Basic studies on the small-sized Josephson superlattices for the occurrence of mesoscopic phenomena and for the application to electronic devices

本研究的目的是阐明层状高温(高温)氧化物超导体(Bi1-x&gt；Pbx)2Sr2CaCu2Oy(0[小于或等于]x[小于或等于]0.2)单晶(转变温度T_c为~85K)，由0.3 nm厚的超导CuO_2层与1.2 nm厚的其他半导体或绝缘层交替堆叠组成，相邻超导层之间通过非金属层进行约瑟夫森耦合。主要研究结果如下：1.由横向尺寸的本征约瑟夫森结构成的小尺寸矩形台面超晶格器件的制备工艺降至…已经为单晶研制出了0.70×0.7μm^2以上的单晶。2.在小尺寸器件的电流-电压特性上观察到了清晰的超导带隙。能隙Δ(4.2K)的值为25-30 meV，与x无关。3.建立了一个理论模型来解释超导能隙的准粒子隧穿电流-电压特性的总体轮廓。4.在电流-电压特性的准粒子分支上也观察到了精细的亚能隙结构，与x、温度和弱磁场无关。这是由于本征约瑟夫森结中拉曼激活的光学声子和交流约瑟夫森振荡的相互作用。5.在4.2K下观察到临界电流密度J_c随横向尺寸减小到亚微米和增加结数而可控地减小，这可能是由于库仑阻塞效应所致。然而，对于这些设备，单库珀对隧道还没有得到明确的确认。较少

The purpose of this study is to clarify the essential tunneling properties and further develop the mesoscopic phenomena of small-sized natural superlattices of intrinsic Josephson junctions with lateral dimensions decreased down to submicron in layered high-temperature (high-T_c) oxide superconductor (Bi_<1-x>Pb_x)_2Sr_2CaCu_2O_y (0【less than or equal】x【less than or equal】0.2) single crystals (with transition temperatures T_c of〜85K), which consist of the stacking of superconducting CuO_2 layers of 0.3nm thickness alternating with other semiconducting or insulating layers of 1.2nm thickness, with the Josephson coupling between every adjacent superconducting layers via nonmetallic layers, and also to get information for the application of the superlattices to electronic devices.The obtained results are as follows :1.A fabrication technology of a small-sized rectangular mesa shape of superlattice devices composed of intrinsic Josephson junctions with lateral dimensions decreased down to … More 0.7×0.7μm^2 has been developed for the single crystals.2.Clear superconducting gaps have been successfully observed on the current-voltage characteristics of the small-sized devices. The values of gaps Δ(4.2K) have been found to be 25-30meV, independent of x.3.A theoretical model to explain the overall profile of the quasiparticle tunneling current-voltage characteristics, which shows the superconducting gap, has been established.4.Fine subgap structures have also been observed on the quasiparticle branches of the current-voltage characteristics, independent of x, temperature and weak magnetic field. This has been interpreted to be due to an interaction between Raman-active optical phonons and ac Josephson oscillation in the intrinsic Josephson junction.5.Critical current densities J_c at 4.2K have been observed to decrease controllably with decreasing the lateral dimensions to submicron and increasing the number of junctions in the small-sized devices of Bi_2Sr_2CaCu_2O_y. This may be considered to be due to the Coulomb blockade effect. However, the single Cooper-pair tunneling has not been clearly confirmed for the devices. Less