Electrons In Quantum Wires Research Articles

Observation of orientation- and k-dependent Zeeman spin-splitting in hole quantum wires on (100)-oriented AlGaAs/GaAs heterostructures

In this paper, We study the Zeeman spin-splitting in hole quantum wires oriented along the [011] and crystallographic axes of a high mobility undoped (100)-oriented AlGaAs/GaAs heterostructure. Our data show that the spin-splitting can be switched ‘on’ (finite g*) or ‘off’ (zero g*) by rotating the field from a parallel to a perpendicular orientation with respect to the wire, and the properties of the wire are identical for the two orientations with respect to the crystallographic axes. We also find that the g-factor in the parallel orientation decreases as the wire is narrowed. This is in contrast to electron quantum wires, where the g-factor is enhanced by exchange effects as the wire is narrowed. This is evidence for a k-dependent Zeeman splitting that arises from the spin- nature of holes.

Resonant tunneling of electrons in quantum wires (Review)

We consider resonant electron tunneling in various nanostructures, including single-wall carbon nanotubes, molecular transistors, and quantum wires, formed in two-dimensional electron gas. The review starts with a textbook description of resonant tunneling of noninteracting electrons through a double-barrier structure. The effects of electron–electron interaction in sequential and resonant electron tunneling are studied by using the Luttinger liquid model of electron transport in quantum wires. The experimental aspects of the problem (fabrication of quantum wires and transport measurements) are also considered. The influence of vibrational and electromechanical effects on resonant electron tunneling in molecular transistors is discussed.

The Nonlinear Absorption Coefficient of Strong Electromagnetic Waves Caused by Confined Electrons in Quantum Wires

The Nonlinear Absorption Coefficient of Strong Electromagnetic Waves Caused by Confined Electrons in Quantum Wires

Quantum teleportation of electrons in quantum wires with surface acoustic waves

We propose and numerically simulate a semiconductor device based on coupled quantum wires, suitable for deterministic quantum teleportation of electrons trapped in the minima of surface acoustic waves. We exploit a network of interacting semiconductor quantum wires able to provide the universal set of gates for quantum information processing with the qubit defined by the localization of a single electron in one of two coupled channels. The numerical approach is based on a time-dependent solution of the three-particle Schr\"odinger equation. First, a maximally entangled pair of electrons is obtained via Coulomb interaction between carriers in different channels. Then, a complete Bell-state measurement involving one electron from this pair and a third electron is performed. Finally, the teleported state is reconstructed by means of local one-qubit operations. The large estimated fidelity explicitly suggests that an efficient teleportation process could be reached in an experimental setup.

Spin–orbit coupling in GaxIn1−xAs/InP two-dimensional electron gases and quantum wire structures

In this work, the effect of spin–orbit coupling in two-dimensional electron gases and quantum wire structures is discussed. First, the theoretical framework is introduced including spin–orbit coupling due to structural inversion asymmetry, the so-called Rashba effect, as well as the Dresselhaus term. The latter originates from bulk inversion asymmetry. With regard to wire structures, special attention is devoted to the influence of the particular shape of the confinement potential on the energy spectrum. As a model system GaxIn1−xAs/InP heterostructures are chosen, where different thicknesses of the strained Ga0.23In0.77As channel layer were introduced, in order to adjust the strength of the spin–orbit coupling. Hall bar structures as well as sets of identical wires with different widths were prepared. In two-dimensional electron gases, the strength of the spin–orbit coupling was extracted by analyzing the characteristic beating pattern in the Shubnikov–de Haas oscillations. In addition, the weak antilocalization was utilized to obtain information on the spin–orbit coupling. It is shown that for decreasing width of the strained layer the Rashba effect, which dominates in our layer systems, is increased. This behavior is attributed to the larger interface contribution if the electron wavefunction is strongly confined. The measurements on the wire structures revealed a transition from weak antilocalization to weak localization if the wire width is decreased. This effect is attributed to an enhanced spin diffusion length for strongly confined systems.

