The heavy-fermion superconductor CeCoIn5 displays an additional transition within its superconducting (SC) state, whose nature is characterized by high-precision studies of the isothermal field dependence of the entropy, derived from combined specific heat and magnetocaloric effect measurements at temperatures T≥100 mK and fields H≤12 T aligned parallel, perpendicular and 18∘ off the tetragonal [100] direction. For any of these conditions, we do not observe an additional entropy contribution upon tuning at constant temperature by magnetic field from the homogeneous SC into the presumed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) SC state. By contrast, for H∥[100] a negative isothermal entropy contribution, compatible with spin-density-wave (SDW) ordering, is found. Our data exclude the formation of a FFLO state in CeCoIn5 for out-of-plane field directions, where no SDW order exists.
@article{arxiv.1203.5567,
title = {Quasiparticle entropy in the high-field superconducting phase of CeCoIn$_5$},
author = {Y. Tokiwa and E. D. Bauer and P. Gegenwart},
journal= {arXiv preprint arXiv:1203.5567},
year = {2013}
}