Observations of Pauli Paramagnetic Effects on the Flux Line Lattice in CeCoIn5

TL;DR

This study uses small-angle neutron scattering to investigate how Pauli paramagnetic effects influence the flux line lattice in CeCoIn5, revealing field-dependent behaviors and a suppression of Cooper pairing near the upper critical field.

Contribution

The paper extends previous neutron scattering measurements to higher temperatures, demonstrating the impact of paramagnetism on the flux line lattice and clarifying the nature of the form factor decrease.

Findings

Paramagnetism increases the FLL form factor with field.

Near H_c2, the form factor decreases due to paramagnetic suppression of pairing.

A crossover to conventional behavior occurs at higher temperatures.

Abstract

From small-angle neutron scattering studies of the flux line lattice (FLL) in CeCoIn5, with magnetic field applied parallel to the crystal c-axis, we obtain the field- and temperature-dependence of the FLL form factor, which is a measure of the spatial variation of the field in the mixed state. We extend our earlier work [A.D. Bianchi et al. 2008 Science 319, 177] to temperatures up to 1250 mK. Over the entire temperature range, paramagnetism in the flux line cores results in an increase of the form factor with field. Near H_c2 the form factor decreases again, and our results indicate that this fall-off extends outside the proposed FFLO region. Instead, we attribute the decrease to a paramagnetic suppression of Cooper pairing. At higher temperatures, a gradual crossover towards more conventional mixed state behavior is observed.