Tuning photoluminescence of single InAs quantum dot by electric field

By using photoluminescence (PL) and time-resolved PL spectra, the optical properties of single InAs quantum dot (QD) embedded in the p-1-n structure have been studied under an applied electric field With the increasing of electric field, the exciton lifetime increases due to the Stark effect. We noticed that the decrease or quenching of PL intensity with increasing the electric field is mainly due to the decrease of the carriers captured by QD.

Magnetic quantum oscillations for the surface states of topological insulator Bi2Se3

We study quantum oscillations of the magnetization in Bi2Se3 (111) surface system in the presence of a perpendicular magnetic field. The combined spin-chiral Dirac cone and Landau quantization produce profound effects on the magnetization properties that are fundamentally different from those in the conventional semiconductor two-dimensional electron gas. In particular, we show that the oscillating center in the magnetization chooses to pick up positive or negative values depending on whether the zero-mode Landau level is occupied or empty. An intuitive analysis of these features is given and the subsequent effects on the magnetic susceptibility and Hall conductance are also discussed.

Spin-dependent transport and spin polarization in coupled quantum wells

We theoretically investigate the electron transport and spin polarization of two coupled quantum wells with Dresselhaus spin-orbit interaction. In analogy with the optical dual-channel directional coupler, the resonant tunneling effect is treated by the coupled-mode equations. We demonstrate that spin-up and -down electrons can be completely separated from each other for the system with an appropriate system geometry and a controllable barrier. Our result provides a new approach to construct spin-switching devices without containing any magnetic materials or applying a magnetic field. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2981204]

Anomalous photogalvanic effect of circularly polarized light incident on the two-dimensional electron gas in AlxGa1-xN/GaN heterostructures at room temperature

Under normal incidence of circularly polarized light at room temperature, a charge current with swirly distribution has been observed in the two-dimensional electron gas in Al0.25Ga0.75N/GaN heterostructures. We believe that this anomalous charge current is produced by a radial spin current via the reciprocal spin Hall effect. It suggests a new way to research the reciprocal spin Hall effect and spin current on the macroscopic scale and at room temperature.

Spin states in semiconductor quantum dot with a single magnetic ion

We investigate theoretically CdTe quantum dots containing a single Mn2+ impurity, including the sp-d exchange interaction between carriers and the magnetic ion and the short-range exchange interaction between electron and hole. We find anticrossing behaviors in the energy spectrum of the electron-hole (e-h) pair that arise from the interplay between exchange interactions and the magnetic field. In addition to the s-d exchange interaction, we find that other mechanisms inducing the anticrossings become important in the strong heavy hole-light hole (hh-lh) mixing regime. The transition strengths between the states with spin projection of Mn2+ ion S-z not equal -5/2 (S-z = -5/2) decrease (increase) with increasing magnetic fields due to the alignment of the Mn2+ spin. The spin splitting of the e-h pair states depends sensitively on the external magnetic and electric field, which reveals useful information about the spin orientation and position of the magnetic ion. Meanwhile, the manipulation of the position of the magnetic ion offers us a way to control the spin splitting of the carriers. (C) 2008 Elsevier B.V. All rights reserved.

Photoluminescence properties of tensile-strained GaAsP/GaInP single quantum wells grown by metal organic chemical vapor deposition

Tensile-strained GaAsP/GaInP single quantum well (QW) laser diode (I-D) structures have been grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) and related photoluminescence (PL) properties have been investigated in detail. The samples have the same well thickness of 16 nm but different P compositions in a GaAsP QW. Two peaks in room temperature (RT) PL spectra are observed for samples with a composition larger than 0.10. Temperature and excitation-power-dependent PL spectra have been measured for a sample with it P composition of 0.15. It is found that the two peaks have a 35 meV energy separation independent of temperature and only the low-energy peak exists below 85 K. Additionally, both peak intensities exhibit a monotonous increase as excitation power increases. Analyses indicate that the two peaks arise from the intrinsic-exciton recombination mechanisms of electron-heavy hole (e-hh) and electron-light hole (e-hh). A theoretical calculation based oil model-solid theory, taking, into account the spin-orbit splitting energy, shows good agreement with our experimental results. The temperature dependence of PL intensity ratio is well explained using the spontaneous emission theory for e-hh and e-hh transitions. front which the ratio can be characterized mainly by the energy separation between the fill and Ill states.

Transmission properties through a double quantum dot with a wave packet

The transmiss on time and tunneling probability of an electron through a double quantum dot are studied using the transfer matrix technique. The time-dependent Schrodinger equation is applied for a Gaussian wave packet passing through the double quantum clot. The numerical calculations are carried out for a double quantum clot consisting of GaAs/InAs material. We find that the electron tunneling resonance peaks split when the electron transmits through the double quantum dot. The splitting energy increases as the distance between the two quantum dots decreases. The transmission time can be elicited from the temporal evolution of the Gaussian wave packet in the double quantum dot. The transmission time increases quickly as the thickness of tire barrier increases. The lifetime of the resonance state is calculated tram the temporal evolution of the Gaussian-state at the centers of quantum dots.

