First order superconducting phase transition in CeCoIn$_5$

TL;DR

This study demonstrates that CeCoIn_5 exhibits a first order superconducting phase transition under high magnetic fields aligned with the crystal's c-axis, caused by Pauli limiting, with no evidence of FFLO state.

Contribution

It provides experimental evidence of a first order transition in CeCoIn_5 driven by Pauli limiting, confirming theoretical predictions and clarifying the nature of its phase transition.

Findings

Transition becomes first order above 4.7 T along [001]

No evidence of FFLO state in CeCoIn_5 with H || [001]

Strong anomalies in specific heat and thermal expansion at transition

Abstract

We investigated the magnetic field dependence of the superconducting phase transition in heavy fermion CeCoIn_5 (T_c = 2.3 K) using specific heat, magneto-caloric effect, and thermal expansion measurements. The superconducting transition becomes first order when the magnetic field is oriented along the 001 crystallographic direction with a strength greater that 4.7 T, and transition temperature below T_0 ~ 0.31 T_c. The change from second order at lower fields is reflected in strong sharpening of both specific heat and thermal expansion anomalies associated with the phase transition, a strong magnetocaloric effect, and a step-like change in the sample volume. The first order superconducting phase transition in CeCoIn_5 is caused by Pauli limiting in type-II superconductors, and was predicted theoretically in the mid 1960s. We do not see evidence for the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state (predicted by an alternative theory also dating back to mid-60's) in CeCoIn_5 with field H || [001].