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

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 $1K/$ C and $H_{c2}(T=0)$ rises at about $5T/$ C. This means that 3.5% C will depress $T_c$ from $39.2K$ to $36.2K$ and raise $H_{c2}(T=0)$ from $16.0T$ to $32.5T$. 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 $40K$ 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 $H\perp c$.