Electrons In Semiconductor Heterostructures Research Articles

On the Speed of Light as a Key Element in the Structure of Quantum Mechanics

We follow the assumption that relativistic causality is a key element in the structure of quantum mechanics and integrate the speed of light, c, into quantum mechanics through the postulate that the (reduced) Planck constant is a function of c with a leading order of the form ℏc∼Λ/cp for a constant Λ>0, and p>1. We show how the limit c→∞ implies classicality in quantum mechanics and explain why p has to be larger than 1. As the limit c→∞ breaks down both relativity theory and quantum mechanics, as followed by the proposed model, it can then be understood through similar conceptual physical laws. We further show how the position-dependent speed of light gives rise to an effective curved space in quantum systems and show that a stronger gravitational field implies higher quantum uncertainties, followed by the varied c. We then discuss possible ways to find experimental evidence of the proposed model using set-ups to test the varying speed of light models and examine analogies of the model based on electrons in semiconductor heterostructures.

Spectral detection of spin-polarized ultra low-energy electrons in semiconductor heterostructures

The circularly polarized cathodoluminescence (CL) technique has been used to study the free spin-polarized electron injection in semiconductor heterostructures with quantum wells (QWs). A polarized electron beam was created by the emission of optically oriented electrons from the p-GaAs(Cs,O) negative electron affinity (NEA) photocathode. The prepared beam was injected in a semiconductor QW target, which was activated by cesium and oxygen to reduce the work function. To study the spin-dependent injection, we developed a spin-detector prototype, which consists of a compact proximity focused vacuum tube with the source and target placed parallel to each other on the opposite ends of the vacuum tube (photodiode). The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range of 0.5-5.0 eV and temperature in the range of 90-300 K was studied. The CL was polarized to 2 % by the injection of 20 % spin-polarized electron beam with the energy of 0.5 eV at room temperature. The asymmetry (Sherman function) of spin detection was estimated. It was shown that the dependence of the CL polarization degree on the injected electron energy is satisfactory described by the model that considers the electron spin relaxation in the heterostructure matrix and QWs. The results demonstrate that semiconductor detectors are promising for the spin-polarimetry applications based on the optical detection of free-electron spin polarization.

Quantum confinement in perovskite oxide heterostructures: Tight binding instead of a nearly free electron picture

Most recently, orbital-selective quantum well states of $d$ electrons have been experimentally observed in SrVO$_3$ ultrathin films [K. Yoshimatsu et. al., Science 333, 319 (2011)] and SrTiO$_3$ surfaces [A. F. Santander-Syro et. al., Nature 469, 189 (2011)]. Hitherto, one tries to explain these experiments by a nearly free electron (NFE) model, an approach widely used for delocalized electrons in semiconductor heterostructures and simple metal films. We show that a tight binding (TB) model is more suitable for describing heterostructures with more localized $d$ electrons. In this paper, we construct from first principles simple TB models for perovskite oxide heterostructures and surfaces. We show that the TB model provides a simple intuitive physical picture and yields, already with only two parameters, quantitatively much more reliable results, consistent with experiment.

Total transmission of electrons in semiconductor heterostructures at oblique incidence

An alternative method to resonant tunneling for obtaining total transmission in semiconductor heterostructures is investigated. This nonresonant total transmission has been shown to be difficult to achieve for electrons at normal incidence. However, if the momentum of the electron is altered so that the component in the direction parallel to the material interface is nonzero, the transmission properties of the electron change, and the potential for total transmission is more promising. In the single-band approximation, for incident electron energies above the crossing points of the dispersion curves of the two materials, there exists an angle between the direction of the electron’s momentum and the material interface at which total transmission is achieved. The experimental demonstration of nonresonant total transmission could potentially open the door to a number of applications. In particular, its potential for the measurement of small band offsets between two materials in a heterostructure and in electronic applications is discussed.

Dynamic pair excitations in 2D Fermi fluids

We apply a theory (Böhm et al 2006 AIP Conf. Proc. 850 111, 2007 Int. J. Mod. Phys. B 21 2055) developed recently on dynamic two-pair fluctuations to layered systems, such as electrons in semiconductor hetero-structures or He on graphite. The theory fulfils the zeroth and first frequency moment sum rule. Results are presented for the static effective particle–hole interaction, the dynamic structure function and the dispersion of the plasmon.

