Publications by Amalia Coldea

Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe

npj Quantum Materials Nature Research (part of Springer Nature) (2020)

L Farrar, M Bristow, AA Haghighirad, A McCollam, SJ Bending, AMALIA Coldea

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 16T and temperatures down to 2K 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. In the thin limit we detect signatures of a crossover towards two-dimensional behaviour from the observation of the vortex-antivortex unbinding transition and strongly enhanced anisotropy. Our study provides detailed insights into the evolution of the superconducting properties from three-dimensional bulk behaviour towards the two-dimensional limit of FeSe in the absence of a dopant substrate.

Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

Physical Review B American Physical Society 101 (2020) 134502

M Bristow, W Knafo, P Reiss, W Meier, PC Canfield, SJ Blundell, A Coldea

<p>The upper critical field of multiband superconductors is an important quantity that can reveal details about&nbsp;the nature of the superconducting pairing. Here we experimentally map out the complete upper-critical-field&nbsp;phase diagram of a stoichiometric superconductor, CaKFe4As4, up to 90 T for different orientations of the&nbsp;magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching&nbsp;values close to &sim;3 Tc at the lowest temperature, and the anisotropy decreases dramatically with temperature,&nbsp;leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the&nbsp;upper critical field can be well described by a two-band model in the clean limit with band-coupling parameters&nbsp;favoring intraband over interband interactions. The large Pauli paramagnetic effects together with the presence&nbsp;of the shallow bands is consistent with the stabilization of an FFLO state at low temperatures in this clean&nbsp;superconductor.</p>

Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1-xSx

Phys. Rev. Research 2, 013309 (2020) (2020)

M Bristow, P Reiss, AA Haghighirad, Z Zajicek, SHIV Singh, T Wolf, D Graf, W Knafo, A McCollam, AMALIA Coldea

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 FeSe$_{1-x}$S$_x$ 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 $T^{1.5}$ resistivity. The transverse magnetoresistance inside the nematic phase has as a $H^{1.55}$ 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

Quantum oscillations probe the Fermi surface topology of the nodal-line semimetal CaAgAs

Physical Review Research American Physical Society 2 (2020) 012055(R)

YH Kwan, P Reiss, Y Han, M Bristow, D Prabhakaran, D Graf, A McCollam, S Ashok Parameswaran, AI Coldea

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 π 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.

Quenched nematic criticality and two superconducting domes in an iron-based superconductor under pressure

Nature Physics 16, 89–94 (2020) Nature Research (2019)

P Reiss, D Graf, AA Haghighirad, W Knafo, L Drigo, M Bristow, AJ Schofield, AI Coldea

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.

Optimization of superconducting properties of the stoichiometric CaKFe4As4

Supercond. Sci. Technol. 33 (2020) 025003 IOP Publishing (2019)

SJ Singh, SJ Cassidy, M Bristow, S Blundell, SJ Clarke, AI Coldea

Evolution of the low-temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition

Nature npj Quantum Materials Springer Nature 4 (2019) 2

AI Coldea, SF Blake, S Kasahara, AA Haghighirad, MD Watson, W Knafo, ES Choi, A McCollam, P Reiss, T Yamashita, M Bruma, SC Speller, Y Matsuda, T Wolf, T Shibauchi, AJ Schofield

An ideal Weyl semimetal induced by magnetic exchange

Physical review B: Condensed matter and materials physics American Physical Society 100 (2019) 201102(R)

J-R Soh, F De Juan, M Vergniory, N Schroeter, M Rahn, DY Yan, J Jiang, M Bristow, P Reiss, J Blandy, Y Guo, Y Shi, T Kim, A McCollam, S Simon, Y Chen, A Coldea, A Boothroyd

Significant change in the electronic behavior associated with structural distortions in monocrystalline SrAg4As2

PHYSICAL REVIEW B 98 (2018) ARTN 235130

B Shen, E Emmanouilidou, X Deng, A McCollam, J Xing, G Kotliar, AI Coldea, N Ni

Ultra-high critical current densities, the vortex phase diagram and the effect of granularity of the stoichiometric high-Tc superconductor, CaKFe4As4

Physical Review Materials American Physical Society 2 (2018) 074802-

S Singh, M Bristow, WR Meier, P Taylor, S Blundell, PC Canfield, A Coldea

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 ∼35 K. 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 108 A/cm2 for single crystals when H(ab) at 5 K, which remains robust in fields up to 16 T, being the largest of any other iron-based superconductor. The critical current density is reduced by a factor 10 in single crystals when Hc 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 Hc. 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.

Multi-band magnetotransport in exfoliated thin films of CuxBi2Se3

Journal of Physics: Condensed Matter IOP Publishing 30 (2018) 155302

JA Alexander-Webber, J Huang, J Beilsten-Edmands, P Cermak, C Drasar, RJ Nicholas, A Coldea

We report magnetotransport studies in thin (&lt;100 nm) exfoliated films of CuxBi2Se3 and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with Tc ∼ 3.5 K 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 &lt; 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 Lφ &gt; 100 nm indicating the presence of topologically protected surface states.

