Systematic effects of carbon doping on the superconducting properties of Mg(B$_{1-x}$C$_x$)$_2$

TL;DR

This study investigates how carbon doping affects the superconducting properties of Mg(B$_{1-x}$C$_x$)$_2$, revealing that increased C content raises the upper critical field while lowering the transition temperature.

Contribution

It provides systematic measurements of how carbon doping influences $H_{c2}$ and $T_c$ in MgB$_2$, offering insights into optimizing superconducting performance through doping.

Findings

Carbon doping depresses $T_c$ by about 1 K per 1% C.

$H_{c2}(0)$ increases by approximately 5 T per 1% C.

Resistivity at 40 K increases significantly with C doping.

Abstract

The upper critical field, $H_{c2}$, of Mg(B$_{1-x}$C$_x$)$_2$ has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped boron filaments are prepared by CVD techniques, and then these fibers are then exposed to Mg vapor to form the superconducting compound. The transition temperatures are depressed about $1 K/%$ C and $H_{c2}(T=0)$ rises at about $5 T/%$ C. This means that 3.5% C will depress $T_c$ from $39.2 K$ to $36.2 K$ and raise $H_{c2}(T=0)$ from $16.0 T$ to $32.5 T$. Higher fields are probably attainable in the region of 5% C to 7% C. These rises in $H_{c2}$ are accompanied by a rise in resistivity at $40 K$ from about $0.5 \mu \Omega cm$ to about $10 \mu \Omega cm$. Given that the samples are polycrystalline wire segments, the experimentally determined $H_{c2}(T)$ curves represent the upper $H_{c2}(T)$ manifold associated with $H\perp c$.