A. D. Levin

Nonlocal transport near charge neutrality point in a two-dimensional electron-hole system.

Nonlocal resistance is studied in a two-dimensional system with a simultaneous presence of electrons and holes in a 20 nm HgTe quantum well. A large nonlocal electric response is found near the charge neutrality point in the presence of a perpendicular magnetic field. We attribute the observed nonlocality to the edge state transport via counterpropagating chiral modes similar to the quantum spin Hall effect at a zero magnetic field and graphene near a Landau filling factor ν=0.

Persistence of a Two-Dimensional Topological Insulator State in Wide HgTe Quantum Wells

Our experimental studies of electron transport in wide (14xa0nm) HgTe quantum wells confirm the persistence of a two-dimensional topological insulator state reported previously for narrower wells, where it was justified theoretically. Comparison of local and nonlocal resistance measurements indicate edge state transport in the samples of about 1xa0mm size at temperatures below 1xa0K. Temperature dependence of the resistances suggests an insulating gap of the order of a few meV. In samples with sizes smaller than 10u2009u2009μm a quasiballistic transport via the edge states is observed.

Viscous electron flow in mesoscopic two-dimensional electron gas

We report electrical and magneto transport measurements in mesoscopic size, two-dimensional (2D) electron gas in a GaAs quantum well. Remarkably, we find that the probe configuration and sample geometry strongly affects the temperature evolution of local resistance. We attribute all transport properties to the presence of hydrodynamic effects. Experimental results confirm the theoretically predicted significance of viscous flow in mesoscopic devices.

Giant microwave photo-conductance of a tunnel point contact with a bridged gate

We study the microwave photo-response of a quantum point contact (QPC) formed on a GaAs/AlGaAs heterostructure with a high-electron-mobility two-dimensional electron gas. The QPCs are fabricated by two types of gates: a traditional split gate and a specially designed bridged gate. We observe a three orders of magnitude enhancement of the dark QPC conductance in the tunneling regime at the incident microwave power density of ∼10u2009mW/cm2. The response of the bridged-gate structure is more than ten times larger than that of the split-gate QPC. This giant effect and the difference between the two types of gates are explained by the influence of microwaves on the steady-state electron distribution function in the vicinity of the tunnel contact. Experimental results are in good quantitative agreement with theoretical calculations. The bridged-gate QPC can be used for the creation of highly sensitive detectors of electromagnetic radiation.

Microwave-Induced Magneto-Oscillations and Signatures of Zero-Resistance States in Phonon-Drag Voltage in Two-Dimensional Electron Systems.

We observe the phonon-drag voltage oscillations correlating with the resistance oscillations under microwave irradiation in a two-dimensional electron gas in perpendicular magnetic field. This phenomenon is explained by the influence of dissipative resistivity modified by microwaves on the phonon-drag voltage perpendicular to the phonon flux. When the lowest-order resistance minima evolve into zero-resistance states, the phonon-drag voltage demonstrates sharp features suggesting that current domains associated with these states can exist in the absence of external dc driving.

Microwave response of a ballistic quantum dot

The microwave response (photovoltage and photoconductance) of a lateral ballistic quantum dot made of a high-mobility two-dimensional electron gas in an AlGaAs/GaAs heterojunction has been studied experimentally in the frequency range of 110–170 GHz. It has been found that the asymmetry of the photovoltage with respect to the sign of the magnetic field has mesoscopic character and depends on both the magnetic field and the microwave power. This indicates the violation of the Onsager reciprocity relations regarding the electron-electron interactions in the mesoscopic photovoltaic effect. A strong increase in the conductance of the quantum dot induced by the microwave radiation and unrelated to heating, as well as the microwave-induced magneto-oscillations, has been discovered.

Magnetocapacitance oscillations and thermoelectric effect in a two-dimensional electron gas irradiated by microwaves

To study the influence of microwave irradiation on two-dimensional electrons, we apply a method based on capacitance measurements in GaAs quantum well samples where the gate covers a central part of the layer. We find that the capacitance oscillations at high magnetic fields, caused by the oscillations of thermodynamic density of states, are not essentially modified by microwaves. However, in the region of fields below 1 T, we observe another set of oscillations, with the period and the phase identical to those of microwave-induced resistance oscillations. The phenomenon of microwave-induced capacitance oscillations is explained in terms of violation of the Einstein relation between conductivity and the diffusion coefficient in the presence of microwaves, which leads to a dependence of the capacitor charging on the anomalous conductivity. We also observe microwave-induced oscillations in the capacitive response to periodic variations of external heating. These oscillations appear due to the thermoelectric effect and are in antiphase with microwave-induced resistance oscillations because of the Corbino-like geometry of our experimental setup.

Microwave-induced nonlocal transport in a two-dimensional electron system

We observe microwave induced nonlocal resistance in magnetotransport in single and bilayer electronic systems. The obtained results provide evidence for an edge state current stabilized by microwave irradiation due to nonlinear resonances. Our observation are closely related to microwave induced oscillations and zero resistance states in a two-dimensional (2D) electron system.

Robust helical edge transport at ν=0 quantum Hall state

Among the most interesting predictions in two-dimensional materials with a Dirac cone is the existence of the zeroth Landau level (LL), equally filled by electrons and holes with opposite chirality. The gapless edge states with helical spin structure emerge from Zeeman splitting at the LL filling factor

Magnetophonon oscillations of thermoelectric power and combined resonance in two-subband electron systems

nu=0