise quantum maps of the electronic structure of different iron-based superconductors that have formed that the basis of different theoretical models to describe and predict how superconductivity can be enhanced. Using extreme conditions of high magnetic fields and applied pressure, we have identified how electrons interact with each other and how different types of superconducting pairing compete with each other to enhance superconductivity. We have identified a new iron-based superconductor with large critical transition temperature and upper critical field as a candidate for high-magnetic field technologies. We have established the key ingredients of the nematic electronic states of iron-chalcogenide superconductors. We have developed and investigated thin flakes of superconductors.
Exploitation Route Our quantum maps of the electronic structure of different iron-based superconductors have formed that the basis of different theoretical models to describe and predict how superconductivity can be enhanced. The tuning of superconductivity under extreme conditions will help to identify the essential pairing mechanism to stabilize high-temperature superconductivity. The quantum devices based on thin flake superconductors are the first step towards tunable superconducting applications.
Sectors Education,Energy,Manufacturing, including Industrial Biotechology,Other

URL https://www2.physics.ox.ac.uk/contacts/people/coldeaa
 
Description The work on novel superconductors has opened the possibility to create a direct link with local industries working on superconducting applications, as a part of the Oxford Centre for Applied Superconductivity. Our group has tested a versatile probe developed by Oxford Instruments using quantum materials developed during the research grant. We have identified a new iron-based superconductor with large critical transition temperature and upper critical field as a candidate for high-magnetic field technologies and pursue the development of superconducting wires. The students and staff trained during the time of this award have developed extensive expertise in developing new experimental projects and computational tools, analysing and presenting complex information and data towards their new jobs either in research, software companies and financial sector. The development of thin flake devices has lead to the research development of superconducting thin flake devices.
First Year Of Impact 2015
Sector Energy,Manufacturing, including Industrial Biotechology
Impact Types Societal,Economic

 
Description Oxford Centre for Applied Superconductivity
Amount £1,550,000 (GBP)
Funding ID http://www.cfas.ox.ac.uk/ 
Organisation Local Enterprise Partnerships 
Sector Charity/Non Profit
Country United Kingdom
Start 04/2015 
End 12/2022
 
Description Oxford Quantum Materials Platform Grant
Amount £1,736,109 (GBP)
Funding ID EP/M020517/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2015 
End 03/2020
 
Title Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1-xSx 
Description These data are raw data as part of the manuscript: "Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1-xSx" (arXiv:1904.02522) https://arxiv.org/abs/1904.02522. The manuscript will be published as an Article in Physical Review Research 2020. The magnetotransport data were collected using high magnetic fields with Helium 3 cryostats either in Nijmegen up to 38T , Tallahahassee up to 45T and pulsed fields close to 65T in Toulouse. The data were collected using lock-in amplifiers. The data here are part of the figures presented in the manuscript were detailed figure captions are provided. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact Understanding superconductivity requires detailed knowledge of the normal electronic state from which it emerges. A nematic electronic state that breaks the rotational symmetry of the lattice can potentially promote unique scattering relevant for superconductivity. Here, we investigate the normal transport of superconducting FeSe1-xSx across a nematic phase transition using high magnetic fields up to 69 T to establish the temperature and field-dependencies. We find that the nematic state is an anomalous non-Fermi liquid, dominated by a linear resistivity at low temperatures that can transform into a Fermi liquid, depending on the composition x and the impurity level. Near the nematic end point, we find an extended temperature regime with T1.5 resistivity. The transverse magnetoresistance inside the nematic phase has as a H1.55 dependence over a large magnetic field range and it displays an unusual peak at low temperatures inside the nematic phase. Our study reveals anomalous transport inside the nematic phase, driven by the subtle interplay between the changes in the electronic structure of a multi-band system and the unusual scattering processes affected by large magnetic fields and disorder 
URL https://ora.ox.ac.uk/objects/uuid:37ed680b-6992-4d97-b929-c9af2559a029
 
Title Evidence for unidirectional nematic bond ordering in FeSe 
Description These data are related to the paper https://arxiv.org/abs/1603.04545 to be published in Physical Review B in 2016. Raw data files are in .nxs format, which are standard HDF5 format files created at I05 at diamond. PDF figures have some basic description in the filename. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Impact n/a 
 
