Magnetic Interactions at the Superconductor-Insulator Transition in Ultrathin Dysprosium BARIUM(2) COPPER(3) OXYGEN(7-X) Films
Abstract
Disorder and magnetic field driven superconductor -insulator transitions were investigated in ultrathin DyBa _2Cu_3O _6 films. The films were fabricated by ozone assisted molecular beam epitaxy and were measured in a dilution refrigerator at temperatures down to 70 mK and in magnetic fields up to 14 T. Both the disorder and magnetic field driven transitions are influenced strongly by granularity in the films. Finite resistance was observed at low temperatures over a broad range of film parameters between clearly superconducting and insulating states. This suggests that the superconductor -insulator transition is not direct in this system. Nonmonotonic temperature and magnetic field dependence of the resistance was also observed at low temperatures and magnetic fields. This unexpected behavior is not due to superconductivity, but instead can be accounted for by interactions of single charge carriers with the localized magnetic Dy^{3+} ions at the magnetic field dependent Neel temperature of the sublattice. Although it is widely believed that interactions between the carriers and the magnetic Dy^ {3+} ions do not occur, it is possible that this insensitivity to the presence of a magnetic sublattice is due to the widely different energy scales of superconductivity (the transition temperature is near 100 K) and Dy ^{3+} antiferromagnetism (the Neel temperature is near 1 K). For disordered films having sheet resistance near the quantum resistance for pairs, superconductivity is nearly quenched, and the effects of magnetic interactions should become apparent. The results presented in this doctoral thesis display interactions between a wide range of phenomena present in ultrathin DyBa_2Cu _3O_6 films. The interaction between superconductivity and localization, both marginal in two dimensions, leads to a superconductor -insulator transition. The presence of a unique vortex state in high temperature superconductors may produce a magnetic field driven superconductor-insulator transition with distinctly different properties than would be found in a conventional superconductor. The magnetic Dy ^{3+} sublattice also plays a role in modulating the low temperature and field dependent resistance, and may indirectly modulate the vortex mobility in the film. Finally, the granularity of the films influences all of the behavior observed in these ultrathin films.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT........65B
- Keywords:
-
- DYSPROSIUM BARIUM COPPER OXIDE;
- ULTRATHIN FILMS;
- Physics: Condensed Matter