Two-dimensional Quantum Wire Research Articles

Topological metals constructed by sliding quantum wire arrays

A general strategy of alternated slide construction to craft topological metals is proposed, where there is a relative slide between the odd and even chains in the trivial spinless quantum wire array. Firstly, taking the three-leg ladder as an example, we find that alternated slide can induce a topological phase transition from the normal metal to topological metal phases, which are protected by inversion symmetry. Remarkably, topological metal without nontrivial edge states is found, and the bulk-boundary correspondence breaks down. Secondly, the two-dimensional quantum wire arrays with alternated slide manifests similar physical behaviors. Two types of topological metal phases emerge, where there are gapless bulk bands with and without nontrivial edge states. These results could be confirmed by current experimental techniques.

Reconfigurable quantum logic gates using Rashba controlled spin polarized currents

A reconfigurable logic gate is proposed in a two-dimensional double quantum wire system with a coupling window enabled by a Rashba field. Manipulating the spin states of incoming electrons several quantum logic gates (OR, AND, XOR, CNOT) can be implemented. The logic gate functionality can be switched by tuning the Rashba parameter only. In this context, we investigate suitable configurations of the device region by embedding a quantum point contact located in the coupling region to obtain all four logic gates. The ballistic spin polarized transmission functions are calculated using an effective mass scattering formalism in the framework of a multi-channel, multi-terminal system. Owing its versatility, the proposed logic gate can be integrated in programmable architectures, able to implement both classical and quantum algorithms.

Variational approach to the soft-Coulomb potential in low-dimensional quantum systems

The variational method is used to obtain the ground- and first-excited states for the soft-Coulomb central potential, 1/r2+d2, characterized by a bias distance d, taken into account as a fixed parameter. Applications are presented for spatially indirect excitons, i.e., photo-generated electron-hole (e-h) bound pairs, where the two charges are kept separated in two different regions of a heterostructure. We consider one- or two-dimensional systems, namely, quantum wires or wells, respectively, and compare the results with numerical calculations obtained by finite-difference diagonalization of the Hamiltonian. An explicit example is given for GaAs-based heterostructures.

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.

PERSISTENT SPIN CURRENT IN SPIN-ORBIT COUPLING SYSTEMS IN THE ABSENCE OF AN EXTERNAL MAGNETIC FIELD

The spin-orbit coupling systems with a zero magnetic field are studied under the equilibrium situation, i.e., without a voltage bias. A persistent spin current is predicted to exist under most circumstances, although the persistent charge current and the spin accumulation are identically zero. In particular, a two-dimensional quantum wire is investigated in detail. Surprisingly, a persistent spin current is found to flow along the confined direction, due to the spin precession accompanied by the particle motion. This provides an interesting example of constant spin flowing without inducing a spin accumulation, contrary to common intuition.

Bound state with negative binding energy induced by coherent transport in a two-dimensional quantum wire

In a two-dimensional quantum wire in a perpendicular magnetic field with a smooth embedded repulsive scattering potential we find in the multimode conductance resonances caused by bound states with negative binding energy. These resonances are the counterexamples to well-known dips in the conductance and evanescent states caused by quasibound states of attractive scattering centers in the wire.

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.

Conducting nanowires in insulating ceramics.

Low-dimensional structures, such as microclusters, quantum dots and one- or two-dimensional (1D or 2D) quantum wires, are of scientific and technological interest due to their unusual physical properties, which are quite different from those in the bulk. Here we present a successful method for fabricating conducting nanowire bundles inside an insulating ceramic single crystal by using unidirectional dislocations. A high density of dislocations (10(9) cm(-2)) was introduced by activating a primary slip system in sapphire (alpha-Al2O3 single crystal) using a two-stage deformation technique. Plate specimens cut out from the deformed sapphire were then annealed to straighten the dislocations. Finally, the plates on which metallic Ti was evaporated were heat-treated to diffuse Ti atoms inside sapphire. As a result of this process, Ti atoms segregated along the unidirectional dislocations within about 5 nm diameter, forming unidirectional Ti-enriched nanowires, which exhibit excellent electrical conductivity. This simple technique could potentially to be applied to any crystal, and may give special properties to commonly used materials.

