标题： $^{11}$B NMR研究纯的和轻度碳掺杂的MgB$_2$超导体 显示英文标题

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

摘要： 我们报告了纯镁硼化物和轻度碳掺杂的 MgB$_{2-x}$C$_{x}$在涡旋态和 23.5 kOe 磁场下的$^{11}$B NMR 线形和自旋-晶格弛豫率 ($1/(T_1T)$) 研究，其中$x=0$、0.02 和 0.04。我们表明，虽然纯 MgB$_2$展示了正常态和阿布里科索夫态叠加的磁场分布，但硼的轻微替代为碳揭示了纯阿布里科索夫态的磁场分布。 这表明与纯 MgB$_2$相比，碳掺杂使$H_{c2}^c$有显著增加。 自旋-晶格弛豫率$1/(T_1T)$清晰地显示了在纯 MgB$_2$中低于$T_c$的位置存在一个相干峰，随后在冷却过程中出现典型的 BCS 减少。 然而，在低于$\approx 10$K 的温度下，观察到与 BCS 行为的强烈偏离，这可能是由于涡旋动力学的残余贡献。 在碳掺杂系统中，相干峰和$1/(T_1T)$的 BCS 温度依赖性都减弱，这一现象归因于电子掺杂下费米面的$\sigma $空穴圆柱体逐渐缩小。 显示英文摘要

摘要： 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.