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The coupling of superconductivity in a two-dimensional electron gas (2DEG) generates a number of magnetotransport oscillations. For instance, an Aharonov–Bohm-type oscillation at intermediate magnetic fields and an Altshuler–Aronov–Spivak-like oscillation around zero magnetic field appear under the circumstance of the coexistence of Andreev and normal reflections from the interface between the normal-conductor and the superconductor. The presence and the characteristics of such magnetotransport oscillations are investigated in this work by carrying out fully quantum-mechanical simulations. The significant role of the quantum interference is thereby demonstrated. It is also shown how the oscillations are affected by the presence of a potential disorder and finite excitation biases. Shubnikow–de Haas oscillations of the 2DEG are not always suppressed by the disorder under the influence of superconductivity, making their distinction from the Aharonov–Bohm-type oscillation possibly ambiguous.

A recent experiment [Hortelano et al., Semicond. Sci. Technol. 32, 125005 (2017)] reported a rectification effect that appeared in curved narrow ballistic channels of a two-dimensional electron gas when the strength of a magnetic field applied to the channels was tuned. The phenomenon was reproduced by classical billiard simulations as resulting from a transmission asymmetry caused by diffuse boundary scattering. However, this manifests breakdown of a commonly used simple model for diffuse boundary since the magnetic-field dependence of the transmission in two-terminal geometries has to be symmetric in equilibrium. We demonstrate here that this tendency of the system predicted by the billiard simulations is a real transmission asymmetry effect that emerges in the nonequilibrium transport. We perform nonequilibrium quantum-mechanical simulations with taking into account Coulomb repulsion. Experimental observations are presented to demonstrate consistencies with the numerical results.

Nanometer-sized channels are fabricated in (In,Ga)As-InP heterostructures using Ar ion milling. The ion milling causes spontaneous creation of nanowires, and moreover, electrical conduction of the surface as carriers is generated by sputtering-induced defects. We demonstrate a method to restore electrical isolation in the etched area that is compatible with the presence of the nanochannels. We remove the heavily damaged surface layer using a diluted HCl solution and subsequently recover the crystalline order in the moderately damaged part by annealing. We optimize the HCl concentration to make the removal stop on its own before reaching the conduction channel part. The lateral depletion in the channels is shown to be almost absent.

Quantum interference in scattering from a potential offset is investigated in narrow strips of two-dimensional systems described by the Bernevig-Hughes-Zhang Hamiltonian. Attention is focused on the situations where the transmission in the scattering region takes place around the Dirac point of topological insulators when the hybridization energy gap is eliminated by utilizing transverse interference. Apart from conventional periodic transmission modulation that takes place when the length of the potential offset region is varied, resonant disappearances of reflection occur for short potential offsets. The anomalous resonance appears not only for the four-band Hamiltonian but also for the two-band Hamiltonian, manifesting the generality of the phenomenon. Evanescent-like waves excited around the potential steps are indicated to be responsible for the anomalous behavior. The interference states can couple with each other and generic reduction in the amplitude of transmission modulation occurs upon coupling with the periodic modulation.

Ab initio calculations have predicted a transition between topological insulators and band insulators in superlattices consisting of Sb2Te3 and GeTe, when the periodicity is varied. We examine the amplitude of the weak antilocalization effect to confirm the transition. In agreement with the predictions, robust surface states are present for (Sb2Te3)1(GeTe)2 but absent when the content of the well-known topological insulator Sb2Te3 is increased as (Sb2Te3)4(GeTe)2, manifesting that the electronic coupling in the superlattices affects the emergence of the helical surface states nontrivially.

The electrostatic potential and the carrier distribution in periodically line‐doped structures are calculated within the semiclassical approximation. When the distance between the doped lines is 8 nm, which is a typical periodicity experimentally realized on the (100) GaAs surface, the system is regarded as a lateral superlattice instead of parallel wires because of strong overlap of the electron charge. Moreover, the modulation is anticipated not to be sufficient to probe the electronic state unambiguously. Alternative possible structures to enhance the modulation, which may provide more suitable systems for experiments, are investigated. It is shown that the confinement is stronger for heavier carriers by virtue of their smaller screening length. The coupling among the wires is suppressed by inserting weak p‐type dopant lines in between the channels. An application of the line doping to a heterojunction is also examined.

