A Point-Contact Study of the Superconducting Gaps in Al-Substituted and C-Substituted MgB_2 Single Crystals

TL;DR

This study uses directional point-contact spectroscopy to analyze how superconducting gaps in MgB2 crystals change with Al and C doping, revealing phase segregation and a transition to single-gap superconductivity.

Contribution

It provides new doping-dependent measurements of superconducting gaps in MgB2 with Al and C substitutions, highlighting phase segregation and a doping-induced gap transition.

Findings

Al substitution beyond x>0.09 may cause phase segregation

C substitution induces a transition to single-gap superconductivity at y=0.132

Doping affects the superconducting gap structure in MgB2

Abstract

We present the results of directional point-contact spectroscopy in state-of-the-art Mg_{1-x}Al_xB_2 and Mg(B_{1-y}C_{y})_2 single crystals produced at ETH, Zurich. Fitting the conductance curves of our point contacts, that always feature Andreev reflection structures, we obtained the doping dependence of the gap amplitudes. The results are discussed in comparison with other experimental findings and relevant theoretical predictions. We conclude that the physics of Al-substituted crystals at x > 0.09 might be governed by phase segregation, while C-substituted crystals unexpectedly show a doping-induced transition to single-gap superconductivity at y=0.132.