A. G. Pogosov

Electron transport in suspended semiconductor structures with two-dimensional electron gas

We study electron transport in suspended semiconductor microstructures fabricated from AlAs/GaAs membranes containing a high mobility two-dimensional electron gas. In quantizing magnetic fields, a reflection of edge current channels from the border of suspended area is observed resulting in the absence of vanishing magnetoresistance in the quantum Hall effect (QHE) regime. Relocation of this border out of the Hall bar revives the QHE. We have also found that the critical current of the breakdown of QHE in suspended samples is three times lower than in non-suspended samples due to the peculiarity of heat transport in the membranes.

Actuation and transduction of resonant vibrations in GaAs/AlGaAs-based nanoelectromechanical systems containing two-dimensional electron gas

Driven vibrations of a nanoelectromechanical system based on GaAs/AlGaAs heterostructure containing two-dimensional electron gas are experimentally investigated. The system represents a conductive cantilever with the free end surrounded by a side gate. We show that out-of-plane flexural vibrations of the cantilever are driven when alternating signal biased by a dc voltage is applied to the in-plane side gate. We demonstrate that these vibrations can be on-chip linearly transduced into a low-frequency electrical signal using the heterodyne down-mixing method. The obtained data indicate that the dominant physical mechanism of the vibrations actuation is capacitive interaction between the cantilever and the gate.

The features of ballistic electron transport in a suspended quantum point contact

A suspended quantum point contact and the effects of the suspension are investigated by performing identical electrical measurements on the same experimental sample before and after the suspension. In both cases, the sample demonstrates conductance quantization. However, the suspended quantum point contact shows certain features not observed before the suspension, namely, plateaus at the conductance values being non-integer multiples of the conductance quantum, including the “0.7-anomaly.” These features can be attributed to the strengthening of electron-electron interaction because of the electric field confinement within the suspended membrane. Thus, the suspended quantum point contact represents a one-dimensional system with strong electron-electron interaction.

High-amplitude dynamics of nanoelectromechanical systems fabricated on the basis of GaAs/AlGaAs heterostructures

Nonlinear vibrations of nanoelectromechanical beam fabricated from the GaAs/AlGaAs heterostructure containing two-dimensional electron gas are investigated. The measurement of time-averaged conductance is shown to be a sensitive method for the nanomechanical motion detection. The possibilities to excite high-amplitude (up to 20u2009nm) mechanical vibrations and to effectively detect them using the simple conductance measurement make such systems promising for practical applications. High-amplitude vibrations of the beam are shown to demonstrate softening nonlinearity. This can be attributed to the bending of the beam due to Euler buckling instability caused by the compressive stress though GaAs/AlGaAs heterostructures are typically considered as stress-free.

Universal behavior of magnetoresistance in quantum dot arrays with different degrees of disorder

The magnetoresistance in a two-dimensional array of Ge/Si quantum dots was studied in a wide range of zero magnetic field conductances, where the transport regime changes from a hopping to a diffusive one. The behavior of the magnetoresistance is found to be similar for all samples--it is negative in weak fields and becomes positive with increasing magnetic field. The result apparently contradicts existing theories. To explain experimental data we suggest that clusters of overlapping quantum dots are formed. These clusters are assumed to have metal-like conductance, the charge transfer taking place via hopping between the clusters. Relatively strong magnetic field shrinks electron wavefunctions, decreasing inter-cluster hopping and, therefore, leading to a positive magnetoresistance. Weak magnetic field acts on metallic clusters, destroying the interference of the electron wavefunctions corresponding to different paths (weak localization) inside clusters. The interference may be restricted either by inelastic processes, or by the cluster size. Taking into account weak localization inside clusters and hopping between them within the effective medium approximation, we extract effective parameters characterizing charge (magneto-) transport.

Ballistic thermopower of suspended semiconductor Hall bars with two dimensional electron gas

We study ballistic electron transport in suspended semiconductor nanostructures containing high mobility two dimensional electron gas structured with periodical square lattice of artificial scatters — antidots. Thermopower demonstrates magnetic field commensurability oscillations resulting from geometrical resonances similar to those earlier observed in magnetoresistance, thus indicating the retaining of the ballistic regime in thermopower measurement on suspended structures and the validity of the Mott rule in this case. In spite of peculiarities of the heat transport in suspended structures leading to the observed anomalies in non-linear effects, the amplitude of thermopower oscillations remains unchanged after the suspension. This can be explained by short Thouless time compared to the time of electron- phonon interaction.

Ballistic electron transport in structured suspended semiconductor membranes

We study ballistic electron transport in freely suspended AlAs/GaAs microstructures containing a high mobility two-dimensional electron gas with square lattice of antidots. We found that the magnetoresistance of the samples demonstrates commensurability oscillations both for the case of non-suspended and suspended devices. The temperature dependence of the commensurability oscillations is similar for both cases. However, the critical dc current, that suppresses these oscillations, in suspended samples is three times lower than in non-suspended ones. The observed phenomenon can be explained by peculiarities of the heat transport in membranes.

Nonequilibrium currents in the quantum Hall effect regime spatially resolved by transport experiment

Eddy currents in a constriction placed in a two-dimensional electron gas (2DEG) have been experimentally studied. The constrictions fabricated by the electron-beam lithography demonstrate quantum Hall effect in high magnetic field. Pronounced hysteresis of magnetoresistance is observed at Hall plateaus corresponding to integer filling factors v = 1, 2 and 4 of a macroscopic 2DEG. The dependence of magnetoresistance hysteresis on the constriction width is obtained. The dependence contains the threshold value of width that points to the edge nature of eddy currents and enables us to determine the eddy currents area width, which is about 0.5 ?m.

Magnetoresistance in two-dimensional array of Ge/Si quantum dots

Magnetoresistance in two-dimensional array of Ge/Si was studied for a wide range of the conductance, where the transport regime changes from hopping to diffusive one. The behavior of magnetoresistance is similar for all samples; it is negative in weak fields and becomes positive with increasing of magnetic field. Negative magnetoresistance can be described in the frame of weak localization approach with suggestion that quantum interference contribution to the conductance is restricted not only by the phase breaking length but also by the localization length.

Inverted hysteresis of magnetoresistance of a 2DEG at integer filling factors

We report on an observation of giant hysteresis of longitudinal magnetoresistance of a constriction placed in a macroscopic two‐dimensional electron gas at integer filling factors. The most striking feature of observed phenomenon is the inverted form of hysteresis loops which is manifested in advanced system response to a small change of external magnetic field. We found that the resistance in the region of hysteresis loop relaxes in a stepwise manner, that is phenomenologically similar to Barkhausen jumps.