Field Dependence of the Superconducting Basal Plane Anisotropy of TmNi2B2C

TL;DR

This study investigates how the basal plane anisotropy in TmNi2B2C superconductors decreases with increasing magnetic field, using neutron scattering to link this behavior to Pauli paramagnetic effects and vortex core expansion.

Contribution

It provides the first direct, quantitative measurement of the field-dependent anisotropy in TmNi2B2C and links it to Pauli paramagnetic effects.

Findings

Anisotropy decreases with increasing field

Reentrance of square vortex lattice phase

Vortex core expansion near Hc2

Abstract

The superconductor TmNi2B2C possesses a significant four-fold basal plane anisotropy, leading to a square Vortex Lattice (VL) at intermediate fields. However, unlike other members of the borocarbide superconductors, the anisotropy in TmNi2B2C appears to decrease with increasing field, evident by a reentrance of the square VL phase. We have used Small Angle Neutron Scattering measurements of the VL to study the field dependence of the anisotropy. Our results provide a direct, quantitative measurement of the decreasing anisotropy. We attribute this reduction of the basal plane anisotropy to the strong Pauli paramagnetic effects observed in TmNi2B2C and the resulting expansion of vortex cores near Hc2.