Washboard Frequency of the Moving Vortex Lattice in YB${\mathrm{a}}_2$C${\mathrm{u}}_3$${\mathrm{O}}_{6.93}$ 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 $\omega /2\mathrm{}\pi =v/a$ where $v$ is the velocity and $a$ the lattice period. By mixing a dc and an ac current (of frequency ${\omega }_{\mathrm{ext}}$), we demonstrate the existence of the washboard effect in the vortex state in an untwinned crystal of YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_{6.93}$. Jumps in the dc current-voltage characteristics are observed when $\omega$ and ${\omega }_{\mathrm{ext}}$ are harmonically related. We show that these jumps vanish when the line at the solid-to-liquid transition is crossed.