Two-component systems with equal concentrations of electrons and holes exhibit nonsaturating, linear magnetoresistance in classically strong magnetic fields. The effect is predicted to occur in finite-size samples at charge neutrality due to recombination. The phenomenon originates in the excess quasiparticle density developing near the edges of the sample due to the compensated Hall effect. The size of the boundary region is of the order of the electron-hole recombination length that is inversely proportional to the magnetic field. In narrow samples and at strong enough magnetic fields, the boundary region dominates over the bulk leading to linear magnetoresistance. Our results are relevant for two-and three-dimensional semimetals and narrow band semiconductors including most of the topological insulators.

We present theoretical and experimental studies of the direct current effect on the detection of subterahertz and terahertz radiation in gated two-dimensional structures. We developed a theory of the current-driven detection both for resonant case, when the fundamental frequency of plasma oscillation is large compared to inverse scattering time, omega(0)tau >> 1, and for the nonresonant case, omega(0)tau << 1, when the plasma oscillations are damped. We predict that, in the nonresonant case, even a very small dc current would increase the detection amplitude up to two orders of magnitude. Physically, this increase is related to an abrupt transition from the linear to saturation region near the knee of the current-voltage characteristic. When the current increases up to the saturation value, the electron concentration near the drain becomes very low and can be strongly affected by a small external field. As a consequence, the two-dimensional channel becomes extremely sensitive to external perturbations. In the resonant case, the detection amplitude has maxima when the radiation frequency is equal to fundamental plasma frequency and its harmonics. We predict that the effective linewidths of the respective resonances would decrease with the increasing current. Physically, this happens because dc current shifts the system towards the plasma wave instability. At some critical current value, the width corresponding to the fundamental frequency would turn to zero, indicating the onset of plasma waves generation. Our experimental measurements performed on GaAs HEMT confirm the theoretical predictions