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We study conductivity of strongly disordered amorphous antimony films under high bias voltages. We observe non-linear current-voltage characteristic, where the conductivity value at zero bias is one of two distinct values, being determined by the sign of previously applied voltage. Relaxation curves demonstrate high stability of these conductivity values on a large timescale. Investigations of the antimony film structure allows to determine the percolation character of electron transport in strongly disordered films. We connect the memory effect in conductivity with modification of the percolation pattern due to recharging of some film regions at high bias voltages., Comment: final version

We investigate charge transport through the junction between a niobium superconductor and the edge of a two-dimensional electron-hole bilayer, realized in an InAs/GaSb double quantum well. For the transparent interface with a superconductor, we demonstrate that the junction resistance is determined by the interlayer charge transfer near the interface. From an analysis of experimental $I-V$ curves we conclude that the proximity induced superconductivity efficiently couples electron and hole layers at low currents. The critical current demonstrates periodic dependence on the in-plane magnetic field, while it is monotonous for the field which is normal to the bilayer plane., Comment: 5 pages

Investigations of topological insulators, which are two- and three-dimensional systems with a gap in the bulk spectrum and topologically protected gapless edge states, are of considerable fundamental interest at present. The experiments confirming the presence of the edge states in two-dimensional systems with inverted bands and problems of determining the nature of such states in these experiments are reviewed. Special attention is focused on spin-sensitive experiments since the topological edge states have a nontrivial spin structure.

We experimentally investigate spin-polarized electron transport between a permalloy ferromagnet and the edge of a two-dimensional electron system with band inversion, realized in a narrow, 8 nm wide, HgTe quantum well. In zero magnetic field, we observe strong asymmetry of the edge potential distribution with respect to the ferromagnetic ground lead. This result indicates that the helical edge channel, specific for the structures with band inversion even at the conductive bulk, is strongly coupled to the ferromagnetic side contact, possibly due to the effects of proximity magnetization. This allows selective and spin-sensitive contacting of helical edge states.

We experimentally investigate spin-polarized electron transport between two ferromagnetic contacts, placed at the edge of a two-dimensional electron system with band inversion. The system is realized in a narrow (8 nm) HgTe quantum well, the ferromagnetic side contacts are formed from a premagnetized permalloy film. In zero magnetic field, we find a significant edge current contribution to the transport between two ferromagnetic contacts. We experimentally demonstrate that this transport is sensitive to the mutual orientation of the magnetization directions of two 200 µm-spaced ferromagnetic leads. This is a direct experimental evidence on the spin-coherent edge transport over the macroscopic distances. Thus, the spin is extremely robust at the edge of a two-dimensional electron system with band inversion, confirming the helical spin-resolved nature of edge currents.

We experimentally investigate electron transport through the interface between a superconductor and the edge of a two-dimensional electron system with band inversion. The interface is realized as a tunnel NbN side contact to a narrow 8~nm HgTe quantum well. It demonstrates a typical Andreev behavior with finite conductance within the superconducting gap. Surprisingly, the conductance is modulated by a number of equally-spaced oscillations. The oscillations are present only within the superconducting gap and at lowest, below 1~K, temperatures. The oscillations disappear completely in magnetic fields, normal to the two-dimensional electron system plane. In contrast, the oscillations' period is only weakly affected by the highest, up to 14~T, in-plane oriented magnetic fields. We interpret this behavior as the interference oscillations in a helical one-dimensional edge channel due to a proximity with a superconductor., Comment: minor corrections - as published

This review presents experimental results on the inter-edge-state transport in the quantum Hall effect, mostly obtained in the regime of high imbalance. The application of a special geometry makes it possible to perform I-V spectroscopy between individual edge channels in both the integer and the fractional regime. This makes it possible to study in detail a number of physical effects such as the creation of topological defects in the integer quantum Hall effect and neutral collective modes excitation in fractional regime. The while many of the experimental findings are well explained within established theories of the quantum Hall effects, a number of observations give new insight into the local structure at the sample edge, which can serve as a starting point for further theoretical studies., 13 pages, 13 figures