Anisotropic spin-dependent electron tunneling in a triple-barrier resonant tunneling diode

The one-band envelope function approximation is used to investigate the spin-dependent tunneling of conduction band electrons in semiconductor heterostructures when both the bulk inversion asymmetry (BIA) and structure inversion asymmetry (SIA) are present. It is shown that under certain conditions the interplay between BIA and SIA may be used to induce a strong dependence of transmission probabilities on the direction of electrons lateral momenta thus offering means to improve the existing designs of nonmagnetic semiconductor spin filters.

Spin-based quantum computing using electrons on liquid helium

Numerous physical systems have been proposed for constructing quantum computers, but formidable obstacles stand in the way of making even modest systems with a few hundred quantum bits (qubits). Several approaches utilize the spin of an electron as the qubit. Here it is suggested that the spin of electrons floating on the surface of liquid helium will make excellent qubits. These electrons can be electrostatically held and manipulated much like electrons in semiconductor heterostructures, but being in a vacuum the spins on helium suffer much less decoherence. In particular, the spin orbit interaction is reduced so that moving the qubits with voltages applied to gates has little effect on their coherence. Remaining sources of decoherence are considered and it is found that coherence times for electron spins on helium can be expected to exceed 100 s. It is shown how to obtain a controlled-NOT operation between two qubits using the magnetic dipole-dipole interaction.

Resonant Rayleigh Scattering from Bilayer Quantum Hall Phases

We observe resonant Rayleigh scattering of light from quantum Hall bilayers at Landau level filling factor nu = 1. The effect arises below 1 Kelvin when electrons are in the incompressible quantum Hall phase with strong interlayer correlations. Marked changes in the Rayleigh scattering signal in response to application of an in-plane magnetic field indicate that the unexpected temperature dependence is linked to formation of a nonuniform electron fluid close to the phase transition towards the compressible state. These results demonstrate a new realm of study in which resonant Rayleigh scattering methods probe quantum phases of electrons in semiconductor heterostructures.

Perturbation theory for isotropic velocity-dependent potentials: Scattering case

The time-independent Schr\odinger equation with an isotropic velocity-dependent potential is considered. Treating the velocity-dependent interaction as a small perturbation, we develop analytical formulas for the changes in the scattering phase shifts and wave functions. It is shown that only the zeroth-order solution and the perturbing potential are needed to determine the phase-shift and wave-function corrections. No prior knowledge of the unperturbed scattering-states continuum is required. In order to test the validity of our approach, we applied it to an exactly solvable model for nucleon-nucleon scattering. The results of the perturbation formalism compare quite well with those of the exactly solvable model. The developed formalism can be applied in problems concerning pion-nucleon, nucleon-nucleon, and electron-atom scattering. It may also be useful in studying the scattering of electrons in semiconductor heterostructures.

Design of electron band pass filters for electrically biased finite superlattices

We design optimal band pass filters for electrons in semiconductor heterostructures, under a uniform applied electric field. The inner cells are chosen to provide a desired transmission window. The outer cells are then designed to transform purely incoming or outgoing waves into Bloch states of the inner cells. The transfer matrix is interpreted as a conformal mapping in the complex plane, which allows us to write constraints on the outer cell parameters, from which physically useful values can be obtained.

Cyclotron Resonance of Wigner Crystal in Semiconductor Heterostructures

The cyclotron resonance absorption of two-dimensional electrons in semiconductor heterostructures in high magnetic fields is investigated. It is assumed that the ionized impurity potential is a dominant scattering mechanism, and the theory explicitly takes the Coulomb correlation effect into account through the Wigner phonons. The cyclotron resonance linewidth is in quantitative agreement with the experiment in the Wigner crystal regime at T=4.2K. Similar to the cyclotron resonance theory of the charge density waves pinned by short-range impurities, the present results for the long-range scattering also show the doubling of the resonance peaks. However, unlike the case of the charge density waves, our theory gives the pinning mode independent of the bulk compressibility of the substrate materials.

