Reversal of thermopower oscillations in mesoscopic Andreev interferometer

We report measurements of thermopower oscillations $vs$ magnetic field in a diffusive Andreev interferometer. Upon the increase of the dc current applied to the heater electrodes, the amplitude of these oscillations first increases then goes to zero as one would expect. Surprisingly, the oscillations reappear at yet higher heater currents with their phase being $\pi$-shifted compared to low current values. From direct measurements of the temperature gradient we estimate the amplitude of the oscillations to be orders of magnitude smaller than predicted by theory.