Thermopower of two-dimensional channels and quantum point contacts in amagnetic field

The thermopower of two-dimensional parabolic quantum wires and quantumcontacts in magnetic field is investigated. We obtain a convenient analytic formulafor the thermopower of these structures. The temperature dependence of thethermopower is studied and the influence of the magnetic field on thethermopower is examined. Oscillations in the thermopower are investigated.

Nonballistic spin-field-effect transistor.

We propose a spin-field-effect transistor based on spin-orbit coupling of both the Rashba and the Dresselhaus types. Different from earlier proposals, spin transport through our device is tolerant against spin-independent scattering processes. Hence the requirement of strictly ballistic transport can be relaxed. This follows from a unique interplay between the Dresselhaus and the Rashba coupling; these can be tuned to have equal strengths, leading to k-independent eigenspinors even in two dimensions. We discuss two-dimensional devices as well as quantum wires. In the latter, our setup presents strictly parabolic dispersions which avoids complications from anticrossings of different bands.

Lippmann‐Schwinger equation approach to scattering in quantum wires

AbstractWe apply the Lippmann‐Schwinger equation for obtaining the scattering amplitudes and conductance as a function of Fermi energy for electrons scattering from one and two point defects in a two‐dimensional quantum wire. Further, we discuss the first and higher‐order Born approximation to the scattering wave function and show that keeping five terms in the Born series can lead to a convergent wave function (except for Fermi energy close to the subband energy where it naturally diverges). It has been stated previously that electron transmission through a single point defect in a quantum wire is perfect at every subband minima independent of where the scatterer is located. However, here we demonstrate that perfect transmission at subband minima is strongly affected by the transversal position of the defect. In particular, we show that the perfect transparency effect is modified when the scatterer is located at the nodes of a normal confinement mode.

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.

Electron transport through two-dimensional quantum wires with flanges

We report a detailed theoretical study on electron transport through a quantum wire connected to the source and drain by flanges. Our results show that the connection flange angle affects significantly the electron transmission through the wire. Resonant reflection and trapping are observed in our calculations. The optimal flange angle that allows maximum transmission corresponds to this resonance condition, which is around $18\ifmmode^\circ\else\textdegree\fi{}$ and is essentially independent of the width or length of the wire. This angle changes by a few degrees when the initial energy, the energy spread of the wave packet, or the length of the flange changes. Such a finding can be used to assist the design of high-conductance quantum wires.

Scaling behavior in a quantum wire with scatterers

We study the conductance properties of a straight two-dimensional quantum wire with impurities modeled by $s$-like scatterers. Their presence can lead to strong inter-channel coupling. It was shown that such systems depend sensitively on the number of transverse modes included. Based on a poor man's scaling technique we include the effect of higher modes in a renormalized coupling constant. We therefore show that the low-energy behavior of the wire is dominated by only a few modes, which hence is a way to reduce the necessary computing power. The technique is successfully applied to the case of one and two $s$-like scatterers.

Elastic scattering as a cause of quantum dephasing: the conductance of two-dimensional imperfect conductors

A method is proposed for studying wave and particle transport in disordered waveguide systems of dimension higher than unity by means of exact one-dimensionalization of the dynamic equations in the mode representation. As a particular case, the T=0 conductance of a two-dimensional quantum wire is calculated, which exhibits ohmic behaviour, with length-dependent conductivity, at any conductor length exceeding the electron quasi-classical mean free path. The unconventional diffusive regime of charge transport is found in the range of conductor lengths where the electrons are commonly considered as localized. In quantum wires with more than one conducting channel, each being identified with the extended waveguide mode, the inter-mode scattering is proven to serve as a phase-breaking mechanism that prevents interference localization without real inelasticity of interaction.

