Material Design for High-T_c Superconductors by MBE and Study for Electronic State Using Cross Sectional STM

人们普遍认为高温超导氧化物具有由CuO_2层为基础的导电块和电荷储存块组成的固有超结构。本课题主要是利用分子束外延技术在原子尺度上生长高温氧化物异质结构，以用于材料设计。研究了PrBa_2Cu_3O_<7-delta>（PrBCO）/YBa_2Cu_3O_<7-delta>（YBCO）/PrBCO和SrTiO_3/SrCuO_2异质结构，发现当终止的CuO_2<1-delta>层被具有MO岩盐型层的氧化物覆盖，形成MO-CuO-BaO电荷储集块时，YBCO的表面单胞层表现出超导电性。我们成功地研究了无限层状结构SrCuO_2薄膜在外加电场作用下电阻率的变化，通过扫描隧道显微镜（STM）观察发现YBCO薄膜和SrRuO_3薄膜分别以二维单胞生长方式和阶梯流动生长方式生长。对所设计的异质结构进行横截面TEM观察，可以揭示电子结构和载流子掺杂到导电层中的机理。近年来，约瑟夫森等离子体振荡毫米波发生器因其在高温超导体中的应用前景而受到广泛关注。我们的研究结果将有助于实现这种器件的制作。

It is widely accepted that high-T_c superconducting oxides have the inherent superstructures comprising of conducting blocks based on CuO_2 layrs and charge reservoir blocks. This subject concerns the growth of heterostructures of high-T_c oxides on atomic scales by molecular beam epitaxy (MBE) for the material design. We have studied PrBa_2Cu_3O_<7-delta> (PrBCO) /YBa_2Cu_3O_<7-delta> (YBCO) /PrBCO and SrTiO_3/SrCuO_2 heterostructures.We have found that the surface one unit cell layr of c-axis oriented YBCO shows a superconductivity when the terminating CuO_<1-delta> layr has covered with an oxide, which has MO rocksalt-type layr, to form MO-CuO-BaO charge reservoir block. This result is of primary importance for the tunneling junctions and electric field devices.We have succeeded the change of electrical resistivity for SrCuO_2 thin films with infinite layr structure by applying electric field.It has been found through scanning tunneling microscope (STM) observation that YBCO and SrRuO_3 grow by a two-dimensional unit cell by unit cell growth manner and a step flow growth manner, respectively. Cross sectional TEM observation for designed heterostructures would reveal the electronic structures and carrier doping mechanism into conducting layrs.We have successfully grown heterostructures consisting of ultrathin layrs of high-T_c superconducting oxides. Recently, milli-meter wave generator using Josephson plasma oscillations have attracted much attentions for a promising applications of high-T_c superconductors. Our result would contribute to realize the fabrication of such devices.