Title FeSe_magneotransport_Watson_PRL_2015 
Description These data constitute the data presented in the plots of Figures 1 and 2 in "Dichotomy between the Hole and Electron Behavior in the Multiband FeSe Probed by Ultrahigh Magnetic Fields" (Watson, PRL, 2015). The figures were created in Origin, but can be reproduced in any graphing software that can import standard ASCII files. The resistivity and Hall effect data has been scaled by the sample geometry into resistivty (not resistance) units. The data were collected in the Clarendon Laboratory, Oxford University and at the EMFL high magnetic field facilities in Toulouse. 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
Impact n/a 
 
Title Magnetotransport CuxBi2Se3 
Description These data were created by recording the transport behaviour on thin flakes devices at low temperatures and in magnetic field. These are all DAT files easily readable. We report magnetotransport studies in thin (<100nm) exfoliated films of CuxBi2Se3 and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with Tc~3.5K and a possible electronic phase transition around 200K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T<30K. This transition between competing conducting channels may be enhanced due to the presence of electronic ordering, and could be affected by the presence of an effective internal stress due to Cu intercalation. By fitting to the weak antilocalisation conductivity correction at low magnetic fields we confirm that the low temperature regime maintains a quantum phase coherence length Lphiv> > 100nm indicating the presence of topologically protected surface states. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact Publication: Multi-band magnetotransport in exfoliated thin films of CuxBi2Se3, by J A Alexander-Webber, J Huang, J Beilsten-Edmands, P Cermák, C Drašar, R J Nicholas and A I Coldea Published 16 March 2018, Journal of Physics: Condensed Matter, Volume 30, Number 15. This work has opened up the design of devices using thin flakes of single crystals to study they magnetotransport and topological signatures. 
URL https://ora.ox.ac.uk/objects/uuid:f9dbd74f-1f42-4d45-a236-456bee6d8d1e
 
Title Optimization of superconducting properties of the stoichiometric CaKFe4As4 
Description These data were generated part of the publication Optimization of superconducting properties of the stoichiometric CaKFe4As4 by Singh et al to appear in Supercond. Sci. Technol. 2019 https://doi.org/10.1088/1361-6668/ab58be 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact CaKFe4As4 (1144) is a unique stoichiometric iron-based superconductor which harbors high upper critical fields and large critical current densities. In this work, we describe a study to optimize the synthesis conditions of stoichiometric polycrystalline samples of CaKFe4As4 and assess their structural, magnetic and transport properties. The samples were prepared over a wide temperature range (900 °C-1100 °C) and the pure phase formation is centered around 955 °C. Outside this temperature region, impurity phases of KFe2As2 and CaFe2As2 can also form. Magnetic susceptibility and resistivity measurements establish that the critical temperature reaches ~34 K for the optimum synthesis conditions and the critical current reaches 2 × 104 A cm-2. The post-annealing process demonstrates the stability of the 1144 phase up to 500 °C, however, under higher temperature annealing, phase degradation occurs. Our study indicates that the formation of phase-pure 1144 occurs over a much narrower window and its highly prone to multi-phase formation as compared with the 122 family. As a result, the superconducting properties are enhanced for the pure 1144 phase but they are likely to be affected by the inter- and intra-granular behavior originating from the microstructural nature of polycrystalline CaKFe4As4, similar to other iron-based superconductors. Based on our study, we construct the phase diagram for polycrystalline 1144 and compared it with that reported for 1144 single crystal. 
URL https://ora.ox.ac.uk/objects/uuid:32be44a2-6850-469c-8c6b-32b26f4e9a03
 
