Width and magnetic-field dependence of the transition temperature in ultranarrow superconducting wires

We calculate the transition temperature in ultranarrow superconducting wires as a function of resistance, wire width, and applied magnetic field. We compare the results of first-order perturbation theory and the nonperturbative resummation technique developed by Oreg and Finkel'stein. The latter technique is found to be superior as it is valid even in the strong disorder limit. In both cases, the predicted additional suppression of the transition temperature due to the reduced dimensionality is strongly dependent upon the boundary conditions used. When we use the correct (zero-gradient) boundary conditions, we find that theory and experiment are consistent, although more experimental data is required to verify this systematically. We calculate the magnetic-field dependence of the transition temperature for different wire widths and resistances in the hope that this will be measured in future experiments. The predicted results have a rich structure-in particular, we find a dimensional crossover which can be tuned by varying either the width of the wire or its resistance per square.