Date of Award
Doctor of Philosophy
Physics and Astronomy
Douglas K. Finnemore
The flux line lattice in superconducting single crystals of YBa [subscript]2Cu[subscript]3O[subscript]7-[delta] is shown to undergo a very abrupt melting transition as the temperature and field approach the normal state. The sharpest transitions from the vortex solid to liquid phase, as seen in resistivity measurements, are of the order of 10mK in a temperature of 85K and of the order of 10 Oe in a field of 6T. The discontinuity in the resistivity associated with vortex lattice melting is asymmetrically hysteretic with applied field in clean crystals. The melting transition width for decreasing applied magnetic field through the melting field H[subscript]m is only 10 Oe, much sharper than for increasing the field through H[subscript]m, about 300 Oe, at fixed temperature. For moderately defect-free crystals and for high current densities in cleaner crystals, the hysteresis becomes symmetric and broader. In samples with moderate amounts of disorder the vortex melting transition width is typically wider. Resistive measurements show hysteresis with applied field upon ramping the magnetic field partially through the melting transition. The resistivity data indicate a first order vortex lattice melting transition in low defect density crystals. At fixed field and temperature, the V-I character changes from ohmic to non-ohmic as the chosen temperature crosses the melting temperature T[subscript]m. Further evidence of flux line lattice melting is a peak in a critical current J[subscript]c seen just below the melting transition. A maximum in the optimization of the lattice softening and of the lattice adjustments to pinning sites occurs at the peak in J[subscript]c. Induced point defects from electron irradiation smears out the first order melting transition. The sharp melting discontinuity becomes a continuous curve without any non-ohmic behavior. The resistivity data taken in the vortex liquid state shows indications of being viscous. The indications of a first order melting transition are returned by subsequent annealing of the irradiated sample. Further work on the exact nature of the vortex viscosity in the vortex liquid state and experiments to study the plastic and elastic behavior of the vortex solid state should be considered.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
John Anton Fendrich
Fendrich, John Anton, "Vortex lattice melting in an anisotropic type II superconductor " (1995). Retrospective Theses and Dissertations. 10899.