Microwave-induced Zero-resistance States Research Articles

Acoustoelectric Study of Microwave-Induced Current Domains.

Surface acoustic waves (SAW) have been utilized to investigate the properties of a two-dimensional electron system subjected to a perpendicular magnetic field and monochromatic microwave radiation in the regime where the so-called microwave-induced zero-resistance states form. Contrary to conventional magnetotransport in Hall bar and van der Pauw geometries, the collimated SAW beam probes only the bulk of the electronic system exposed to this wave. Clear signatures appear in the SAW propagation velocity, corroborating that neither contacts nor sample edges are a root source for their emergence. By virtue of the directional nature of this probing method and with the assistance of theoretical modeling, we were able to demonstrate that the SAW response depends on the angle between its propagation vector and the orientation of domains that spontaneously form when zero-resistance is observed in transport. This confirms in unprecedented manner the formation of an inhomogeneous phase under these nonequilibrium conditions.

Self-oscillations and noise-induced flips of spontaneous electric field in microwave-induced zero resistance state

Experiments demonstrate that microwave illumination of a two-dimensional electron system in a perpendicular magnetic field can produce a state with vanishingly small dissipative response at low frequency. Such zero resistance state is attributed to the spontaneous formation of static electrical domains in an unstable uniform system with negative absolute conductivity. We show that the electrostatic coupling of this system to a weakly conducting doping layer leads to additional dynamic instabilities. In particular, the two-layer system can develop a limit cycle where the domain electric field periodically flips in time. The proposed model provides a natural explanation for quasiperiodic flips of internal spontaneous voltages observed in recent experiments.

Microwave-induced zero-resistance states in a high-mobility two-subband electron system

In this study, a selectively-doped GaAs/AlAs heterostructure was used to fabricate a high-mobility two-subband electronic system with a substantially different concentration of electrons in the subbands. We observe microwave photoresistance at high numbers of magneto-intersubband oscillations (MISO). The system under study demonstrates microwave-induced resistance oscillations (MIRO) and MISO interference. The MIRO in the studied two-subband system appears in lower magnetic fields compared to the MISO. This is an indication of some additional mechanism that exists in the two-subband system and is responsible for the MISO amplitude damping in low magnetic fields, while it does not affect the MIRO amplitude. Zero resistance states appear in the system under study under microwave irradiation in a narrow range of magnetic fields near the MISO maxima.

Microwave-induced zero-resistance state in two-dimensional electron systems with unidirectional periodic modulation

In this study we fabricated lateral superlattices (LSLs) based on the selectively doped GaAs/AlAs heterostructures with a high-mobility two-dimensional (2D) electron gas. The LSLs were formed using the electron-beam lithography and lift-off techniques, which produced a set of metallic strips on top of a heterojunction. The amplitude of the 2D electron gas modulation in the LSL was controlled by the gate voltage applied to the metallic strips. The LSLs with two different periods (a = 200 nm and 500 nm) were used to investigate the influence of microwave radiation with the frequency of 110–150 GHz on the 2D electron transport at the temperature T = 1.6 K in the magnetic field B < 1 T. We have found that zero-resistance states (ZRSs) appear under the microwave radiation in the 2D systems with a unidirectional periodic modulation. These ZRSs are located at the minima of commensurability oscillations.

Comment on “Theory of microwave-induced zero-resistance states in two-dimensional electron systems” and on “Microwave-induced zero-resistance states and second-harmonic generation in an ultraclean two-dimensional electron gas”

Recent papers by S. A. Mikhailov [Phys. Rev. B 83, 155303 (2011) and Phys. Rev. B 89, 045410 (2014)] claim to explain ``all experimental facts'' pertinent to microwave-induced resistance oscillations and zero-resistance states in terms of ``the ponderomotive force theory.'' This Comment shows that the analysis of the results obtained in the above-mentioned papers, in fact, leads to opposite conclusions, i.e., that they cannot explain even the most basic experimental facts.

Reply to “Comment on ‘Theory of microwave-induced zero-resistance states in two-dimensional electron systems’ and on ‘Microwave-induced zero-resistance states and second-harmonic generation in an ultraclean two-dimensional electron gas’ ”

We show that all questions raised in the Comment can be easily answered within the theory formulated in the commented papers. It is also shown that the bulk theory promoted by the commenter fails to explain the most important fundamental features of the discussed phenomenon.

