Interference Effects in a Superconducting Aluminum Film; Vortex Structure and Interactions

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Abstract
Interference transitions are observed in the mixed state of a granular oxygen-doped superconducting aluminum film when vortex-lattice motion is driven by applying superimposed rf and dc electric currents. An analysis of the properties of the transitions in the small-signal rf resistivity is made according to a recent theory of vortex motion in an inhomogeneous superconductor by Schmid. The theory satisfactorily explains the effect, which is due to the dissipative excitation of vortex-lattice fluctuations. New information on vortex structure and interactions at low magnetic field is obtained. The vortex-lattice shear-constant measurements agree with the theoretical work of Fetter, Hohenberg, and Pincus and of Brandt. A pinning-potential correlation function also comes out of the measurements and its component factors are identified: the structural-defect correlation function and the vortex-free-energy distribution. The maximum in the free-energy density at the vortex axis, which is larger than Ginzburg-Landau-theory predictions, suggests the presence of the core excitations discussed in a recent microscopic theory of vortex structure.