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

利用美国佛罗里达NHMFL的33 T水冷磁体在1.4 K下对场致超导体λ-（BETS）_2FeCl_4进行了磁光测量。采用反向行波振荡器（BWO）作为光源，InSb热电子测辐射热计作为探测器。旋转的样品保持器，这是为这种测量，是用来使角度依赖性测量。首先，我们在没有掩模的情况下用一个针状单晶进行测量，但我们无法检测到信号。因此，我们用掩模将几个针状单晶平行放置，让光通过样品之间的间隙。事实证明，施加平行于所有样品的c轴的磁场是非常困难的，我们无法完全消除ESR信号。结果，我们观察到不寻常的频率依赖性，其中8 T附近的传输随着频率的增加而变得更强。此外，在8 T以上的透射率变得非常小，并且在20 T以上的S/N极限内没有观察到显著的变化，其中场致超导性是预期的。然而，由于有机超导体处于清洁极限并且Drude尾小于超导带隙，因此存在难以通过频率依赖性测量来估计超导带隙的可能性，因为超导状态和金属状态之间的透射差很小。另一方面，为了了解高频区域中8 T附近传输的非常不寻常的增加，计划在今年夏天在塔林举行的高场国际会议上进行讨论。

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.