Microwave-induced zero-resistance states and second-harmonic generation in an ultraclean two-dimensional electron gas

Microwave-induced resistance oscillations (MIRO) and "zero-resistance" states (ZRS) were discovered in ultraclean two-dimensional electron systems in 2001-2003 and have attracted great interest from researchers. A comprehensive theory of these phenomena was developed in 2011: It was shown that all experimentally observed dependencies can be naturally explained by the influence of the ponderomotive forces which arise in the near-contact regions of the two-dimensional electron gas under the action of microwaves. Now we show that the same near-contact physical processes should lead to another nonlinear electrodynamic phenomenon - the second-harmonic generation. We calculate the frequency, magnetic field, mobility, and power dependencies of the second-harmonic intensity and show that it can be as large as ~ 0.5 mW/cm2 under realistic experimental conditions. A part of this paper is devoted to a further discussion of the MIRO/ZRS phenomena: we explain how the ponderomotive-force theory explains different experimental details, including those which were not known in 2011, and critically discuss alternative theories.

Towards a synchronization theory of microwave-induced zero-resistance states

We develop the synchronization theory of microwave induced zero-resistance states (ZRS) for two-dimensional electron gas in a magnetic field. In this theory the dissipative effects lead to synchronization of cyclotron phase with driving microwave phase at certain resonant ratios between microwave and cyclotron frequencies. This synchronization produces stabilization of electron transport along edge channels and at the same time it gives suppression of dissipative scattering on local impurities and dissipative conductivity in the bulk, thus creating the ZRS phases at that frequency ratios. The electron dynamics along edge and around circular disk impurity is well described by the Chirikov standard map. The theoretical analysis is based on extensive numerical simulations of classical electron transport in a strongly nonlinear regime. We also discuss the value of activation energy obtained in our model and the experimental signatures that could establish the synchronization origin of ZRS.

Zero-resistance states in bilayer electron systems induced by microwave irradiation

We experimentally demonstrate the existence of microwave-induced zero-resistance states (ZRS) in bilayer electron systems created in wide quantum wells. In contrast to single-layer two-dimensional electron systems, ZRS are developed from the strongest magneto-intersubband oscillation peaks inverted by microwave radiation. Our experimental work is discussed within the theories of microscopic mechanisms of photoresistance, and the calculations reveal that the condition for absolute negative resistivity correlates with the appearance of ZRS.

Negative conductivity and anomalous screening in two-dimensional electron systems subjected to microwave radiation

A 2D electron system in a quantized magnetic field can be driven by microwave radiation into a non-equilibrium state with strong magnetooscillations of the dissipative conductivity. We demonstrate that in such system a negative conductivity can coexist with a positive diffusion coefficient. In a finite system, solution of coupled electrostatic and linear transport problems shows that the diffusion can stabilize a state with negative conductivity. Specifically, this happens when the system size is smaller than the absolute value of the non-equilibrium screening length that diverges at the point where the conductivity changes sign. We predict that a negative resistance can be measured in such a state. Further, for a non-zero difference between the work functions of two contacts, we explore the distribution of the electrostatic potential and of the electron density in the sample. We show that in the diffusion-stabilized regime of negative conductivity the system splits into two regions with opposite directions of electric field. This effect is a precursor of the domain structure that has been predicted to emerge spontaneously in the microwave-induced zero-resistance states.

Theory of microwave-induced zero-resistance states in two-dimensional electron systems

The phenomena of the microwave induced zero resistance states (MIZRS) and the microwave induced resistance oscillations (MIRO) were discovered in the ultraclean two-dimensional electron systems in 2001 -- 2003 and have attracted great interest of researchers. In spite of numerous theoretical efforts the true origin of these effects remains unknown so far. We show that the MIRO/ZRS phenomena are naturally explained by the influence of the ponderomotive forces which arise in the near-contact regions of the two-dimensional electron gas under the action of microwaves. The proposed analytical theory is in agreement with all experimental facts accumulated so far and provides a simple and self-evident explanation of the microwave frequency, polarization, magnetic field, mobility, power and temperature dependencies of the observed effects.

Microwave-induced zero-resistance states are not necessarily static

We study the effect of inhomogeneities in Hall conductivity on the nature of the zero resistance states seen in the microwave irradiated two-dimensional electron systems in weak perpendicular magnetic fields and show that time-dependent domain patterns may emerge in some situations. For an annular Corbino geometry, with an equilibrium charge density that varies linearly with radius, we find a time-periodic nonequilibrium solution, which might be detected by a charge sensor, such as a single electron transistor. For a model on a torus, in addition to static domain patterns seen at high and low values of the equilibrium charge inhomogeneity, we find that, in the intermediate regime, a variety of nonstationary states can also exist. We catalog the possibilities we have seen in our simulations. Within a particular phenomenological model, we show that linearizing the nonlinear charge continuity equation about a particularly simple domain-wall configuration and analyzing the eigenmodes allows us to estimate the periods of the solutions to the full nonlinear equation.

