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.

Magnetic order and disorder in a quasi-two-dimensional quantum Heisenberg antiferromagnet with randomized exchange

PHYSICAL REVIEW B 102 (2020) ARTN 174429

F Xiao, WJA Blackmore, BM Huddart, M Gomilsek, TJ Hicken, C Baines, PJ Baker, FL Pratt, SJ Blundell, H Lu, J Singleton, D Gawryluk, MM Turnbull, KW Kramer, PA Goddard, T Lancaster

Observation of a neutron spin resonance in the bilayered superconductor CsCa<sub>2</sub>Fe<sub>4</sub>As<sub>4</sub>F<sub>2</sub>.

Journal of physics. Condensed matter : an Institute of Physics journal 32 (2020) 435603-

DT Adroja, SJ Blundell, F Lang, H Luo, Z-C Wang, G-H Cao

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa<sub>2</sub>Fe<sub>4</sub>As<sub>4</sub>F<sub>2</sub> above and below its superconducting transition temperature T <sub>c</sub> ≈ 28.9 K to investigate the presence of a neutron spin resonance. This compound crystallises in a body-centred tetragonal lattice containing asymmetric double layers of Fe<sub>2</sub>As<sub>2</sub> separated by insulating CaF<sub>2</sub> layers and is known to be highly anisotropic. Our INS study clearly reveals the presence of a neutron spin resonance that exhibits higher intensity at lower momentum transfer (Q) at 5 K compared to 54 K, at an energy of 15 meV. The energy E <sub>R</sub> of the observed spin resonance is broadly consistent with the relationship E <sub>R</sub> = 4.9k <sub>B</sub> T <sub>c</sub>, but is slightly enhanced compared to the values observed in other Fe-based superconductors. We discuss the nature of the electron pairing symmetry by comparing the value of E <sub>R</sub> with that deduced from the total superconducting gap value integrated over the Fermi surface.

Magnetically driven loss of centrosymmetry in metallic Pb2CoOsO6

PHYSICAL REVIEW B 102 (2020) ARTN 104410

AJ Princep, HL Feng, YF Guo, F Lang, HM Weng, P Manuel, D Khalyavin, A Senyshyn, MC Rahn, YH Yuan, Y Matsushita, SJ Blundell, K Yamaura, AT Boothroyd

Extremely well isolated two-dimensional spin-1/2 antiferromagnetic Heisenberg layers with a small exchange coupling in the molecular-based magnet CuPOF

PHYSICAL REVIEW B 102 (2020) ARTN 064431

D Opherden, N Nizar, K Richardson, JC Monroe, MM Turnbull, M Polson, S Vela, WJA Blackmore, PA Goddard, J Singleton, ES Choi, F Xiao, RC Williams, T Lancaster, FL Pratt, SJ Blundell, Y Skourski, M Uhlarz, AN Ponomaryov, SA Zvyagin, J Wosnitza, M Baenitz, I Heinmaa, R Stern, H Kuhne, CP Landee

Information and Decoherence in a Muon-Fluorine Coupled System


J Wilkinson, S Blundell

Strong in-plane anisotropy in the electronic structure of fixed-valence $β$-LuAlB$_4$

Physical Review B: Condensed Matter and Materials Physics American Physical Society (2020)

P Reiss, J Baglo, H Tan, X Chen, S Friedemann, K Kuga, FM Grosche, S Nakatsuji, M Sutherland

The origin of intrinsic quantum criticality in the heavy-fermion superconductor $\beta$-YbAlB$_4$ has been attributed to strong Yb valence fluctuations and its peculiar crystal structure. Here, we assess these contributions individually by studying the isostructural but fixed-valence compound $\beta$-LuAlB$_4$. Quantum oscillation measurements and DFT calculations reveal a Fermi surface markedly different from that of $\beta$-YbAlB$_4$, consistent with a `large' Fermi surface there. We also find an unexpected in-plane anisotropy of the electronic structure, in contrast to the isotropic Kondo hybridization in $\beta$-YbAlB$_4$.

