Development of high-frequency impedance measurement under magnetic fields over wide range of temperature

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To study the low energy excitation in the highly correlated electrons and vortices in superconductors, we have developed two kinds of apparatuses that can measure the high-frequency impedance under magnetic fields over wide range of temperature. First, we constructed a surface resistance measurement system based on a bolometric technique. In this system, samples were located inside the cavity resonator to specify the RF field direction. Although the sensitivity of the bolometer decreases with increasing temperature, we could extend the upper limit of temperature over 80 K by the optimization of various conditions. Furthermore, we succeeded in measuring the-surface resistance as function of temperature instead of applied field. This is important for superconductors because the field inhomogeneity, which is originated from the pinning effect, can be excluded. Using the above system, we investigated the Josephson plasma resonance in high-T_c superconductors. Secondly, we constructed a system that can measure the surface resistance and the surface reactance simultaneously by applying a cavity perturbation technique. We prepared several cavities whose resonant frequencies were up to 98 GHz. Typical Q-factors of the cavities were about 10^4 and the reproducibility of the resonant frequency and the Q-factor were 10^<-6> and 10^<-3>, respectively. This system should also be useful to determine the absolute value of the surface impedance.