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

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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.