Fabrication of Ultra Low Noise Quasiparticle Mixer for Submillimeter Wave

在本项目中，我们的目标是首先制造具有自身消失约瑟夫森电流的磁势垒超导隧道结，然后利用这些结，我们的最终目标是开发工作在亚毫米波区域的超低噪声温度的准粒子混频器。特别是，我们制造了由Nb制成的隧道结，它坚固耐用，具有高临界温度，Tc。我们得到了以下结果。1. 从理论上评价了准粒子混合器的性能。我们发现，即使结的I-V特性不优越，也可以实现超低噪声性能。为了建立结制造的基本技术，我们尝试制造Pb/TaOx/Nb Josephson隧道结。然而，制造的结有太大的漏电流。我们尝试用磁势垒制造隧道结，Pb/TaOx/NiOx/TaOx/Nb结和Nb/TaOx/Nb josephson结。在这两种情况下，连接的质量都很差。因此，我们需要进一步发展结的制造技术。为了研究高Tc超导体结的高频特性，我们测量了Nb/YBaCuO结在525 GHz频率下的亚毫米波响应。这些结被证明是桥型约瑟夫森结，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.