Title Quantum oscillations in the nematic superconductors FeSe1-xSx 
Description These are the Raw data used to generate the figures related to the publication: "Evolution of the Fermi surface of the nematic superconductors FeSe1-xSx" by A. I. Coldea et al. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact The existence of a nematic phase transition in iron-chalcogenide superconductors poses an intriguing question about its impact on superconductivity. To understand the nature of this unique quantum phase transition, it is essential to study how the electronic structure changes across this transition at low temperatures. Here, we investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition of FeSe1-xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 T in the low temperature regime down to 0.4 K. Most of the Fermi surfaces of FeSe1-xSx monotonically increase in size except for a prominent low frequency oscillation associated with a small, but highly mobile band, which disappears at the nematic phase boundary near x ~ 0.17, indicative of a topological Lifshitz transition. The quasiparticle masses are larger inside the nematic phase, indicative of a strongly correlated state, but they become suppressed outside it. The experimentally observed changes in the Fermi surface topology, together with the varying degree of electronic correlations, will change the balance of electronic interactions in the multi-band system FeSe1-xSx and promote different kz-dependent superconducting pairing channels inside and outside the nematic phase. 
URL https://ora.ox.ac.uk/objects/uuid:dad66609-f461-4f7d-9373-190e309a2ed9
 
Title Quantum oscillations probe the Fermi surface topology of the nodal-line semimetal CaAgAs 
Description These data accompany the manuscript entitled: "Quantum oscillations probe the Fermi surface topology of the nodal-line semimetal CaAgAs" arXiv:2001.02434 by Y. Kwan et al to be published in Physical Review Research 2020. The quantum oscillations data were obtained using the torque magnetometry technique and were collected at high magnetic field facilities, at HFML in Nijmegen up to 35T and NHMFL Tallahassee up to 45T. Measurements were performed using Helium 3 cryostat and single-axis rotators. Band structure calculations were performed using the experimental details of CaAgAs and Wien2k. Other modelling of the data is described in detail in the Supplemental Material of the paper (arXiv:2001.02434). 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact Nodal semimetals are a unique platform to explore topological signatures of the unusual band structure that can manifest by accumulating a nontrivial phase in quantum oscillations. Here we report a study of the de Haas-van Alphen oscillations of the candidate topological nodal line semimetal CaAgAs using torque measurements in magnetic fields up to 45 T. Our results are compared with calculations for a toroidal Fermi surface originating from the nodal ring. We find evidence of a nontrivial p phase shift only in one of the oscillatory frequencies. We interpret this as a Berry phase arising from the semiclassical electronic Landau orbit which links with the nodal ring when the magnetic field lies in the mirror (ab) plane. Furthermore, additional Berry phase accumulates while rotating the magnetic field for the second orbit in the same orientation which does not link with the nodal ring. These effects are expected in CaAgAs due to the lack of inversion symmetry. Our study experimentally demonstrates that CaAgAs is an ideal platform for exploring the physics of nodal line semimetals and our approach can be extended to other materials in which trivial and nontrivial oscillations are present. 
URL https://ora.ox.ac.uk/objects/uuid:422df41e-14ed-43b0-818f-e65b885071f6
 
Title Quenched nematic criticality separating two superconducting domes in an iron-based superconductor under pressure 
Description The data reflect experimental transport and tunnel diode oscillator measurements collected in zero field or in magnetic fields either in Oxford or at the NHMFL Tallahassee Florida. Measurements were performed at different applied pressuresusing a piston pressure cell. The data set contain raw data plotted in Figures of the manuscript arXiv:1902.11276 (https://arxiv.org/abs/1902.11276). 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact The nematic electronic state and its associated critical fluctuations have emerged as a potential candidate for the superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with magnetically ordered phases poses a significant challenge in determining their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we access a nematic quantum phase transition isolated from any other competing magnetic phases. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure and detect a Lifshitz transition that separates two distinct superconducting regions. One emerges from the nematic phase with a small Fermi surface and strong electronic correlations, while the other one has a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically ordered phase at high pressures. The absence of mass divergence at the nematic quantum phase transition suggests that the nematic fluctuations could be quenched by the strong coupling to the lattice or local strain effects. A direct consequence is the weakening of superconductivity at the nematic quantum phase transition in the absence of magnetically driven fluctuations. 
URL https://ora.ox.ac.uk/objects/uuid:b739b72e-b6a4-4c4b-bc5f-6848546ae274
 
Title Simulations_AMRO_hexagonal 
Description These data were created using Matlab over the last few years. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Impact n/a 
URL https://ora.ox.ac.uk/objects/uuid:ffe7d5fe-00da-497e-b94c-3735eb2edf7d
 