The helical edge states of two-dimensional topological insulators (TIs) experience appreciable quantum mechanical scattering in narrow channels when the width changes abruptly. The interference of the geometry scattering in narrow-wide-narrow waveguide structures is shown to give rise to the strong suppression of transmission when the incident energy is barely above the propagation threshold. Periodic resonant transmission takes place in this high reflection regime while the length of the wide section is varied. The resonance condition is governed by the transverse confinement in the wide section, where the form of quantization is manifested to differ for the two orthogonal directions. The confined energy levels in TI quantum dots are derived based on this observation. In addition, the off-diagonal spin-orbit term is found to produce an anomalous resonance state, which merges with the bottom ordinary resonance state to annihilate.

We investigate the transmission properties in quantum wires with a superconducting island placed in the center of the channel. The conductance quantization is destroyed, unless the Fermi energies in the quantum wire and the superconductor are identical as a consequence of the single-particle reflection from the normal-conductor–superconductor interface. The conductance exhibits fluctuations due to the quantum interference effect in the island. The fluctuation amplitude is found to depend strongly on the pair potential amplitude in the superconductor.

The quantum transport of electrons in a T‐shaped waveguide structure is investigated using the tight‐binding model. We take into account electron‐electron interaction in the resonator region within the mean‐field approximation. The scaling rule breaks down in the interacting system and the effect of the interaction is significant when the sample dimensions become large compared to the effective Bohr radius of electrons. The nearly perfect reflection due to the transmission resonances is shifted to higher energy as the repulsive interaction is increased. The interaction effect in the presence of short‐range disorder is also examined.

A tunneling current across a thin wall of a narrow‐wide‐narrow (NWN) cavity region in an electron waveguide is calculated. A lead is attached to the cavity region at right angle through a thin tunneling barrier to drain the current. The transmission coefficient through the NWN waveguide shows successive dips due to resonances through quasibound states in the cavity region as the length of the cavity region or the Fermi energy of an incident electron is varied. The transmission coefficient leaking out the tunneling barrier shows peaks that line up with the dips in the forward transmission probability, demonstrating that a tunneling spectroscopy of the quasibound state levels is provided through sweeping the Fermi energy.

Magnetic-field dependence of the resistivities in MnAs(11¯00) films on GaAs(001) and MnAs(0001) films on GaAs(111)B is investigated at low temperatures. Correspondence of the features in the longitudinal and transverse resistivities under reorientations of the magnetization enables us to distinguish the anisotropic magnetoresistance effect from the effects originating from the band structure of MnAs. Simultaneous contributions of holes and electrons are evidenced for the (11¯00)-oriented films, whereas the transport is almost completely dominated by holes for the (0001)-oriented films. The magnetization flip by an in-plane magnetic field applied along the magnetic easy axis generates jumps in the longitudinal resistivity owing to the magnetoresistance originating from the band-structure effect.

On the surface capped by an AlN-GaN bilayer, the particle displacements of surface acoustic waves (SAWs) are localized in the short-wavelength regime in the buried GaN layer instead of the overlaying AlN layer as a consequence of the significantly faster propagation of sound in AlN than in GaN. We numerically explore this relocation effect for Al2O3(0001) substrates, which are the commonly used substrate for group-III nitrides. We show that the critical wavelength for the relocation effect can be manipulated in a wide range by varying the thickness of the AlN layer owing to the moderate sound velocity in Al2O3. We also investigate the influences of the polarization mixing on the dispersion of SAWs. The SAW dispersion in the layered system is interpreted in terms of the anticrossing of relevant acoustic modes.

We numerically investigate the characteristics of surface acoustic waves (SAWs) in AlN∕GaN∕γ‐LiAlO2 heterostructures. The markedly large sound velocity in AlN in comparison to that in GaN leads to an expulsion of SAWs from the top AlN layer and their resultant relocation to the middle GaN layer in the short-wavelength regime. The SAW velocity in the limit of zero wavelength is given by a bulk sound velocity of GaN, owing to the capping by the AlN barrier layer. The extra confinement of the SAW power in the acoustic well is advantageous in manipulating the operation of GaN-based devices by SAWs. The threshold velocity for the appearance of guided Rayleigh-like modes is found to be smaller than the bulk transverse sound velocity in the substrate. The present system exhibits furthermore unusual bowing behavior in the SAW dispersion.