Effect of dc excitation on microwave-induced resistance oscillations and zero-resistance states in a two-dimensional electron system

We study the effect of dc current on microwave photoresistance in a high-mobility two-dimensional electron system. We find that dc current leads to the evolution of the microwave photoresistance maxima into minima and vice versa. Observed quenching of microwave-induced zero resistance states by a dc current is consistent with the domain model but cannot be unambiguously linked to it. We derive a modified resonant condition for photoresistance oscillations under dc current and suggest interpretation in terms of indirect cyclotron resonances in Hall field tilted Landau levels. Detailed understanding of the evolution requires a systematic theory treating both excitation types within a single framework.

Magnetotransport in a two-dimensional electron system in dc electric fields

We report on nonequilibrium transport measurements in a high-mobility two-dimensional electron system subject to weak magnetic field and dc excitation. Detailed study of dc-induced magneto-oscillations, first observed by Yang et al., reveals a resonant condition that is qualitatively different from that reported earlier. In addition, we observe dramatic reduction of resistance induced by a weak dc field in the regime of separated Landau levels. These results demonstrate similarity of transport phenomena in dc-driven and microwave-driven systems and have important implications for ongoing experimental search for predicted quenching of microwave-induced zero-resistance states by a dc current.

Influence of a parallel magnetic field on microwave-induced zero-resistance states and magnetoplasmon resonance

In an ultra-clean two-dimensional electron system (2DES), we have observed the magnetoplasmon resonance (MPR) through its microwave-photoconductivity, concurrently with the microwave-induced resistance oscillations (MIRO) and zero-resistance states (ZRS). It is found that the MIRO/ZRS is strongly suppressed by a moderate magnetic field ( < 1 T ) parallel to the 2DES, while the MPR is robust against a parallel magnetic field. These findings have not been addressed by current models proposed to explain the MIRO/ZRS.

Domain Patterns in the Microwave-Induced Zero-Resistance State

It has been proposed that the microwave-induced ``zero-resistance'' phenomenon, observed in a GaAs two-dimensional electron system at low temperatures in moderate magnetic fields, results from a state with multiple domains, in which a large local electric field $\bE(\br)$ is oriented in different directions. We explore here the questions of what may determine the domain arrangement in a given sample, what do the domains look like in representative cases, and what may be the consequences of domain-wall localization on the macroscopic dc conductance. We consider both effects of sample boundaries and effects of disorder, in a simple model, which has a constant Hall conductivity, and is characterized by a Lyapunov functional.

Microwave-induced zero-resistance states on 2D electron gas: theoretical explanation and temperature dependence

A new theoretical model is presented in which the radiation-induced zero-resistance states and resistivity oscillations are analyzed. The basis of our model is an exact solution for the harmonic oscillator wave function in the presence of radiation and a perturbation treatment for elastic scattering due to randomly distributed charged impurities. Following this model most experimental results are reproduced: zero-resistance states, resistivity oscillations and the dependence of resistivity with temperature, where a physical model of interaction with acoustic phonon is presented, and an activation energy is calculated.

Absolyutnaya otritsatel'naya provodimost', indutsirovannaya mikrovolnovym izlucheniem, i sostoyaniya s nulevym soprotivleniem v dvumernykh elektronnykh sistemakh: istoriya i sovremennoe sostoyanie

Absolyutnaya otritsatel'naya provodimost', indutsirovannaya mikrovolnovym izlucheniem, i sostoyaniya s nulevym soprotivleniem v dvumernykh elektronnykh sistemakh: istoriya i sovremennoe sostoyanie

Physics. Cooking a two-dimensional electron gas with microwaves.

Two-dimensional electron gases (2DEGs) are rich test systems for a variety of phenomena in condensed matter physics. In their Perspective, Durst and Girvin discuss a surprising new phenomenon by which the electrical resistance of a 2DEG in a magnetic field can be driven nearly all the way to zero when the system is subjected to microwave radiation. This microwave-induced zero-resistance state may correspond to a new collective state of matter, or alternatively, it may represent an exciting new example of the nonequilibrium physics of pattern formation.