Atomic Layer Growth of High Tc (Ca,Sr)CuO 2 Thin Films by Laser MBE
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ATOMIC LAYER GROWTH OF HIGH Tc (Ca,Sr)CuO LASER MBE M.KANA1, T.KAWAI and S.KAWAI The Institute of Scientific and Industrial Mihogaoka, Ibaraki, Osaka 567, Japan
Research,
2
THIN FILMS
Osaka
BY
University,
8-1,
ABSTRACT Resistive and magnetic anomalies were observed at 150-170K in Ca 0 .3Sr 0 .701O2 thin films prepared by atomic-layer stacking using laser molecular beam epitaxy. Systematic current density dependence of the resistivity was observed below these temperatures with clear resistivity drops. At the same temperature range, the films showed diamagnetic response and hysteresis of magnetization between zero-field cooling and field cooling process. These behaviors suggest that the anomalies are due to high temperature superconductivity around 150170K in the Ca 0 .3 S r0 .70uO 2 thin films. INTRODUCTION The atomically and molecularly layered growth of artificial lattices forms a fascinating new research topic, allowing unprecedented structural and compositional control at the atomic level. This sophisticated method recently has been applied to fabricate superconducting Bi 2 S r2 Can - lCun 2n+4 artificial lattices with n=1 to 5 [1] and even n=7,8 [2]. As an extension of this work, thin films of Ca 1-xSrxaQ2 have been grown, representing the core part of the highTc cuprates, by atomic-layer stacking using laser molecular beam epitaxy (laser MBE) [3]. This technique has enabled us to form films of the infinite-layer compound Cal~xSrxOJO2 with x=0.2 - 1.0 under low pressure synthesis conditions[4], although Sr rich bulk samples of this compound have been synthesized only at very high pressure [5]. During these experiments, it was observed that some of the CaixSrxQaO2 films (x=0.6 - 0.8) made by laser MBE exhibited pronounced resistive and magnetic anomalies at temperatures higher than 130K, implying the existence of superconductivity in this temperature range [6]. To further evaluate this possibility, the properties of newly formed Ca 0 .3S r 0 .7 00 2 films have been investigated through detailed resistive and magnetic measurements. Designed to test the criteria for high-Tc superconductivity, these measurements indeed suggest the onset of the superconductivity at temperatures as high as 150K - 170K in the (CaSr)CuO2 thin films grown by atomic-layer stacking using laser MBE. EXPERIMENTAL The Ca 0 .3 S r 0 .7 0O 2 thin films were grown on SrTiO 3 (100) substrates by a computer-controlled laser MBE method [3,4,6]. Briefly, disks of Ca, Sr and CuO were successively ablated in an ambient of NO 2 at 8xl0 6 mbar to form atomiclayers of Ca 0 .3Sr 0 .7c and CuOx, and the infinite-layer structure was constructed by the accumulation of these layers. The substrate temperature was 500" C. Details of the film formation have been described previously in Mat. Res. Soc. Symp. Proc. Vol. 285. 01993 Materials Research Society
258
references 3,4 and 6. Following the deposition, the films were in-siAu annealed 4 by slow cooling (8' C/min) in lxlO- mbar of NO 2 as strong oxidizing agent. The film thickness was typically 300A
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