O. Estibals

Gated wires and interferometers based on Si/SiGe heterostructures

Abstract We report the fabrication and study of gated quantum wires and interferometers based on a Si/SiGe heterostructure fabricated by electron lithography and anisotropic ion etching. In the wires, negative magnetoresistance connected with weak localisation effects and in the ring, Aharonov–Bohm oscillations were investigated in the temperature range 30 mK – 5 K . The phase coherence time was found to be due to electron–electron scattering with small energy transfer and magnetic impurity scattering. High magnetic field Aharonov–Bohm oscillations connected with the interference of edge states current have been observed in Si/SiGe ring for the first time.

Resonance backscattering in submicron rings

Quasiperiodic peaks of the resistance as a function of gate voltage were observed in submicron rings fabricated on the basis of a two-dimensional electron gas in a GaAs quantum well with the AlAs/GaAs superlattice barriers. In magnetic fields higher than 1 T, the peaks disappeared. The negative magnetoresistance observed in the peak maxima is explained by the magnetic-field-induced suppression of the resonance backscattering that appears in the triangular quantum dots situated in the branching regions of a ring interferometer.

Single-electron conductance oscillations of small open quantum dot

Abstract We study the conductance of three-lead small open quantum dots fabricated on the basis of a high mobility two-dimensional (2D) electron gas in an AlGaAs/GaAs heterostructure. Large conductance oscillations have been observed. It is shown that the gate voltage period of these oscillations corresponds to a change of the average dot charge by one electron. We suggest that they are the manifestation of spin-charge separation in small open quantum dots.

Small ring interferometer on the basis of a GaAs quantum well with a high density 2D electron gas

Abstract Magneto-transport properties of small ring interferometers with narrow electronic channels made on the basis of a 2D electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers are studied. It was found that the amplitude of the h / e -oscillations is correlated to the Shubnikov–de Haas oscillations in the area where the 2D electron gas is connected to the ring. A possible mechanism of this effect is a change of the confining potential which forms the geometry of the regions located at the ring input and output, in the quantized Hall regime.

Coulomb oscillations of conductance in an open ring interferometer in a strong magnetic field

Under the conditions corresponding to tunnel-coupled edge current states in an open ring interferometer, oscillations of conductance as a function of gate voltage with two noticeably different periods are observed. The large-period oscillations are attributed to the electron tunneling between the source and drain regions via a closed edge state of the ring, when an integral number of magnetic flux quanta passes through its contour at the Fermi level. The small-period oscillations are explained by the effect of single-electron variations of the ring potential on the transparency of the barriers between the localized and delocalized edge states of the interfer-ometer.

Landau levels in two and three-dimensional electron gases in a wide parabolic quantum well

Shubnikov-de Haas oscillations are measured in a wide parabolic quantum well with 6 subbands in a tilted magnetic fi eld. We find two types of oscillations. The oscillations at low magnetic field are shifted towards higher fields with tilted angles, and can be attributed to the two-dimensional Landau state at the bottom subband. The position of the second type oscillations do not shift with tilted angles indicating a three-dimensional character of the Landau state formed by the highest subbands. The bottom level in the quantum well is not overlapped with the highest subbands due to the enhanced quantum scattering time of the lowest subbands.

Influence of Fermi-System Chirality on the Temperature Dependence of the Aharonov-Bohm Effect

The Aharonov-Bohm effect in submicron rings with narrow electron channels was studied in the range of magnetic fields from 0 to 15 T and temperatures from 0.1 to 10 K. It is found that the temperature dependences of the h/e-oscillation amplitude at low magnetic fields and in the situation of tunnel-coupled edge current states are different. The obtained experimental data are explained by the influence of Fermi-system chirality on the coherent transport in a ring interferometer.

Coherent transport in an ensemble of connected quasi-ballistic rings

Abstract Magneto-transport properties of ensembles of connected quasi-ballistic rings based on a 2D gas in a GaAs quantum well are studied. It has been found that h /2 e and h / e oscillations in the ensembles have the same temperature dependence, similar to the h / e dependence in the single ring. We find the phase-coherence length L φ ∼ T −1 , close to what has been reported for single quasi-one-dimensional rings. It is shown that the amplitude of the Aharonov–Bohm oscillations in the ensemble of quasi-ballistic-rings does not decreases as N −0.5 when N rings are connected in ensemble, as has been observed previously for ensembles of diffusive rings. It means that in the ensemble of connected quasi-ballistic-rings, the stochastic nature of the wave function phase in each individual ring is broken.

Ring interferometer on the basis of 2D electron gas in a double quantum well

Magnetotransport properties of submicron rings fabricated on the basis of 2D electron gas in a GaAs double quantum well are studied. It is shown that, in such interferometers, the Aharonov-Bohm effect is caused by coherent processes in two weakly coupled rings, which have different widths of electron channels. In these interferometers, a phase inversion of h/e oscillations is observed under the action of the parallel component of a tilted magnetic field. This phenomenon is qualitatively explained by a redistribution of charge carriers in the two rings.

Transport of a two-dimensional electron gas in a lattice of diffusive and thermalizing scatterers

Abstract Classical and quantum transport in a new kind of two-dimensional (2D) systems with artificial scatterers—2D electron gas in an array of diffusive and thermalizing scatterers (AuGeNi alloy dots)—was studied experimentally. It is shown that this array leads to a strong diffusive and inelastic scattering of the 2D electrons. As a result, several features in transport properties were observed: commensurability magnetoresistance peak, anomalies of the Hall resistance, unusual behavior (linear temperature dependence, saturation of the negative magnetoresistance) of the quantum interference corrections to the conductivity, which is due to the nontrivial dephasing in the system.