11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors |
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| Authors: | M Karayanni G Papavassiliou M Pissas M Fardis K Papagelis K Prassides T Takenobu and Y Iwasa |
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| Affiliation: | (1) Institute of Materials Science, NCSR, Demokritos, 153 10 Aghia Paraskevi, Athens, Greece;(2) Department of Materials Science, University of Patras, 26110 Patra, Greece;(3) Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, UK;(4) Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan;(5) CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan; |
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| Abstract: | We report a 11B NMR line shape and spin-lattice relaxation rate (1/(T1T))(1/(T_1T)) study of pure and lightly carbon-doped MgB2-xCx_{2-x}{\rm C}_x for x = 0,0.02x = 0,0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB2_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 Hcc2H^c_{c2} with carbon doping with respect to pure MgB2_2. The spin-lattice relaxation rate 1/(T1T)1/(T_1T) demonstrates clearly the presence of a coherence peak right below TcT_{\rm c} in pure MgB2_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/(T1T)1/(T_1T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping. |
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