New High Field State of Flux Line Lattice in Unconventional Superconductor CeCoIn_5

TL;DR

This study uncovers a novel high-field flux line lattice phase in the unconventional superconductor CeCoIn_5, characterized by a structural transition likely corresponding to the FFLO phase with spatially modulated order parameter.

Contribution

It provides experimental evidence of a high-field flux line lattice phase transition in CeCoIn_5, supporting the existence of the FFLO phase with a unique structural transformation.

Findings

Identification of a flux line lattice transition near the upper critical field

Collapse of the FLL tilt modulus at the transition

Evidence for the FFLO phase with spatially modulated order parameter

Abstract

Ultrasound velocity measurements of the unconventional superconductor CeCoIn_5 with extremely large Pauli paramagnetic susceptibility reveal an unusual structural transformation of the flux line lattice (FLL) in the vicinity of the upper critical field. The transition field coincides with that at which heat capacity measurements reveal a second order phase transition. The lowering of the sound velocity at the transition is consistent with the collapse of the FLL tilt modulus and a crossover to quasi two-dimensional FLL pinning. These results provide a strong evidence that the high field state is the Fulde-Ferrel-Larkin-Ovchinikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices.