Occupied Subbands Research Articles

The role of Rashba spin-orbit induced spin textures in the anomalous Josephson effect

This work reports the theoretical investigation into the mechanism underpinning the anomalous Josephson effect within ballistic systems; currently, there is no agreed-upon microscopic mechanism behind the origin of this effect. The prototypical system we study is a ballistic two-dimensional junction containing a two-dimensional Rashba spin-orbit interaction. In this paper, we demonstrate how this two-dimensional Rashba interaction mixes the spins of adjacent transverse subbands, leading to significant spin-asymmetry within the junction. Under an external magnetic field, applied perpendicular to both the axis of transport and the normal vector of the junction, the sinusoidal Josephson current can then experience an anomalous phase shift. The role of this spin mixing in the limit of a single sub-band is initially explored by deriving an analytical expression for the resulting anomalous phase shift. The analysis is then extended to systems with multiple occupied sub-bands; in this later section, starting from a microscopic model, we derive an analytic formula for the resulting anomalous phase shift indicating it is linear in both magnetic field and spin-orbit strength. We then verify and validate all findings by comparing them with numerical results evaluated by a tight-binding model.

Intense high-frequency laser-field control of spin-orbit coupling in GaInAs/AlInAs quantum wells: A laser dressing effect

The Rashba spin-orbit (SO) coupling can flexibly be controlled via an external gate, while the Dresselhaus term, which mainly depends on quantum confinement, is in general immune to electrical control. Here we theoretically report optical manipulation of the SO coupling by resorting to an intense high-frequency laser field, which ``dresses'' the confining potential for electrons as a result of optical stark effect and enables a flexible and simultaneous control of the Rashba and Dresselhaus couplings. Focusing on ordinary GaInAs/AlInAs quantum wells with two occupied subbands subject to both laser and gate fields, we perform a self-consistent Poisson-Schr\"odinger calculation in the Hartree approximation to determine electro-optical control of the intrasubband (intersubband) Rashba ${\ensuremath{\alpha}}_{\ensuremath{\nu}}$ ($\ensuremath{\eta}$) and Dresselhaus ${\ensuremath{\beta}}_{\ensuremath{\nu}}$ ($\mathrm{\ensuremath{\Gamma}}$) SO terms with $\ensuremath{\nu}=1,2$. Under the impact of laser field, we find that the Rashba terms of the two subbands ${\ensuremath{\alpha}}_{1}$ and ${\ensuremath{\alpha}}_{2}$ may remain locked to equal strength in a broad gate range, providing a means for unified manipulation of the two-subband Rashba couplings. Further, as the laser field varies, we observe that ${\ensuremath{\alpha}}_{1}$ and ${\ensuremath{\alpha}}_{2}$ may have either the same or opposite signs, or even ${\ensuremath{\alpha}}_{2}$ vanishes while ${\ensuremath{\alpha}}_{1}$ is finite, greatly fascinating for selective SO control of distinct subbands. For the Dresselhaus coupling, we disclose two distinct scenarios depending on the interplay of the well width and the laser field strength, and reveal that ${\ensuremath{\beta}}_{2}$ may decrease rapidly when the laser field strengthens, even though ${\ensuremath{\beta}}_{1}$ remains essentially constant. Regarding the intersubband Rashba ($\ensuremath{\eta}$) and Dresselhaus ($\mathrm{\ensuremath{\Gamma}}$) terms, which mainly depend on the overlap and parity of the wave functions of the two subbands, they have relatively weak dependence on the laser field. Moreover, the combined effect of intra- and intersubband SO terms may lead to crossings and avoided crossings of the energy dispersion of multiband spin branches and may even trigger the spin polarization of an originally spin degenerate (unpolarized) band, tunable by the laser field. Our results should stimulate experiments probing the laser field mediated multiband SO control and further enables its spintronic applications.

Quantum Hall effect and Shubnikov–de Haas oscillations in a high-mobility p -type PbTe quantum well

We present here the magnetotransport experiment performed with a magnetic field up to 30 T on a 10 nm-wide $p$-type PbTe quantum well. Pronounced Shubnikov--de Haas oscillations and the integer quantum Hall effect are clearly observed, demonstrating the transport of a confined two-dimensional hole gas with negligible parallel conduction at low temperatures. The sequence of filling factors can be explained by considering the magnetic field evolution of Landau levels derived from two longitudinal and one oblique occupied subbands and taking Zeeman spin splitting into account. During illumination, the total carrier density of the system is raised by 15%, which leads to a plateau sequence in the integer quantum Hall effect with higher filling factors in comparison to the dark condition.

