Radiation-induced Magnetoresistance Oscillations Research Articles

Radiation-induced magnetoresistance oscillations with massive Dirac fermions

We report on a theoretical study on the rise of radiation-induced magnetoresistance oscillations in two-dimensional (2D) systems of massive Dirac fermions. We study the bilayer system of monolayer graphene and hexagonal boron nitride (h-BN/graphene) and the trilayer system of hexagonal boron nitride encapsulated graphene (h-BN/graphene/h-BN). We extend the radiation-driven electron orbit model that was previously devised to study the same oscillations in 2D systems of Schrödinger electrons (GaAs/AlGaAS heterostructure) to the case of massive Dirac fermions. In the simulations we obtain clear oscillations for radiation frequencies in the terahertz and far-infrared bands. We investigate also the power and temperatures dependence. For the former we obtain similar results as for Schrödinger electrons and predict the rise of zero resistance states. For the latter we obtain a similar qualitatively dependence but quantitatively different when increasing temperature. While in GaAs the oscillations are wiped out in a few degrees, interestingly enough, for massive Dirac fermions, we obtain observable oscillations for temperatures above 100 K and even at room temperature for the higher frequencies used in the simulations.

Microwave resonance in a high-quality GaAs/AlGaAs two-dimensional electron system in the low-density and low-mobility condition

Radiation induced magnetoresistance oscillations are well known and readily observable in the high-quality two-dimensional (2D) electron system. Here, we report unexpected photoexcited transport, under a low-carrier density, low-mobility condition, in the high-quality GaAs/AlGaAs devices that exhibit the above-mentioned oscillations. This study reveals enhanced nonresonant magnetoresistance as well as microwave resonance that appear unrelated to the above-mentioned radiation-induced magnetoresistance oscillations. The positions of the resonance along the $B$ axis depends on the microwave power, the microwave frequency, and the direction of sweep of the magnetic field, as the photoexcited diagonal resistance is prominently enhanced over the dark value. Most remarkably, the observed microwave-induced resonance becomes unobservable below a characteristic cutoff frequency. These features suggest a bulk magnetoplasmon origin for the observed resonances. A picture including bottlenecks in transport within 2D electron systems and microwave modified tunneling at bottlenecks is suggested to help understand some aspects of experiment.

Microscopic model for radiation-induced magnetoresistance oscillations excited by circularly polarized radiation

We develop a microscopic model to explain the striking result of immunity to the sense of circularly polarized radiation of the photo-excited resistance oscillations in high-mobility 2D electron systems. Our model is based on the radiation-driven electron orbit model, previously developed to explain the photo-induced resistance oscillations and zero resistance states in these systems. According to it, the guiding center of the Landau states when irradiated by circularly polarized radiation performs a circular path driven by radiation. In principle, in an infinite sample, this path is different according to the the sense of circular polarization (left or right). However, the limited size of the sample with the essential role of the edges and the concurrent presence of the Hall electric field tend to quench the displacement of the driven guiding center making nearly equal both trajectories. In the end and in the presence of scattering, the longitudinal irradiated magnetoresistance turns out nearly the same irrespective of the sense of circular radiation.

Radiation-induced magnetoresistance oscillations in monolayer and bilayer graphene

We examine the characteristics of the microwave/mm-wave/terahertz radiation-induced magnetoresistance oscillations in monolayer and bilayer graphene and report that the oscillation frequency of the radiation-induced magnetoresistance oscillations in the massless, linearly dispersed monolayer graphene system should depend strongly both on the Fermi energy, and the radiation frequency, unlike in the case of the massive, parabolic, GaAs/AlGaAs 2D electron system, where the radiation-induced magnetoresistance oscillation frequency depends mainly on the radiation frequency. This possible dependence of the magnetoresistance oscillation frequency on the Fermi level at a fixed radiation frequency also suggests a sensitivity to the gate voltage in gated graphene, which suggests an in-situ tunable photo-excitation response in monolayer graphene that could be useful for sensing applications. In sharp contrast to monolayer graphene, bilayer graphene is expected to show radiation-induced magnetoresistance oscillations more similar to the results observed in the GaAs/AlGaAs 2D system. Such expectations for the radiation-induced magnetoresistance oscillations are presented here to guide future experimental studies in both of these modern atomic layer material systems.

