No abstract

No abstract

Experimental evidence is presented for a supercurrent oscillation arising from vortex motion in the flux-flow regime of superconducting films with periodically modulated thickness. An essential condition for detecting the rf electric field associated with the oscillation is matching of the vortex lattice to the periodic pinning structure represented by the thickness modulation.This Letter reports the observation of radiofrequency (rf) electric fields in the flux-flow regime of type-II superconducting films with periodically modulated thickness when vortex motion is driven by a dc transport current. These oscillating voltages are a manifestation of a Josephson-like supercurrent oscillation arising from coherent vortex motion in the one-dimensional periodic pinning potential represented by the thickness modulation. 1 Evidence for the oscillation is found when the value of the transverse magnetic field H-B corresponds to matching of the vortex lattice to the periodic film structure.Several years ago, Kulik 2 and Schwartz 3 suggested that there is a close analogy between the flux-flow state in type-II superconductors and the ac Josephson effect in superconducting weak: links. In fact, a moving vortex lattice can be thought of as a supercurrent density distribution oscillating both in space and time in a way very similar to that sometimes observed in weak links in connection with the ac Josephson effect. 4 According to this picture, one would expect the existence of electromagnetic radiation from type-II superconductors in the flux-flow regime. 2,3 As pointed out by Meincke, 5 however, the dramatic mismatch between the flux-flow velocity (or phase velocity of the supercurrent pattern) and the phase velocity of the electromagnetic field almost excludes the possibility of detecting ac Josephson effects in type-II superconductors. Meincke's argument, however, is only valid for uniform motion of an extended vortex lattice. Actually, by considering flux flow in the presence of pinning, Schmid and Hauger 6 have shown that the pinning potential introduces the necessary mechanism for coupling electromagnetic fields to the supercurrent oscillations. This coupling can be significant only when the effect of pinning results in a sufficiently coherent modulation of the vortex velocity. In this case the corresponding modulation of the supercurrent density distribution leads to a net supercurrent oscillation which can therefore interact with the electromagnetic field. This interaction was first demonstrated experimentally for weak random pinning by Fiory 7 and recently for a periodic pinning structure by Martinoli et al.,8 who observed quantum-interference phenomena when the flux-flow state was driven by superimposed dc and rf currents.At this point, the problem of the direct detection of the supercurrent oscillation associated with the moving lattice arises in a quite natural way. In this connection, we note that Clem 9 predicts the existence of characteristic structures, related to the oscillation, in the noise power spectrum. Suc...

No abstract