Subband occupation in semiconductor-superconductor nanowires

Subband occupancy (i.e. the number of occupied subbands or energy levels in the semiconductor) is a key physical parameter characterizing the topological properties of superconductor-semiconductor hybrid systems in the context of the search for non-Abelian Majorana zero modes. We theoretically study the subband occupation of semiconductor nanowire devices as function of the applied gate potential, the semiconductor-superconductor (SM-SC) work function difference, and the surface charge density by solving self-consistently the Schr\"{o}dinger-Poisson equations for the conduction electrons of the semiconductor nanowire. Realistic surface charge densities, which are responsible for band bending, are shown to significantly increase the number of occupied subbands, making it difficult or impossible to reach a regime where only a few subbands are occupied. We also show that the energy separation between subbands is significantly reduced in the regime of many occupied subbands, with highly detrimental consequences for the realization and observation of robust Majorana zero modes. As a consequence, the requirements for the realization of robust topological superconductivity and Majorana zero modes should include a low value of the chemical potential, consistent with the occupation of only a few subbands. Finally, we show that the local density of states on the exposed nanowire facets provides a powerful tool for identifying a regime with many occupied subbands and is capable of providing additional critical information regarding the feasibility of Majorana physics in semiconductor-superconductor devices. In our work, we address both InAs/Al and InSb/Al superconductor-nanowire hybrid systems of current experimental interest.

Microwave-Induced Magneto-Intersubband Scattering in a Square Lattice of Antidots

The effect of microwave radiation on low-temperature electron magnetotransport in a square antidot lattice with a period of d = 0.8 micrometer based on a GaAs quantum well with two occupied energy subbands E1 and E2 is investigated. It is shown that, owing to a significant difference between the electron densities in the subbands, commensurability oscillations of the resistance in the investigated antidot lattice are observed only for the first subband. It is found that microwave irradiation under the cyclotron resonance condition results in the formation of resistance oscillations periodic in the inverse magnetic field in the region of the main commensurability peak. It is established that the period of these oscillations corresponds to the period of magneto-intersubband oscillations. The observed effect is explained by the increase in the rate of intersubband scattering caused by the difference between the electron heating in the subbands E1 and E2.

Magnetotransport property of graded AlGaN/GaN heterostructure

Abstract This paper reports magnetotransport property of a graded AlGaN/GaN heterostructure. When temperature (T) is higher than 10 K, resistivity as a function of T exhibits a metallic conduction behavior, from which the Debye temperature of 1074.4 K is extracted. At low Ts, clear Shubnikov-de-Haas (SdH) oscillations in magnetoresistance (MR) curve are observed when magnetic field is larger than 2 T. The observed SdH oscillations originate from the two-dimensional electrons. From fast Fourier transform power spectrums of the SdH oscillations, it is found that there are five occupied subbands, in agreement with the results obtained from the energy band calculation. In the intermediate field range of MR curve, a parabolic negative MR is observed, which can be attributed to the two-dimensional electron-electron interaction effect.

Analysis of Pisarenko Harmonic Distortion (PHD) based subNyquist rate spectrum sensing for broadband Cognitive Radio

The essential part of Cognitive Radio (CR) is spectrum sensing, so that the underutilized spectrum could be detected to improve the spectrum efficiency. For this purpose, a wide range of frequency bands are considered and locations of multiple occupied spectrum subbands are focused. A major challenge related with such broadband spectrum sensing is that it is either extravagant or impracticable to perform Nyquist sampling on the broadband signal. In this study, a broadband spectrum sensing method, that takes advantage of subNyquist sampling wherein the sampling rate is considerably reduced. The correlation matrix of a limited number of noisy samples is computed and is used to estimate the frequency function of the PHD method to detect the occupied and unoccupied channels. The salient feature of this approach as compared to other methods is that, no prior knowledge of signal properties (which would lead to uncertainly problems) is necessary. Further, the efficiency of this method is assessed by calculating the detection probability of the occupied channel as a function of the limited number of samples and the signal to noise ratio of random input signals. The simulation results demonstrate a reliable detection, even with limited samples and a low SNR.

Persistent Skyrmion Lattice of Noninteracting Electrons with Spin-Orbit Coupling.

A persistent spin helix (PSH) is a robust helical spin-density pattern arising in disordered 2D electron gases with Rashba α and Dresselhaus β spin-orbit (SO) tuned couplings, i.e., α=±β. Here, we investigate the emergence of a persistent Skyrmion lattice (PSL) resulting from the coherent superposition of PSHs along orthogonal directions-crossed PSHs-in wells with two occupied subbands ν=1, 2. For realistic GaAs wells, we show that the Rashba α_{ν} and Dresselhaus β_{ν} couplings can be simultaneously tuned to equal strengths but opposite signs, e.g., α_{1}=β_{1} and α_{2}=-β_{2}. In this regime, and away from band anticrossings, our noninteracting electron gas sustains a topologically nontrivial Skyrmion-lattice spin-density excitation, which inherits the robustness against spin-independent disorder and interactions from its underlying crossed PSHs. We find that the spin relaxation rate due to the interband SO coupling is comparable to that of the cubic Dresselhaus term as a mechanism of the PSL decay. Near anticrossings, the interband-induced spin mixing leads to unusual spin textures along the energy contours beyond those of the Rahsba-Dresselhaus bands. Our PSL opens up the unique possibility of observing topological phenomena, e.g., topological and Skyrmion Hall effects, in ordinary GaAs wells with noninteracting electrons.

