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Abstract

The magneto-resistance of high-mobility two-dimensional electron systems exposed to microwaves exhibits radiation-induced oscillations with some minima approaching zero within experimental accuracy. Consensus has been reached that they originate from disorder-assisted indirect optical transitions and a non-equilibrium population of the electronic states. Both mechanisms capture the hall-marks of the observed oscillations except for the appearance of zero resistance. Theory has predicted that in the minima the resistivity can become negative. Then a homogeneous system turns unstable and current domains with large internal Hall electric fields pointing in opposite directions spontaneously form to produce zero resistance. Direct evidence for such domains has remained elusive. Here we introduce time as an unexplored parameter. Probing internal Hall voltages reveals random telegraph signals in the zero-resistance regime. They provide compelling evidence for spontaneous switching between two different distributions of the electric field, which is attributed to two distinct current domain configurations.