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氧化物超导体（Bi_ Pb_x）_2Sr_2CaCu_2O_y中横向尺寸减小到亚微米的本征约瑟夫森结的小尺寸自然超晶格的基本隧穿性质，并进一步发展其介观<1-x>现象（0[小于或等于]x[小于或等于]0.2）单晶（转变温度T_c为1085 K），其由0.3nm厚的CuO_2超导层与1.2nm厚的其它半导体或绝缘层交替堆叠而成，研究了超导层间通过非金属层的Josephson耦合，为超晶格在电子器件中的应用提供了理论依据，取得了如下结果：1.本征Josephson结组成的小尺寸矩形梅萨超晶格器件的制作工艺， 关于我们 2.在小尺寸器件的电流-电压特性上，成功地观察到了清晰的超导带隙。实验结果表明，能隙Δ（4.2K）为25- 30 meV，与x无关; 3.建立了一个理论模型，解释了准粒子隧穿电流-电压特性的总体轮廓，表明了超导能隙; 4.在电流-电压特性的准粒子分支上也观察到精细的亚能隙结构，与x、温度和弱磁场无关。这可能是由于本征Josephson结中的交流Josephson振荡与喇曼激活光学声子相互作用的结果。5.在小尺寸Bi_2Sr_2CaCu_2O_y器件中，观察到4.2K下的临界电流密度J_c随横向尺寸减小到亚微米和结数增加而可控地减小。这可能被认为是由于库仑阻塞效应造成的。然而，单库珀对隧穿尚未明确证实的设备。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