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

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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.