Transport in quantum wires

With a brief introduction to one-dimensional channels and conductance quantization in mesoscopic systems, we discuss some recent experimental puzzles in these systems, which include reduction of quantized conductances and an interesting odd-even effect in the presence of an in-plane magnetic field. We then discuss a recent non-homogeneous Luttinger liquid model proposed by us, which addresses and gives an explanation for the reduced conductances and the odd-even effect. We end with a brief summary and discussion of future projects.     

0.7 structure in long quantum wires

While quantized conductance steps in short quantum wires are understood through a single electron picture, additional structure often observed in high-quality one-dimensional systems near g=0.7×(2e²/h) is commonly interpreted as arising due to many-body interactions. Most studies of conductance structure below 2e²/h use short one-dimensional wires where transport is known to be ballistic. We report transport measurements for both short (0.5 μm) and long (5 μm) quantum wires, and use both conductance and nonlinear transport to explore the behavior of one-dimensional wires.     

Direct evidence for quantum contact resistance effects in V-groove quantum wires

The effect of quantum contact resistance on one-dimensional (1D) electrical conductance was investigated in quantum wires (QWR) realized with V-shaped GaAs/AlGaAs heterostructure. The transition length between the electron reservoir and the QWR was controlled by employing an electric field. The required transition length is found to decrease with increasing overlap between the 2D states in the reservoir and the 1D states in the QWR.     

Exchange coupling for excitons in quantum wires

We consider theoretically the exchange coupling for independent heavy excitons in both weakly and strongly confining quantum wires. We discuss different contributions for the spin depolarization in the presence of spin-conserving scatterings and show that localization effects weaken the spin depolarization rate.     

Modeling of Coulomb interaction in parabolic quantum wires

We study the exciton states in a parabolic quantum wire. An exactly solvable model is introduced for calculating the exciton state and the binding energy as a function of the radius of the quantum wire within the envelope-function approximation. In the calculation, we replace the actual Coulomb interaction between the electron and the hole by a Gaussian nonlocal separable potential and obtain closed expressions for both the envelope-function and the binding energy. Results are compared with those obtained by perturbative methods.     

Contact correlations in one-dimensional systems of interacting fermions and the photoluminescence from quantum wires

We calculate the dependence of the carriers lifetime with the wire width in quantum wires by considering a strictly one-dimensional system of interacting electrons and holes. Confinement effects are taken into account through a width-dependent pair-potential proposed by Hu and Das Sarma. The carriers lifetime is then obtained from the inverse of the contact electron–hole correlations. We explain the change in the sign of the derivative at a critical temperature, as it is observed in photoluminescence experiments from In Ga As/InP quantum wires, by taking into account the carriers density dependence with temperature and assuming that the contact correlations are either just a two-body quantity or a many-body one for the lower and higher densities, respectively. In the former case, the system is viewed as an ionized excitonic gas, the pair correlation being the square of the two-body wave function for unbound states. In the latter, we have a metallic electron–hole system and we calculate the contact pair correlation in the many-body ladder approximation.     

Strong Fermi-edge singularity in ultra-high-quality AlGaAs/GaAs quantum wires

We report the observation of a strong Fermi-edge singularity (FES), with the complete suppression of the band-edge peak, in the photoluminescence spectra of ultra-high-quality modulation-doped AlGaAs/GaAs quantum wires (QWRs). We find that the FES effect is very sensitive to the Fermi energy. The strong FES is observed only in QWRs having a Fermi energy of the order of a few meV, and disappears almost completely when the Fermi energy exceeds 10 meV. These results are expected to spark new research activities, both experimentally and theoretically, on many-body effects in one-dimensional electron gas.     

Exciton effects on nonlinear electro-optic effects in semi-parabolic quantum wires

The nonlinear electro-optic effects in quasi-one-dimensional semi-parabolic quantum wires are studied, in which the exciton effects are taken into account. The analytical expression of the electro-optic co-efficient is derived by compact density-matrix approach. Finally, the numerical results are presented for GaAs/AlGaAs semi-parabolic quantum wires. The results show that the electro-optic coefficient is over two times bigger than that obtained by without considering exciton effects. Furthermore, the electro-optic coefficient is related to the relaxation time.     

Far-infrared spectroscopy of quantum wires and dots, breaking Kohn’s theorem

We review far-infrared experiments on quantum wires and dots. In particular, we show that with tailored deviations from a parabolic external lateral confinement potential one can break Kohn’s theorem. This allows a detailed investigation of the internal relative motion in quantum dots and wires and the study of electron–electron interaction effects, for example, the formation of compressible and incompressible states in quantum dots and antidots.     

Binding energy of localized trions in quantum wires

We present a finite difference calculation of the binding energies of localized trions in quasi-one-dimensional quantum wires (QWRs). It is found that both the lateral confinement and the localization potential have a strong effect on the relative stability of the trions. It is confirmed that a weak localization potential not only enhances the binding energy but also changes the relative stability of the positive and negative trions. Our theoretical model is in good accord with a recent experiment regarding photoluminescence in disordered QWRs.     