We use a quasi-Corbino sample geometry with independent contacts to different edge states in the quantum Hall effect regime to investigate a charge redistribution between cyclotron-split edge states at high imbalance. We also modify Buttiker formalism by introducing local transport characteristics in it and use this modified Buttiker picture to describe the experimental results. We find that charge transfer between cyclotron-split edge states at high imbalance can be described by a single parameter, which is a transferred between edge states portion of the available for transfer part of the electrochemical potential imbalance. This parameter is found to be independent of the particular sample characteristics, describing fundamental properties of the inter-edge-state scattering. From the experiment we obtain it in the dependence on the voltage imbalance between edge states and propose a qualitative explanation to the experimental findings., 5 pages, 4 figures, to appear in JETP Letters

Combining a quasi-Corbino geometry with the cross-gate technique, we developed a sample geometry that allows us to separately contact edge states in the integer and fractional quantum Hall regime. The energy barriers between edge states at integer filling factors give rise to pronounced steps in the I-V characteristics that directly reflect the gap structure of the reconstructed edge. The traces can readily be interpreted in terms of the Landauer-Buttiker formalism and the compressible/incompressible liquid picture. At a temperature of 30 mK and for the fractional filling factor combinations I : 2/3 and 1 : 1/3, the slopes of the obtained I-V traces at currents up to 50 nA are all in very good agreement with the predictions of the Landauer-Buttiker formalism, assuming edge states of fractional charge 1/3. From the nonlinearity of the I-V characteristics we estimate the energy barrier between fractional edge states of charge 1/3 to be of the order of 40 μeV.

We experimentally investigate transport across a single incompressible strip at the sample edge in the fractional quantum Hall regime under high-imbalance conditions. We show that the I – V dependence exhibits a power-law behavior, which is the characteristic feature of a Luttinger-type tunnel density of states. The obtained results give the strong evidence for the existence of the so-called neutral collective modes at the sample edge. We observe the influence of the collective modes on the equilibration process across a single incompressible strip in the fractional quantum Hall regime.

We employ a magnetocapacitance technique to study the spectrum of the soft two-subband (or double-layer) electron system in a parabolic quantum well with a narrow tunnel barrier in the centre. In this system unbalanced by gate depletion, at temperatures $T\agt 30$ mK we observe two sets of quantum oscillations: one originates from the upper electron subband in the closer-to-the-gate part of the well and the other indicates the existence of common gaps in the spectrum at integer fillings. For the lowest filling factors $\nu=1$ and $\nu=2$, both the common gap presence down to the point of one- to two-subband transition and their non-trivial magnetic field dependences point to magnetic-field-induced hybridization of electron subbands., Comment: Major changes, added one more figure, the latest version to be published in JETP Lett

We experimentally demonstrate the simultaneous reconstruction of the bulk and the edge energy spectra caused by strong spin-orbit interaction, at the two lowest filling factors $\nu=1$ and $\nu=2$ for $In_xGa_{1-x}As$ two-dimensional electron system with $x = 0.75$. The observed reconstruction is of different character at these filling factors: it is characterized by zeroing of the energy gap at the levels crossing point at $\nu=2$, while the reconstruction at $\nu=1$ goes through the coexistence of two $\nu=1$ quantum Hall phases with different spin projections. An analysis indicates a strong influence of many-body interaction on the reconstruction at $\nu=1$., Comment: 6 pages

We apply a novel inductive coupling technique to study edge magnetoplasmons (EMP) in two-dimensional electron gases in the time domain. This technique is mostly appropriated for measurements on gated samples in the low-magnetic-field limit. We obtain time delay, amplitude, and broadening of the EMP as a function of the magnetic field.

Strong electron–electron interactions in dilute two-dimensional electron systems in silicon lead to Pauli spin susceptibility growing critically at low electron densities. This effect originates from renormalization of the effective mass rather than the g-factor. The relative mass enhancement is system and disorder independent, which suggests that it is determined by electron–electron interactions only.