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

Abstract

We report a ${}^{11}$B NMR line shape and spin-lattice relaxation rate ($1/(T_{1}T)$) 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 $1/(T_{1}T)$ 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 $1/(T_{1}T)$ weaken, an effect attributed to the gradual shrinking of the $\sigma$ hole cylinders of the Fermi surface with electron doping.

8 pages, 6 figures, submitted to Phys. Rev. B

References in corpus (19)

• Two-Band Superconductivity in MgB2
• Properties of the Ideal Ginzburg-Landau Vortex Lattice
• Vortex Imaging in the pi-Band of Magnesium Diboride
• Carbon-substituted MgB2 single crystals
• Crystal Chemistry of Carbon-Substituted MgB2
• Multiband model for penetration depth in MgB2
• Carbon doping of superconducting magnesium diboride
• Mixed state of a dirty two-band superconductor: application to MgB_2
• Carbon solubility and superconductivity in MgB2
• Macroscopic anisotropy in superconductors with anisotropic gaps
• Anisotropy of the upper critical field in superconductors with anisotropic gaps. Anisotropy parameters of MgB2
• Enhancement of the irreversibility field by carbon substitution in single crystal MgB${}_2$
• Retention of Two-Band Superconductivity in Highly Carbon-Doped MgB2
• Two band/two gap superconductivity in carbon-substituted MgB2 evidenced by point-contact spectroscopy
• The Magnetic-field Dependence of the Gaps in a Two-band Superconductor: A Point-contact Study of MgB_2 Single Crystals
• Evidence for High-frequency Phonon Mediated S-wave Superconductivity : 11B-NMR Study of Al-doped MgB2
• The effect of magnetic field on the two superconducting gaps in MgB2
• ${}^{11}$B and ${}^{27}$Al NMR spin-lattice relaxation and Knight shift study of Mg${}_{1-x}$Al${}_x$B${}_2$. Evidence for anisotropic Fermi surface
• muSR study of carbon-doped MgB2 superconductors

Cited by in corpus (1)

• Anisotropy and Penetration Depth of MgB${}_2$ from ${}^{11}$B NMR