11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors
11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors
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纯轻碳掺杂 MgB2 超导体的 11B NMR 研究
DOI：
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发表时间：
2004
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通讯作者：
Y. Iwasa
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作者：
M. Karayanni;G. Papavassiliou;M. Pissas;M. Fardis;K. Papagelis;K. Prassides;T. Takenobu;Y. Iwasa
本文报道了纯的和轻碳掺杂的MgB_{2-x}的~（11）B NMR谱线形状和自旋-晶格弛豫速率的研究。
在23.5kOe的磁场中，对x = 0，0.02和0.04的情况，在涡旋态下，得到了最大磁致伸缩系数m C}_x.我们发现，虽然纯MgB$$_2$$表现出的磁场分布从正常的和Abrikosov状态的叠加，轻微的硼与碳的替代揭示了纯Abrikosov状态的磁场分布。这表明相对于纯MgB$$_2$$，碳掺杂的$$H^c_{c2}$$显著增加。自旋-晶格弛豫速率$$1/（T_1T）$$清楚地表明在$$T_{
m c}$$在纯MgB$$_2$$中，随后是典型的BCS在冷却时降低。然而，在温度低于$$约$10 K强偏离BCS行为观察到，可能是从残余贡献的涡旋动力学。在碳掺杂体系中，相干峰和BCS温度依赖性都减弱，这是由于电子掺杂使费米面的σ空穴柱逐渐收缩所致。
We report a 11B NMR line shape and spin-lattice relaxation rate $$(1/(T_1T))$$ study of pure and lightly carbon-doped MgB$$_{2-x}{
m 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^c_{c2}$$ with carbon doping with respect to pure MgB$$_2$$. The spin-lattice relaxation rate $$1/(T_1T)$$ demonstrates clearly the presence of a coherence peak right below $$T_{
m 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 σ hole cylinders of the Fermi surface with electron doping.