Using innovative millimeterwave photoconductivity spectroscopy, our original experiments on moderate-mobility two-dimensional electron systems (2DES) from GaAs-Al_xGa_1-xAs heterostructures have revealed novel giant amplitude microwave-induced resistance oscillations (MIRO) when a system is subjected to crossed microwave (f = 30--150 GHz) and weak (B < 0.5 T) magnetic fields(Zudov et al, Phys. Rev. B 64, 201311(R), (2001)). Indifferent to the filling factor, MIRO were found to be periodic in 1/B with the period governed by the ratio of the microwave frequency to the electron cyclotron frequency. More importantly, it was noticed that microwaves suppress the resistance at the minim a and that the mobility of 2DES favors oscillation amplitude, implying that in higher quality samples the resistance could eventually approach zero. Following these experiments, two research groups(Mani et al, Nature 420, 646 (2002)), (Zudov et al, Phys. Rev. Lett. 90, 46807 (2003)) have recently reported an observation of a periodic series of apparently zero-resistance states (ZRS), developed from the MIRO minima in ultra-high mobility 2DES under similar experimental conditions. ZRS are positioned about the magnetic fields where the microwave frequency somewhat exceeds integer multiple of the cyclotron frequency and are characterized by an exponentially small low-temperature (T\sim1 K) diagonal resistance and a mostly classical Hall resistance. The activation energies associated with ZRS appear to exceed the Landau level spacing by an order of magnitude. Based on the experimental results, we conclude that our findings represent a new dissipationless effect in 2D electronic transport, possibly signaling a new non-equilibrium state of matter. This talk will briefly survey the experimental development from MIRO to ZRS as the sample mobility is improved, present experimental results on temperature, frequency and power dependence, and discuss open issues.
[A1.002] Zero-resistance states induced by electromagnetic waves in a 2DEG
Ramesh G. Mani (Harvard University, Cambridge, MA and Max-Planck-Institut FkF, Stuttgart, Germany)
We report the experimental detection of novel zero-resistance states induced by electromagnetic wave excitation in ultra high mobility GaAs/AlGaAs heterostructure devices, at low magnetic fields, B, in the large filling factor limit. Vanishing resistance is observed in the vicinity of B = [4/(4j+1)] B_f, where B_f =2\pi f m^*/e, where m^* an the effective mass, e is the charge, and f is the microwave frequency. The dependence of the effect is reported as a function of f, the temperature, the power, the current, and other experimental variables.[1]
[1] R. G. Mani, J. H. Smet, K. von Klitzing, V.
Narayanamurti, W. B. Johnson, and V. Umansky, Nature 420,
646 (2002); cond-mat/0303034, 0305507, 0306388, 0310474,
0311010.
[A1.003] Radiation-Induced Magnetoresistance Oscillations in a 2D Electron Gas
Adam Durst (Yale University)
Recent measurements of a 2D electron gas subjected to
microwave radiation reveal a magnetoresistance with an
oscillatory dependence on the ratio of radiation frequency
to cyclotron frequency. Oscillations grow with radiation
intensity, with the minima saturating at zero resistance. We
have performed a diagrammatic calculation which yields
radiation-induced resistivity oscillations with the correct
period and phase. Results are understood via a simple
picture of photoexcited disorder-scattered electrons
contributing to the dc conductivity. Sufficient intensity
drives the calculated minima to negative resistivity, a
situation shown by Andreev, Aleiner, and Millis to be
unstable to the development of an inhomogeneous current
distribution with zero resistivity. Hence, our result, taken
together with theirs, provides an explanation for the
experiments.
[A1.004] Dynamical Symmetry Breaking as the Origin of the Zero-Resistance State in an ac-Driven System
Anton Andreev (University of Colorado)
Under a strong ac drive the zero-frequency linear response
dissipative resistivity \rho_d(j=0) of a homogeneous state
is allowed to become negative. We show that such a state is
absolutely unstable. The only time-independent state of a
system with \rho_d(j=0)<0 is characterized by a current
which almost everywhere has a magnitude j_0 fixed by the
condition that the nonlinear dissipative resistivity
\rho_d(j_0)=0. As a result, the dissipative component of
the dc-electric field vanishes. The total current may be
varied by rearranging the current pattern appropriately with
the dissipative component of the dc-electric field remaining
zero. This result, together with the calculation of Durst et
al., indicating the existence of regimes of applied ac
microwave field and dc magnetic field where \rho_d(j=0)<0,
explains the zero-resistance state observed by Mani et al.
and Zudov et al..
[A1.005] Current flow anomalies in the irradiated 2DEG
R.L. Willett (Bell Laboratories, Lucent Technologies)
Simple irradiation in the GHz range on 2D electron systems has been shown to have remarkable consequences on the transport at low magnetic fields. It was found by Zudov, et. al.(M.A. Zudov, R.R. Du, J.A. Simmons, J.L. Reno, Phys. Rev. B. 64, 201311 (2001).) that radiation from 30 to 120GHz imposed on a high quality heterostructure resulted in a series of magnetoresistance oscillations periodic in amp;#61472;the radiation frequency/cyclotron frequency, using bare GaAs electron mass. Subsequently it was observed by Mani, et al(R.G. Mani, J.H. Smet, K. von Klitzing, V. Narayanamurty, W.B. Johnson, V. Umansky, cond-mat/0303034; Nature 420, 646 (2002).) and Zudov, et al(M.A. Zudov, R.R. Du, L.N. Pfeiffer, K.W. West, cond-mat/0210034; Phys. Rev. Lett. 90, 046807 (2003).) that in high mobility samples the minima can form apparent zeroes, with the temperature dependence activated. Addressing this same issue, we report experimental results of low temperature magnetotransport in high mobility 2D electron systems exposed to radiation up to 20GHz frequency using a simple dipole configuration. Magnetoresistance oscillations are observed as in the previously reported results using higher frequency radiation on 2D systems, however minima here can be seen to extend to negative biases. Also, zeroes previously reported are not observed persistently around the full sample perimeters. In addition, under radiation, voltages are observed from internal to external contacts in the absence of applied driving currents and are apparently not due to simple rectification. These voltages, generated in the absence of applied driving currents, correspond in amplitude and B-field position to the original radiation induced oscillations, and can demonstrate dependence upon B-field sweep direction. Overall these findings are consistent with micro- and macroscopic theoretical pictures of radiation induced transport and current instabilities due to local negative resistivities.(A.C. Durst, S. Sachdev, N. Read, S.M. Girvin, cond-mat/0301569; Phys. Rev. Lett. 91, 086803 (2003).), (A.V. Andreev, I.L. Aleiner, A. J. Millis, cond-mat/0302063; Phys. Rev. Lett. 91, 056803 (2003)). However, an important property of the radiation-induced minima is the temperature dependence: the previous experimental work\footnote[2]R.G. Mani, J.H. Smet, K. von Klitzing, V. Narayanamurty, W.B. Johnson, V. Umansky, cond-mat/0303034; Nature 420, 646 (2002). \footnote[3]M.A. Zudov, R.R. Du, L.N. Pfeiffer, K.W. West, cond-mat/0210034; Phys. Rev. Lett. 90, 046807 (2003). showed activated transport consistent with an energy gap in the transport spectrum. We examined the transport minima and found a complicated temperature dependence outside of present theoretical understanding. The temperature development is not necessarily activated over a large temperature range, with hysteresis possible in the minima. In addition, we find all temperature development is severely dependent upon the incident radiation power.
collaborators L.N. Pfeiffer, K.W. West, Bell Laboratories, Lucent Technologies