$^{11}$B NMR study of pure and lightly carbon doped MgB$_2$ superconductors

TL;DR

This study uses $^{11}$B NMR to analyze the effects of light carbon doping on the magnetic and superconducting properties of MgB$_2$, revealing increased upper critical field and changes in electronic behavior.

Contribution

It provides new insights into how light carbon doping alters the magnetic field distribution and electronic properties of MgB$_2$ superconductors using NMR techniques.

Findings

Carbon doping increases $H_{c2}^c$ in MgB$_2$.

The coherence peak in $1/(T_1T)$ weakens with doping.

Doping causes a shrinking of the $\sigma$ hole cylinders in the Fermi surface.

Abstract

We report a $^{11}$B NMR line shape and spin-lattice relaxation rate ($1/(T_1T)$) study of pure and lightly carbon doped MgB$_{2-x}$C$_{x}$ for $x=0$, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB$_2$ exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase of $H_{c2}^c$ with carbon doping with respect to pure MgB$_2$. The spin-lattice relaxation rate $1/(T_1T)$ demonstrates clearly the presence of a coherence peak right below $T_c$ in pure MgB$_2$, followed by a typical BCS decrease on cooling. However, at temperatures lower than $\approx 10$K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon doped systems both the coherence peak and the BCS temperature dependence of $1/(T_1T)$ weaken, an effect attributed to the gradual shrinking of the $\sigma $ hole cylinders of the Fermi surface with electron doping.