Observation of a superconducting gap in a magnetic-field-induced two-dimensional organic superconductor by multi-frequency ESR spectroscopy

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Magneto-optical measurements of field-induced superconductor λ-(BETS)_2FeCl_4 have been performed at 1.4 K using the 33 T water cooling magnet at NHMFL, Florida, USA. The Backward traveling wave oscillators (BWO) are used as the light sources, and the InSb hot electron bolometer is used as a detector. The rotational sample holder which is developed for this measurement, is used to enable the angular dependence measurement. Firstly we performed the measurement with one needle like single crystal without a mask but we were not able to detect the signal. Therefore, we set several needle like single crystals parallel to each other with a mask and let the light transmit through the spacing between samples. It turned out to be very difficult to apply the magnetic field parallel to the c-axes of all samples and we were not able to eliminate ESR signals completely. As a result, we observed unusual frequency dependence where the transmission aound 8 T became stronger as the frequency increased. Moreover, the transmission above 8T became very small and no significant change was observed within the S/N limit above 20, T where the field-induced superconductivity was expected. As, however the organic superconductor is in the clean limit and the Drude tail is smaller than the superconducting gap, there is a possibility that it is difficult to estimate the superconducting gap by the frequency dependence measurements because the transmission difference between the superconducting state and the metal state is small. On the other hand, in order to understand the very unusual increase of transmission around 8 T in the high frequency region, the discussion at the high field international conference, which will be held in Tallin this summer, is planned.