One-dimensional Semiconductor Quantum Wires Research Articles

Nonperturbative approach for the electronic Casimir-Polder effect in a one-dimensional semiconductor

We present the electronic Casimir-Polder effect for a system consisting of two impurities on a one-dimensional semiconductor quantum wire. Due to the charge transfer from the impurity to a one-dimensional conduction band, the impurity states are dressed by a virtual cloud of the electron field. The attractive electronic Casimir force arises due to the overlap of the virtual clouds. The Van Hove singularity causes the persistent bound state (PBS) to appear below the band edge even when the bare impurity state energy is above the band edge. Since the decay rate of the virtual cloud of the PBS in space is small, the Casimir force can be of a very long range. While the overlap of the electronic virtual cloud is consistent with the idea of the radiation reaction, it is shown that also vacuum fluctuations play a role in the electronic Casimir force as a result of the fermionic anticommutation relations. We introduce an effective mass, different from the effective band mass of the conduction band, which is associated with the distance of the energy of the PBS from the band edge where the Van Hove singularity is located and determines the decay rate of the electronic Casimir-Polder force.

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

AbstractA novel adaptive approach to compute the eigenenergies and eigenfunctions of the two-particle (electron-hole) Schrödinger 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 Schrödinger equation, i.e., the one-dimensional 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.

Electron trapping in a one-dimensional semiconductor quantum wire with multiple impurities

We demonstrate the trapping of a conduction electron between two identical adatom impurities in a one-dimensional semiconductor quantum-dot array system (quantum wire). Bound steady states arise even when the energy of the adatom impurity is located in the continuous one-dimensional energy miniband. The steady state is a realization of the bound state in continuum (BIC) phenomenon first proposed by von Neuman and Wigner [Phys. Z. 30, 465 (1929)]. We analytically solve the dispersion equation for this localized state, which enables us to reveal the mechanism of the BIC. The appearance of the BIC state is attributed to the quantum interference between the impurities. The Van Hove singularity causes another type of bound state to form above and below the band edges, which may coexist with the BIC.

Spin filtering implemented through Rashba spin-orbit coupling and weak magnetic modulations

We present two theoretical schemes for spin filters in one-dimensional semiconductor quantum wires with spatially modulated Rashba spin-orbit couplings (SOCs) and weak magnetic potentials. In the first scheme, the SOC is periodic and the weak magnetic potential is applied uniformly along the wire. Full spin polarizations with opposite signs are obtained within two separated energy intervals. In the second scheme, the weak magnetic potential is periodic while the SOC is uniform. An ideal negative/positive switching effect for spin polarization is realized by tuning the strength of SOC. The roles of SOC, magnetic potential, and their joint action on the spin filters are analyzed.

Absorption and gain spectra of optically excited semiconductor quantum wires: Effects of Coulomb correlation and screening

We present a systematic study on optical spectra of one-dimensional semiconductor quantum wires by solving the semiconductor Bloch equations. Many-body Coulomb interactions are substantially enhanced by strong geometrical confinement in quantum wire systems. Our theoretical calculation reveals that phase-space filling has strong influence on the absorption spectra. Increasing carrier density bleaches the exciton peak in low-density regime, and induces considerable optical gain in very high density. Band-gap renormalization is partially cancelled by reduction of exciton binding energy, which increases as the cross-section of quantum wire decreases.

Biexcitonic effect on optical nutation in semiconductor quantum wires

We have proposed a simplified theoretical model to examine the optical nutation in one-dimensional semiconductor quantum wires. We explored the role of excitons, biexcitons, and the effect of confinement in enhancing transient optical processes in such semiconductor nanostructures. We apply the time dependent perturbation technique to the coherent radiation-semiconductor interaction for a three-level system in the near band gap resonant excitation regime. We have incorporated the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated theoretically and the role of biexciton density on the polarization is analyzed. Numerical estimations of refractive and absorptive optical nutation have been made for a GaAs∕AlxGa1−xAs quantum wire duly shined by femtosecond pulse Ti: sapphire laser. Our results agree qualitatively with available experimental observations.

Many-body effects on excitonic optical properties of photoexcited semiconductor quantum wire structures

We study carrier-interaction-induced many-body effects on the excitonic optical properties of highly photoexcited one-dimensional semiconductor quantum wire systems by solving the dynamically screened Bethe-Salpeter equation using realistic Coulomb interaction between carriers. Including dynamical screening effects in the electron-hole self-energy and in the electron-hole interaction vertex function, we find that the excitonic absorption is essentially peaked at a constant energy for a large range of photoexcitation density $(n=0--6\ifmmode\times\else\texttimes\fi{}{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}),$ above which the absorption peak disappears without appreciable gain; i.e., no exciton to free electron-hole plasma Mott transition is observed, in contrast to previous theoretical results but in agreement with recent experimental findings. This absence of gain (or the nonexistence of a Mott transition) arises from the strong inelastic scattering by one-dimensional plasmons or charge density excitations, closely related to the non-Fermi-liquid nature of one-dimensional systems. Our theoretical work demonstrates a transition or a crossover in one-dimensional photoexcited electron-hole systems from an effective Fermi liquid behavior associated with a dilute gas of noninteracting excitons in the low-density region $(n<{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1})$ to a non-Fermi liquid in the high-density region $(n>{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}).$ The conventional quasistatic approximation for this problem is also carried out to compare with the full dynamical results. Numerical results for exciton binding energy and absorption spectra are given as functions of carrier density and temperature.

