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  Campillo, E.; Riyat, R.; Pollard, S.; Jefferies, P.; Holmes, A.T.; Cubitt, R.; White, J.S.; Gavilano, J.; Huesges, Z.; Stockert, O.; Forgan, E.M.; Blackburn, E.: Observations of the effect of strong Pauli paramagnetism on the vortex lattice in superconducting CeCu₂Si₂. Physical Review B 104 (2021), p. 184508/1-8

10.1103/physrevb.104.184508
Open Access Version

Abstract:
We present the results of a study of the vortex lattice in the heavy fermion superconductor CeCu2Si2, using small-angle neutron scattering (SANS). In this material at temperatures well below Tc∼0.6 K, the value of the upper critical field Bc2∼2.2 T is strongly limited by the Pauli paramagnetism of the heavy fermions. In this temperature region, our SANS data show an increase in the magnetization of the flux line cores with field, followed by a rapid fall near Bc2. This behavior is the effect of Pauli paramagnetism and we present a theory-based model, which can be used to describe this effect in a range of materials. The pairing in CeCu2Si2 appears to arise from the effect of magnetic fluctuations, but the evidence for a d-wave order parameter is rather weak. We find that the vortex lattice structure in CeCu2Si2 is close to regular hexagonal. There are no phase transitions to square or rhombic structures; such transitions are expected for d-wave superconductors and observed in CeCoIn5; however, the temperature dependence of the SANS intensity indicates that both large and small gap values are present, most likely due to multiband s-wave superconductivity, rather than a nodal gap structure.