CV

Postdoctoral Research Associate

Department of Physics, University of Bath

Postdoctoral Research Associate

School of Physics, University of Bristol

PhD

University of Bristol

Condensed Matter Physics

MSci

University of Bristol

Physics

Superconducting enegry gap structure of CsV3Sb5 from magnetic penetration depth measurements

Journal of Physics: Condensed Matter

Experimental determination of the structure of the superconducting order parameter in the kagome lattice compound CsV3Sb5 is an essential step towards understanding the nature of the superconducting pairing in this material. Here we report measurements of the temperature dependence of the in-plane magnetic penetration depth, λ(T), in crystals of CsV3Sb5 down to ~60 mK. We find that λ(T) is consistent with a fully-gapped state but with significant gap anisotropy. The magnitude of the gap minima are in the range ~0.2–0.3 Tc  for the measured samples, markedly smaller than previous estimates. We discuss different forms of potential anisotropy and how these can be linked to the V and Sb Fermi surface sheets. We highlight a significant discrepancy between the calculated and measured values of λ(T=0) which we suggest is caused by spatially suppressed superconductivity.

Observation of the Non-Linear Meissner Effect

Nature Communications

A long-standing theoretical prediction is that in clean, nodal unconventional superconductors the magnetic penetration depth λ, at zero temperature, varies linearly with magnetic field. This non-linear Meissner effect is an equally important manifestation of the nodal state as the well studied linear-in-T dependence of λ, but has never been convincingly experimentally observed. Here we present measurements of the nodal superconductors CeCoIn5 and LaFePO which clearly show this non-linear Meissner effect. We further show how the effect of a small dc magnetic field on λ(T) can be used to distinguish gap nodes from non-nodal deep gap minima. Our measurements of KFe2As2 suggest that this material has such a non-nodal state.

Full-Gap Superconductivity Robust against Disorder in Heavy-Fermion CeCu2Si2

Physical Review Letters

A key aspect of unconventional pairing by the antiferromagnetic spin-fluctuation mechanism is that the superconducting energy gap must have the opposite sign on different parts of the Fermi surface. Recent observations of non-nodal gap structure in the heavy-fermion superconductor CeCu2⁢Si2 were then very surprising, given that this material has long been considered a prototypical example of a superconductor where the Cooper pairing is magnetically mediated. Here we present a study of the effect of controlled point defects, introduced by electron irradiation, on the temperature-dependent magnetic penetration depth 𝜆⁡(𝑇) in CeCu2⁢Si2. We find that the fully gapped state is robust against disorder, demonstrating that low-energy bound states, expected for sign-changing gap structures, are not induced by nonmagnetic impurities. This provides bulk evidence for 𝑠++-wave superconductivity without sign reversal.

Fully gapped superconductivity with no sign change in the prototypical heavy-fermion CeCu2Si2

Science Advances

In exotic superconductors, including high-Tc copper oxides, the interactions mediating electron Cooper pairing are widely considered to have a magnetic rather than a conventional electron-phonon origin. Interest in this exotic pairing was initiated by the 1979 discovery of heavy-fermion superconductivity in CeCu2Si2, which exhibits strong antiferromagnetic fluctuations. A hallmark of unconventional pairing by anisotropic repulsive interactions is that the superconducting energy gap changes sign as a function of the electron momentum, often leading to nodes where the gap goes to zero. We report low-temperature specific heat, thermal conductivity, and magnetic penetration depth measurements in CeCu2Si2, demonstrating the absence of gap nodes at any point on the Fermi surface. Moreover, electron irradiation experiments reveal that the superconductivity survives even when the electron mean free path becomes substantially shorter than the superconducting coherence length. This indicates that superconductivity is robust against impurities, implying that there is no sign change in the gap function. These results show that, contrary to long-standing belief, heavy electrons with extremely strong Coulomb repulsions can condense into a fully gapped s-wave superconducting state, which has an on-site attractive pairing interaction.