The optical Stark effect in semiconductor quantum wires

A new approach for controlling the optical emission wavelength of semiconductor quantum wires is proposed. The wavelength control resides upon the effect of an intense, long-wavelength laser field radiation applied to the semiconductor structure. Under such condition a strong optical Stark effect leads to optical tunability. Calculation of the optical Stark effect was carried out in the frame of the nonperturbative theory and finite difference method. Different geometries concerning the size of GaAs–AlGaAs quantum wires as well as the polarization direction and the strength of the applied laser field with respect to the quantum structure were considered.     

Critical point estimation and long-range behavior in the one-dimensional XY model using thermal quantum and total correlations

We investigate the thermal quantum and total correlations in the anisotropic XY spin chain in transverse field. While we adopt concurrence and geometric quantum discord to measure quantum correlations, we use measurement-induced non-locality and an alternative quantity defined in terms of Wigner–Yanase information to quantify total correlations. We show that the ability of these measures to estimate the critical point at finite temperature strongly depend on the anisotropy parameter of the Hamiltonian. We also identify a correlation measure which detects the factorized ground state in this model. Furthermore, we study the effect of temperature on long-range correlations.     

Spatial electric field effect in GaAs–AlAs quantum wires

The response of an electron to a three-dimensional electric field in an infinite quantum well wire of square cross-section is investigated within a variational scheme. The ionization energy and the polarization are calculated for different locations of impurity ion. It is found that the results for the spatial electric field differ from the previous results found for the electric field applied in the direction perpendicular to the wire axis. The ionization energy weakens rapidly with the axial component of the field as the polarization of the carrier distribution intensifies.     

Exciton states and optical absorption in quantum wires under laser radiation

We analyze the exciton states in a quantum wire under intense laser radiation. Electrons and holes are confined by the parabolic potential of the quantum wire. An exactly solvable model is introduced for calculating the exciton binding energy, replacing the actual Coulomb interaction between the electron and the hole by a projective operator.     

A systematic study of the electron mobility in V-shaped quantum wires at low temperatures

We present a systematic study of the electron mobility in V-shaped AlGaAs/GaAs quantum wires taking into account the impurity (background, remote and interface) and the acoustic-phonon scattering. The electron scattering rates are calculated for wires with electron concentrations up to 10⁶ cm⁻¹ and temperatures up to 40 K by using Fermi’s golden rule. The effects of the interface roughness scattering and the alloy scattering are also discussed. The energy eigenstates and eigenvalues of the system under study are calculated using a finite difference method. We analyze the importance of each scattering mechanism on the mobility of several quantum wires of different qualities as a function of the electron concentration and the temperature.     

Carrier recombination kinetics in (3 1 1)A InGaAs sidewall quantum wires

We present a study of the optical properties and carrier dynamics in strained InGaAs sidewall quantum wires (QWR) on patterned GaAs (3 1 1)A substrates by means of picosecond time-resolved photoluminescence (PL). A pronounced dynamical red shift of the QWR-PL band when increasing the delay time after the pulse excitation is observed. In addition, time-resolved data show a significant shortening of the PL decay time from the wire at short delay and when high excitation power is used. The data are compared with theoretical predictions. The results, i.e. the dynamical red shift observed in the wire emission and the shortening of the PL decay with increasing the excitation density, are interpreted in terms of a dynamical screening effect of the piezoelectric field.     

Donor impurity states in coupled quantum well wires under hydrostatic pressure and applied electric field

In this work we study the binding energy of the ground state for a hydrogenic donor impurity in laterally coupled GaAs/Ga_1−xAl_xAs quantum well wires, considering the simultaneous effects of hydrostatic pressure and applied electric field. We have used a variational method and the effective mass and parabolic band approximations. The low dimensional structure consists of two quantum well wires with rectangular transverse section coupled by a central Ga_1−xAl_xAs barrier. Our results are reported for several sizes of the structure and we have taken into account variations of the impurity position along the growth direction of the heterostructure.     

Dynamics of uniform quantum gases, I: Density and current correlations

A unified approach valid for any wavenumber q, frequency ω, and temperature T is presented for uniform ideal quantum gases allowing for a comprehensive study of number density and particle-current density response functions. Exact analytical expressions are obtained for spectral functions in terms of polylogarithms. Also, particle-number and particle-current static susceptibilities are presented which, for fugacity less than unity, additionally involve Kummer functions. The q- and T-dependent transverse-current static susceptibility is used to show explicitly that current correlations are of long range in a Bose-condensed uniform ideal gas but for bosons at T>T_c and for Fermi and Boltzmann gases at all temperatures these correlations are of short range. Contact repulsive interactions for systems of neutral quantum particles are considered within the random phase approximation. The expressions for particle-number and transverse-current susceptibilities are utilized to discuss the existence or nonexistence of superfluidity in the systems under consideration.     

Polarizability of a hydrogenic donor impurity in a ridge quantum wire

We use in this paper the variational method to calculate the polarizability of a hydrogenic donor impurity, in the presence of electric field, in a V-groove GaAs/Al_xGa_1−xAs quantum wire. The carrier ground states are analytically obtained by an effective potential scheme together with a suitable coordinate transformation that allows the decoupling of the two-dimensional Schrodinger equation. According to the results obtained from the present work for polarizability and binding energy reveals that the impurity position and field direction play important roles.