Fate of the Peak Effect in a Type-II Superconductor: Multicriticality in the Bragg-Glass Transition

TL;DR

This study investigates the complex phase transitions in a Nb superconductor, revealing how the peak effect disappears at a multicritical point where multiple phase boundaries converge, using neutron scattering and magnetic susceptibility.

Contribution

It provides the first detailed phase diagram showing the multicritical behavior of the Bragg-glass transition in a superconductor, linking the peak effect disappearance to this multicriticality.

Findings

Identification of four phase boundary lines in the H-T plane.

Disappearance of the peak effect correlates with multicriticality.

Convergence of phase boundaries at a multicritical point.

Abstract

We have used small-angle-neutron-scattering (SANS) and ac magnetic susceptibility to investigate the global magnetic field H vs temperature T phase diagram of a single crystal Nb in which a first-order transition of Bragg-glass melting (disordering), a peak effect, and surface superconductivity are all observable. It was found that the disappearance of the peak effect is directly related to a multicritical behavior in the Bragg-glass transition. Four characteristic phase boundary lines have been identified on the H-T plane: a first-order line at high fields, a mean-field-like continuous transition line at low fields, and two continuous transition line associated with the onset of surface and bulk superconductivity. All four lines are found to meet at a multicritical point.