用临界涨落研究磁场调谐柱中MgB2的超导电性
Probing superconductivity in MgB2 confined to magnetic field tuned cylinders by means of critical fluctuations
摘要：
本文报道并分析了高质量MgB_2单晶在零场转变温度Tc = 38.83 K附近，沿c轴方向施加几个高达300 Oe的磁场时的可逆磁化测量结果。虽然MgB2是一个两个间隙超导体，我们的标度分析揭示了显着的一致性与3D-xy临界行为，揭示了接近临界的序参量是一个单一的复杂标量在4He。这打开了探索磁场诱导的有限尺寸效应的窗口，因此横向于沿i轴施加的所施加的磁场H_i的相关长度不能增长超过与涡旋线之间的平均距离相关的限制磁性长度L_Hi。我们找到明确的证据，这种有限的尺寸效应。这意味着在第二类超导体（如MgB_2）中，存在着3D到1D的交叉线H_pi，xi表示沿相应轴的T_c上下相关长度的临界振幅。因此，在H_pi（T）以上和T <T_c时，超导性被限制在直径为L_Hi（1D）的圆柱体中。相反，在T_c以上，非凝聚电子对被限制在圆柱体中.因此，在（H，T）平面中不存在如通过平均场处理预测的沿着H_c2线的连续相变。
英文摘要：
We report and analyze reversible magnetization measurements on a high quality MgB2 single crystal in the vicinity of the zero field transition temperature, T_c=38.83 K, at several magnetic fields up to 300 Oe, applied along the c-axis. Though MgB2 is a two gap superconductor our scaling analysis uncovers remarkable consistency with 3D-xy critical behavior, revealing that close to criticality the order parameter is a single complex scalar as in 4He. This opens up the window onto the exploration of the magnetic field induced finite size effect, whereupon the correlation length transverse to the applied magnetic field H_i applied along the i-axis cannot grow beyond the limiting magnetic length L_Hi, related to the average distance between vortex lines. We find unambiguous evidence for this finite size effect. It implies that in type II superconductors, such as MgB2, there is the 3D to 1D crossover line H_pi and xi denotes the critical amplitudes of the correlation lengths above and below T_c along the respective axis. Consequently, above H_pi(T) and T<T_c superconductivity is confined to cylinders with diameter L_Hi (1D). In contrast, above T_c the uncondensed pairs are confined to cylinders. Accordingly, there is no continuous phase transition in the (H,T)-plane along the H_c2-lines as predicted by the mean-field treatment.
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