**A Hint of Negative Electrical Resistance**

A hint of negative electrical resistance emerges from a new experiment in which microwaves of two different frequencies are directed at a 2-dimensional electron gas. The electrons, moving at the interface between two semiconductor crystals, are subjected to an electric field in the forward (longitudinal) direction and a faint magnetic field in the direction perpendicular to the plane. In such conditions the electrons execute closed-loop trajectories which will, in addition, drift forward depending on the strength of the applied voltage.

A few years ago, two experimental groups observed that when the electrons were exposed to microwaves, the overall longitudinal resistance could vary widely - for example, increasing by an order of magnitude or extending down to zero, forming a zero-resistance state. This would depend on the relationship between microwave frequency and the strength of the applied magnetic field (for background, see Physics Today, April 2003).

Some theorists proposed that in such zero-resistance state, the resistance would actually have been less than zero: the swirling electrons would have drifted backwards against the applied voltage. However, this rearward motion would be difficult to observe due to an instability in the current flow. In this case the current distribution becomes inhomogeneous so as to yield a vanishing voltage drop.

Prof. Rui-Rui Du's group has now tested this hypothesis in a clever bichromatic experiment using microwaves at the two frequencies, as reported by Zudov et al. in a recent Phys. Rev. Lett. By irradiating the electrons with microwaves of two different frequencies, they observed that for nonzero-resistance states the resultant resistance was the average of the values corresponding to the two frequencies separately. On the other hand, when the measurements included frequencies that had yielded a zero resistance, the researchers observed a dramatic reduction of the signal. Judging from the average resistance observed for non-zero measurements, they deduce that whenever zero resistance was detected, the true microscopic resistance had actually been less than zero. In other words, an observed zero resistance was masking what was in fact an unstable negative- resistance state.

The research is funded by NSF. Experiments on the microwave-induced zero-resistances states and other non-equilibrium quantum transport phenomena are actively pursued by Postdoctoral Associate Changli Yang, Graduate Students Zhouquen Yuan and Kristjan Stone.

Further Readings: M.A. Zudov, R.R. Du, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. 96, 236804 (2006); Physics News Update, Number780#1, by Phil Schewe and Ben Stein; A.C. Durst, News &Views, Nature 442, 752(2006); Physics Today, August 2006, Physics Update

a, b, Schematic depictions of micorwave-induced resistance oscillations and zero-resistance states for applied radiation of two distinct frequencies. Minima of the oscillations are cut off at zero resistance. c, If microwavesof both frequencies are applied at once, it is unclear how their effects should be combined. If the Zeros are Fundamental, each curve should be cut off at zero first, and then averaged (red curve). If the zeros are masking negatice resistance, the full oscillations should be averaged first (including the dashed negative parts), and the result cut off at zero (blue curve). The measurements of Zudov et al. support the second approach. Courtsey A.C. Durst, Nature 442, 752(2006) |