Dynamic spin fluctuations in the frustrated A-site spinel CuAl2O4

PHYSICAL REVIEW B 102 (2020) ARTN 014439

H Cho, R Nirmala, J Jeong, PJ Baker, H Takeda, N Mera, SJ Blundell, M Takigawa, DT Adroja, J-G Park

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>

Enhancing easy-plane anisotropy in bespoke Ni(II) quantum magnets

Polyhedron 180 (2020)

JL Manson, ZE Manson, A Sargent, DY Villa, NL Etten, WJA Blackmore, SPM Curley, RC Williams, J Brambleby, PA Goddard, A Ozarowski, MN Wilson, BM Huddart, T Lancaster, RD Johnson, SJ Blundell, J Bendix, KA Wheeler, SH Lapidus, F Xiao, S Birnbaum, J Singleton

© 2020 The Authors We examine the crystal structures and magnetic properties of several S = 1 Ni(II) coordination compounds, molecules and polymers, that include the bridging ligands HF2−, AF62− (A = Ti, Zr) and pyrazine or non-bridging ligands F−, SiF62−, glycine, H2O, 1-vinylimidazole, 4-methylpyrazole and 3-hydroxypyridine. Pseudo-octahedral NiN4F2, NiN4O2 or NiN4OF cores consist of equatorial Ni-N bonds that are equal to or slightly longer than the axial Ni-Lax bonds. By design, the zero-field splitting (D) is large in these systems and, in the presence of substantial exchange interactions (J), can be difficult to discriminate from magnetometry measurements on powder samples. Thus, we relied on pulsed-field magnetization in those cases and employed electron-spin resonance (ESR) to confirm D when J ≪ D. The anisotropy of each compound was found to be easy-plane (D > 0) and range from ≈ 8–25 K. This work reveals a linear correlation between the ratio d(Ni-Lax)/d(Ni-Neq) and D although the ligand spectrochemical properties may play an important role. We assert that this relationship allows us to predict the type of magnetocrystalline anisotropy in tailored Ni(II) quantum magnets.

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

Phase transitions for beginners


SJ Blundell

Group theory for physicists, 2nd edition


SJ Blundell

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.

SquidLab-A user-friendly program for background subtraction and fitting of magnetization data.

The Review of scientific instruments 91 (2020) 023901-

MJ Coak, C Liu, DM Jarvis, S Park, MJ Cliffe, PA Goddard

We present an open-source program free to download for academic use with a full user-friendly graphical interface for performing flexible and robust background subtraction and dipole fitting on magnetization data. For magnetic samples with small moment sizes or sample environments with large or asymmetric magnetic backgrounds, it can become necessary to separate background and sample contributions to each measured raw voltage measurement before fitting the dipole signal to extract magnetic moments. Originally designed for use with pressure cells on a Quantum Design MPMS3 SQUID magnetometer, SquidLab is a modular object-oriented platform implemented in Matlab with a range of importers for different widely available magnetometer systems (including MPMS, MPMS-XL, MPMS-IQuantum, MPMS3, and S700X models) and has been tested with a broad variety of background and signal types. The software allows background subtraction of baseline signals, signal preprocessing, and performing fits to dipole data using Levenberg-Marquardt non-linear least squares or a singular value decomposition linear algebra algorithm that excels at picking out noisy or weak dipole signals. A plugin system allows users to easily extend the built-in functionality with their own importers, processes, or fitting algorithms. SquidLab can be downloaded, under Academic License, from the University of Warwick depository (

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

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

Exsolution of SrO during the topochemical conversion of LaSr3CoRuO8 to the oxyhydride LaSr3CoRuO4H4

Inorganic Chemistry American Chemical Society 58 (2019) 14863-14870

L Jin, M Batuk, FKK Kirschner, F Lang, SJ Blundell, J Hadermann, M Hayward

Reaction of the n = 1 Ruddlesden-Popper oxide LaSr3CoRuO8 with CaH2 yields the oxyhydride phase LaSr3CoRuO4H4 via a topochemical anion exchange. Close inspection of the X-ray and neutron powder diffraction data in combination with HAADF-STEM images reveals that the nanoparticles of SrO are exsolved from the system during the reaction, with the change in cation stoichiometry accommodated by the inclusion of n &gt; 1 (Co/Ru)nOn+1H2n "perovskite" layers into the Ruddlesden-Popper stacking sequence. This novel pseudotopochemical process offers a new route for the formation of n &gt; 1 Ruddlesden-Popper structured materials. Magnetization data are consistent with a LaSr3Co+Ru2+O4H4 (Co+, d8, S = 1; Ru2+, d6, S = 0) oxidation/spin state combination. Neutron diffraction and μ+SR data show no evidence for long-range magnetic order down to 2 K, suggesting the diamagnetic Ru2+ centers impede the Co-Co magnetic-exchange interactions.

A review of modern ophthalmic optics

CONTEMPORARY PHYSICS 60 (2019) 330-331

SJ Blundell