Resistance fluctuations in integer quantum-Hall transitions

We have studied the resistance fluctuations (RF) in integer quantum-Hall transitions for high-mobility two-dimensional electron gas systems. The role of coherence in RF is examined by investigating the conductance through two scattering regions, that are spatially separated but interfere quantum-mechanically with each other. The phase coherence does not play a substantial role in determining the pattern of RF, whereas it affects the amplitude of RF.     

New class of magnetoresistance oscillations: interaction of a two-dimensional electron gas with leaky interface phonons

We report on a new class of magnetoresistance oscillations observed in a high-mobility two-dimensional electron gas (2DEG) in GaAs-Al(x)Ga(1--x)As heterostructures. Appearing in a weak magnetic field ( B < 0.3 T) and only in a narrow temperature range ( 2 K < T < 9 K), these oscillations are periodic in 1/B with a frequency proportional to the electron Fermi wave vector, k(F). We interpret the effect as a magnetophonon resonance of the 2DEG with leaky interface-acoustic phonon modes carrying a wave vector q = 2k(F). Calculations show a few branches of such modes existing on the GaAs-Al(x)Ga(1--)xAs interface, and their velocities are in quantitative agreement with the observation.     

Oscillatory nonlinear differential magnetoresistance of highly mobile 2D electrons in high Landau levels

We examine the current-induced magnetoresistance oscillations in high-mobility two-dimensional electron systems using the balance-equation scheme for nonlinear magnetotransport. The reported analytical expressions for differential magnetoresistivity at high filling factors in the overlapping Landau-level regime show good agreement with the experimental observation and the numerical calculation.     

Photon-Stimulated Transport in a Quantum Point Contact (Brief Review)

JETP Letters - Studies of photon-stimulated transport through a quantum point contact based on a high-mobility two-dimensional electron gas in a GaAs quantum well are reviewed. This review includes...     

Two-dimensional quantum oscillations of the conductance at LaAlO3/SrTiO3 interfaces

We report on a study of magnetotransport in LaAlO3 /SrTiO3 interfaces characterized by mobilities of the order of several thousands cm2/V s. We observe Shubnikov-de Haas oscillations whose period depends only on the perpendicular component of the magnetic field. This observation directly indicates the formation of a two-dimensional electron gas originating from quantum confinement at the interface. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass m* ? 1.45 m(e). An electric field applied in the back-gate geometry increases the mobility, the carrier density, and the oscillation frequency.     

Low-Frequency Microwave Response of a Quantum Point Contact

JETP Letters - The low-frequency microwave photoconductance of a short (100 nm) quantum point contact based on a high-mobility two-dimensional electron gas in the frequency range of 2–3 GHz...     

Electron interaction and ‘memory’ effects in the presence of mixed disorder

We have studied the effect of electron–electron interaction in the presence of mixed disorder on the conductivity and Hall effect of a high-mobility two-dimensional electron gas in a GaAs/AlGaAs heterostructure. A parabolic, negative, temperature-dependent magnetoresistance (MR) and temperature-dependent Hall effect are observed. We show that these effects can be explained in terms of the interaction theory. In addition, a temperature independent, positive MR is observed. This classical MR is also shown to be a consequence of the mixed disorder.     

Magnetoresistance oscillations due to Zener tunneling and microwave radiation in a 2D electron gas in GaAs quantum well with AlAs/GaAs superlattices barriers

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces magnetoresistance oscillations periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the covered frequency range depends on the microwave radiation power.     

Experimental observation of coherent backscattering in open ballistic microstructures

We report on observations of coherent backscattering in open devices consisting of a quantum point contact and two reflector gates. The devices are fabricated in a high-mobility two-dimensional electron gas system, and magnetotransport data are taken at a temperature of 385 mK. Unlike ballistic microstructures in which this effect has previously been measured, our devices are designed to yield many backscattered trajectories enclosing very similar areas. An array of these devices has been measured in order to average away magnetoconductance fluctuations.     

Thermodynamic properties of a magnetically modulated graphene monolayer

The effect of magnetic modulation on thermodynamic properties of a graphene monolayer in the presence of a constant perpendicular magnetic field is reported here. One-dimensional spatial electric or magnetic modulation lifts the degeneracy of the Landau levels and converts into bands and their bandwidth oscillates with magnetic field, leading to Weiss-type oscillations in the thermodynamic properties. The effect of magnetic modulation on the thermodynamic properties of a graphene sheet is studied and then compared with electrically modulated graphene and magnetically modulated conventional two-dimensional electron gas (2DEG). We observe Weiss-type and de Haas-van Alphen oscillations at low and high magnetic fields, respectively. There is a definite phase difference in Weiss-type oscillations in thermodynamic quantities of magnetically modulated graphene compared to electrically modulated graphene. On the other hand, the phase remains the same and the amplitude of the oscillation is large when compared with the magnetically modulated two-dimensional electron gas (2DEG). Explicit asymptotic expressions of the density of states and the Helmholtz free energy are provided to understand the phase and amplitude of the Weiss-type oscillations qualitatively. We also study thermodynamic properties when both electric and magnetic modulations are present. The Weiss-type oscillations still exist when the modulations are out-of-phase.     