Theory of cyclotron resonance of correlated electron systems

The cyclotron resonance (CR) absorption of 2D electrons in semiconductor heterostructures GaAs/AlGaAs in high magnetic fields is investigated. The theory explicitly takes the Coulomb correlation into account through the Wigner phonons. The CR line width is in quantitative agreement with the experiment in the Wigner crystal regime at T=4.2 K . The diagonal conductivity has the two-peak structure as predicted by Fukuyama and Lee.

Spin-galvanic effect.

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a 'spin-galvanic' effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.

Sampling a terahertz dipole transition with subcycle time resolution

We present a time-resolved technique to measure optical excitation processes with a time resolution shorter than the oscillation period of the exciting light. Our terahertz (THz) experiments fully resolve the polarization dynamics of electrons in semiconductor heterostructures when they are excited by a THz pulse. The time resolution of the polarization enables us to deduce the population dynamics of the excited state, which includes the dynamics of a virtual population in the case of off-resonant excitation.

Hamiltonian and Boundary Conditions for Electrons in Semiconductor Heterostructures

AbstractIn using the equivalent Hamiltonian approach for the simulation of semiconductor heterostructures two aspects are discussed controversially in the present literature: firstly the form of the equivalent Hamiltonian and secondly the appropriate boundary conditions. In this paper both aspects are investigated in the framework of the envelope‐function formalism especially for non‐parabolic band structures. Appropriate forms of the equivalent Hamiltonian and the boundary conditions are suggested and compared. The expected deviations are presented using theoretical calculations for type I and type II heterostructures. In accordance to theoretical evidence, the differences may be important in the case of type II heterostructures. From a theoretical point of view the use of the energy mass should be preferred. But for final evidence further experimental investigations and comparison with calculations based on different models are necessary.

Comment on "Light-induced drift of quantum-confined electrons in semiconductor heterostructures"

A comment on the Letter by M. I. Stockman, L. N. Pandey, T. F. George Phys. Rev. Lett. 65, 3433 (1990).Received 28 January 1991DOI:https://doi.org/10.1103/PhysRevLett.67.156©1991 American Physical Society

Light-induced drift of quantum-confined electrons in semiconductor heterostructures.

The effect of light-induced drift of quantum-confined electrons in semiconductor heterostructures is predicted. The effect manifests itself as the electric current in the heterostructure plane in response to optical excitation with the frequency close, but not exactly equal, to a transition frequency between levels in the heterostructure. The current reverses its direction with a change in the detuning sign, and vanishes if the radiation polarization is normal to the heterostructure plane.

Quantum reflections and inelastic scattering of electrons in semiconductor heterostructures.

We study the contribution of polar optic phonons to the inelastic scattering rate for the two distinct cases of a nonequilibrium electron initially moving parallel and perpendicular to semiconductor heterojunctions. Quantum mechanical reflections from the abrupt change in potential at the heterointerface are found to change the inelastic scattering rate by spatially separating initial and final electron wave functions. In this paper we explore this effect for typical heterojunction geometries and discuss the influence of the phenomenon on the performance of devices, such as field-effect and unipolar hot-electron transistors.

Novel Landau level laser in the quantum Hall regime

A novel laser is proposed for the two-dimensional electrons in semiconductor heterostructures in strong magnetic fields. The laser operates under the two-dimensional Landau quantization with population inversion in the Landau levels. The laser frequency, which is directly tunable by the magnitude of the external magnetic field (H), falls on the far infrared for H∼1 T.

Nonlocal exchange-correlation energy in very inhomogeneous systems: Quasi-two-dimensional electron layers

A simple nonlocal approximation for the exchange-correlation energy of an inhomogeneous electron gas is developed and applied to density distributions representative of quasi-two-dimensional inversion and accumulation layers, and electrons in semiconductor heterostructures. Unlike the usual methods based on the local-density approximation, this technique interpolates correctly between three and two dimensions. We suggest that these usual methods may contain large errors for a range of density distributions typical of quasi-two-dimensional electron layers. We also apply the nonlocal approximation to a simple model of the metal surface.