A study for the cartography of the interface roughness of V-shaped AlGaAs/GaAs quantum wires

We present a theoretical study for the cartography of the interface roughness of AlGaAs/ GaAs V-shaped quantum wires which is reflected on the photoluminescence and micro-photoluminescence spectra of these structures. The model developed is based on the existence of microscopic compositional fluctuations at the interfaces. The fine structure of the micro-photoluminescence spectrum is attributed to localized excitonic states in island-like fluctuations which act as quantum boxes distributed along the free direction of the wire. The fluctuation of the concentration of these boxes together with the estimation of their sizes are used to explain the evolution of the signals along the wire axis and to produce the overall photoluminescence spectrum. The model is applied to a V-shaped Al_0.3Ga_0.7As/GaAs quantum wire and reproduces successfully the observed photoluminescence and micro-photoluminescence characteristics.     

Micro-Photoluminescence Confocal Mapping of Single V-Grooved GaAs Quantum Wire

We perform the micro-photoluminescence measurement at low temperatures and a scanning optical mapping with high spatial resolution of a single V-grooved GaAs quantum wire modified by the selective ion-implantation and rapid thermally annealing. While the mapping shows the luminescences respectively from the quantum wires and from quantum well areas between quantum wires in general, the micro-photoluminescence at liquid He temperatures reveals a plenty of spectral structures of the PL band for a single quantum wire. The spectral structures are attributed to the inhomogeneity and non-uniformity of both the space structure and compositions of real wires as well as the defects nearby the interface between quantum wire and surrounding quantum well structures. All these make the excitons farther localized in quasi-zero-dimensional quantum potential boxes related to these non-uniformity and/or defects. The results also demonstrate the ability of micro-photoluminescence measurement and mapping for the characterization of both opto-electronic and structural properties of real quantum wires.     

Fine structure of highly charged excitons in semiconductor quantum dots

An exciton in a symmetric semiconductor quantum dot has two possible states, one dark and one bright, split in energy by the electron-hole exchange interaction. We demonstrate that for a doubly charged exciton, there are also two states split by the electron-hole exchange, but both states are now bright. We also uncover a fine structure in the emission from the triply charged exciton. By measuring these splittings, and also those from the singly charged and doubly charged biexcitons, all on the same quantum dot, we show how the various electron-hole exchange energies can be measured without having to break the symmetry of the dot.     

Localized end states in density modulated quantum wires and rings

We study finite quantum wires and rings in the presence of a charge-density wave gap induced by a periodic modulation of the chemical potential. We show that the Tamm-Shockley bound states emerging at the ends of the wire are stable against weak disorder and interactions, for discrete open chains and for continuum systems. The low-energy physics can be mapped onto the Jackiw-Rebbi equations describing massive Dirac fermions and bound end states. We treat interactions via the continuum model and show that they increase the charge gap and further localize the end states. The electrons placed in the two localized states on the opposite ends of the wire can interact via exchange interactions and this setup can be used as a double quantum dot hosting spin qubits. The existence of these states could be experimentally detected through the presence of an unusual 4π Aharonov-Bohm periodicity in the spectrum and persistent current as a function of the external flux.     

Correlated photons from multi-carrier complexes in GaAs quantum dots

We have studied micro-photoluminescence spectra of a self-assembled single GaAs quantum dot under 8 K. With strong pulsed excitation, the micro-photoluminescence spectrum shows bright emission lines originated from an exciton, a positively charged exciton, and a biexciton, together with weak lower energy emissions reflecting multi-excitonic structures with more carriers. We have identified the origins of these weak emission lines, and showed the existence of charged biexciton states, through single photon correlation measurements and excitation power dependence of the photoluminescence intensity. In addition, investigating the radiative recombination process of the charged biexciton, we have determined the electron–hole exchange energy in the GaAs quantum dot.     

Proposal for measuring mechanically equilibrium spin currents in the rashba medium

We demonstrate that an equilibrium spin current in a 2D electron gas with Rashba spin-orbit interaction (Rashba medium) results in a mechanical torque on a substrate near an edge of the medium. If the substrate is a cantilever, the mechanical torque displaces the free end of the cantilever. The effect can be enhanced and tuned by a magnetic field. Observation of this displacement would be an effective method to prove the existence of equilibrium spin currents. The analysis of edges of the Rashba medium demonstrates the existence of localized edge states. They form a 1D continuum of states. This suggests a new type of quantum wire: spin-orbit quantum wire.     

