Abstract
We have studied the activation energy of thermally activated vortex motion using (Pr/sub 0.6/Y/sub 0.4/)Ba/sub 2/Cu/sub 3/O/sub 7//YBa/sub 2/Cu/sub 3/O/sub 7//(Pr/sub 0.6/Y/sub 0.4/)Ba /sub 2/Cu/sub 3/O/sub 7/ trilayer structures. The dependence of the activation energy on the thickness of the YBa/sub 2/Cu/sub 3/O/sub 7/ layer and magnetic field was studied for YBa/sub 2/Cu/sub 3/O/sub 7/ layer thicknesses or 1.2/spl sim/120 nm and fields up to 7 T. We obtain a longitudinal vortex correlation length of 15/spl sim/20 mm in YBa/sub 2/Cu/sub 3/O/sub 7/. When the YBa/sub 2/Cu/sub 3/O/sub 7/ layer is a few unit-cells thick, the activation energy shows a logarithmic field dependence. In the thickness range between 10 to 30 nm, the dependence of the activation energy on magnetic field shows a crossover behavior from logarithmic dependence at low fields to one of power law, U/sub o//spl sim/H/sup -/spl alpha// (with /spl alpha//spl sim/0.5), at high fields. This crossover field does not depend on the thickness of the YBa/sub 2/Cu/sub 3/O/sub 7/ layer.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
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