Observation of Spin Susceptibility Enhancement in the Possible FFLO State in CeCoIn$_5$

TL;DR

This study uses NMR measurements to identify an FFLO state in CeCoIn$_5$, revealing a significant increase in spin susceptibility in this inhomogeneous superconducting phase, which enhances understanding of unconventional superconductivity.

Contribution

First direct NMR evidence of spin susceptibility enhancement in the FFLO state of CeCoIn$_5$, clarifying the nature of this inhomogeneous superconducting phase.

Findings

Detection of two phase transitions near $H_{c2}$

Significant spin susceptibility increase in the FFLO state

Confirmation of inhomogeneous superconductivity in CeCoIn$_5"

Abstract

We report $^{115}$In nuclear magnetic resonance (NMR) measurements of the heavy-fermion superconductor CeCoIn$_5$ in the vicinity of the superconducting critical field $H_{c2}$ for a magnetic field applied perpendicular to the $\hat c$-axis. A possible inhomogeneous superconducting state, Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), is stabilized in this part of the phase diagram. In 11 T applied magnetic field, we observe clear signatures of the two phase transitions: the higher temperature one to the homogeneous superconducting state and the lower temperature phase transition to a FFLO state. It is found that the spin susceptibility in the putative FFLO state is significantly enhanced as compared to the value in a homogeneous superconducting state. Implications of this finding for the nature of the low temperature phase are discussed.