Title Suppression of electronic correlations by chemical pressure from FeSe to FeS 
Description ARPES data were created at the Diamod Light Source and transport data were collected in Oxford. These data are part of the publication with the same title to appear in Phys Rev B, Rapid Communication 2017. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Impact Iron-based chalcogenides are complex superconducting systems in which orbitally-dependent electronic correlations play an important role. Here, using high-resolution angle-resolved photoemission spectroscopy, we investigate the effect of these electronic correlations outside the nematic phase in the tetragonal phase of superconducting FeSe1-xSx (x = 0; 0:18; 1). With increasing sulfur substitution, the Fermi velocities increase significantly and the band renormalizations are suppressed towards a factor of 1.5-2 for FeS. Furthermore, the chemical pressure leads to an increase in the size of the quasi-two dimensional Fermi surface, compared with that of FeSe, however, it remains smaller than the predicted one from first principle calculations for FeS. Our results show that the isoelectronic substitution is an effective way to tune electronic correlations in FeSe1-xSx, being weakened for FeS with a lower superconducting transition temperature. This suggests indirectly that electronic correlations could help to promote higher-Tc superconductivity in FeSe. 
URL https://ora.ox.ac.uk/objects/uuid:6423d3c7-d4a4-443f-9fbc-b2ff254863bc
 
Title Suppression of orbital ordering by chemical pressure in FeSe1-xSx 
Description This data was collected using Diamond Light Source, beamline i05. Some of the raw data is in the nxs format of the beamline. Data were analyzed using Matlab and Origin. Each folder contains the data in different figure of the paper posted on archive: http://arxiv.org/abs/1508.05016. We report a high-resolution angle-resolved photoemission spectroscopy study of the evolution of the electronic structure of FeSe1-xSx single crystals. Isovalent S substitution onto the Se site constitutes a chemical pressure which subtly modifies the electronic structure of FeSe at high temperatures and induces a suppression of the tetragonal-symmetry-breaking structural transition temperature from 87 to 58 K for x = 0.15. With increasing S substitution, we find smaller splitting between bands with dyz and dxz orbital character and weaker anisotropic distortions of the low-temperature Fermi surfaces. These effects evolve systematically as a function of both S substitution and temperature, providing strong evidence that an orbital ordering is the underlying order parameter of the structural transition in FeSe1-xSx . Finally, we detect the small inner hole pocket for x = 0.12, which is pushed below the Fermi level in the orbitally ordered low-temperature Fermi surface of FeSe. 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
Impact Understanding of electronic structure and superconductivity of FeSe1-xSx 
URL https://ora.ox.ac.uk/objects/uuid:be7c2ba3-b358-4840-b77e-7a8b70d32340
 
Title Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe 
Description These data are part of the manuscript "Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe" on https://arxiv.org/abs/1907.13174 which will appear in npj Quantum Materials 2020. The data are magnetotransport data on FeSe thin flakes. These data were mainly generated using a 16T PPMS in Oxford and the thin flakes were preparated at the University of Bath. The magnetotransport data were mainly funded by the Oxford Centre for Applied Superconductivity (CFAS) at Oxford University (www.cfas.ox.ac.uk). 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes, while its monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16 T and temperatures down to 2 K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. The observation of the vortex-antivortex unbinding transition in different flakes provide a direct signature of a dominant two-dimensional pairing channel. However, the upper critical field reflects the evolution of the multi-band nature of superconductivity in FeSe becoming highly two-dimensional and strongly anisotropic only in the thin limit. Our study provides detailed insights into the evolution of the superconducting properties of a multi-band superconductor FeSe in the thin limit in the absence of a dopant substrate. 
URL https://ora.ox.ac.uk/objects/uuid:7ffea1eb-b584-488e-9653-aa9e009186d2
 
