Polar interface and surface optical vibration spectra in multi-layer wurtzite quantum wires： transfer matrix method

The polar interface optical （IO） and surface optical （SO） phonon modes and the corresponding Froehlich electron phonon-interaction Hamiltonian in a freestanding multi-layer wurtzite cylindrical quantum wire （QWR） are derived and studied by employing the transfer matrix method in the dielectric continuum approximation and Loudon＇s uniaxial crystal model. A numerical calculation of a freestanding wurtzite GaN/AlN QWR is performed. The results reveal that for a relatively large azimuthal quantum number m or wave-number kz in the free z-direction, there exist two branches of IO phonon modes localized at the interface, and only one branch of SO mode localized at the surface in the system. The degenerating behaviours of the IO and SO phonon modes in the wurtzite QWR have also been clearly observed for a small kz or m. The limiting frequency properties of the IO and SO modes for large kz and m have been explained reasonably from the mathematical and physical viewpoints. The calculations of electron-phonon coupling functions show that the high-frequency IO phonon branch and SO mode play a more important role in the electron phonon interaction.

Band structure and absorption coefficient in GaN/AlGaN quantum wires

The band structures of rectangular GaN/AlGaN quantum wires are modeled by using a parabolic effective-mass theory. The absorption coefficients are calculated in a contact-density matrix approach based on the band structure. The results obtained indicate that the peak absorption coefficients augment with the increase of the injected carrier density, and the optical gain caused by interband transition is polarization anisotropic. For the photon energy near 1.55 eV, we can obtain relatively large peak gain. The calculations support the previous results published in the recent literature.

Mobility limited by cluster scattering in ternary alloy quantum wires

The mobility limited by cluster scattering in ternary alloy semiconductor quantum wire （QWR） is theoretically inves- tigated under Born approximation. We calculate the screened mobility due to clusters （high indium composition lnGaN） scattering in the InxGal_xN QWR structure. The characteristics of the cluster scattering mechanism are discussed in terms of the indium composition of clusters, the one-dimensional electron gas （1DEG） concentration, and the radius of QWR. We find that the density, breadth of cluster, and the correlation length have a strong effect on the electron mobility due to cluster scattering, Finally, a comparison of the cluster scattering is made with the alloy-disorder scattering. It is found that the cluster scattering acts as a significant scattering event to impact the resultant electron mobility in ternary alloy QWR.

Spin texturing in quantum wires with Rashba and Dresselhaus spin-orbit interactions and in-plane magnetic field

In this work, we investigate the effects of interplay of spin-orbit interaction and in-plane magnetic fields on the electronic structure and spin texturing of parabolically confined quantum wire. Numerical results reveal that the competing effects between Rashba and Dresselhaus spin--orbit interactions and the external magnetic field lead to a complicated energy spectrum. We find that the spin texturing owing to the coupling between subbands can be modified by the strength of spin- orbit couplings as well as the magnitude and the orientation angle of the external magnetic field.

Influence of InAs deposition thickness on the structural and optical properties of InAs quantum wires

The influence of InAs deposition thickness on the structural and optical properties of InAs/InA1As quantum wires （QWR） superlattices （SLS） was studied. The transmission electron microscopy （TEM） results show that with increasing the InAs deposited thickness, the size uniformity and spatial ordering of InAs QWR SLS was greatly improved, but threading dislocations initiated from InAs nanowires for the sample with 6 monolayers （MLs） InAs deposition. In addition, the zig-zag features along the extending direc- tion and lateral interlink of InAs nanowires were also observed. The InAs nanowires, especially for the first period, were laterally compact. These structural features may result in easy tunneling and coupling of charge carders between InAs nanowires and will hamper their device applications to some extent. Some suggestions are put forward for further improving the uniformity of the stacked InAs QWRs, and for suppressing the formation of the threading dislocations in InAs QWR SLS.

Effects of electron-and impurity-ion-LO phonon couples on the impurity states in cylindrical quantum wires

The variational method and the effective mass approximation are used to calculate the phonon effects on the hydrogenic impurity states in a cylindrical quantum wire with finite deep potential by taking both the couplings of the electron-confined bulk longitudinal optical（LO） phonons and the impurity-ion-LO phonons into account.The binding energies and the phonon contributions are calculated as functions of the transverse dimension of the quantum wire.The results show that the polaronic effect induced by the electron-LO phonon coupling and the screening effect induced by the impurity-ion-LO phonon coupling tend to compensate each other and the total effects reduce the impurity binding energies.

Binding energy of the donor impurities in GaAs-Ga_(1-x)Al_xAs quantum well wires with Morse potential in the presence of electric and magnetic fields

The behavior of a donor in the GaAs–GaAlAs quantum well wire represented by the Morse potential is examined within the framework of the effective-mass approximation. The donor binding energies are numerically calculated for with and without the electric and magnetic fields in order to show their influence on the binding energies. Moreover, how the donor binding energies change for the constant potential parameters(De, re, and a) as well as with the different values of the electric and magnetic field strengths is determined. It is found that the donor binding energy is highly dependent on the external electric and magnetic fields as well as parameters of the Morse potential.

