  The mixed-state resistive response of a superconductor thin film with an
asymmetric washboard pinning potential subject to superimposed dc and ac
currents of arbitrary amplitudes and frequency at finite temperature is
theoretically investigated. The problem is considered in the single-vortex
approximation, relying upon the exact solution of the Langevin equation in
terms of a matrix continued fraction. The dc voltage response and the absorbed
power in ac response are analyzed as functions of dc bias, ac current amplitude
and frequency, in a wide range of corresponding dimensionless parameters.
Predictions are made of (i) a reversal of the rectified voltage at small dc
biases and strong ac drives and (ii) a non-monotonic enhancement of the
absorbed power in the nonlinear ac response at far sub-depinning frequencies.
It is elucidated how and why both these effects appear due to the competition
of the fixed internal and the tunable, dc bias-induced external asymmetry of
the potential as the only reason. This is distinct from other scenarios used
for explaining the vortex ratchet reversal effect so far.
