Surface acoustic wave detection of robust zero-resistance states under microwaves

Microwave-induced resistance oscillations (MIROs) and zero-resistance states (ZRSs) occur in high-mobility two-dimensional electron gas exposed to microwave (MW). We observe that the velocity shift (Δv/v) oscillates in anticorrelation with MIRO, and Δv/v shows peaks at the minimal resistance of MIRO or at ZRS. The SAW velocity features of ZRS remain robust even in the absence of external driving current, which suggests the involvement of intrinsic mechanism in the nonequilibrium phase. In addition, under high-power MW, the phase (φac) of ZRS stays constant at about 1/4, whereas the phase of the transitions in MIRO is reduced to below 0.10. We argue that the peaks of SAW velocity at ZRS may result from the inhomogeneity of superposed current domain structures. Moreover, a multiphoton process around =1/2 is observed in the SAW measurements.