EPR spectrum via entangled states for an exchange-coupled dimer of single-molecule magnets

Multi-high-frequency electron paramagnetic resonance(EPR) spectrum for a supermolecular dimer of single-molecule magnets recently reported [S. Hill, R.S. Edwards, N. Aliaga-Alcalde and G. Christou (HEAC), Science 302, 1015 (2003)] is studied in terms of the perturbation method in which the high-order corrections to the level splittings of degenerate states are included. It is shown that the corresponding eigenvectors are composed of entangled states of two molecules. The EPR-peak positions are calculated in terms of the eigenstates at various frequencies. From the best fit of theoretical level splittings with the measured values we obtain the anisotropy constant and exchange coupling which are in agreement with the corresponding values of experimental observation. Our study confirms the prediction of HEAC that the two Mn4 units within the dimer are coupled quantum mechanically by the antiferromagnetic exchange interaction and the supermolecular dimer behaviors in analogy with artificially fabricated quantum dots.     

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.     

Spatially inhomogeneous states of charge carriers in graphene

The interaction of two-dimensional quasiparticles characterized by a linear dispersion E = ±u|p| (graphene) with impurity potentials is studied. It is shown that discrete levels corresponding to localized states are present in a one-dimensional potential well (quantum wire), whereas such states are absent in a two-dimensional well (quantum dot). The cross section for the scattering of electrons (holes) of graphene by an axially symmetric potential well is determined. It is shown that the cross section tends to a constant value in the limit of infinite particle energy. The effective Hamiltonian is derived for a curved quantum wire of graphene.     

Excited states and multi-exciton complexes in single CdTe quantum dots

We present an optical study of excited states in single CdTe quantum dot (QDs). Using micro-photoluminescence excitation spectroscopy, absorption up to two confined excited levels have been observed in some dots. Power-dependent micro-photoluminescence is then used to study the occupation of excited states. The emission pattern is characteristic of the increase of the exciton number in the QD (shell-filling). A clear identification of the different multi-exciton complexes has been obtained in a highly symmetric dot. The evolution of the different multi-exciton intensities can then be reproduced by solving the rate equations for multi-exciton state occupancy and the fit by this simple model provides an estimate of the radiative lifetime of the different multi-exciton complexes.     

Localized states in InxGa1−xN epitaxial film

This study investigated localized states from In_0.36Ga_0.64N epitaxial film grown on a Si (111) substrate by performing macro-photoluminescence, micro-photoluminescence and time-resolved micro-photoluminescence experiments. Experimental data revealed two localized states — single and extended. The single localized state is a single-quantum-dot-like deep confined energy state, which is responsible for the bright line emissions. The extended localized state is a shallow confined energy state, which is related to a broad background emission. This work suggests that the origin of single and extended localized states is the indium-rich In_xGa_1?xN cluster and the spatial indium concentration fluctuation, respectively.     

Spatial structure of an incompressible quantum Hall strip

The incompressible quantum Hall strip is sensitive to charging of localized states in the cyclotron gap. We study the effect of localized states by a density functional approach and find electron density and the strip width as a function of the density of states in the gap. Another important effect is electron exchange. By using a model density functional which accounts for negative compressibility of the QH state, we find electron density around the strip. At large exchange, the density profile becomes nonmonotonic. Both effects, localized states and exchange, lead to a substantial increase of the strip width.     

Spectrum of localized states in graphene quantum dots and wires

We developed semiclassical method and show that any smooth potential in graphene describing elongated a quantum dot or wire may behave as a barrier or as a trapping well or as a double barrier potential, Fabry–Perot structure, for 1D Schrödinger equation. The energy spectrum of quantum wires has been found and compared with numerical simulations. We found that there are two types of localized states, stable and metastable, having finite life time. These life times are calculated, as is the form of the localized wave functions which are exponentially decaying away from the wire in the perpendicular direction.     

