Study on Submilimeterwave Spectroscopy of Superconducting quasi-one-dimensional Quantum Wires

报道了超导体-收缩-超导体(S-c-S)准一维介观器件中输运噪声的性质和射频特性的实验结果。这些器件是通过在液氦(4.2K)中对绝缘Nb/绝缘体/Nb(或NbN)三明治施加电压脉冲来制造的。通过这种方法，我们可以连续地改变准粒子电流-电压(I-V)曲线，从绝缘体到隧道接触，最后到理想的金属导电界面。我们发现，I，dV/Di-V特性与Blonder，Tinkham and Kapwijk(BTK)，Octabio和BTK(OTBK)理论的预言是一致的。我们还发现噪声电压功率谱密度Sv(F)随着器件品质因数Q^lt；-1&gt；的增加而迅速减小，并在Q^lt；-1&gt；-2区域达到饱和。这种饱和表明在理论模型(Hooge模型)的框架内，与缺陷相关的载流子数不再随着Q^&lt；-1&gt；2区域的变化而变化。在我们的样品中，只有清洁的金属器件才能观察到清晰的周期dv/di峰。这些在带隙电压以上的周期性dv/di峰的产生可能与量子尺寸效应或考虑了压缩中态密度量子化的几何共振模型有关。在微波辐照下器件的电流-电压特性中观察到了新的微波感应台阶。观察到的台阶与众所周知的“夏皮罗台阶”的特征不同。这种微波诱导步骤的总体特征不能用现有模型来解释。虽然这些峰的来源尚不清楚，但这一现象的一个可能的来源是微波场与收缩中Andreev反射的电子/空穴的相互作用。

We repotr the experimental results concerning the nature of transport noise, and rf-properties in Superconductor-constrictions-Superconductor (S-c-S) quasi-one-dimensional mesoscopic device. These devices were fabricated by applying voltage pulses to an insulating Nb/Insulator/Nb (or NbN) sandwich in liquid helium (4.2K). By this method, we could continuously vary the quasiparticle current-voltage (I-V) curve from insulator to tunnel-contact and finally up to perfectly metallic conducting interface. We found that I,dV/dI vs. V characteristics is colsely with the predicrions of the Blonder, Tinkham and Klapwijk (BTK), and Octabio and BTK (OTBK) theories. We also found that noise voltage power spectral density Sv (f) decreases quickly with increasing device quality factor Q^<-1> and saturates for Q^<-1>>2. This saturation suggests that within the framework of the theoretical model (Hooge model) the defect related number of carriers near the constrictions no longer changes for Q^<-1>>2 regime. In our samples, clear and periodie dV/dI peaks were observed only for clean metallic devices. The origin of these periodic dV/dI peaks above gap voltage may be associated to the quantum size effect, or the geometrical resonance model which takes into account the quantization of the density of states in the constriction. New microwave-induced steps were observed in the current-voltage characteristics of the devices with microwave irradiation. The observed steps are different from the features of the well-known "Shapiro steps". Overall features of such microwave-induced steps can not be explained using existing models. Although the origin of these peaks is not clear yet, one possible origin of this phenomena is the interaction of microwave field with Andreev-reflected electron/hole in the constriction.