Superconducting Transition and Phase Diagram of Single Crystal MgB2

TL;DR

This study maps the superconducting phase diagram of MgB2 using various measurements, revealing anisotropy increase with temperature and evidence of surface superconductivity, advancing understanding of its superconducting properties.

Contribution

It provides detailed phase diagram data and insights into anisotropy and surface effects in MgB2's superconductivity, which were not fully characterized before.

Findings

An in-plane coherence length of 8 nm at zero temperature.

Superconducting anisotropy increases from ~2 near Tc to above 4.5 at 22 K.

Presence of a surface superconducting state for H||c.

Abstract

The superconducting phase diagram of MgB2 was determined from magnetization, magneto-transport and the first single-crystal specific heat measurements. A zero-temperature in-plane coherence length of 8 nm is determined. The superconducting anisotropy increases from a value around 2 near Tc to above 4.5 at 22 K. For H||c a pronounced peak effect in the critical current occurs at the upper critical field. Evidence for a surface superconducting state is presented for H||c which might account for the wide spread in reported values of the anisotropy.