Measurements of the temperature dependence of the resistivity r a , r b , resistance R c , and thermoelectric power a a , a b , a c for a YBa 2 Cu 4 O 8 crystal under high pressure verify that the CuO 2 sheets are oxidized by pressure-induced electron transfer from the CuO 2 sheets to the Cu(1)-O double chains. The anomaly in the CuO 2 -sheet resistivity r a at T ഠ 170 K, which moves to a higher temperature on oxidation of the CuO 2 sheets, is correlated to a change in the character of R c . Our data also demonstrate unambiguously the contribution to the transport properties from Cu-O chains. [S0031-9007 (96)01634-1] PACS numbers: 74.25.Fy, 72.15.Jf
We report the transport properties of the La 0.67 Sr 0.33 MnO 3 (LSMO)/YBa 2 Cu 3 O 7+d (YBCO) heterostructures in the vicinity of the superconducting transition. The bottom deposited manganite was an LSMO ferromagnet with a Curie temperature of 350 K, while the top deposited cuprate was a nearly optimally doped YBCO superconductor with T c ∼90 K. The samples used in these studies had fixed a thickness of LSMO layer (90 nm), whereas the thickness of the YBCO layer was varied between 20 and 100 nm. Single YBCO layers with respective thicknesses were also measured for comparison. The mixed state properties of the bilayers and single layers were characterized by means of the Nernst effect and electrical resistance measurements in magnetic fields of up to B=12.5 T. All investigated samples show a zero-resistance state below T=60 K in a field of 12.5 T and demonstrate gradual degradation of the superconducting properties with decreasing thickness of the YBCO layer in bilayer structures.
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