Soft superconducting gap in semiconductor-based Majorana nanowires

We develop a theory for the proximity effect in superconductor-semiconductor-normal metal tunneling structures, which have recently been extensively studied experimentally, leading to the observation of transport signatures consistent with the predicted zero-energy Majorana bound states. We show that our model for the semiconductor nanowire having multiple occupied subbands with different transmission probabilities through the barrier reproduces the observed "soft-gap" behavior associated with substantial subgap tunneling conductance. We study the manifestations of the soft gap phenomenon both in the tunneling conductance and in local density of states measurements and discuss the correlations between these two quantities. We emphasize that the proximity effect associated with the hybridization between low-lying states in the multiband semiconductor and the normal metal states in the lead is an intrinsic effect leading to the soft gap problem. In addition to the intrinsic contribution, there may be extrinsic effects, such as, for example, interface disorder, exacerbating the soft gap problem. Our work establishes the generic possibility of an ubiquitous presence of an intrinsic soft gap in the superconductor-semiconductor-normal metal tunneling transport conductance induced by the inverse proximity effect of the normal metal.

Two-dimensional electron gas in a modulation-doped SrTiO3/Sr(Ti, Zr)O3 heterostructure

A two-dimensional electron gas (2DEG) in SrTiO3 is created via modulation doping by interfacing undoped SrTiO3 with a wider-band-gap material, SrTi1−xZrxO3, which is doped n-type with La. All layers are grown using hybrid molecular beam epitaxy. Using magnetoresistance measurements, we show that electrons are transferred into the SrTiO3, and a 2DEG is formed. In particular, Shubnikov-de Haas oscillations are shown to depend only on the perpendicular magnetic field. Experimental Shubnikov-de Haas oscillations are compared with calculations that assume multiple occupied subbands.

Shubnikov-de Haas effect in tilted magnetic fields in wide quantum well

We study the magneto-resistance of a two-dimensional electron systems in a wide quantum well. We have considered two occupied subbands subjected to the tilted magnetic field in the regime of the magneto-inter-subband oscillations. We report on both experimental and theoretical studies of such a phenomenon and deduced information on the inter-subband energy gap in the presence of the in-plane magnetic field.

Charge-Induced Coherence between Intersubband Plasmons in a Quantum Structure

In this Letter we investigate a low dimensional semiconductor system, in which the light-matter interaction is enhanced by the cooperative behavior of a large number of dipolar oscillators, at different frequencies, mutually phase locked by Coulomb interaction. We experimentally and theoretically demonstrate that, owing to this phenomenon, the optical response of a semiconductor quantum well with several occupied subbands is a single sharp resonance, associated with the excitation of a bright multisubband plasmon. This effect illustrates how the whole oscillator strength of a two-dimensional system can be concentrated into a single resonance independently from the shape of the confining potential. When this cooperative excitation is tuned in resonance with a cavity mode, their coupling strength can be increased monotonically with the electronic density, allowing the achievement of the ultrastrong coupling regime up to room temperature.

Impurity and edge roughness scattering in graphene nanoribbons: the Boltzmann approach

The conductivity of graphene nanoribbons in the presence of bulk impurities and edge roughness is studied theoretically using the Boltzmann transport equation for quasi-one-dimensional systems. As the number of occupied subbands increases, the conductivity due to bulk impurities converges towards the two-dimensional case. It is shown that the dependence of the conductivity generated by edge roughness scattering depends in a distinctly different way on the sample parameters than the conductivity due to bulk scattering. The Boltzmann model furthermore predicts the amplitude of the edge-roughness-induced magnetoconductance dip as a function of the amplitude and the correlation length of the edge roughness.

Cyclotron resonance study in InAs/AlSb quantum well heterostructures with two occupied electronic subbands

We report on the cyclotron resonance (CR) study in InAs/AlSb (001) quantum well (QW) heterostructures with two occupied electronic subbands. Experimental results are compared with the CR energy calculations in the self-consistent Hartree approximation. Our theoretical approach is based on the 8-band k · p Hamiltonian and takes into account the band nonparabolicity, lattice-mismatch deformation, and spin-orbit coupling. We find out a large splitting of CR line associated with a difference in cyclotron energies in the first and second electronic subbands. The results of CR study in InAs/AlSb QW heterostructures reveal pronounced effect of the “built-in” electric field on CR spectra in the samples with two occupied electronic subbands.