Magnetoconductance of interacting electrons in quantum wires: Spin density functional theory study

We present a systematic quantitative description of the magnetoconductance of split-gate quantum wires focusing on formation and evolution of the odd spin-resolved conductance plateaus. We start from the case of spinless electrons where the calculated magnetoconductance in the Hartree approximation shows the plateaus quantized in units of 2e 2 /h separated by transition regions, whose width grows as the magnetic field is increased. We show that the transition regions are related to the formation of the compressible strips in the middle of the wire occupied by electrons belonging to the highest spin-degenerate subband. Accounting for the exchange and correlation interactions within the spin density functional theory DFT leads to the lifting of the spin degeneracy and formation of the spin-resolved plateaus at odd values of e 2 /h. The most striking feature of the magnetoconductance is that the width of the odd conductance steps in the spin DFT calculations is equal to the width of the transition intervals between the conductance steps in the Hartree calculations. A detailed analysis of the evolution of the Hartree and the spin DFT subband structure provides an explanation of this finding. Our calculations also reveal the effect of the collapse of the odd conductance plateaus for lower fields. We attribute this effect to the reduced screening efficiency in the confined wire geometry when the width of the compressible strip in the center becomes much smaller than the extent of the wave function. A detailed comparison to the experimental data demonstrates that the spin DFT calculations reproduce not only qualitatively but also quantitatively all the features observed in the experiment. This includes the dependence of the width of the odd and even plateaus on the magnetic field as well as the estimation of the subband index corresponding to the last resolved odd plateau in the magnetoconductance.

Functional methods in the theory of magnetoimpurity states of electrons in quantumwires

Functional methods are used to study magnetoimpurity states of electrons innanostructures. The Keldysh formalism is applied to these states. The theory isillustrated using a quantum wire sample with impurity atoms capable of localizingelectrons in a magnetic field. The characteristics of magnetoimpurity states ofelectrons in the wire are calculated using the model of a Gaussian separable potential.

Pinning structure of dimensional phonon resonance in quantum wires

The dimensional phonon resonances in dimensionally-quantum semiconductor wires are considered. It is supposed that the absorption of light is caused by electron transitions between dimensionally-quantum subbands in potential wells with the participation of long-wavelength phonons. The influence of resonance electron-phonon interaction on a spectrum of one-dimensional electrons in quantum wires is investigated. It is shown that the spectrum contains two branches and also the splitting of the peak of the dimensional phonon resonance into two components; the distance between them is determined by the resonance electron-phonon interaction.

0.7 Structure and Zero Bias Anomaly in Ballistic Hole Quantum Wires

We study the anomalous conductance plateau around G=0.7(2e2/h) and the zero bias anomaly in ballistic hole quantum wires with respect to in-plane magnetic fields applied parallel B parallel and perpendicular B perpendicular to the quantum wire. As seen in electron quantum wires, the magnetic fields shift the 0.7 structure down to G=0.5(2e2/h) and simultaneously quench the zero bias anomaly. However, these effects are strongly dependent on the orientation of the magnetic field, owing to the highly anisotropic effective Landé g-factor g* in hole quantum wires. Our results highlight the fundamental role that spin plays in both the 0.7 structure and zero bias anomaly.

Roughness effect on electron transport through quantum wires

AbstractIn the present work we study, by means of numerical simulations, the coherent propagation of electrons in quantum wires and focus on the effect of the introduction of roughness along the channel. We study the electron evolution both in single and coupled quantum wires in the case where the electron freely move through the channel and in the case where transport is assisted by surface acoustic waves. It has been shown that systems of coupled quantum wires can realize a complete set of quantum logic gates.However, its physical implementation is severely limited by the spreading of the wave packet and by the presence of interface roughness. Our results show that the potential of the surface acoustic wave succeeds in confining the electron wavefunction and in driving it along the channels also when interface roughness is introduced. This suggests that electron transport assisted by surface acoustic waves could be a very efficient way to realize quantum solid state devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum transport in one-dimensional GaAs hole systems

In many advanced semiconductor devices, the physical dimensions are sufficiently small that quantum physics becomes important in determining the device behaviour. A celebrated example is the quantum wire, where in the absence of scattering the conductance is quantised in units of 2e²/h. Although electron quantum wires have been studied extensively for almost two decades, the development of hole quantum wires has been a significant challenge, limiting studies of hole-based devices. Here we review our recent work on hole quantum wires, and show how they can be used to probe the spin properties of hole systems. The ability to fabricate ballistic quantum wires, and control their spin properties using electrical gate biases, may have implications for future spintronic devices.