The key ingredients of the electronic structure of FeSe

Annual Reviews of Condensed Matter Physics, Vol. 9, 125-146, 2018 (2018)

AI Coldea, MD Watson

FeSe is a fascinating superconducting material at the frontier of research in condensed matter physics. Here we provide an overview on the current understanding of the electronic structure of FeSe, focusing in particular on its low energy electronic structure as determined from angular resolved photoemission spectroscopy, quantum oscillations and magnetotransport measurements of single crystal samples. We discuss the unique place of FeSe amongst iron-based superconductors, being a multi-band system exhibiting strong orbitally-dependent electronic correlations and unusually small Fermi surfaces, prone to different electronic instabilities. We pay particular attention to the evolution of the electronic structure which accompanies the tetragonal-orthorhombic structural distortion of the lattice around 90 K, which stabilizes a unique nematic electronic state. Finally, we discuss how the multi-band multi-orbital nematic electronic structure has an impact on the understanding of the superconductivity, and show that the tunability of the nematic state with chemical and physical pressure will help to disentangle the role of different competing interactions relevant for enhancing superconductivity.

Suppression of electronic correlations by chemical pressure from FeSe to FeS

Phys. Rev. B 96, 121103(R) (2017) American Physical Society (2017)

P Reiss, MD Watson, TK Kim, AA Haghighirad, DN Woodruff, M Bruma, SJ Clarke, AI Coldea

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.

Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe

PHYSICAL REVIEW B 95 , 081106(R) (2017) (2017)

MD Watson, S Backes, AA Haghighirad, M Hoesch, TK Kim, AI Coldea, R Valenti

Evidence for unidirectional nematic bond ordering in FeSe

Physical Review B - Condensed Matter and Materials Physics American Physical Society (2016)

MD Watson, TK Kim, LC Rhodes, M Eschrig, M Hoesch, AA Haghighirad, AI Coldea

The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations.

Modeling the angle-dependent magnetoresistance oscillations of Fermi surfaces with hexagonal symmetry

Physical Review B American Physical Society 93 (2016) 245105

JCA Prentice, A Coldea

By solving the Boltzmann transport equation we investigate theoretically the general form of oscillations in the resistivity caused by varying the direction of an applied magnetic field for the case of quasi-two dimensional systems on hexagonal lattices. The presence of the angular magnetoresistance oscillations can be used to map out the topology of the Fermi surface and we study how this effect varies as a function of the degree of interplane warping as well as a function of the degree of isotropic scattering. We find that the angular dependent effect due to in-plane rotation follows the symmetry imposed by the lattice whereas for inter-plane rotation the degree of warping dictates the dominant features observed in simulations. Our calculations make predictions for specific angle-dependent magnetotransport signatures in magnetic fields expected for quasi-two dimensional hexagonal compounds similar to PdCoO2 and PtCoO2.

Publisher's Note: Dichotomy between the Hole and Electron Behavior in Multiband Superconductor FeSe Probed by Ultrahigh Magnetic Fields [Phys. Rev. Lett. 115, 027006 (2015)].

Physical review letters 115 (2015) 219902-

MD Watson, T Yamashita, S Kasahara, W Knafo, M Nardone, J Béard, F Hardy, A McCollam, A Narayanan, SF Blake, T Wolf, AA Haghighirad, C Meingast, AJ Schofield, H V Löhneysen, Y Matsuda, AI Coldea, T Shibauchi

Suppression of orbital ordering by chemical pressure in FeSe1-xSx

Phys. Rev. B 92, 121108(R) (2015) (2015)

MD Watson, TK Kim, AA Haghighirad, SF Blake, NR Davies, M Hoesch, T Wolf, AI Coldea

We report a high-resolution angle-resolved photo-emission 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 87K to 58K 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.

Dichotomy between the hole and electrons behavior in the multiband FeSe probed by ultra high magnetic fields

Phys. Rev. Lett. 115, 027006 (2015) (2015)

MD Watson, T Yamashita, S Kasahara, W Knafo, M Nardone, J Beard, F Hardy, A McCollam, A Narayanan, SF Blake, T Wolf, AA Haghighirad, C Meingast, AJ Schofield, HV Lohneysen, Y Matsuda, AI Coldea, T Shibauchi

Magnetoresistivity \r{ho}xx and Hall resistivity \r{ho}xy in ultra high magnetic fields up to 88T are measured down to 0.15K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel and independent approach to identify the sign of corresponding cyclotron orbit in a compensated metal from magnetotransport measurements. The observed significant differences in the relative amplitudes of the quantum oscillations between the \r{ho}xx and \r{ho}xy components, together with the positive sign of the high-field \r{ho}xy , reveal that the largest pocket should correspond to the hole band. The low-field magnetotransport data in the normal state suggest that, in addition to one hole and one almost compensated electron bands, the orthorhombic phase of FeSe exhibits an additional tiny electron pocket with a high mobility.

Emergence of the nematic electronic state in FeSe (vol 91, 155106, 2015)

PHYSICAL REVIEW B 91 (2015) ARTN 199905

MD Watson, TK Kim, AA Haghighirad, NR Davies, A McCollam, A Narayanan, SF Blake, YL Chen, S Ghannadzadeh, AJ Schofield, M Hoesch, C Meingast, T Wolf, AI Coldea