Detection of the vortex-liquid phase in superconducting films by Nernst effect

To clearly detect the vortex-liquid state of a superconducting film, we constructed an experimental system for measuring the Nernst signals in a film sample. We measured Nernst signals N in an amorphous Mo_xGe_1−x film with a thickness of 300 nm. As the field is increased in the vortex-solid phase, N rises from zero at the boundary between the vortex-solid state and the vortex-liquid state, showing a peak in the middle of the liquid phase, and finally decreases. From a linear extrapolation of N after the peak to N = 0, we roughly estimated the upper bound of the liquid state. These results imply that the Nernst measurement could be an alternative to a resistivity measurement for detecting the vortex-liquid state. It would be helpful when the resistivity measurement is inconclusive in detecting vortices such as in an insulator phase in the vicinity of the superconductor-insulator transition in two dimensions. (paper)