Absorption Coefficient of Weak Electromagnetic Waves Caused by Confined Electrons in Quantum Wires

Absorption Coefficient of Weak Electromagnetic Waves Caused by Confined Electrons in Quantum Wires

Spin relaxation of “upstream” electrons in quantum wires: Failure of the drift diffusion model

The classical drift diffusion (DD) model of spin transport treats spin relaxation via an empirical parameter known as the ``spin diffusion length.'' According to this model, the ensemble averaged spin of electrons drifting and diffusing in a solid decays exponentially with distance due to spin dephasing interactions. The characteristic length scale associated with this decay is the spin diffusion length. The DD model also predicts that this length is different for ``upstream'' electrons traveling in a decelerating electric field than for ``downstream'' electrons traveling in an accelerating field. However, this picture ignores energy quantization in confined systems (e.g., quantum wires) and therefore fails to capture the nontrivial influence of subband structure on spin relaxation. Here we highlight this influence by simulating upstream spin transport in a multisubband quantum wire, in the presence of D'yakonov-Perel' spin relaxation, using a semiclassical model that accounts for the subband structure rigorously. We find that upstream spin transport has a complex dynamics that defies the simplistic definition of a ``spin diffusion length.'' In fact, spin does not decay exponentially or even monotonically with distance, and the drift diffusion picture fails to explain the qualitative behavior, let alone predict quantitative features accurately. Unrelated to spin transport, we also find that upstream electrons undergo a ``population inversion'' as a consequence of the energy dependence of the density of states in a quasi-one-dimensional structure.

Single Electron Transport and Entanglement Induced by Surface Acoustic Waves versus Free Ballistic Propagation in Coupled Quantum Wires

In this work we study the coherent propagation of electrons in quantum wires driven by surface acoustic waves, as a part of a feasibility study on a coupled quantum wires device, able to realize the basic operations needed for quantum computing. We demonstrate that the introduction of the surface acoustic wave is able to improve the devices performance. In particular, single- and double-qubit quantum gates have been simulated and better results are obtained, with respect to the free electron propagation. A further analysis reveals a strong reduction of the final undesired spatial entanglement and confirms the effectiveness of the dynamics driven by surface acoustic wave.

Energy relaxation studies in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As two-dimensional electron gases and quantum wires

We present Joule heating measurements carried out over a wide temperature range on two-dimensional electron gases (2DEGs) and quantum wires of varying widths fabricated in an In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As heterostructure system that has a 25 nm wide In0.53Ga0.47As quantum well region. The power dissipated per electron is extracted and the electron–phonon coupling processes in these systems are studied. The temperature decay of the power loss at the 2DEG points towards unscreened piezoelectric coupling to the acoustic modes over temperatures of 1–30 K, with boundary scattering in the ohmic contacts gaining importance at very low temperatures. In the wires, we observe different behaviour and the effect of wire width and carrier density on the observed energy-loss rates. Possible phonon confinement and exponential suppression in these structures are also looked at.

DYNAMICAL CONDUCTIVITY OF LOW-DIMENSIONAL SYSTEMS

A calculation of dynamical conductivity is performed for low-dimensional systems, by taking into account the screening of field. Our calculation is valid for all value of wave vector and frequency. The Drude conductivity of three, two and one-dimensional free electron gas, layered electron gas and quantum wire system can be deduced from our calculation. However, our calculation suggests that the use of Drude formulae of conductivity to explain experimental result on microwave and infra-red conductivity, in long wave length limit, can be highly erroneous in case of low-dimensional system that offer larger value of relaxation time. It is found that; (i) screening of a dynamical field becomes less significant on reduction in dimensionality and (ii) unlike the case of three dimensional electron gas, transverse electric field cannot excite collective excitation modes (penetration depth cannot be defined) in a two-dimensional electron gas and quantum wire system. In comparison with prior reported calculation ours is more rigorous calculation as it includes the possibility of propagation of collective excitation modes in all direction. The plasmons in a low-dimensional system cannot be excited for negligibly small value of momentum transfer.

