11B NMR study of pure and lightly carbon-doped MgB2 superconductors

Abstract

We report a 11 B NMR line shape and spin-lattice relaxation rate (1/(T1T )) study of pure and lightly carbon doped MgB2−xCx 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 MgB2 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 c c2 with carbon doping with respect to pure MgB2. The spin-lattice relaxation rate 1/(T1T ) demonstrates clearly the presence of a coherence peak right below Tc in pure MgB2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ≈ 10K 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/(T1T ) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.PACS numbers: 74.25.-q., 76.60.Es