Cyclotron Resonance of Free Magnetopolaron in Quantum Well Wires

The cyclotron resonance of magnetopolaron in quantum well wires (QWWs) have been studied with the use of variational solutions to the effective mass equation. The results show that both the abso- lute value of the electron-phonon interaction energy and the cyclotron resonance frequency de- crease with the increase of the sizes of QWWs, and also that the cyclotron resonance frequency in- creases with the external magnetic field.

Excitonic Eigenstates of Disordered Semiconductor Quantum Wires:Adaptive Wavelet Computation of Eigenvalues for the Electron-Hole Schrodinger Equation

A novel adaptive approach to compute the eigenenergies and eigenfunctions of the two-particle(electron-hole)Schrodinger equation including Coulomb attraction is presented.As an example,we analyze the energetically lowest exciton state of a thin one-dimensional semiconductor quantum wire in the presence of disorder which arises from the non-smooth interface between the wire and surrounding material.The eigenvalues of the corresponding Schrodinger equation,i.e.,the onedimensional exciton Wannier equation with disorder,correspond to the energies of excitons in the quantum wire.The wavefunctions,in turn,provide information on the optical properties of the wire.We reformulate the problem of two interacting particles that both can move in one dimension as a stationary eigenvalue problem with two spacial dimensions in an appropriate weak form whose bilinear form is arranged to be symmetric,continuous,and coercive.The disorder of the wire is modelled by adding a potential in the Hamiltonian which is generated by normally distributed random numbers.The numerical solution of this problem is based on adaptive wavelets.Our scheme allows for a convergence proof of the resulting scheme together with complexity estimates.Numerical examples demonstrate the behavior of the smallest eigenvalue,the ground state energies of the exciton,together with the eigenstates depending on the strength and spatial correlation of disorder.

Spin current and its heat effect in a multichannel quantum wire with Rashba spin-orbit coupling

Using the perturbation method, we theoretically study the spin current and its heat effect in a multichannel quantum wire with Rashba spin-orbit coupling. The heat generated by the spin current is calculated. With the increase of the width of the quantum wire, the spin current and the heat generated both exhibit period oscillations with equal amplitudes. When the quantum-channel number is doubled, the oscillation periods of the spin current and of the heat generated both decrease by a factor of 2. For the spin current js,xy, the amplitude increases with the decrease of the quantum channel; while the amplitude of the spin current js,yx remains the same. Therefore we conclude that the effect of the quantum-channel number on the spin current js,xy is greater than that on the spin current js,yx. The strength of the Rashba spin-orbit coupling is tunable by the gate voltage, and the gate voltage can be varied experimentally, which implies a new method of detecting the. spin current. In addition, we can control the amplitude and the oscillation period of the spin current by controlling the number of the quantum channels. All these characteristics of the spin current will be very important for detecting and controlling the spin current, and especially for designing new spintronic devices in the future.

Electron transport for a laser-irradiated quantum channel with Rashba spin-orbit coupling

We investigate theoretically the electron transport for a two-level quantum channel （wire） with Rashba spinorbit coupling under the irradiation of a longitudinally-polarized external laser field at low temperatures. Using the method of equation of motion for Keldysh nonequilibrium Green function, we examine the time-averaged spin polarized conductance for the system with photon polarization parallel to the wire direction. By analytical analysis and a few numerical examples, the interplay effects of the external laser field and the Rashba spin-orbit coupling on the spinpolarized conductance for the system are demonstrated and discussed. It is found that the longitudinally-polarized laser field can adjust the spin polarization rate and produce some photon sideband resonances of the conductance for the system.

Tunneling conductance in quantum wire/insulator/d_(x2-y2) + id_(xy) mixed wave superconductor junctions

Using the extended Blonder-Tinkham-Klapwijk （BTK） theory, this paper calculates the tunnelling conductance in quantum wire/insulator/dx2-y2 ＋ idly mixed wave superconductor （q/I/dx2-y2 ＋ idly） junctions. That is different from the case in d- and p-wave superconductor junctions. When the angle α between a-axis of the dx2-y2 wave superconductor and the interface normal is π/4, there follows a rather distinctive tunnelling conductance. The zero-bias conductance peak （ZBCP） may or may not appear in the tunnelling conductance. Both the interface potential z and the quasi-particle lifetime factor F are smaller, there is no ZBCP. Otherwise, the ZBCP will appear. The position of bias conductance peak （BCP） depends strongly on the amplitude ratio of two components for dx2-y2 ＋ idxy mixed wave. The low and narrow ZBCP may coexist with the BCP in the tunnelling conductance. Using those features in the tunnelling conductance of q/I/dx2-y2 ＋ idxy junctions, it can distinguish dx2-y2 ＋ idxy mixed wave superconductor from d- and p-wave one.

