Study of pressure-induced-superconductivity with improvements of NMR technique under steady high pressure up to 4GPa

自旋梯形铜酸盐Sr2Ca&lt；12&gt；Cu_&lt；24&gt；O_&lt；41&gt；是一种在3 Gpa以上的高压下表现出超导电性的材料。然而，由于超导电性是通过施加高压才能实现的，它的物理性质至今仍未被很好地了解。建立了一种适用于高压下电阻率等体相测量的三面顶法。然而，这种方法不适用于包括核磁共振在内的微观层面的测量，因为需要大量的样品。在目前的核磁共振研究中，我们使用了一种特殊的NiCrAl合金，成功地获得了3.5 Gpa的压强，这使得我们能够研究自1996年超导电性被发现以来一直是一个悬而未决的问题。我们测量了3.5 Gpa的奈特位移和驰豫速率(1/T_1)，发现实现了完全带隙的超导态。即使在接近泡利极限的高压下，超导状态也是相当稳定的，并且在转变温度(T_C)没有观察到对应于单重态对的奈特位移的减小。事实表明，三胞胎配对是可能的。在T_c以上的正常态和超导能隙中也观察到了自旋能隙，这意味着实现了具有局域化特征的反常金属态。在分离的温区观察到自旋能隙和超导能隙，这与高T_c铜酸盐的情况形成了对比。

A spin ladder cuprate Sr_2Ca_<12>Cu_<24>O_<41> is well known as a material which exhibits superconductivity under high pressure above 3GPa. However, it's physical properties are not well understood even nowadays since the superconductivity is realized only by applying high pressure. A method with using a cubic anvil is well established for the bulk measurements such as the resistivity under high pressure. However, such a method is not appliable for measurements on the microscopic level including nuclear magnetic resonance (NMR) since a large amount of samples are required. Only a clamp-type pressure cell is available for these measurements, however, the maximum pressure of the conventional cell is at most 2.5GPa, which has prevented further investigation about the superconductivity.In the present NMR study we used a special NiCrAl alloy in the cell and succeeded to attain 3.5GPa, which enabled us to investigate the microscopic properties which had been an open problem since the discovery of the superconductivity going back to 1996..We measured Knight shift and relaxation rate (1/T_1) at 3.5GPa and found that the superconducting state with a full gap is realized. The superconducting state is quite stable even at high pressure close to Pauli limit, and the decrease of Knight shift corresponding to singlet pairing were not observed at the transition temperature (T_c). The facts show a possibility of triplet-pairing. The spin gap was also observed at the normal state above T_c as well as superconducting gap, which implies that anomalous metallic state with a localized character is realized. The spin gap and the superconducting gap were observed at separated temperature regions, which contrasts with the case of high-T_c cuprates.