Title Ultra-high critical current densities, the vortex phase diagram and the effect of granularity of the stoichiometric high-T c superconductor, CaKFe4As4 
Description These data make up the figures of the paper with the same name to be published in Physical Review Materials. The data are raw .DAT file created mainly by measuring magnetization in magnetic field at different temperatures and analysing the raw data, as described in the paper. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact We present a comprehensive study of the critical current densities and the superconducting vortex phase diagram in the stoichiometric superconductor CaKFe4As4 which has a critical temperature of ~35K. We performed detailed magnetization measurements both of high quality single crystals for different orientations in an applied magnetic field up to 16 T and for a powder sample. We find an extremely large critical current density, Jc, up to 108A/cm2 for single crystals when H?(ab) at 5K, which remains robust in fields up to 16T, being the largest of any other iron-based superconductor. The critical current density is reduced by a factor 10 in single crystals when H?c at 5 K and significantly suppressed by the presence of grain boundaries in the powder sample. We also observe the presence of the fishtail effect in the magnetic hysteresis loops of single crystals when H?c. The flux pinning force density and the pinning parameters suggest that the large critical current could be linked to the existence of point core and surface pinning. Based on the vortex phase diagram and the large critical current densities, CaKFe4As4 is now established as a potential iron-based superconductor candidate for practical applications. 
URL https://ora.ox.ac.uk/objects/uuid:e25d5cab-1a08-4d13-9554-a1b4aa4c406e
 
Title Upper critical field in a stoichiometric iron-based superconductor, CaKFe4As4 
Description These data are part of the manuscript (arXiv:2003.02888) entitled: "Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor, CaKFe4As4". The data represent resistivity data collected at low temperatures and in magnetic fields up 16T using a superconducting magnet in Oxford as well as in pulsed fields up to 80T at the LNCMI in Toulouse, France. The measurements were performed between 2K and 300K either at constant temperature and varying the magnetic field or keeping the field constant and varying the temperature. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact The upper critical field of multiband superconductors is an important quantity that can reveal details about the nature of the superconducting pairing. Here we experimentally map out the complete upper-critical-field phase diagram of a stoichiometric superconductor, CaKFe4As4, up to 90T for different orientations of the magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching values close to ~3Tc at the lowest temperature, and the anisotropy decreases dramatically with temperature, leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the upper critical field can be well described by a two-band model in the clean limit with band-coupling parameters favoring intraband over interband interactions. The large Pauli paramagnetic effects together with the presence of the shallow bands is consistent with the stabilization of an FFLO state at low temperatures in this clean superconductor. 
URL https://ora.ox.ac.uk/objects/uuid:ea841ce1-0e22-412d-a717-4d269b1e47ed
 
Title de Haas--van Alphen study of the role of 4f electrons in antiferromagnetic CeZn11 as compared to its nonmagnetic analogue LaZn11 
Description These are the data generated to produce the publication entitled "de Haas--van Alphen study of the role of 4f electrons in antiferromagnetic CeZn11 as compared to its nonmagnetic analogue LaZn11" by S. F. Blake, H. Hodovanets, A. McCollam, S. L. Bud'ko, P. C. Canfield, and A. I. Coldea published in Physical Review B. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Impact We present a de Haas-van Alphen study of the Fermi surface of the low temperature antiferromagnet CeZn11 and its non-magnetic analogue LaZn11, measured by torque magnetometry up to fields of 33T and at temperatures down to 320 mK. Both systems possess similar de Haas-van Alphen frequencies, with three clear sets of features - ranging from 50T to 4kT - corresponding to three bands of a complex Fermi surface, with an expected fourth band also seen weakly in CeZn11. The effective masses of the charge carriers are very light (<1 me) in LaZn11 but a factor of 2 - 4 larger in CeZn11, indicative of stronger electronic correlations. We perform detailed density functional theory (DFT) calculations for CeZn11 and find that only DFT+U calculations with U=1.5 eV, which localize the 4f states, provide a good match to the measured de Haas-van Alphen frequencies, once the presence of magnetic breakdown orbits is also considered. Our study suggests that the Fermi surface of CeZn11 is very close to that of LaZn11 being dominated by Zn 3d, as the Ce 4f states are localised and have little influence on its electronic structure, however, they are responsible for its magnetic order and contribute to enhance electronic correlations. 
URL https://ora.ox.ac.uk/objects/uuid:e3251001-47a6-405d-9359-02bec24ba363
 