Persistent spin current in a quantum wire with weak Dresselhaus spin-orbit coupling

The spin current in a parabolically confined semiconductor hcterojunction quantum wire with Drcsselhaus spinorbit coupling is theoretically studied by using the perturbation method. The formulae of the elements for linear and angular spin current densities are derived by using the recent definition for spin current based on spin continuity equation. It is found that the spin current in this Dresselhaus spin-orbit coupling quantum wire is antisymmetrical, which is different from that in Rashba model due to the difference in symmetry between these two models. Some numerical examples for the result are also demonstrated and discussed.

Electron Raman scattering in semiconductor quantum well wire of cylindrical ring geometry

We study the electron states and the differential cross section for an electron Raman scattering process in a semi- conductor quantum well wire of cylindrical ring geometry. The electron Raman scattering developed here can be used to provide direct information about the electron band structures of these confinement systems. We assume that the system grows in a GaAs/Al0.35Ga0.65As matrix. The system is modeled by considering T = 0 K and also a single parabolic con- duction band, which is split into a sub-band system due to the confinement. The emission spectra are discussed for different scattering configurations, and the selection rules for the processes are also studied. Singularities in the spectra are found and interpreted.

Thermal conductance in a two-slit quantum waveguide

Using the scattering-matrix method, we investigate the thermal conductance in a two-slit quantum waveguide at low temperature. The results show that the total thermal conductance decreases monotonically with temperature increasing. Moreover, we find that the behaviours of the thermal conductance versus temperature are different for different types of slits.

Resonant Transmission Through Two Impurities in a Narrow Quantum Wire

We study electron transmission through two impurities in a narrow quantum wire by solving Dyson's equations for single electron Green functions. We have verified that, for the delta-function potential of two impurities, the Green function can be factorized into a product of the ‘free’ Green function and current transmission amplitude. Meanwhile Green function and current transmission amplitude obey Fisher-Lee's relation. An analytical expression of the electron transmission amplitude for intrasubband and intersubband is obtained as a function of Fermi energy and the distance between two impurities. The resonant behavior of the current transmission amplitude are detail discussed.

Spin texturing in a parabolically confined quantum wire with Rashba and Dresselhaus spin-orbit interactions

In this study, we investigate theoretically the effect of spin-orbit coupling on the energy level spectrum and spin texturing of a quantum wire with a parabolic confining potential subjected to the perpendicular magnetic field. Highly accurate numerical calculations have been carried out using a finite element method. Our results reveal that the interplay between the spin-orbit interaction and the effective magnetic field significantly modifies the band structure, producing additional subband extrema and energy gaps. Competing effects between external field and spin-orbit interactions introduce comp｜ex features in spin texturing owing to the couplings in energy subbands. We obtain that spatia~ modulation of the spin density along the wire width can be considerably modified by the spin-orbit coupling strength, magnetic field and charge carrier concentration.

Spin-Dependent Transport through a Quantum Wire with Magnetic Impurity

Spin-dependent transmission coefficients as a function of Fermi energy is calculated for electron scattering from magnetic impurity in a narrow quantum wire, in which spin-exchange interaction between conducting electron and the impurity leads to exchange spin-flip scattering. Transmission in the spin-flipped channels is explicitly calculated. It has been found that spin-up and spin-down transmission coefficients for intrasubband and intersubband enhance Fano-resonance profiles, which have asymmetric behaviors, whenever Fermi energy crosses bottom of every subband below. Meanwhile due to dephasing of electron wave caused by spin-flip scattering, the entanglement between spin states of electron and magnetic impurity obviously destroys the global step structure of quantized conductance and suppresses the height of the conductance step.

Electrical-Controlled Transport for Surface States in a Dirac Semimetal Quantum Wire

The transport properties of a Dirac semimet^l quantum wire with two side gates are theoretically studied by adopting the lattice Green function method. It is found that a residual conductance quantum contributed from the surface states can be switched on or off by tuning the electron energy or the side gates voltage. This ideal switching effect for the surface Dirac electron results from the transversal quantum confinement of the quantum wire in combination with the electrostatic potential induced by the side gates. These findings may provide useful guidance for designing all-electrical topological nanoelectronic devices.

Supercell Approach in Tight-Binding Calculation of Si and Ge Nanowire Bandstructures

Energy bandstructures of [100] oriented Si and Ge quantum nanowires with various cross-sections are calculated by using the sp^3d^5s^＊ tight-binding model with a supercell approach. Results are compared with those obtained by the first principles method （i.e., density functional theory, or DFT）. The differences in the bandstructure between silicon and germanium nanowires are analysed and it is shown that germanium keeps indirect-bandgap and the silicon nanowire along the [100] direction becomes direct-bandgap when the wire diameter shrinks. It is shown in comparison with the available experimental data that the tight-binding method is adequate in predicting the bandstructure parameters relevant to the carrier transport in mesoscopic nanowire devices and is far superior to the DFT method in terms of computational cost.