Observations of the effect of strong Pauli paramagnetism on the vortex lattice in superconducting CeCu2Si2

Probing superconducting order in overdoped Ca$_{x}$Y$_{1-x}$Ba$_{2}$Cu$_{3}$O$_{7}$ by neutron diffraction measurements of the vortex lattice

“…This would increase the field contrast between the cores and the bulk which is observed as a corresponding increase in the VL form factor. This has also been observed in heavy-fermion superconductors [52][53][54], a borocarbide [55] and an iron-based superconductor [56]. We have speculated that in YBa 2 Cu 3 O 7 the continued deviation of the VL form factor in the parent compound at the highest measured fields [32] could have a similar origin.…”

“…We have speculated that in YBa 2 Cu 3 O 7 the continued deviation of the VL form factor in the parent compound at the highest measured fields [32] could have a similar origin. While the models constructed to describe the behaviour of the heavy fermion system, CeCoIn 5 [57,58], are not quantitatively appropriate for our results, we can draw several qualitative conclusions from them, and from more recent work [52]. Firstly, since we are at relatively small fractions of H/H c2 , we expect the Pauli contribution to be small compared to the orbital component, leading to a deviation from the London model as we see here rather than the dominance of Pauli effects.…”

“…Pauli paramagnetic pair breaking is one of two possible mechanisms by which superconductivity is destroyed at H c2 , so at such low fields it is not expected to be relevant in the bulk. However, within the vortex core region the Cooper pairs are much less strongly bound, and so this can lead to the formation of a paramagnetic moment and the corresponding alteration of the vortex core structure [51,52]. This would increase the field contrast between the cores and the bulk which is observed as a corresponding increase in the VL form factor.…”

Deviations from the extended London model at high magnetic fields in YBa$_2$Cu$_3$O$_7$

“…Such effects must be present in all singlet-pairing superconductors, but will be negligible unless µ B B c2 ≥ k B T c , so that the Zeeman energy of the electron spins is comparable with the zero-field energy gap. Pauli paramagnetic effects have been observed in heavy-fermion materials such as CeCoIn 5 [28,29], CeCu 2 Si 2 [30], a borocarbide [31] and an iron-based superconductor [32], but not to our knowledge in a high-T c cuprate. Nevertheless, Pauli-paramagnetic effects are expected in our sample, because it satisfies µ B B c2 k B T c .…”