Quasiparticle entropy in the high-field superconducting phase of CeCoIn$_5$

TL;DR

This study investigates the entropy behavior of CeCoIn$_5$ under high magnetic fields, finding no evidence for the FFLO state but detecting SDW order for certain field orientations, refining understanding of its superconducting phases.

Contribution

The paper provides high-precision entropy measurements that exclude the formation of the FFLO state in CeCoIn$_5$ for out-of-plane fields and identify SDW order for in-plane fields.

Findings

No additional entropy contribution indicating FFLO state.

Detection of SDW order for H//[100].

Exclusion of FFLO formation for out-of-plane fields.

Abstract

The heavy-fermion superconductor CeCoIn$_5$ 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 $T\geq 100$ mK and fields $H\leq 12$ T aligned parallel, perpendicular and $18^\circ$ 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 $H\parallel [100]$ a negative isothermal entropy contribution, compatible with spin-density-wave (SDW) ordering, is found. Our data exclude the formation of a FFLO state in CeCoIn$_5$ for out-of-plane field directions, where no SDW order exists.