Description ARPES 
Organisation Diamond Light Source
Country United Kingdom 
Sector Private 
PI Contribution Performing, analyzing and publishing work together. We provided samples and developed our own software to analyze ARPES data on Fe based superconductors.
Collaborator Contribution Provide technical help and complementary analysis tools.
Impact Beamtime awarded on the Diamond I05 beamline. Training of a PhD student which now is employed by the Diamond Light Source Title: Emergence of the nematic electronic state in FeSe Author(s): Watson, M. D.; Kim, T. K.; Haghighirad, A. A.; et al. Source: Physical Review B Volume: 91 Issue: 15 Published: 2015 DOI: 10.1103/PhysRevB.91.155106 Title: Suppression of orbital ordering by chemical pressure in FeSe1-xSx Author(s): Watson, M. D.; Kim, T. K.; Haghighirad, A. A.; et al. Source: Physical Review B Volume: 92 Issue: 12 Published: 2015 DOI: 10.1103/PhysRevB.92.121108
Start Year 2014
 
Description DFT collaboration 
Organisation Goethe University Frankfurt
Country Germany 
Sector Academic/University 
PI Contribution DFT and DMFT calculations
Collaborator Contribution Experimental and computational studies to understand the electronic structure and electronic correlations of FeSe measured using angle-resolved photoemission spectroscopy.
Impact Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe Matthew D. Watson, Steffen Backes, Amir A. Haghighirad, Moritz Hoesch, Timur K. Kim, Amalia I. Coldea, and Roser Valentí Phys. Rev. B 95, 081106(R) - Published 22 February 2017
Start Year 2011
 
Description Institute of High Pressure Physics 
Organisation Polish Academy of Sciences
Department Institute of High Pressure Physics
Country Poland 
Sector Public 
PI Contribution Development of superconducting wires based on fundamental research on new candidate superconductors.
Collaborator Contribution Institute of High Pressure Physics (IHPP), also known as "Unipress", was founded in 1972 by Polish Academy of Science and contributes to the development of superconducting wires.
Impact Development of superconducting wires for magnet development technology.
Start Year 2018
 
Description Oxford Quantum Materials 
Organisation University of Oxford
Department Quantum Materials
Country United Kingdom 
Sector Academic/University 
PI Contribution Oxford Quantum Materials Platform Grant
Collaborator Contribution Oxford Quantum Materials Platform Grant
Impact Oxford Quantum Materials Platform Grant (1,736,109) Physics, Materials, Chemistry Recent relevant publication: Ideal Weyl semimetal induced by magnetic exchange J.-R. Soh, F. de Juan, M. G. Vergniory, N. B. M. Schröter, M. C. Rahn, D. Y. Yan, J. Jiang, M. Bristow, P. Reiss, J. N. Blandy, Y. F. Guo, Y. G. Shi, T. K. Kim, A. McCollam, S. H. Simon, Y. Chen, A. I. Coldea, and A. T. Boothroyd Phys. Rev. B 100, 201102(R) - Published 13 November 2019
Start Year 2013
 
Description OUTREACH Schools 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Talks and demonstrations to primary school children

The children got excited about the science presented. Parents confirmed the children interest in the presented talk and demonstrations.
Year(s) Of Engagement Activity 2011,2012,2013,2014
 
Description Open days 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Laboratory visits from public and prospective students to present active research;
Year(s) Of Engagement Activity 2014
 
Description Oxford Symposium on Quantum Materials 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? Yes
Type Of Presentation workshop facilitator
Geographic Reach Regional
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Research activities were presented. Collaborations were formed. New experiments performed.

This is an annual interdisciplinary forum to bring together physicists, chemists, materials scientists and theoreticians in and around Oxford to advance the science and promote direct collaboration between groups interested in novel quantum materials and phenomena. This is an annual forum which has generated discussions with scientists and industry and collaborations.
Year(s) Of Engagement Activity 2011,2012,2013,2014
URL https://www2.physics.ox.ac.uk/research/quantum-materials/group-activities/oxford-symposium-on-quantu...
 
Description Superconductivity -laboratory visits 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Annual involvement in laboratory tours to explain to the public the scientific significance and the practical applications of superconductivity.
Year(s) Of Engagement Activity 2017,2018,2019
URL https://www2.physics.ox.ac.uk/about-us/outreach