Evidence for Single-gap Superconductivity in Mg(B_{1-x}C_x)_2 Single Crystals with x=0.132 from Point-Contact Spectroscopy

TL;DR

This study provides evidence of single-gap superconductivity in Mg(B_{1-x}C_x)_2 single crystals at higher carbon doping levels, showing the merging of two gaps into one at x=0.132 through point-contact spectroscopy.

Contribution

First directional point-contact measurements on Mg(B_{1-x}C_x)_2 crystals revealing the transition from two-gap to single-gap superconductivity with increased carbon doping.

Findings

Two-gap superconductivity persists up to x=0.105.

Gaps merge into one at x=0.132 with T_c=19 K.

Confirmed gap merging using BTK models.

Abstract

We report the results of the first directional point-contact measurements in Mg(B_{1-x}C_{x})_2 single crystals with 0.047 <= x <= 0.132. The two-gap superconductivity typical of MgB_2 persists up to x=0.105. In this region, the values of the gaps Delta_{sigma} and Delta_{pi} were determined by fitting the Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk (BTK) model, and confirmed by the single-band BTK fit of the sigma- and pi-band conductances, separated by means of a magnetic field. At x=0.132, when T_{c}=19 K, we clearly observed for the first time the merging of the two gaps into one of amplitude Delta~3 meV.