Washboard frequency of the moving vortex lattice in YBa2Cu3O6.93 detected by ac-dc interference

The depinned vortex lattice in a type II superconductor represents a periodic structure moving in a random distribution of pinning centers. Such a system displays a washboard frequency given by /2=v/a where v is the velocity and a the lattice period. By mixing a dc and an ac current (of frequency ext), we demonstrate the existence of the washboard effect in the vortex state in an untwinned crystal of YBa2Cu3O6.93. Jumps in the dc current-voltage characteristics are observed when and ext are harmonically related. We show that these jumps vanish when the line at the solid-to-liquid transition is crossed.