Tunable coupled plasmon modes via nanoshell particle chains

Coupled plasmon modes have been studied theoretically in periodic chains of nanoshell particles embedded in a graded dielectric host. These chains not only sustain a variety of localized modes as unshelled nanoparticle chains, but also offer precise control of the localization-delocalization transition among these modes by varying the permittivity contrast and/or core-shell radius ratio. By optimizing these parameters, the upper band can be tuned into higher frequencies while the lower band can be tuned into the optical communication frequencies for practical application. We also discuss the Ohmic loss effects in the metallic component of the nanoshells.     

Electronic states of a hydrogenic impurity in a zinc-blende GaN/AlGaN quantum well

Binding energies of ground and a few low lying excited states of a hydrogenic donor confined in a zinc-blende GaN/AlGaN quantum well are investigated. They are computed within the framework of single band effective mass approximation, by means of a variational approach. The donor states are investigated with the various impurity positions as a function of well width. The calculations have been carried out with the inclusion of conduction band non-parabolicity through the energy dependent effective mass. The variational solutions have been improved by using a two-parametric trial wavefunction. The results seem better and good agreement with the other investigators. To support our results, we observe that the values of variational parameters are consistent when two parameter wave function is used. We find that the inclusion of non-parabolic effects leads to more binding for all the values of well width and is significant for narrow wells. The results are compared with the existing available literature.     

Self-interaction correction in a simple model

We discuss various ways to handle self-interaction corrections (SIC) to Density Functional Theory (DFT) calculations. To that end, we use a simple model of few particles in a finite number of states together with a simple zero-range interaction for which full Hartree-Fock can easily be computed as a benchmark. The model allows to shed some light on the balance between orthonormality of the involved states and energy variance.     

Optoelectronic Properties for Armchair-Edge Graphene Nanoribbons

We study theoretically the electronic and transport property for an armchair-edge graphene nanoribbon （GNR.） with 12 and 11 transversal atomic lines, respectively. The ONR. is irradiated under an external longitudinal polarized high-frequency electromagnetic field at low temperatures. Within the framework of linear response theory in the perturbative regime, we examine the joint density of states and the real conductance of the system. It is demonstrated that, by numerical examples, some new photon-assisted intersubband transitions over a certain range of field frequency exist with different selection rules from those of both zigzag-edge GNR. and single-walled carbon nanotube. This opto-electron property dependence of armchair-edge GNR. on field frequency may be used to detect the high-frequency electromagnetic irradiation.     

Strengthened Hyper-Raman Lines with the Coherent Superposition State

High-order harmonic generations from a one-dimensional Coulomb potential atom are calculated with the initial state prepared as a coherent superposition between its ground and first excited states. When the energy difference of the two states is small, we can choose proper laser pulse such that the first excited state can be excited only to other bound states instead of being ionized. We show that only the hyper-Raman lines are observable instead of the harmonics. The energy difference of the ground and the first excited state can be deduced from the highest peak of the hyper-Raman lines. We further show that the similar results can be obtained by using a combination of two laser pulses with different frequencies interacting with the atom initially at the ground state.     

Spectral line suppression and sideband narrowing via quantum interference

We reveal that for a realistic system, interference effects are obtained such as the suppression of central line and inner sidebands and the narrowing of the outer fluorescence sidebands. For this purpose, we consider a spontaneous decay from an excited state to a metastable state when the excited and metastable states are resonantly coupled to an auxiliary metastable state by a laser field and a microwave field, respectively. The fluorescence spectrum evolves from a five-peaked structure into a doublet of ultrasharp lines as the ratio of the laser field Rabi frequency to the microwave Rabi frequency is decreased. The physical origin is presented in terms of dressed states.     

Influence of electronic excitations on the thermodynamics of small carbon clusters: the example of C3 +

We illustrate the influence of the thermalization of electrons on the linear-cyclic isomerization of small carbon clusters by model Monte Carlo calculations of C₃ ⁺ at 3500 K. It is shown that the inclusion of the electronic degrees of freedom, in such systems with several low-lying excited states, can significantly change the relative isomer populations and energy distributions, as compared with equivalent electronic ground-state simulations.     

Laser-induced operations with charge qubits in a double-well nanostructure

We present the results of theoretical studies on operations with charge qubits in the system composed of two tunnel-coupled semiconductor quantum dots whose two lowest states (localized in different dots) define the logical qubit states while two excited states (delocalized between the dots) serve for the electron transfer from one dot to another under the influence of the laser pulse. It is shown that in the case of small energy separation between the excited levels, the optimal (from the viewpoint of minimal single-qubit operation time and maximum operation fidelity) strategy is to tune the laser frequency between the excited levels. The pulse parameters for implementation of the quantum NOT operation are determined. Analytical results obtained in the rotating-wave approximation are confirmed by rigorous numerical calculations.     