Temperature-dependent transport and spin accumulation in a quantum wire with Rashba spin-orbit interaction

We study the theory of temperature-dependent electron transport, spin polarization, and spin accumulation in a Rashba spin-orbit interaction (RSOI) quantum wire connected nonadiabatically to two normal conductor electrode leads. The influence of both the wire-lead connection and the RSOI on the electron transport is treated analytically by means of a scattering matrix technique and by using an effective free-electron approximation. Through analytical analysis and numerical examples, we demonstrate a simple way to design a sensitive spin-transfer switch that operates without applying any external magnetic fields or attaching ferromagnetic contacts. We also demonstrate that the antisymmetry of the spin accumulation can be destroyed slightly by the coupling between the leads and the wire. Moreover, temperature can weaken the polarization and smear out the oscillations in the spin accumulation.

Coherence loss and recovery of an electron spin coupled inhomogeneously to a one-dimensional interacting spin bath: An adaptive time-dependent density-matrix renormalization group study

Coherence evolution and echo effect of an electron spin, which is coupled inhomogeneously to an interacting one-dimensional finite spin bath via hyperfine-type interaction, are studied using the adaptive time-dependent density-matrix renormalization group method. It is found that the interplay of the coupling inhomogeneity and the transverse intrabath interactions results in two qualitatively different coherence evolutions, namely, a coherence-preserving evolution characterized by periodic oscillation and a complete decoherence evolution. Correspondingly, the echo effects induced by an electron-spin flip at time tau exhibit stable recoherence pulse sequence for the periodic evolution and a single peak at root 2 tau for the decoherence evolution, respectively. With the diagonal intrabath interaction included, the specific feature of the periodic regime is kept, while the root 2 tau-type echo effect in the decoherence regime is significantly affected. To render the experimental verifications possible, the Hahn echo envelope as a function of tau is calculated, which eliminates the inhomogeneous broadening effect and serves for the identification of the different status of the dynamic coherence evolution, periodic versus decoherence.

Electrical control of dynamic spin splitting induced by exchange interaction as revealed by time-resolved Kerr rotation in a degenerate spin-polarized electron gas

Manipulation of the spin degree of freedom has been demonstrated in a spin-polarized electron plasma in a heterostructure by using exchange-interaction-induced dynamic spin splitting rather than the Rashba and Dresselhaus types, as revealed by time-resolved Kerr rotation. The measured spin splitting increases from 0.256 meV to 0.559 meV as the bias varies from -0.3 V to -0.6 V. Both the sign switch of the Kerr signal and the phase reversal of Larmor precessions have been observed with biases, which all fit into the framework of exchange-interaction-induced spin splitting. The electrical control of it may provide a new effective scheme for manipulating spin-selected transport in spin FET-like devices. Copyright (C) EPLA, 2008.

Influence of electron irradiation on hydrothermally grown zinc oxide single crystals

The resistivity of hydrothermally grown ZnO single crystals increased from similar to 10(3) Omega cm to similar to 10(6) Omega cm after 1.8 MeV electron irradiation with a fluence of similar to 10(16) cm(-2), and to similar to 10(9) Omega cm as the fluence increased to similar to 10(18) cm(-2). Defects in samples were studied by thermally stimulated current (TSC) spectroscopy and positron lifetime spectroscopy (PLS). After the electron irradiation with a fluence of 10(18) cm(-2), the normalized TSC signal increased by a factor of similar to 100. A Zn vacancy was also introduced by the electron irradiation, though with a concentration lower than expected. After annealing in air at 400 degrees C, the resistivity and the deep traps concentrations recovered to the levels of the as-grown sample, and the Zn vacancy was removed.

Spectroscopy of long wavelength coupled quantum dots

We obtained a low density of coupled InAs/GaAs quantum dots (QDs) with an emission wavelength of around 1.3 mu m at room temperature. Atomic force microscopy and transmission electronic microscopy reveal that the dot size difference and the lateral displacement between the two dots are related to the spacer thickness. Spectroscopy of the coupled QD ensembles is considerably influenced by the spacer thickness.

Electronic states of InAs/GaAs tyre-shape quantum ring

In the framework of the effective mass theory, this paper calculates the electron energy levels of an InAs/GaAs tyre-shape quantum ring (TSQR) by using the plane wave basis. The results show that the electron energy levels are sensitively dependent on the TSQR's section thickness d, and insensitively dependent on TSQR's section inner radius R-1 and TSQR's inner radius R-2. The model and results provide useful information for the design and fabrication of InAs/GaAs TSQRs.

Positive magnetoresistance in In0.53Ga0.47As/In0.52Al0.48As quantum well

Magnetotransport measurements have been carried out on In0.53Ga0.17As/In0.52Al0.48 As quantum wells in a temperature range between 1.5 and 77 K. We have observed a large positive magnetoresistance in the low magnetic field range, but saturating in high magnetic fields. The magnetoresistance results from two occupied subbands in the two-dimensional electron gas. With the intersubband scattering considered, we obtained the subband mobility by analyzing the positive magnetoresistance. It is found that the second subband mobility is larger than that of the first due to the existence of the intersubband scattering.

Voltage tunable two-color InAs/GaAs quantum dot infrared photodetector

We report a bias voltage tunable two-color InAs/GaAs quantum dot infrared photodetector working under the normal incidence infared irradiation. The two-color detection of our device is realized by combining a photovoltaic and a photoconductive response by bias voltage tuning. The photovoltaic response is attributed to the transition of electron from the ground state to a high continuum state. The photoconductive response arises from the transition of electron from the ground state to the wetting layer state through the barrier via Fowler-Nordheim tunneling evidenced by a broad feature of the photocurrent peak on the high energy side. (C) 2008 American Institute of Physics.