A hint of negative electrical resistance

At a planar interface between two different semiconductors, electrons can form a two-dimensional electron gas. When placed in a magnetic field and illuminated with microwaves, such a 2DEG can show oscillations in its resistance as the magnetic field is varied. In some cases, the oscillations get so large that the resistance minima reach zero; for still larger oscillations, though, the resistance does not go negative but instead stays at zero over an extended region (see Physics Today, April 2003, page 24 ). Of the many theories that have been proposed to explain the zero-resistance state, new experiments support the most popular: Microwaves do induce negative resistance, but because a negative-resistance state is unstable, the current flow breaks up into different domains in such a way that the total voltage across the sample vanishes. In the new work, the researchers simultaneously illuminate the 2DEG with microwaves at two different frequencies and compare the bichromatic photoresistance with the two monochromatic photoresistances. At fields for which the two monochromatic responses are either both positive or both zero, the bichromatic response is the average of the two. But at fields for which only one of the microwave frequencies induces a zero-resistance state, the averaging no longer holds—the bichromatic response is always lower than the average. Such behavior, claim the researchers, can be viewed as qualitative evidence of absolute negative resistance in the monochromatic behavior. (M. A. Zudov et al., Phys. Rev. Lett. 96 , 236804, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.236804 .)