Anisotropic distributions of electrical currents in high-T c grain-boundary junctions

Abstract

A decisive role in electrical transport in high T c superconducting materials with anisotropic pairing, like cuprate superconductors [1,2] and Fe based compounds [3][4][5][6], belongs to grain boundaries (GB). Among a variety of possible crystallographic orienta tions of grains forming a boundary, the [001] tilt GB (see Fig. 1a) had been studied the most intensively with an emphases on the effect of order parameter symmetry and the mechanisms of charge transfer across the GB interface [1,2,[7][8][9][10][11][12][13][14][15].Recently, [100] tilt high T c GB junctions with mutually tilted c axis's (see Fig. 1b) have been fabri cated [16][17][18][19][20][21][22][23][24][25] with an order of magnitude lower GB meandering and up to a threefold increase of the I c R n values [16][17][18][19]. A complete antiphase correlation between low frequency fluctuations of the resistance R n and the critical current I c has been observed in these junctions, thus showing that both quasiparticles and Cooper pairs flow directly through the same regions of the barrier [24]. In spite of intensive efforts, the mech anisms of electrical transport through grain bound aries in high T c superconductors are still far from understanding. This is true not only in respect of phys ics of conductivity of GB itself, but also in respect of influence of normal and supercurrent redistribution in the vicinity of GB on their properties.It is well known that strong anisotropy of resistivity of high T c materials must be taken into account during ¶ The article is published in the original.interpretations of electrical data. A simple assumption that electrical resistivity of the material is proportional to the measured resistance with some coefficient of proportionality given by sample geometry may give an incorrect result [26-36].We developed a self consistent method for the calculation of spatial current distributions in high T c grain boundary junctions. It is found that crystallographic anisotropy of high T c superconducting electrodes results in the effects, which previously were not taken into account for interpretations of experimental data. Among them is a significant redistribution of electrical currents in superconducting electrodes in the vicinity of a grain boundary. In particular in the case of [100] tilt bicrystal junctions, this current redistribution results in a substantial focusing to the top or bottom part of a thickness of the grain boundary, depending on "roof" or "valley" type of the grain boundary. This redistribution is accompanied by generation of vortex currents around the grain boundary, which leads to self biasing of grain boundary junctions by magnetic field nucle ated by these vortex currents. It is shown that twinning or variation of geometrical shape of the high T c elec trode may also result in intensive redistribution of electrical currents and nucleation of local magnetic fields inside a high T c superconducting electrodes. Fig. 1. Schematic diagram showing the crystallography of a [001] tilt boundary (a), a [100] tilt b...

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