Crystal growth, characterization, and point-contact Andreev-reflection spectroscopy of the noncentrosymmetric superconductor Mo3Al2C
Crystal growth, characterization, and point-contact Andreev-reflection spectroscopy of the noncentrosymmetric superconductor
Mo3Al2C
复制标题
非中心对称超导体 Mo3Al2C 的晶体生长、表征和点接触安德烈夫反射光谱
作者：
Nikolai Zhigadlo;D. Logvinovich;R. Gonnelli;V. A. Stepanov;D. Daghero
本文报道了用压砧高温高压技术首次成功地生长出非中心对称超导体Mo $_3 $Al $_2 $C单晶。通过X-射线衍射、能量色散X-射线光谱、磁化率和电阻率随温度的变化测量，研究了晶体的组成、结构和正常状态下的输运性质。观察到临界温度（$T_c$）在8.6和9.3 K之间的变化，这可能是由于样品的碳化学计量略有不同。单晶X射线精炼证实了生长晶体的高结构完美性。值得注意的是，使用$P4_132$空间群模型测量的所有晶体的精细Flack参数值始终接近0或1，因此表明所考虑的晶体属于两个对映体空间群，$P4_132$和$P4_332$。在电阻率的异常观察到在约130 K，最有可能与电荷密度波阶段的发病。的超导性能（特别是对称性，振幅和温度依赖性的超导间隙）进行了研究，通过使用-第一次在这个化合物-点接触Andreev反射光谱。结果证实了Mo_3Al_2C是一个中等强耦合的超导体，其δ = 2Delta/kBT_c = 4，并明确地证明了尽管由于缺乏反演对称性而产生了非对称的自旋-轨道耦合，但序参量仍具有s波对称性。
We report on the first successful growth of single crystals of the noncentrosymmetric superconductor Mo$_3$Al$_2$C obtained by means of a cubic-anvil, high-pressure and high-temperature technique. Composition, structure, and normal-state transport properties of the crystals were studied by means of X-ray diffraction, energy-dispersive X-ray spectroscopy, magnetic susceptibility and resistivity measurements as a function of temperature. Variations in critical temperature ($T_c$) between 8.6 and 9.3 K were observed, probably due to the slightly different carbon stoichiometry of the samples. Single-crystal X-ray refinement confirmed the high structural perfection of the grown crystals. Remarkably, the refined Flack parameter values for all the measured crystals using a $P4_132$ space-group model were consistently close to either 0 or 1, hence indicating that the considered crystals belong to two enantiomorphic space groups, $P4_132$ and $P4_332$. An anomaly in the resistivity is observed at about 130 K, most likely associated with the onset of a charge-density-wave phase. The superconducting properties (and in particular the symmetry, the amplitude and the temperature dependence of the superconducting gap) were studied by using - for the first time in this compound - point contact Andreev-reflection spectroscopy. The results confirm that Mo$_3$Al$_2$C is a moderately strongly-coupled superconductor with $2Delta/k_BT_c simeq 4$ and unambiguously prove that the order parameter has an s-wave symmetry despite the asymmetric spin-orbit coupling arising from the lack of inversion symmetry.