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

为了研究超导体中高度相关的电子和涡流的低能激发，我们研制了两种可以测量宽温度范围内磁场下高频阻抗的仪器。首先，我们构建了一个基于热容测量技术的表面电阻测量系统。在该系统中，样品被放置在腔腔谐振器内以指定射频场方向。虽然测热计的灵敏度随温度的升高而降低，但通过各种条件的优化，我们可以将测温上限延长到80 K以上。此外，我们成功地测量了表面电阻随温度的变化，而不是电场的变化。这对超导体来说很重要，因为可以排除由钉住效应引起的场不均匀性。利用上述系统，我们研究了高t_c超导体中的约瑟夫森等离子体共振。其次，我们利用空腔微扰技术构建了一个可以同时测量表面电阻和表面电抗的系统。我们制备了几个谐振频率高达98 GHz的空腔。谐振腔的典型q因子约为10^4，谐振频率的再现性和q因子分别为10^<-6>和10^<-3>。该系统还可用于确定表面阻抗的绝对值。

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.