Classical Effects in the Weak-Field Magnetoresistance of InGaAs/InAlAs Quantum Wells

We observe an unusual behavior of the low-temperature magnetoresistance of the high-mobility two-dimensional electron gas in InGaAs/InAlAs quantum wells in weak perpendicular magnetic fields. The observed magnetoresistance is qualitatively similar to that expected for the weak localization and antilocalization but its quantity exceeds significantly the scale of the quantum corrections. The calculations show that the obtained data can be explained by the classical effects in electron motion along the open orbits in a quasiperiodic potential relief manifested by the presence of ridges on the quantum well surface.     

Terahertz time-domain magnetospectroscopy of a high-mobility two-dimensional electron gas

We have observed cyclotron resonance in a high-mobility GaAs/AlGaAs two-dimensional electron gas by using the techniques of terahertz time-domain spectroscopy combined with magnetic fields. From this, we calculate the real and imaginary parts of the diagonal elements of the magnetoconductivity tensor, which in turn allows us to extract the concentration, effective mass, and scattering time of the electrons in the sample. We demonstrate the utility of ultrafast terahertz spectroscopy, which can recover the true linewidth of cyclotron resonance in a high-mobility (>10(6) cm(2)V(-1)s(-1)) sample without being affected by the saturation effect.     

Zener tunneling between landau orbits in a high-mobility two-dimensional electron gas

Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs -Al Ga 0.3As (0.7) heterostructures, and can be well explained by the Zener mechanism.     

Oscillations of the magnetoresistance of a two-dimensional electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers in a microwave field

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces oscillations of this magnetoresistance, which are periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the frequency range under study depends on the microwave radiation power.     

Electrostatic potential, energy spectrum, and Fano resonances in a ballistic ring interferometer based on an AlGaAs/GaAs heterojunction

For a ballistic ring interferometer based on high-mobility two-dimensional electron gas in an AlGaAs/GaAs heterojunction, the electrostatic potential and the energy spectrum are determined. It is shown that the splitting points in such an interferometer have the form of triangular potential wells. Calculation is performed for the two-dimensional electron transmission through the ring, and the Fano resonances caused by the coupling of the transmitted waves with the levels of higher transverse modes in triangular wells are predicted. These resonances are observed in the experiment.     

Oscillatory magnetothermopower and resonant phonon drag in a high-mobility 2D electron gas

Experimental and theoretical evidence is presented for new low-magnetic-field (B<5 kG) 1/B oscillations in the thermoelectric power of a high-mobility GaAs/AlGaAs two-dimensional (2D) electron gas. The oscillations result from inter-Landau-level resonances of acoustic phonons carrying a momentum equal to twice the Fermi wave number at B=0. Numerical calculations show that both 3D and 2D phonons can contribute to this effect.     

Resistance oscillations in two-dimensional electron systems induced by both ac and dc fields

We report on magnetotransport measurements in a high-mobility two-dimensional electron system subject simultaneously to ac (microwave) and dc (Hall) fields. We find that dc excitation affects microwave photoresistance in a nontrivial way. Photoresistance maxima (minima) evolve into minima (maxima) and back, reflecting strong coupling and interplay of ac- and dc-induced effects. Most of our observations can be explained in terms of indirect electron transitions using a new, combined resonant condition. Observed quenching of microwave-induced zero resistance by a dc field cannot be unambiguously linked to a domain model, at least before a systematic theory treating both excitation types within a single framework is developed.     

Magnetophonon resonance in a GaAs quantum well with AlAs/GaAs superlattice barriers at high filling factors

The magnetotransport of a high-mobility 2D electron gas in single GaAs quantum wells with AlAs/GaAs superlattice barriers is studied at high filling factors. For the selectively doped structures under study in the temperature range from 10 to 25 K, magnetoresistance oscillations periodic in the inverse magnetic field are observed with their frequency being proportional to the Fermi wave vector of the 2D electron gas. The experimental results are explained by the interaction of the 2D electron gas with leaky interface acoustic phonons.     

Quantization of the Hall conductivity well beyond the adiabatic limit in pulsed magnetic fields.

We measure the Hall conductivity, sigma(xy), on a Corbino geometry sample of a high-mobility AlGaAs/GaAs heterostructure in a pulsed magnetic field. At a bath temperature about 80 mK, we observe well expressed plateaux in sigma(xy) at integer filling factors. In the pulsed magnetic field, the Laughlin condition of the phase coherence of the electron wave functions is strongly violated and, hence, is not crucial for sigma(xy) quantization.     

Observation of a light-induced nonohmic current in a toroidal-moment-possessing nanostructure

We report on the first observation of light-induced nonohmic current in a semiconductor nanostructure. The effect is revealed in an unbiased asymmetric InAs quantum well under excitation by far-infrared laser radiation in the presence of a tilted magnetic field. It is interpreted in terms of a nonzero toroidal moment of a two-dimensional electron gas.