Abstract
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.
Highlights
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
We present measurements of λ(H) for two putative nodal superconductors, CeCoIn5 and LaFePO, which show convincingly that the predicted field dependence of λ is present in nodal superconductors and this field dependence decreases with increasing temperature as expected
We show how the combination of the effect of field and temperature on λ can be used to distinguish between the case where there is a small density of impurities in a nodal superconductor from the case where there is a small but finite energy gap
Summary
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 important manifestation of the nodal state as the well studied linear-in-T dependence of λ, but has never been convincingly experimentally observed. The change in λ with the field, Δλ(H), was found to be approximately linear in the cuprate superconductor YBa2Cu3O6+x (Y123)[5,6], Δλ(H) had a very weak temperature and angle dependence The latter might be explained by the orthorhombicity of Y1237 but the lack of temperature dependence of Δλ(H) is in serious disagreement with theory and strongly suggests that the observed effects in Y123 were of extrinsic origin[5,6]. KFe2As2 is more controversial; λ(T) has a strong, quasi-linear dependence consistent with gap nodes[24], specific heat measurements[25] show evidence for very small gaps which may be difficult to distinguish from true nodes. κ(T) measurements have been argued to show universal behaviour consistent with d-wave order[26], this has been disputed[27]
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