Structural Defects in Thin Films Of High T C Superconductors
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STRUCTURAL DEFECTS IN THIN FILMS OF HIGH Tc SUPERCONDUCTORS R.Ramesh, D.M.Hwang, P.England, T.S.Ravi, C.Y.Chen, A.Inam#, B.Dutta*, L.Nazar and T.Venkatesan Bellcore, Red Bank, NJ 07701. # Rutgers University, Piscataway, NJ *Middlebury College, Vermont, VT. ABSTRACT In this short report, we describe some of the structural defects present in laser deposited Y-Ba-Cu-O thin films. Many of the defects observed are polytypoidic variants, related to the layered structure of these cuprates. One possible model by which flux pinning can be achieved is presented. INTRODUCTION Currently, considerable interest is focussed on obtaining good quality thin films of the recently discovered high Tc cuprate superconductors for electronic applications. The Y-Ba-Cu-O system, with a Tc of about 92K, has received a large part of the attention. The physics involved in dissipation in these materials is progressively revealing itself to be very complex, partly due to the high superconducting transition temperatures and partly due to the large structural anisotropy. Since these cuprates are type 11superconductors with a lower critical field, Hcl of about 100 Oe, invariably one has to consider the role of fluxons, either in an ordered lattice or in a disordered and amorphous state. It is established that in the case of the low temperature superconductors, it is essential to strongly pin the fluxons in order to prevent dissipation due to flux motion or creep[l]. Pinning of the flux lattice or fluxons can be achieved by structural defects in the crystal lattice or, in general, regions where the free energy of the vortex lattice is lower than that in the superconducting matrix. It is progressively becoming clear that the high density of structural defects in thin films of YBCO are largely responsible for the much larger critical currents compared to the bulk or single crystals. Thus, in order to understand the origin of the higher Jc in thin films compared to the single crystals, a detailed study of structural defects in the thin films has been initiated. The objective of this study is to characterize the structural defects as a function of key processing parameters and examine the interaction of the flux lattice with the structural defects in order to understand the physics behind the high critical currents obtained in the films. Clearly, such an understanding will ultimately enable us to design not only thin films, but also bulk superconductors, with the potential to carry larger Jc than currently obtained. In recent papers, a variety of structural defects, in thin films of Y-Ba-Cu-O deposited on single crystal MgO, were described[2,3]. EXPERIMENTAL The thin films were synthesised by in-situ pulsed laser deposition on to single crystal MgO. Details of the thin film deposition process are reported in earlier publications[4]. In general, the films are found to be aligned with their c-axis normal to the substrate surface. From dc resistivity measurements the samples were found to have zero resistance at 88K. Samples for electron microscopy were pr
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