The superconducting gap of \QTR{em}{in situ} $MgB_{2}$ thin films by microwave surface impedance measurements

TL;DR

This study uses microwave surface impedance measurements to determine the superconducting gap in in situ MgB₂ thin films, revealing a fully-gapped order parameter with a small gap ratio, consistent with BCS theory.

Contribution

First direct microwave measurement of the superconducting gap in in situ MgB₂ films showing a fully-gapped state with a small gap ratio.

Findings

Exponential behavior of surface resistance at low temperature

Surface resistance fits Mattis-Bardeen formalism with a small gap ratio

Superconducting gap Δ(0) approximately 1.9 meV

Abstract

Precision measurements of the microwave surface resistance $R_{s}$ of in situ $MgB_{2}$ films directly reveal an exponential behavior of $R_{s}$ at low temperature indicating a fully-gapped order parameter. The entire temperature dependence of $R_{s}$ is well described by a Mattis-Bardeen formalism but with a small gap ratio of $\Delta (0)/kT_{c}=0.72$, corresponding to $\Delta (0)=1.9meV$.