Critical Currents at Grain Boundaries in High Temperature Superconductors
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Critical Currents at Grain Boundaries in High Temperature Superconductors
D. AGASSIa, S. J. PENNYCOOKb, D. K. CHRISTENb, G. DUSCHERb,c, A. FRANCESCHETTIa,d and S. T. PANTELIDESa,d a Naval Surface Warfare Center, Carderock Division, Bethesda, MD b Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee c Department of Material Science and Engineering, North Carolina State University, Raleigh, NC d Department of Physics and Astronomy, Vanderbilt University, Nashville TN ABSTRACT We present atomic resolution Z-contrast images, electron energy loss spectroscopy (EELS) and theoretical calculations in support of a band-bending model for the effect of grain boundaries on critical currents. In the high angle regime, dislocation cores are closely spaced and the boundary is modeled as a continuous junction, with a width determined by the dislocation density per unit boundary length. This quantitatively explains the approximately exponential reduction in critical current. In the low angle regime, where dislocations are separated by substantial good passages, explicit calculations of flux pinning are presented. Significant differences are found between a strain and band-bending mechanism. Recent data fit the band-bending model and suggest substantial improvement is possible through doping to a flat band condition. INTRODUCTION Dislocation cores in the perovskite SrTiO3 have recently been shown to be intrinsically non-stoichiometric1. Evidence is accumulating that similar effects occur in structurally-related materials, including the high-temperature superconductor YBa2Cu3O7-x (YBCO). We present atomic resolution Z-contrast images, electron energy loss spectroscopy (EELS) and densityfunctional total energy calculations to support this idea. Non-stoichiometry leads to band bending and a non-superconducting zone surrounding each dislocation core. For closely spaced dislocation cores the non-superconducting zones overlap, and supercurrent can only cross the boundary by tunneling. The exponential reduction in critical current observed for grain boundaries above ~ 10˚ 2,3 can be quantitatively explained by assuming a fixed charge per dislocation core. For low angle boundaries the dislocation spacing is sufficiently large that supercurrent can pass between the dislocation cores. In this regime therefore the supercurrent is not limited by tunneling across the barrier but by the critical current of the good passages between the cores. We present calculations of the pinning potential at low angle grain boundaries using a linear Josephson junction array model which predicts the angular and temperature dependence of critical current, both perpendicular and parallel to the boundary. Results are in good agreement with experiment and suggest that recent data are limited by band bending with significant potential for improvement through appropriate doping. Our key experimental probe of grain boundary atomic and electronic structure is the scanning transmission electron microscope (STEM). Such microscopes are now routinely av
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