TIME-DEPENDENT CALCULATION FOR THE TRANSMISSION COEFFICIENT OF TWO-DIMENSIONAL QUANTUM WIRE STRUCTURES IN THE PRESENCE OF MAGNETIC FIELD

We present a simple and efficient method for calculating transmission coefficient of two-dimensional quantum wire structures in the presence of magnetic field. The time evolution of a wave packet is first obtained by solving the time dependent Schrödinger equation. Transmission coefficient is then extracted from the wave function by an intergal transform. This method is easier to implement than traditional time-independent methods such as mode matching method and it can be used to study the time evolution of wave functions for systems with arbitrary shape.

Semiconductor Switching Devices. .Future Trends

A variety of semiconductor devices and circuits have been successfully developed usingconduction properties of electrons and holes in a number of elemental and compound semiconductors.Carriers confinement in a potential well, formed out of a thin layer of lower band gap materialsandwitched between two layers of a higher band gap material, has been extended from one to two andthree dimensions. Resultant of two-dimensional carrier sheet, quantum wire and quantum dot havingdiscrete energy levels arising out of quantisation are being presently explored for possible device applications. A number of devices have been fabricated using resonant tunneling across a thin potentialbarrier. This has opened up several newer possibilities of using such structures for various electronicand optoelectronic devices and circuits applications as tunneling is relatively faster than conductionprocess. While looking into the interband tunneling between two quantum dots, possibility of a singleelectron switching has also been examined carefully. The idea of a single electron switching isconceptually being extended from quantl,lm dots to molecules and atoms ultimately. Simulations basedon transmission of electrons through a chain of molecules and atoms have shown that tens of THz speed and functional device density 1012 devices/mm2 are possible with such schemes. Devices basedon atom relay transistor (ART) will be ultimate in its performance of switching speed. A brief onpresent-day situation followed by future proposals of fast switching devices for informationelectronics has been discussed.

Resonant light scattering induced by Coulomb interaction in semiconductor microstructures

We consider inelastic light scattering by electron excitations in a two-dimensional electron system, which is induced by the Coulomb coupling between the Fermi sea and the interband excitons. The distinguishing feature of this mechanism of inelastic light scattering is a strong enhancement of the intensity in resonance with high two-dimensional subbands. Such resonant behaviour of the light scattering was observed in recent experiments for a two-dimensional electron plasma, quantum wires, and dots. In this paper we focus on the effect of the normal electric field on the intensity of the light scattering. We show that the interference between various virtual processes leads to the specific electric field dependence of the intensity. The intensity of the light scattering by charge-density excitations as a function of the normal electric field has a maximum at non-zero field. This maximum arises from the Coulomb direct interaction between the polarized exciton and the electron gas. Light scattering by spin-density excitations is assisted by the exchange interaction, and its intensity decreases with increasing electric field.

Current conservation in two-dimensional ac transport

The electric current conservation in a two-dimensional quantum wire under a time-dependent field is investigated. Such a conservation is obtained as the global density of states contribution to the emittance is balanced by the contribution due to the internal charge response inside the sample. However when the global partial density of states is approximately calculated using scattering matrix only, correction terms are needed to obtain precise current conservation. We have derived these corrections analytically using a specific two-dimensional system. We found that when the incident energy E is near the first subband, our result reduces to the one-dimensional result. As E approaches to the nth subband with n>1, the correction term diverges. This explains the systematic deviation to precise current conservation observed in a previous numerical calculation.

Weakly nonlinear quantum transport: An exactly solvable model

We have studied the weakly non-linear quantum transport properties of a two-dimensional quantum wire which can be solved exactly. The non-linear transport coefficients have been calculated and interesting physical properties revealed. In particular we found that as the incoming electron energy approaches a resonant point given by energy $E=E_r$, where the transport is characterized by a complete reflection, the second order non-linear conductance changes its sign. This has interesting implications to the current-voltage characteristics. We have also investigated the establishment of the gauge invariance condition. We found that for systems with a finite scattering region, correction terms to the theoretical formalism are needed to preserve the gauge invariance. These corrections were derived analytically for this model.