Phase Diagram of CeCoIn_5 in the Vicinity of H_{c2} as Determined by NMR

TL;DR

This study uses ^{115}In NMR to map the phase diagram of CeCoIn_5 near H_{c2}, revealing phase transitions and the emergence of the FFLO state, aligning with thermodynamic data.

Contribution

First NMR-based phase diagram of CeCoIn_5 near H_{c2} identifying phase transitions and the FFLO state.

Findings

Identification of two phase transitions near H_{c2}

Observation of a continuous transition to the FFLO state

Agreement with thermodynamic phase diagram

Abstract

We report ^{115}In nuclear magnetic resonance (NMR) measurements in the heavy-fermion superconductor CeCoIn_5 as a function of temperature in different magnetic fields applied parallel to the $(\hat a, \hat b)$ plane. The measurements probe a part of the phase diagram in the vicinity of the superconducting critical field H_{c2} where a possible inhomogeneous superconducting state, Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), is stabilized. We have identified clear NMR signatures of two phase transitions occurring in this part of the phase diagram. The first order phase transitions are characterized by the sizable discontinuity of the shift. We find that a continuous second order phase transition from the superconducting to the FFLO state occurs at temperature below which the shift becomes temperature independent. We have compiled the first phase diagram of CeCoIn_5 in the vicinity of H_{c2} from NMR data and found that it is in agreement with the one determined by thermodynamic measurements.