Beating pattern in radiation-induced oscillatory magnetoresistance in 2DES: Coupling of plasmon-like and acoustic phonon modes

We present a microscopic theory on the observation of a beating pattern in the radiation-induced magnetoresistance oscillations at a very low magnetic field. We consider that such a beating pattern develops as a result of the coupling between two oscillatory components: the first is a system of electron Landau states being harmonically driven by radiation. The second is a lattice oscillation, i.e., an acoustic phonon mode. We analyze the dependence of the beating pattern on temperature, radiation frequency, and power. We conclude that the beating pattern is an evidence of the radiation-driven nature of the irradiated Landau states that makes them behave as a collective plasma oscillation at the radiation frequency. Thus, the frequency of such plasmons could be tuned from microwave to terahertz in the same nanodevice with an apparent technological application.

Power dependence of the amplitude of the microwave radiation induced magneto-Resistance oscillations and linear polarisation

We have studied the power dependence of the amplitude of the microwave radiation induced magneto resistance oscillations and the linear polarization sensitivity of these oscillations. For the power dependence, some experimental results have indicated that the amplitude of the microwave radiation induced oscillations increase nonlinearly with the microwave power as the radiation driven electron orbit model has theoretically confirmed a nonlinear power relation. The microwave polarization dependence is concerned; the microwave radiation induced oscillations are in dependent of the polarization orientation for both linearly and circularly polarized microwaves. Microwave radiation induced oscillations are independent of the polarization orientation for both linearly and circularly polarized microwaves. Microwave radiation induces oscillation amplitudes do depend on the polarization angle of linearly polarized microwaves and that they follow a cosine square function of the polarization angle. We found that microwave photo excited electrons are scattered by impurities and this gives rise to an additional current density due to radiation. The inelastic model for magneto oscillations in the photo conductivity of the two dimensional which is governed by the microwave induced change in the distribution function, suggests a linear dependence in the amplitude with the microwave power that is independent of the linear microwave polarization.

Radiation-induced resistance oscillations in 2D electron systems with strong Rashba coupling

We present a theoretical study on the effect of radiation on the mangetoresistance of two-dimensional electron systems with strong Rashba spint-orbit coupling. We want to study the interplay between two well-known effects in these electron systems: the radiation-induced resistance oscillations and the typical beating pattern of systems with intense Rashba interaction. We analytically derive an exact solution for the electron wave function corresponding to a total Hamiltonian with Rashba and radiation terms. We consider a perturbation treatment for elastic scattering due to charged impurities to finally obtain the magnetoresistance of the system. Without radiation we recover a beating pattern in the amplitude of the Shubnikov de Hass oscillations: a set of nodes and antinodes in the magnetoresistance. In the presence of radiation this beating pattern is strongly modified following the profile of radiation-induced magnetoresistance oscillations. We study their dependence on intensity and frequency of radiation, including the teraherzt regime. The obtained results could be of interest for magnetotransport of nonideal Dirac fermions in 3D topological insulators subjected to radiation.

High-temperature observation and power modulation of radiation-induced resistance oscillations in the terahertz band

We report on a theoretical work on magnetotransport under terahertz radiation with high-mobility two-dimensional electron systems focussing on the radiation power and temperature dependence. On the one hand, we study the interaction between the obtained radiation-induced magnetoresistance oscillations (RIRO) and the Shubnikov-de Haas oscillations (SdHO) from the power dependence standpoint. We obtain strong modulation of the SdHO at sufficient terahertz radiation power. On the other hand from the temperature dependence standpoint we obtain an important result: the range of temperature where RIRO are observed can be largely extended by using terahertz radiation. In the terahertz region we still obtain RIRO up to 20 K at a radiation frequency of 400 GHz. Since an increasing T gives rise in turn to more disorder in the sample, we would expect also the observation of RIRO with terahertz radiation when using low-mobility (high-disorder) samples at low T.

