Current-voltage characteristics and the zero-resistance state in a two-dimensional electron gas

We study the current-voltage characteristics (CVC) of two-dimensional electron gases with microwave-induced negative conductance in a magnetic field. We show that due to the Hall effect, strictly speaking, there is no distinction between N- and S-shaped CVC's. Instead, the observed CVC depends on the experimental setup with, e.g., an N-shaped CVC in a Corbino disk geometry corresponding to an S-shaped CVC in a Hall bar geometry in a strong magnetic field. We argue that instabilities of homogeneous states in regions of negative differential conductivity lead to the observation of zero resistance in Hall bars and zero conductance in Corbino disks, and we discuss the structure of current and electric-field domains.