V形GaAs/AlGaAs量子线结构的微区光致发光谱研究

在室温下用显微光致发光的方法对单根V形GaAs/AlGaAs量子线进行了沿垂直于量子线方向的 空间分辨扫描测试,观察到各种量子结构的光致发光谱随空间位置的变化.在量子线区域附 近观察到来自量子线(QWR)、颈部量子阱(NQWL)和垂直量子阱(VQWL)等各种结构的发光,而 在距离量子线约1μm以远的发光光谱表现出侧面量子阱(SQWL)的发光.对全部发光光谱用高 斯线形进行了拟合,发现QWR和SQWL的发光包含了两个荧光峰,将它们分别归诸为电子到轻 、重空穴的跃迁.拟合后发光强度的空间变化直接确定了与量子线 关键词： V形GaAs/AlGaAs量子线 显微光致发光 空间分辨扫描     

Quantum phase transition in the two-band hubbard model

The interaction between itinerant and Mott localized electronic states in strongly correlated materials is studied within dynamical mean field theory in combination with the numerical renormalization group method. A novel nonmagnetic zero temperature quantum phase transition is found in the bad-metallic orbital-selective Mott phase of the two-band Hubbard model, for values of the Hund's exchange which are relevant to typical transition metal oxides.     

Bound and Scattering States of Extended Calogero Model with an Additional PT Invariant Interaction

Here we discuss two many-particle quantum systems, which are obtained by adding some nonhermitian but PT (i.e. combined parity and time reversal) invariant interaction to the Calogero model with and without confining potential. It is shown that the energy eigenvalues are real for both of these quantum systems. For the case of extended Calogero model with confining potential, we obtain discrete bound states satisfying generalised exclusion statistics. On the other hand, the extended Calogero model without confining term gives rise to scattering states with continuous spectrum. The scattering phase shift for this case is determined through the exchange statistics parameter. We find that, unlike the case of usual Calogero model, the exclusion and exchange statistics parameters differ from each other in the presence of PT invariant interaction.     

Energy sublevels in an Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As quantum wire

We report the successful fabrication of a V-grooveAl_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As quantum wire system and the temperature-dependent photoluminescence (PL) measurement. The PL spectra are dominated by four features at 681, 642, 635 and 621 nm attributed to the luminescences from quantum wire, top, vertical and side-wall well regions by micro-PL measurements. By the calculations of the energy structure, discrete states (localized sublevels) in the quantum wire region and continuum states (extended along the side-wall and vertical quantum wells) in side-wall and vertical quantum wells have been obtained in both the conduction and valence bands. The calculated excitation energies explain very well the peak positions and their temperature dependence in the photoluminescence measurements.     

Drag effect of one-dimensional electrons upon photoionization of D<Superscript>(−)</Superscript> centers in a longitudinal magnetic field

A theory is elaborated for the impurity photon drag effect in a semiconductor quantum wire exposed to a longitudinal magnetic field B directed along the axis of the quantum wire. The phonon drag effect is associated with the transfer of the longitudinal photon momentum to localized electrons in optical transitions from D^(?) states to hybrid-quantized states of the quantum wire, which is described by a confinement parabolic potential. An analytical expression for the drag current density is derived within the model of a zero-range potential in the effective mass approximation, and the spectral dependence of the drag current density is examined at different magnitudes of B and parameters of the quantum wire upon electron scattering by a system of impurities with short-range potentials. It is established that the spectral dependence of the drag current density exhibits a Zeeman doublet with a clear beak-shaped peak due to optical transitions of electrons from D^(?) states to states with the magnetic quantum number m=1. The possibility of using the photon drag effect in a longitudinal magnetic field for the development of laser radiation detectors is analyzed.     

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.     

Entangled spin states of a Bose condensate in an electromagnetic field

We consider the formation of entangled quantum states for an atomic Bose condensate interacting with an external electromagnetic field in a single-particle state under conditions of change in various regimes for exchange interaction processes. These states of the Bose system have high phase coherence and are accompanied by the generation of squeezed states of a new type in terms of the parameters defined by a combination of transition operators for the condensate atoms and external-field photons with an appropriate polynomial deformation of the algebra SU(2). We show that localized quantum structures corresponding to stable elementary excitations of the atoms and the field in the condensate can be formed in principle. We also analyze the purely quantum effects of collapse and revival for the level populations of the Bose condensate and the change in atomic statistics as well as determine the conditions for the formation of superstructure of these unsteady states for the Bose system.