Where is the luttinger liquid in one-dimensional semiconductor quantum wire structures?

We present the theoretical basis for analyzing resonant Raman scattering experiments in one-dimensional systems described by the Luttinger-liquid fixed point. We make experimentally testable predictions for distinguishing Luttinger liquids from the Fermi liquid and argue that presently available quantum wire systems are not in the regime where Luttinger-liquid effects are important.

Observation of photoinduced intersubband transitions in one-dimensional semiconductor quantum wires

We report on midinfrared intersubband transitions between the quantized levels of undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wires that are grown on nonplanar substrates. Using photoinduced infrared absorption spectroscopy, we were able to resolve two absorption lines in the 120--160 meV range. The first absorption line follows the polarization selection rules for intersubband transitions and exhibits photoinduced excitation profile that resembles the photoluminescence excitation spectrum of the quantum wires. The second absorption resonance, at a higher energy, is not polarized and resembles the photoluminescence spectrum of the side quantum wells. Based on these results and energy level calculations, we assign the first absorption resonance to intersubband transition between the quantized subbands of the quantum wires. The origin of the second resonance is less clear. We propose a model that attributes this resonance to an interface localized exciton mode that arises from interface roughness of the side quantum wells and the wires.

Recombination kinetics and intersubband relaxation in semiconductor quantum wires

We present a detailed and systematic investigation of carrier capture, relaxation, cooling and radiative recombination in a one-dimensional semiconductor quantum wire of high structural perfection and optical quality over a large range of excitation (carrier) densities. Experimental evidence for a complete lack of 1D bandgap renormalization is found. Even up to high carrier densities, >106 cm-1, where strong band filling is already present and directly visible in the luminescence, no shift of bandgap to low energy is found. The carrier cooling in 1D is appreciably slower than in comparable 2D structures, thus leading to high carrier temperatures. This confirms theoretical predictions of reduced phonon scattering probability in one-dimensional structures. The temperature dependence of the radiative lifetime of the 1D carriers is investigated. The theoretically predicted T dependence is not found. On the contrary an empirical tau rad=0.02 T ns K-1 law is fulfilled.

LO-phonon emission by hot electrons in one-dimensional semiconductor quantum wires.

We provide a theoretical study of hot-electron energy loss via LO-phonon emission in GaAs quantum wires. Calculations are done for a model in which the hot-electron gas is described by a temperature T which is much higher than the lattice temperature. We take into account several relevant physical mechanisms such as quantum degeneracy, dynamic screening, quantum confinement, and hot-phonon bottleneck effect. We include the effects of quantum-wire phonon confinement using two prevailing boundary conditions, namely, the so-called electrostatic and the mechanical macroscopic models. The energy-loss rate calculated for the bulk LO-phonon emission is comparable to, but somewhat higher than, that due to slab (electrostatic) phonon emission, whereas the guided (mechanical) modes produce more than an order of magnitude slower energy relaxation than that due to the slab (electrostatic) or the bulk modes. Our results show that in the experimentally interesting electron-temperature range of 50--300 K, the hot-phonon bottleneck is the single most important quantitative effect provided the hot-phonon lifetime in GaAs quantum wires is comparable to that (\ensuremath{\sim}5--7 ps) in the bulk GaAs. The numerical magnitudes of the quantum-wire hot-electron energy-loss rate due to LO-phonon emission are comparable to those in quantum-well structures under comparable conditions.

Dielectric response of one-dimensional semiconductor quantum wires

We develop a non-singular theory for the dielectric response of one dimensional quantum wires as occuring, for example, in semiconductor quantum well and metal-insulator-semiconductor inversion layer systems. The theory includes collisional broadening and realistic wavefunction effects. The dynamical response of a lateral two-dimensional superlattice (made from a periodic array of such quantum wires) is calculated to obtain the elementary excitation spectrum. The plasmon spectrum in the superlattice forms a band, and the characteristic one-dimensional to two-dimensional plasmon transition is seen as the superlattice period is decreased.

Dielectric response of one-dimensional semiconductor quantum wires

Abstract We develop a non-singular theory for the dielectric response of one dimensional quantum wires as occuring, for example, in semiconductor quantum well and metal-insulator-semiconductor inversion layer systems. The theory includes collisional broadening and realistic wavefunction effects. The dynamical response of a lateral two-dimensional superlattice (made from a periodic array of such quantum wires) is calculated to obtain the elementary excitation spectrum. The plasmon spectrum in the superlattice forms a band, and the characteristic one-dimensional to two-dimensional plasmon transition is seen as the superlattice period is decreased.