The effect of twin boundaries on the resistive transition of single crystal yttrium barium copper oxide
Abstract
The resistive transitions of $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ span a temperature regime of a few degrees and feature a sharp kink below which the resistance drops rapidly to zero when the magnetic field is oriented parallel to the layers. In contrast, when the magnetic field is applied normal to the layers along the crystallographic c axis the broadening of the transition is enhanced substantially and a more gradual downturn to zero resistance is observed. In a traditional view of dissipation in a type II superconductor it is natural to interpret the broadening as due to flux motion and associate these downturns as the onset of some pinning effect which retards flux motion and drives the resistance to zero. One of the prevalent defects which could be responsible for such a pinning effect in the $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ system are the planar twin boundaries which result from cooling through a tetragonal to orthorhombic transition in the crystal structure. It has also been proposed that these downturns are signatures of a freezing transition of the vortex configuration into a conventional Abrikosov lattice or a vortex glass state. The temperature, magnetic field, and angular dependence of the magneto-resistance were measured on twinned and thermomechanically detwinned single crystals of $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ to determine the effect of twin boundaries on vortex motion and the resistive transition. Pinning by twin boundaries is evidenced as a sharp drop in the resistance for magnetic fields oriented within a "depinning angle" of the twin boundaries. The shoulder in the resistive transition is found to coincide with the onset of twin boundary pinning for H $\Vert$ c. For magnetic fields misaligned with the twin boundaries at angles larger than the depinning angle the behavior of twinned and detwinned crystals is essentially identical. For such orientations a sharp kink in the resistive transition characterizing the onset of nonohmic behavior is observed and is associated with freezing of the vortex liquid. For magnetic fields directed parallel to the twin boundaries the pinning effect is strong enough to drive the resistance to zero before the vortex structure can freeze and thus, mask the freezing transition.
Degree
Ph.D.
Advisors
Reifenberger, Purdue University.
Subject Area
Condensation
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