Carbon-substitution effect on the electronic properties of MgB$_2$ single crystals

TL;DR

This study investigates how carbon substitution in MgB$_2$ single crystals affects their electronic properties, revealing significant changes in resistivity, phonon behavior, and critical fields, and discusses the implications for superconductivity.

Contribution

It provides new insights into the effects of carbon substitution on the electronic structure and superconducting properties of MgB$_2$ single crystals.

Findings

Carbon substitution lowers T$_c$ below 2 K.

Residual resistivity increases markedly with C-content.

Second critical fields increase and anisotropy decreases.

Abstract

The electronic properties of the carbon substituted MgB$_2$ single crystals are reported. The carbon substitution drops T$_c$ below 2 K. In-plane resistivity shows a remarkable increase in residual resistivity by C-substitution, while the change of in-plane/out-of-plane Hall coefficients is rather small. Raman scattering spectra indicate that the E$_{2g}$-phonon frequency radically hardens with increasing the carbon-content, suggesting the weakening of electron-phonon coupling. Another striking C-effect is the increases of the second critical fields in both in-plane and out-of-plane directions, accompanied by a reduction in the anisotropy ratio. The possible changes in the electronic state and the origin of T$_c$-suppression by C-substitution are discussed.