J.A. Bonetti
Roughly 10 years ago, at least one research group studied the magneto-transport properties of 2DEG’s while irradiating with microwaves. The only effect seen in such studies was a large peak in the resistivity at the magnetic field corresponding to cyclotron resonance. Quite recently (2003), such experiments were performed again, but now with mobilities which are at least a factor of 10 greater. Surprisingly, no cyclotron resonance peak was observed. Instead, large magnitude resistivity oscillations were seen. These oscillations resemble the Shubnikov de-Haas(SdH) oscillations, in that they are both 1/B periodic. However, the SdH oscillations are governed by the electron density, while the microwave induced oscillations are governed by the ratio of applied frequency to cyclotron frequency. In very high mobility samples, the minima of the microwave induced oscillations can reach zero, and remain pinned there over a broad range of magnetic field. These zero resistance states are very similar in appearance to the zero resistance states observed in the quantum Hall effect (QHE). In the QHE when a zero resistance state occurs in the longitudinal resistance, a quantized plateau is seen in the Hall resistance. No corresponding Hall plateau is seen for the microwave induced zeroes. The discovery of the microwave induced zero resistance states has led to a flood of theoretical activity. To date, there is no consensus about their origin. There are many remaining questions. We hope to provide some answers. All of the microwave experiments are perfomed in our 3He refrigerator. A homebuilt insert was designed and fabricated specifically for these experiments. The microwaves are generated by a series of Gunn oscicallators, and then travel down the length of the insert in a stainless steel waveguide. An example of our data is shown below demonstrating the effect. The figure below shows the longitudinal resistance with and without microwaves.
The green curve is without microwaves and only shows SdH oscillations. The darker trace was obtained under the same conditions except the sample was irradiated with 60 GHz microwaves. The resistance is pinned at zero from 0.10 to 0.13 T. At higher fields, where the microwaves have little effect, the two curves lie on top of one another. The amplitude of the SdH oscillations in the higher field regime (>0.2 T) are slightly smaller due to electron heating from the microwaves.