SiN passivation layer effects on un-gated two-dimensional electron gas density in AlGaN/AlN/GaN field-effect transistors

In this paper, we present a study of SiN passivation layer effects on the two-dimensional electron gas (2DEG) sheet density in unintentionally doped AlGaN/AlN/GaN heterostructures, using a polarization model based on interface charges and a fully numerical calculation. The analysis of our results clearly indicates that there are at least two occupied sub-bands in the 2DEG for passivated AlGaN/AlN/GaN heterostructures, and with increasing passivation layer thickness and AlN interlayer thickness the 2DEG density increases. The comparison of our calculated results with published experimental data is shown to be in a very good agreement.

Zero differential resistance in a double quantum well at high filling factors

The differential resistance rxx in a GaAs double quantum well with two occupied size-quantization subbands have been studied at temperatures T = 1.6–4.2 K in magnetic fields B < 0.5 T. It has been found that differential resistance rxx vanishes at the maxima of magneto-intersubband oscillations with an increase in the direct current Idc. It has been shown that the discovered rxx ≈ 0 state appears under the condition 2RcEH/ħωc < 1/2, where Rc is the cyclotron radius of electrons at the Fermi level, EH is the Hall electric field induced by the current Idc, and ωc is the cyclotron frequency.

Resonance microwave photoresistance of a two-subband electron system at large filling factors

The microwave photoresistance of a double GaAs quantum well with two occupied size-quantization sub-bands E 1 and E 2 has been studied at the temperatures T = 1.6–4.2 K in the magnetic fields B < 0.5 T. The microwave photoresistance of such a system has been found to have a maximum amplitude when the maximum of the magneto-intersubband oscillations with the number k = (E 2 − E 1)ℏωc coincides with the maximum or minimum of the ω/ωc oscillations, where ω is the microwave frequency and ωc is the cyclotron frequency. It has been shown that the resonance photoresistance that appears in the kth maximum of the magneto-intersubband oscillations is determined by the condition ℏω/(E 2 − E 1) = (j ± 0.2)/k, where k and j are positive integers.

Magnetoresistance oscillations in multilayer systems: Triple quantum wells

Magnetoresistance of two-dimensional electron systems with several occupied subbands oscillates owing to periodic modulation of the probability of intersubband transitions by the quantizing magnetic field. In addition to previous investigations of these magnetointersubband (MIS) oscillations in two-subband systems, we report on both experimental and theoretical studies of such a phenomenon in three-subband systems realized in triple quantum wells. We show that the presence of more than two subbands leads to a qualitatively different MIS oscillation picture, described as a superposition of several oscillating contributions. Under a continuous microwave irradiation, the magnetoresistance of triple-well systems exhibits an interference of MIS oscillations and microwave-induced resistance oscillations. The theory explaining these phenomena is presented in the general form, valid for an arbitrary number of subbands. A comparison of theory and experiment allows us to extract temperature dependence of quantum lifetime of electrons and to confirm the applicability of the inelastic mechanism of microwave photoresistance for the description of magnetotransport in multilayer systems.

Magnetoabsorption in InSb quantum-well heterostructures

Spin-resolved cyclotron resonance is observed in InSb/AlInSb quantum-well heterostructures. Spin-resolved transitions are identified for fields greater than 2.2 T and confirmed by k⋅p modeling. The cyclotron effective mass is calculated for a range of fields up to 11.5 T and is indicative of the strong nonparabolicity in this material. Resonant coupling is observed between the ground and first occupied subbands for normal-incidence excitation. This is explained in the context of conduction band-valance band mixing due to the narrow bandgap. Further resonant coupling is observed via interaction with a hybrid intersubband plasmon-LO phonon mode. This allows a quantitative estimation of the subband spacing and of the electron-phonon interactions in the quantum well.

Zener tunneling between Landau levels in a double quantum well at high filling factors

Differential resistance r xx in a double GaAs quantum well with two occupied size-quantization subbands has been studied at a temperature of 4.2 K in magnetic fields B < 2 T. The oscillations of r xx with a period in the inverse magnetic field determined by the value of a dc bias current I dc have been discovered in the electron system under investigation at high filling factors in the presence of I dc. The amplitude of magneto-intersubband oscillations has been shown to increase in the r xx oscillation maxima, while the oscillation reversal has been observed in the minima. The discovered oscillations have been shown to be due to Zener tunneling of electrons between Landau levels tilted by a Hall electric field. The experimental data are qualitatively explained by the effect of intersubband transitions on the I dc-dependent component of the electron distribution function.