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We have found for the first time that superconductivity can occur in a one-unit-cell-thick layer of YBazCusO? (YBCO). The layer is grown, while monitoring with RHEED oscillations, on a nonsuperconducting PrBa2Cu307 (PrBCO) buffer layer and covered by a PrBCO layer by reactive evaporation. A reduction of the onset temperature is found to be mainly due to a decrease of the hole carriers but not due to the absence of interlayer couplings. The covering PrBCO layer has been found to provide the YBCO layer with carrier holes sufficient to induce superconductivity.

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Diagonal and Hall resistivities (pxx and pxy) in the normal and superconducting states are studied in one-unit-cell-thick YBa2Cu307-5 artificial materials. A number of transport properties in bulk YBa2Cu307-5 are shown to be inherent properties of the individual conducting Cu02 bilayers. Evidence of Kosterlitz-Thouless resistive transition as well as the excitation of free vortex and antivortex is presented.PACS numbers: 74.60.Ge, 74.70.Vy In high-temperature superconductors (HTS), twodimensional (2D) conducting Cu02 planes are weakly coupled by the Josephson effect and magnetic interactions. HTS show unusual transport properties in the normal state, such as T-linear-dependent resistivity and the r~'-dependent Hall resistivity [1,2]. It may be of importance for the study of pairing mechanisms to clarify whether these transport properties are intrinsic to the nature of a single 2D conducting layer or are a consequence of interlayer coupling. In addition, the problem of how the 2D superconducting sheets achieve the zero-resistance state has been a subject of much interest. Although the Kosterlitz-Thouless (KT) model [3][4][5] has been discussed for bulk samples [6-9], unknown interlayer coupling makes the interpretation ambiguous. Recently, superconducting transition has been shown to occur at finite temperatures in one-unit-cell-thick (l-UCT) YBCO [10]. It may be of decisive importance to clarify whether the superconducting transition in a single 2D CuOi layer is describable by KT theory.Here, we carry out detailed studies of diagonal and Hall resistivities, pxx and pxy, of l-UCT YBCO layers. The experimental results show that a number of physical properties of bulk YBCO are intrinsic to the individual Cu02 bilayers, being substantially unaffected by the interlayer coupling. We will show that, nevertheless, the temperature at which the resistance vanishes is remarkably suppressed in the absence of interlayer coupling. We present, for the first time, definite evidence of the occurrence of KT resistive transition in HTS. We study two types of samples, both epitaxially grown on the (100) surface of SrTiOs by the activated reactive evaporation method, in which the crystal growth is controlled on oneunit-cell scale through the observation of reflection highenergy electron diffraction (RHEED) oscillations [10]. The first type is l-UCT ultrathin films consisting of welldefined l-UCT layers of YBCO sandwiched by semiconducting 6-UCT PrBCO and 1 l-UCT PrBCO layers [Pr (6)/Y(l)/Pr(ll)] [10]. The second-type samples are YBCO/PrBCO superlattices, where a multiple layer of 1 -UCT YBCO and m-UCT PrBCO with m ==

We developed two types of magnetic tunnel junctions (MTJs) that showed high thermal stability. One is a PtMn exchange-biased spin-valve MTJ with a CoFe/Al-oxide (AlOx)/NiFe free layer and a CoFeTaOx/CoFe pinned layer, and the other is a pseudo-spin-valve (PSV) MTJ with a CoFe/AlOx/NiFe soft layer, where AlOx and CoFeTaOx act as barriers for Ni and Mn diffusion toward the tunnel barrier, respectively. After 390 °C-1H annealing, the PSV MTJs maintained 28% and the SV MTJs 39% of tunnel magnetoresistance. Transmission electron microscopy observation of the SV MTJs after 380 °C-1H annealing revealed that the migrated Mn atoms were trapped at the CoFeTaOx layer.