Field-angle dependent vortex lattice phase diagram in MgB2

TL;DR

This study uses small-angle neutron scattering to explore how the vortex lattice phase diagram in MgB2 changes with magnetic field orientation, revealing the suppression of an intermediate phase and a transition from continuous to discontinuous behavior.

Contribution

It provides the first detailed field-angle dependent vortex lattice phase diagram in MgB2, showing how anisotropy influences phase transitions.

Findings

Intermediate vortex lattice phase is suppressed with field rotation.

Transition from continuous to discontinuous occurs at a critical angle.

Twelvefold anisotropy term vanishes at the transition.

Abstract

Using small-angle neutron scattering we have studied the superconducting vortex lattice (VL) phase diagram in MgB2 as the applied magnetic field is rotated away from the c axis and towards the basal plane. The field rotation gradually suppresses the intermediate VL phase which exists between end states aligned with two high symmetry directions in the hexagonal basal plane for H || c. Above a critical angle, the intermediate state disappears, and the previously continuous transition becomes discontinuous. The evolution towards the discontinuous transition can be parameterized by a vanishing twelvefold anisotropy term in the VL free energy.