Characteristics of transmission of electrons in a quantum wire tangentially attached to a superconductor ring threaded by magnetic flux

We present numerical investigations of the transmission properties of electrons in a normal quantum wire tangentially attached to a superconductor ring threaded by magnetic flux. A point scatterer with a δ -function potential is placed at node to model scattering effect. We find that the transmission characteristics of electrons in this structure strongly depend on the normal or superconducting state of the ring. The transmission probability as a function of the energy of incident electrons, in the case of a superconductor ring threaded by one quantum magnetic flux, emerges one deep dip, imposed upon the first broad bump in spectrum. This intrinsic conductance dip originates from the superconductor state of the ring. When increasing the magnetic flux from one quantum magnetic flux to two, the spectrum shifts toward higher energy region in the whole. This conductance dip accordingly shifts and appears in the second bump. In the presence of a point-scatterer at the node, the spectrum is substantially modified. Based on the condition of the formation of the standing wave functions in the ring and the broken of the time-reserve symmetry of Schrodinger equation after switching magnetic flux, the characteristics of transmission of electrons in this structure can be well understood.

Spontaneous spin polarization of electrons in quantum wires

The quantum conductance staircase of the one-dimensional (1D) channel is analyzed for a weak filling of the lower 1D subbands, when the exchange electron-electron interaction of carriers dominates over their kinetic energy. Main attention is paid to considering the behavior of the “0.7(2e 2/h)” feature split off from the first quantum step, which is identified as a result of the spontaneous spin polarization of the 1D electronic gas through exchange interaction at zero magnetic field. The critical linear concentration of electrons above which fully polarized electron gas starts to depolarize, with the resulting evolution of the split-off substep height from e 2/h to 2e 2/h, is determined within the framework of phenomenological theory. Moreover, the temperature dependence of the height of this substep at the range 0.5(2e 2/h)–0.75(2e 2/h), resulting from the partial depolarization of the electron gas near the 1D subband bottom, is predicted. The quantum-mechanical consideration carried out analytically in the context of the Hartree-Fock-Slater approximation with localized exchange potential shows that consideration of the electron-electron interaction in a quantum wire with an arbitrary carrier density leads to spontaneous polarization of the quasi-1D electronic gas in zero magnetic field at small linear concentrations of carriers.

Etching trenches to effectively create electron quantum wires for single-electron-transistor applications

By self-consistently solving two-dimensional Schrödinger and Poisson equations, we have studied electron quantum wires formed by etching two shallow trenches in GaAs/InGaAs/AlGaAs and GaAs/AlGaAs heterostructures for single-electron-transistor applications. Two-dimensional electron gases (2DEGs) form in the InGaAs layer in GaAs/InGaAs/AlGaAs and at the GaAs/AlGaAs interface in GaAs/AlGaAs heterostructure, the two etched trenches deplete the carriers of the 2DEGs underneath and thus effectively define an electron quantum wire. It has been determined that two etched trenches with a depth of 50 nm and a spatial separation of 250 nm are optimal to create an electron quantum wire in a GaAs/InGaAs/AlGaAs heterostructure with a cross section of 80×10 nm2 and a peak electron concentration of 1.5×1018 cm−3 at 4.2 K.

Influence of Nonequilibrium LO Phonons on High-frequency Performance of One-dimensional Hot Electrons in Quantum Wires of Polar Semiconductors

Small-signal ac transport of degenerate one-dimensional hot electrons in quantum wires of GaAs and In0.53Ga0.47As is studied for lattice temperatures of 77 K and 300 K. The carrier energy loss via polar optic phonons and momentum losses via polar optic phonons, acoustic phonons and ionized impurities are included in the calculations. Alloy disorder scattering in momentum loss is additionally incorporated for (In,Ga)As. The consideration of nonequilibrium optical phonons or hot phonons is found to enhance the 3dB cut-off frequency (f3dB) considerably, where the ac mobility falls to 0.707 of its low frequency value. f3dB is generally higher for (In,Ga)As quantum wire than for GaAs.