Mutual influence between current-induced giant magnetoresistance and radiation-induced magnetoresistance oscillations in the GaAs/AlGaAs 2DES

Radiation-induced magnetoresistance oscillations are examined in the GaAs/AlGaAs 2D system in the regime where an observed concurrent giant magnetoresistance is systematically varied with a supplementary dc-current, Idc. The Idc tuned giant magnetoresistance is subsequently separated from the photo-excited oscillatory resistance using a multi-conduction model in order to examine the interplay between the two effects. The results show that the invoked multiconduction model describes the observed giant magnetoresistance effect even in the presence of radiation-induced magnetoresistance oscillations, the magnetoresistance oscillations do not modify the giant magnetoresistance, and the magnetoresistance oscillatory extrema, i.e., maxima and minima, disappear rather asymmetrically with increasing Idc. The results suggest the interpretation that the Idc serves to suppress scattering between states near the Fermi level in a strong magnetic field limit.

Extraction of overlapping radiation-induced magnetoresistance oscillations and bell-shaped giant magnetoresistance in the GaAs/AlGaAs 2DES using a multiconduction model

We present an experimental study aimed at extracting the microwave radiation-induced magnetoresistance oscillations from the bell-shape giant magnetoresistance in high mobility GaAs/AlGaAs devices using a multi-conduction model. The results show that the multi-conduction model describes the observed giant magnetoresistance effect and the model helps to extract radiation-induced magnetoresistance oscillations, over a wider parameter space.

Comparative study of microwave radiation-induced magneto-resistance oscillations induced by circularly- and linearly- polarized microwaves

A systematic comparative study of radiation-induced magneto-resistance oscillations using circularly polarized- and linearly polarized microwaves was carried out on the high mobility GaAs/AlGaAs heterostructure two dimensional electron system (2DES). The results showed that, the sinusoidal sensitivity in the amplitude of the radiation-induced magnetoresistance oscillations observed under launcher rotation for linearly polarized microwaves, is remarkably absent in the similar experiment carried out with circularly polarized microwaves.

Evolution of the frequency-dependent polarization-angle phase-shift in the microwave radiation-induced magnetoresistance oscillations

We report the evolution of the phase shift, θ0, extracted from traces of the diagonal resistance, Rxx, vs. the linear polarization angle, θ, at oscillatory extrema of the microwave radiation induced magnetoresistance oscillations over the 36 ≤ f ≤ 40 GHz band in GaAs/AlGaAs system. A reference phase shift for the linear polarization angle in the vicinity of the specimen is obtained with the help of a sensitive carbon resistor. We fit an empirical cosine square law to the sinusoidal responses of Rxx vs. θ to extract the phase shift θ0. The quasi-continuous variation θ0 vs. f trace suggests a preferable polarization orientation for the specimen, and the f− and B− independence of overall average of θ0.

Behaviour of Bichromatic Microwave Induced Magnetoresistance Oscillations in the High Mobility GaAs/AlGaAs 2D electron System

Microwave radiation-induced magneto-resistance oscillations are examined under bichromatic excitation for various frequency combinations in order to obtain a better understanding of the lineshape observed in the dual excitation experiment of the high mobility GaAs/AlGaAs 2D electron system. Here, we examine superposition- or lack thereof- in the lineshape observed in the bichromatic experiment, and report a trend observed between the monochromatic and bichromatic responses of the oscillatory diagonal resistance.

Millimeter wave radiation-induced magnetoresistance oscillations in the high quality GaAs/AlGaAs 2D electron system under bichromatic excitation

Millimeter wave radiation-induced magnetoresistance oscillations are examined in the GaAs/AlGaAs 2D electron system under bichromatic excitation in order to study the evolution of the oscillatory diagonal magnetoresistance, ${R}_{xx}$ as the millimeter wave intensity is changed systematically for various frequency combinations. The results indicate that at low magnetic fields, the lower frequency millimeter wave excitation sets the observed ${R}_{xx}$ response, as the higher frequency millimeter wave component determines the ${R}_{xx}$ response at higher magnetic fields. The observations are qualitatively explained in terms of the order of the involved transitions. The results are also modeled using the radiation-driven electron orbit theory.

