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

自旋阶梯铜酸盐Sr_2Ca_<12>Cu_<24>O_<41>是一种在3GPa以上高压下表现出超导性的材料。然而，直到今天，由于超导性只能通过施加高压才能实现，因此人们对它的物理性质还不是很了解。在高压下电阻率等体积测量中，已经建立了一种使用立方砧的方法。然而，由于需要大量的样品，这种方法不适用于包括核磁共振（NMR）在内的微观水平的测量。目前只有钳型压力电池可用于这些测量，然而，传统电池的最大压力最多为2.5GPa，这阻碍了对超导性的进一步研究。在目前的核磁共振研究中，我们在电池中使用了一种特殊的NiCrAl合金，并成功地达到了3.5GPa，这使我们能够研究自1996年发现超导性以来一直是一个开放问题的微观性质。在3.5GPa下测量了Knight位移和弛豫速率（1/T_1），发现实现了全间隙超导态。即使在接近泡利极限的高压下，超导态也相当稳定，并且在转变温度（T_c）下没有观察到单线态配对导致的Knight位移减小。事实表明存在三联体配对的可能性。在T_c以上的正常状态下也观察到自旋间隙和超导间隙，这意味着实现了局域化的异常金属态。在不同的温度区观察到自旋间隙和超导间隙，这与高t_c铜酸盐的情况相反。

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.