Quantum Transport Research Lab

		Quantum Transport Research Lab
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>> Research		Irradiating a high-mobility, two-dimensional electron system (2DES) with GHz microwaves causes microwave-induced resistance oscillations (MIRO) at He3 temperatures when a magnetic field B is swept at low fields. In addition to MIRO and Shubnikov-de Haas (SdH) oscillations, a resistance oscillation with B-periodic magneto-oscillations has recently been discovered. These new oscillations have been attributed to the interference of edge magnetoplasmons emitted from the potential probe leads along the length of the 2DES Hall bar a distance L apart. These edge magnetoplasmon oscillations (EMPO) had previously not been reported in samples where a MIRO signal was strong enough to study. We explored the relationship between these two signals by taking photovoltage and photoresistance measurements on a 2DES AlGaAs/GaAs heterointerface over microwave frequencies ranging from 27 - 130 GHz.
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		Contrary to MIRO, the EMPO signal has weak temperature dependence and the amplitude drops away logarithmically with decreasing microwave power. By optimizing the sample’s temperature and the microwave irradiation power we were able to observe both MIRO and EMPO. Depending on the frequency of irradiation, we also observed the interaction between the MIRO and EMPO as their signals overlapped, indicating that they are independent phenomena. Our results also indicate that the period of the EMPO does not depend on the distance L between the potential probes, contrary to what the current literature states.

		Quantum Transport in 2DEG with Antidot lattices
		Two dimensional electron gas (2DEG) is a very good and clean system for research on electron transport. In the past years, there were many amazing discoveries on it, such as……. To have better understanding on the electron scattering on 2DEG, artificial 1D or 2D periodic scattering centers were introduced on this clean electron system. The artificial 2D periodic scattering centers are the so-called antidote lattices. Usually, there are three ways to introduce antidote lattices on 2DEG in GaAs samples. The first one is that the antidote lattices are fabricated by Ga focused ion beam implantation. The energy Ga ions beam hits on the sample surface resulting in Ga ions dwell on the surface to form antidote lattices. The second one is to use electrostatically induction. With lithography technique, a grid of gate electrode can be defined on the sample surface. By changing the gate voltage, the local electrons can be depleted so that an antidot pattern is transferred on the 2DEG. The third method is make local damages on the sample surface to form antidote lattices. As this is the way we are using, we will describe it in more details. We coat the sample with PMMA photoresist and define an antidot lattice pattern on the photoresist with e-beam lithography. Then we use reactive ion etching to etch the sample surface. These local damages on the surface will deplete the electrons below them so that antidot lattices can be achieved on 2DEG.



		By measuring the magnetoresistance, a new kind of oscillation, so-called geometrical resonance, was found on 2DEG with antidot lattices by D. Weiss [Phys. Rev. Lett. 66, 2790 (1991)]. The magnetoresistance will show a series of peaks when the radius of cyclotron is commensurate to the period of the antidot lattices. The following figure is a typical trace we got from our sample. The insert shows the cyclotron orbits corresponding to geometrical resonance peaks.


		The subject of electronic transport in a high-mobility 2DEG under microwave irradiation and a small magnetic field has attracted much recent attention, partly because of the spectacular microwave-induced magneto-resistance oscillations (MIRO) and the subsequent zero-resistance states observed in very clean samples. But the role of scatters play in MIRO is an unsettled issue. A 2DEG with antidots provides us a very good system to investigate this problem. So far, we measured the MIRO on 2DEG with triangular antidot lattices. The antidot diameter is 300nm and the lattice constant is 1500nm. In this sample, we found that the antidots affected the MIRO only by decreasing the quality of the sample and the GR, MIRO and magnetoplasmon resonance decouple each other on this sample. Ths result was published on Phys. Rev. B 74, 075313 (2006). Main reason for this decoupling behavior should be due to the large lattice constant of the antidot lattice. In future, we will shorten the antidot diameter and lattice constant of the antidot lattice. We believe that periodical modulation with short period approaching magnetic length should lead to characteristically new behavior in the MIRO and ZRS.
Dr. Rui-Rui Du rrd@rice.edu Rice University Physics & Astronomy Dell Butcher Hall Rm. 170 1900 Rice Blvd. Ent. 20 Houston, TX 77005 >Office Phone: 1-713-348-5780 >Lab Phone/Fax: 1-713-348-5719




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