Microscopic theory for radiation-induced zero-resistance states in 2D electron systems: Franck-Condon blockade

We present a microscopic model on radiation-induced zero resistance states according to a novel approach: Franck-Condon physics and blockade. Zero resistance states rise up from radiation-induced magnetoresistance oscillations when the light intensity is strong enough. The theory begins with the radiation-driven electron orbit model that proposes an interplay of the swinging nature of the radiation-driven Landau states and the presence of charged impurity scattering. When the intensity of radiation is high enough, the driven-Landau states (vibrational states) involved in the scattering process are spatially far from each other and the corresponding electron wave functions no longer overlap. As a result, a drastic suppression of the scattering probability takes place and current and magnetoresistance exponentially drop. Finally, zero resistance states rise up. This is an application to magnetotransport in two-dimensional electron systems of the Franck-Condon blockade, based on the Franck-Condon physics which in turn stems from molecular vibrational spectroscopy.

Comparative study of microwave radiation-induced magnetoresistive oscillations induced by circularly- and linearly- polarized photo-excitation.

A comparative study of the radiation-induced magnetoresistance oscillations in the high mobility GaAs/AlGaAs heterostructure two dimensional electron system (2DES) under linearly- and circularly- polarized microwave excitation indicates a profound difference in the response observed upon rotating the microwave launcher for the two cases, although circularly polarized microwave radiation induced magnetoresistance oscillations observed at low magnetic fields are similar to the oscillations observed with linearly polarized radiation. For the linearly polarized radiation, the magnetoresistive response is a strong sinusoidal function of the launcher rotation (or linear polarization) angle, θ. For circularly polarized radiation, the oscillatory magnetoresistive response is hardly sensitive to θ.

Evolution of the linear-polarization-angle-dependence of the radiation-induced magnetoresistance-oscillations with microwave power

We examine the role of the microwave power in the linear polarization angle dependence of the microwave radiation induced magnetoresistance oscillations observed in the high mobility GaAs/AlGaAs two dimensional electron system. The diagonal resistance Rxx was measured at the fixed magnetic fields of the photo-excited oscillatory extrema of Rxx as a function of both the microwave power, P, and the linear polarization angle, θ. Color contour plots of such measurements demonstrate the evolution of the lineshape of Rxx versus θ with increasing microwave power. We report that the non-linear power dependence of the amplitude of the radiation-induced magnetoresistance oscillations distorts the cosine-square relation between Rxx and θ at high power.

Combined study of microwave-power/linear-polarization dependence of the microwave-radiation-induced magnetoresistance oscillations in GaAs/AlGaAs devices

We report the results of a combined microwave polarization-dependence and power-dependence study of the microwave-radiation-induced magnetoresistance oscillations in high mobility GaAs/AlGaAs heterostructure devices at liquid helium temperatures. The diagonal resistance was measured with the magnetic field fixed at the extrema of the radiation-induced magnetoresistance oscillations, as the microwave power was varied at a number of microwave polarization angles. The results indicate a nonlinear relation between the oscillatory peak or valley magnetoresistance and the microwave power, as well as a cosine square relation between the oscillatory peak or valley magnetoresistance and the microwave polarization angle. The main features are briefly compared with the predictions of existing models.

Interaction between two-dimensional quantum oscillators and time-dependent forces: Case of a harmonic force

We solve analytically the time dependent Schrödinger equation of a two-dimensional quantum oscillator subjected to a time-varying force. We apply the results to the case of a linearly and circularly polarized harmonic force. The main result is that the quantum oscillator orbit center performs a two-dimensional closed loop (elliptical) driven by the force. This theory has been specifically applied to the problem of a two-dimensional electron system subjected to a static and uniform magnetic field and radiation to explain the striking effects of radiation-induced magnetoresistance oscillations and zero resistance states.

Remote sensor response study in the regime of the microwave radiation-induced magnetoresistance oscillations

A concurrent remote sensing and magneto-transport study of the microwave excited two dimensional electron system (2DES) at liquid helium temperatures has been carried out using a carbon detector to remotely sense the microwave activity of the 2D electron system in the GaAs/AlGaAs heterostructure during conventional magneto-transport measurements. Various correlations are observed and reported between the oscillatory magnetotransport and the remotely sensed reflection. In addition, the oscillatory remotely sensed signal is shown to exhibit a power law type variation in its amplitude, similar to the radiation-induced magnetoresistance oscillations.