Magneto-optical reflectivity of superconducting MgB2 single crystals

TL;DR

This study investigates the magneto-optical reflectivity of MgB2 single crystals, revealing a superconducting gap below the weak-coupling value and analyzing the upper critical field's temperature dependence.

Contribution

First detailed magneto-optical measurements of MgB2 single crystals across a broad spectrum and magnetic fields, providing insights into its superconducting gap and critical field behavior.

Findings

Superconducting gap ratio 2Δ/k_B T_c ≈ 1.2, below weak-coupling value

Superconducting gap suppressed by magnetic field depending on temperature

Upper critical field H_c2(T) follows Helfand-Werthamer behavior

Abstract

We present magneto-optical reflectivity results in the basal-plane of the hexagonal $MgB_2$. The data were collected on a mosaic of $MgB_2$ single crystals with $T_{c}=38 ~K$ from the ultraviolet down to the far infrared as a function of temperature and magnetic field oriented along the c-axis. In the far infrared there is a clear signature of the superconducting gap with a gap-ratio $2\Delta/k_{B}T_{c}\sim 1.2$, well below the weak-coupling value. The gap is suppressed in an external magnetic field, which is a function of temperature. We extract the upper critical field $H_{c2}$ along the c-axis. The temperature dependence of $H_{c2}$ is compatible with the Helfand-Werthamer behaviour.