ABSTRACT
High-rc superconducting oxides have planer crystalline structures which consist of cupratebased Perovskite units (AOCu02, where AO = Ca, SrO, BaO, Y, LaO, etc.) [1]. The crystal structure of the monumental high-7^ compound (La2_xBaJCCu04) was found to be a repetition of [LaO-Cu02-LaO] trilayers [2]. The second one (YBa2Cu307_5) can be considered to be a infinite Perovskite structure having ordered tri-units as BaO-Cu02-Y-Cu02-BaOCuO [3]. The infinite layer (Sr,Ca)Cu02, which can be synthesized under high pressure and has a superconducting transition temperature (Tc) as high as 110 K, is also a typical example of this category [4]. The variety of the high-Tc superconductors is extended by so-called multiple rock-salt-type slabs [5], for instance (BiO)2 [6], TIO, (T10)2 [7], and HgO [8] layers, with which the Perovskite slabs are interleaved. The compounds in this group show superconductivity at above 100 K. By devising the combination of the various Perovskite units and rock-salt-type slabs, a number of assorted compounds could be designed and some of them have already been synthesized via powder routes. Although the combination is numerous, the compounds which can be synthesized via the powder route are limited because of the entropy reason; namely, an alternating structure having a larger repeating period (A > 25 Â) cannot be synthesized, nor can an ordered superstructure be formed. A lattice mismatch between the layers is also a crucial problem when one tries to synthesize a designed compound by the thermodynamically equilibrium process (i.e., powder route). Contrary to the powder route, the thermodynamically metastable structure can be realized by the nonequilibrium processes. For these reasons, vapor-phase depositions, especially the atomically layer-by-layer synthesis technique, can be considered as the most suitable tech-
nique to realize the extended combination, designed structures, and heterostructures having a larger period, ordered structures, and thermodynamically metastable structures. By synthesizing a series of assorted structures systematically, this technique contributes to the field which ranges from the basic solid-state physics to the application on the device technology. In another view, it ranges from the venturous material hunting to the systematic data library.
homologous structure series. The members of each family differ principally in the number of Cu02 + Ca layers within each formula unit. They realize the above-mentioned variable combinations of the alternating structure in nature, simply by changing the number of Perovskite layers in a molecular unit. These compounds have strong two dimensionality in the crystal structure because the superconducting cuprate-based Perovskite layers are separated by the rock-salt-type slabs with a certain distance (> 9 Á) which is longer than the in-plane, Cu-Cu distance (3.8 Á). For this reason, these compounds can be considered as intrinsic SNS or SIS arrays along the c directions where S, N, and I express superconductor, normal metal and insulator, respectively. Especially in the Bi case, the bonding mechanism between the molecular units, namely, between the adjacent BiO layers, is a van der Waals type and the distance between the Cu02 layers in the adjacent molecular units is the largest (12.3 A) [9]. Additionally, the BiO double layers show semiconducting transport properties [10]. For these reasons, the Bi2Sr2CaAl_1Cu/T02Ai+4 have the strongest two-dimensional character and are considered to be one of the most suitable series to grow by the layer-bylayer mode. Several attempts have been made for this series by molecular beam epitaxy [1115], alternate sputter deposition [16-19], and pulsed laser ablation [20]. Artificial structures having more than four Cu02 planes in a molecular unit have been synthesized [11-20]. The superstructures consisting of alternate Bi2Sr2Ca0CujO6 and Bi2Sr2CajCu208 are also widely synthesized [21-27]. The major interest of these artificial structures is their Tc. The maximum Tc of the structure is considered to depend on the structure of Cu02 planes, especially Cu05 pyramidal structure of the outmost Cu02 layers [28,29]. The optimum hole doping is necessary for each structure in the family by changing the oxygen content or by substituting the cationic element [30]. The systematic study including both the structural analysis and carrier concentration measurement is rarely found in the artificially layered Bi2Sr2Caw_,Cu/702A?+4 compounds.
