Carbon doping of superconducting magnesium diboride

TL;DR

This study explores the synthesis and physical properties of carbon-doped MgB2 superconductors, revealing a suppressed transition temperature, altered critical field behavior, and preserved two-gap superconductivity.

Contribution

It provides detailed synthesis optimization and characterization of heavily carbon-doped MgB2, highlighting changes in critical fields and anisotropy compared to pure MgB2.

Findings

Superconducting transition temperature ~22 K, lower than pure MgB2.

Steeper temperature dependence of the upper critical field.

Two-gap superconductivity remains in carbon-doped MgB2.

Abstract

We present details of synthesis optimization and physical properties of nearly single phase carbon doped MgB2 with a nominal stoichiometry of Mg(B{0.8}C{0.2})2 synthesized from magnesium and boron carbide (B4C) as starting materials. The superconducting transition temperature is ~ 22 K (~ 17 K lower than in pure MgB2). The temperature dependence of the upper critical field is steeper than in pure MgB2 with Hc2(10K) ~ 9 T. Temperature dependent specific heat data taken in different applied magnetic fields suggest that the two-gap nature of superconductivity is still preserved for carbon doped MgB2 even with such a heavily suppressed transition temperature. In addition, the anisotropy ratio of the upper critical field for T/Tc ~ 2/3 is gamma ~ 2. This value is distinct from 1 (isotropic) and also distinct from 6 (the value found for pure MgB2).