Density-dependent spin polarization in ultra-low-disorder quantum wires

There is controversy as to whether a one-dimensional (1D) electron gas can spin polarize in the absence of a magnetic field. Together with a simple model, we present conductance measurements on ultra-low-disorder quantum wires supportive of a spin polarization at B=0. A spin energy gap is indicated by the presence of a feature in the range (0.5-0.7)x2e(2)/h in conductance data. Importantly, it appears that the spin gap is not constant but a function of the electron density. Data obtained using a bias spectroscopy technique are consistent with the spin gap widening further as the Fermi level is increased.     

Anisotropic linear-polarization luminescence in CdTe/CdMnTe quantum wires

We theoretically studied anisotropic linear optical polarization properties in CdTe/Cd_0.75Mn_0.25Te quantum wires (QWRs) by using the multi-band effective mass method. In this QWR system, the spatial distribution of the Mn composition influences both the lateral quantum confinement and the sp-d exchange coupling. The calculated expectation value of the hole spin demonstrates that the hole spin is reoriented along the external magnetic field when applying the magnetic field parallel to the QWR. The hole-spin reorientation causes anisotropic behavior in the Zeeman shift and the linearly polarized optical transitions, which sensitively depends on the Mn spatial distribution. Such characteristic features appeared in the QWR have been demonstrated experimentally and compared with the theoretical calculations.     

Spin polarization and temperature dependence of electron effective mass in quantum wires

The electron effective mass in GaAs quantum wires has been estimated by using a full dynamical random-phase approximation to examine its properties versus spin polarization, temperature, and carrier density. A decrease of mass with spin polarization is seen. The minority mass increases with the polarization while the majority mass decreases and this behaviour is seen for all densities. A maximal enhancement of mass at moderate temperature around 25 K is also presented. These calculations show a qualitative consistence with results in two-dimensional systems and help to control the electronic transport in quantum wires.     

Polarization spectra of excitonic luminescence of bare ZnCdSe/ZnSe quantum wires

Linearly polarized luminescence spectra of bare (unburied) semiconductor structures with ZnCdSe/ZnSe quantum wires, obtained by reactive ion etching, were investigated. It was found that, regardless of the orientation of the linear polarization of the exciting light, the luminescence radiation of the quantum wires is polarized parallel to the axis of the wires, while the radiation of the buffer layer of the isotropic ZnSe barrier material is oriented perpendicular to the axis of the wires. The polarization features found are due to the modification of the modes of the electromagnetic field near open quantum wires, which occurs as a result of the presence of the vertical interfaces between media with strongly different permittivities. It was also found that, when linearly polarized excitation is used, the alignment of exciton dipole moments strongly influences the polarization properties of the luminescence. Fiz. Tverd. Tela (St. Petersburg) 40, 1559–1562 (August 1998)     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

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.     

Excitons in quantum wires

This paper deals with excitons in quantum wires. We first study these excitons as the limit of excitons in D dimensions when . In order to do it, we have had to find a new resolution of the hydrogen atom Schrödinger equation: besides the fact that the usual resolution found in textbooks is not valid for D exactly equal to 1, it is, surprisingly enough, inconsistent since it relies on two hypergeometric functions which are not independent for the parameters of physical interest! In a second part, we write down the exact potential felt by the exciton relative motion along the wire in terms of the wire confinement. This allows a quite precise determination of the effective Coulomb potential for this 1D motion, which is of crucial importance to obtain a meaningfull finite value for the exciton ground state energy. In a last part, we study the dependence of the exciton energies on the wire area and anisotropy. While the quantitative results are here given for cylindrica l and rectangular wires with infinite barriers, we show how they can easily be extended to any particular wire shape and barrier height.Received: 5 August 2002, Published online: 23 July 2003PACS: 71.35.-y Excitons and related phenomena - 73.21.Hb Quantum wires     

Oscillatory magnetic anisotropy in one-dimensional atomic wires

One-dimensional Co atomic wires grown on Pt(997) have been investigated by x-ray magnetic circular dichroism. Strong changes of the magnetic properties are observed as the system evolves from 1D- to 2D-like. The easy axis of magnetization, the magnetic anisotropy energy, and the coercive field oscillate as a function of the transverse width of the wires, in agreement with theoretical predictions for 1D metal systems.     

Spin-incoherent transport in quantum wires

When a quantum wire is weakly confined, a conductance plateau appears at e;{2}/h with decreasing carrier density in zero magnetic field accompanied by a gradual suppression of the 2e;{2}/h plateau. Applying an in-plane magnetic field B_{ parallel} does not alter the value of this quantization; however, the e;{2}/h plateau weakens with increasing B_{ parallel} up to 9 T, and then strengthens on further increasing B_{ parallel}, which also restores the 2e;{2}/h plateau. Our results are consistent with spin-incoherent transport in a one-dimensional wire.     

Free-induction decay in quantum wires

Using the time-dependent perturbation treatment, we report the occurrence of first-order free-induction decay in GaAs/GaAlAs quantum wire structures. The results of the theoretical investigation reveal oscillations in the decay profile of the polarization. The transient transmitted intensity has been found to decay in a time comparable to the inhomogeneous broadening time, which is much shorter than the dephasing time. We have also studied the transmitted intensity as a function of wire width and found the results to be in good qualitative agreement with that available in the literature.