STM Study on the Organic Superconductor

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We have developed the apparatus of the scanning tunneling microscope (STM) for the investigation of the electronic state of the organic superconductor and carried out the tunneling spectroscopy measurement with this apparatus. We have constructed the apparatus for the measurement under the magnitic field by combining the STM unit with the superconducting magnet. We have also performed the tunneling spectroscopic measurement in the superconducting phase of the organic superconductor (BEDT-TTF)_2Cu (NCS)_2For the construction of the STM apparatus, we purchased the STM unit and the superconducting solenoid and combined them with the possessed STM control unit. We also made the insert metal dewar for the tunneling unit. We have confirmed the operation of this apparatus up to the maximum field 7 T and down to the lowest temperature 1.4K in the preliminary measurement.We carried out the tunneling spectroscopic measurement on the organic superconductor (BEDT-TTF)_2Cu (NCS)_2 and investigated the anisotropy of the superconducting gap in the wave number space with changing the direction of surface of a single crystal sample. The obtained tunneling differential conductance clearly shows the superconducting gap structure and its functional form varies depending on the direction of the surface. It is understood that the electrons with the wave vector parallel to the tunneling tip direction dominantly contribute to the tunneling current in the STM spectroscopy measurement as well. At the same time it is clarified that the superconducting gap of (BEDT-TTF)_2Cu (NCS)_2 has a large anisotropy in the two-dimensional plane (b-c plane). It is indicated from the analysis of the angular dependence of the tunneling spectra that the superconducting gap has the d-wave symmetry where the gap disappears at the line on the Fermi surface. We are now progressing the STM measurement under the magnetic field.