Studies of Vortex States in Anisotropic Superconductors by Low Temperature Scanning Spectroscopy and Muon Spin Rotation

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We have developed low temperature high magnetic field scanning tunneling microscope (LT/HM-STM) and studied the vortex states of anise-tropic superconductors.(1)Construction of LT/HM-STM : We have constructed two low temperature scanning tunneling microscopes : one is able to be operated in magnetic fields up to 15T at 4.2K and the other, combined with ^3He-^4He dilution refrigerator, I is able to be operated at temperatures down to 0.16K in 6T. We have found that S/N ration of scanning tunneling spectroscopy bu using dilution-refrigerator STM can be greatly improved by modifying the cooling method of the 1.5K stage ; this will help the improvement of LT-STM in the future.(2)Scanning tunneling spectroscopy of vortex states of anisotropic superconductors in real time and space : The vortex states of YNi2B2C have been studied by Lt-STS/STM at 0.46K. The bound states of the vortex in YNi2B2C have been found by measuring the local density of states of quasi-particles by LT-STS/STM at 0.46K : the fourfold symmetric star-shaped and quantum-regime vortex core have been observed for the first time. This will a breakthrough to microscopic understanding of the vortex states in type-II superconductors. The collective creep of vortices has been successfully observed at temperatures from 6K down to 0.46K in 1T. This is the first visualization of the vortex movement below 1K. Vortex motion in high^temperature superconductors Bi2Sr2CaCu2Ox in 6T at 4.2K was also observed in real time and space. These will open a frontier of research of vortex dynamics including pinning effects in type-II superconductors. By using muon spin rotation anad relaxation method, we ha ve found that the superconducting state of PrOs4Sb12 is time-reversal symmetry breaking. The vortex states of PrOs4b12 have been studied by LT-STS at 0.46K and the results are now in analysis.