Fabrication of Ultra Low Noise Quasiparticle Mixer for Submillimeter Wave

在这个项目中，我们的目标是首先制作具有磁垒的超导隧道结，它本身就有约瑟夫森电流的消失，然后利用这些结，我们最终的目标是研制一种工作在亚毫米波的超低噪声温度的准粒子混合器。特别是用Nb制作了具有较高临界温度T_c和韧性的隧道结。我们得到了以下结果。1.从理论上对准粒子混合器的性能进行了评价。我们发现，即使结的I-V特性不是很好，也可以实现超低噪声性能。为了建立结制作的基本技术，我们尝试制作了Pb/TaOx/Nb约瑟夫森隧道结。然而，制造的结具有太大的泄漏电流。我们尝试制作了带有磁势垒的隧道结、Pb/TaOx/Niox/TaOx/Nb结和Nb/TaOx/Nb约瑟夫森结。在这两种情况下，结点的质量都很差。因此，我们需要进一步发展结制作技术。为了研究高温超导体结的高频特性，我们测量了频率高达525 GHz的Nb/YBaCuO结的亚毫米波响应。结果表明，这些结是桥型约瑟夫森结，它们的响应与普通超导体的结相似。

In this project, we aim firstly to fabricate superconducting tunnel junctions with a magnetic barrier, Which have vanishing Josephson current by itselr, and then using these Junctions, we aim finnaly to develop a quasiparticle mixer working in the submillimeter wave region with ultra low noise temperature. Especailly, we fabricate tunnel junctions made of Nb, which is robust and has high critical temperature, Tc. We obtained following results. 1. We evaluated the performance of quasiparticle mixer theoretically. We found that ultra low noise performnce can be realized, even if the junction I-V characteristics is not superior.2. To establish basic techniques of junction fabrication, we tried to fabricate Pb/TaOx/Nb Josephson tunnel junctions. The fabricated junctions, however, have too large leak current. We tried to fabricate tunnel junctions with a magnetic barrier, Pb/TaOx/NiOx/TaOx/Nb junctions, and Nb/TaOx/Nb josephson junctions. In both cases, the junctions are of poor quality. Thus, we need to develop techniques of junction fabrication further.3. To investigate high frequency characteristics of junctions made of high Tc superconductors, we measured submillimeter wave response of Nb/YBaCuO junctions up to frequency of 525 GHz. The junctions turned out to be bridge type Josephson junctions and their response is simmilar to taht of junctions made of ordinary superconductors.