Controllable Kerr Nonlinearity with Perfect Transparency in the Existence of Spontaneously Generated Coherence

We investigate the Kerr nonlinearity of a V-type three-level atomic system where the upper two states decay outside to another state and hence spontaneous generated coherence may exist. It is shown that dark state and hence perfect transparency present under certain conditions. Meanwhile, the Kerr nonlinearity can be controlled by manipulation of the decay rates and the splitting of the two excited states. Therefore, enhanced Kerr nonlinearity without absorption can be obtained under proper parameters.     

Optical Generation of Single- or Two-Mode Excited Entangled Coherent States

With nonlinear Mach-Zehnder interferometer （NLMZI） and a typed beta-barium borate （BBO） crystal, we optically generate single-mode excited entangled coherent states. This scheme can be easily generalized to generate two-mode excited entangled coherent states. We simply analyse different influences of single- and two-mode photon excitations on entangled coherent states.     

A Scheme for Atomic Entangled States and Quantum Gate Operations in Cavity QED

We propose a scheme for controllably entangling the ground states of five-state W-type atoms confined in a cavity and realizing swap gate and phase gate operations. In this scheme the cavity is only virtually excited and the atomic excited states are almost not occupied, so the produced entangled states and quantum logic operations are very robust against the cavity decay and atomic spontaneous emission.     

Preparation of Fock states and quantum entanglement via stimulated Raman adiabatic passage using a four-level atom

We study the behaviour of an atom-cavity system exposed to a stimulated Raman adiabatic passage (STIRAP) process in a four-level system, with a coupling scheme which generate two degenerate dark states. We find that the non-adiabatic interaction of the two dark states guarantees that the cavity Fock states can always be generated by both intuitively and counterintuitively ordered pulses. Furthermore, we propose a method to entangle two atoms. Depending on the ordering of the pulses two orthogonal entangled states can be prepared. Since these entangled states do not have component of the excited states included, the technique is robust against the detrimental consequences of spontaneous emission. Received 20 March 2001     

Modeling vacancies and hydrogen impurities in graphene: A molecular point of view

We have followed a ‘molecular’ approach to study impurity effects in graphene. This is thought as the limiting case of an infinitely large cluster of benzene rings. Therefore, we study several carbon clusters, with increasing size, from phenalene, including three benzene rings, up to coronene 61, with 61 benzene rings. The impurities considered were a chemisorbed H atom, a vacancy, and a substitutional proton. We performed HF and UHF calculations using the STO-3G basis set. With increasing cluster size in the absence of impurities, we find a decreasing energy gap, here defined as the HOMO-LUMO difference. In the case of H chemisorption or a vacancy, the gap does not decrease appreciably, whereas it is substantially reduced in the case of a substitutional proton. The presence of an impurity invariably induces an increase of the density of states near the HOMO level. We find a zero mode only in the case of a substitutional proton. In agreement with experiments, we find that both the chemisorbed H, the substitutional proton, and the C atom near a vacancy acquire a magnetic moment. The relevance of graphene clusters for the design of novel electronic devices is also discussed.     

Effect of toroidal moment on a macroscopic self-organization of electrons in the quantum Hall regime

We have studied CR lineshape of terahertz-light-induced current in InAs quantum wells in tilted quantizing magnetic fields. We have observed dramatic modification of the lineshape with increasing of in-plane component of magnetic field as well as with increasing of transverse built-in electric field in the well. Scenario of the modification shows that the energy spectrum asymmetry is determined by so-called toroidal moment of the system and is a function of Landau quantum number. Macroscopic self-organization of electrons under the conditions of quantum Hall effect has also been directly demonstrated in both linear and saturation regimes of the light absorption.     

Ultrahigh Harmonic Generation from an Atom with Superposition of Ground State and Highly Excited States

We investigate the high-order harmonic generation from an atom prepared in a superposition of ground state and highly excited state. When the atom is irradiated by an ultrashort pulse, the cutoff position of the plateau in the harmonic spectrum is largely extended compared with the case that the atom is initially in the ground state. The physics of the extension of the high-order harmonic plateau can be interpreted by the spatial structure of the atomic initial wave packet. We can optimize the generation of high-order harmonics by substituting the excited state for a particular coherent superposition of some highly excited states to form a spatially localized excited wave packet.     

A quantum chemical approach towards the electronically excited states of helium clusters

We have investigated the electronic energies of the ground and excited states of an octahedral helium cluster by quantum chemical ab initio calculations. The excited levels were calculated for the central atom for a set of different inter atomic separations. Our approach yields potential energy curves which are suited to describe a density dependence of the spectral features as previously observed in photo excitation experiments. The potential energy curves of the 2s and 2p states show a hump at ? caused by the strong perturbation of neighbouring atoms. The existence of this hump explains the experimentally observed blue shift and its dependence on the cluster size or density, respectively. The potential curves of the higher levels show almost constant energies. Perturbations of these levels are small, because the overlap between the Rydberg orbital and the orbitals of the surrounding atoms is small. This is the case for both small R values where the Rydberg orbital is well outside the cluster as well as for large R where the density drastically decreases. These findings coincide with the un-shifted features of small clusters observed in experiments.     

Angular momentum population in fragmentation reactions

Using a relativistic transport model followed by a statistical sequential binary emission model, the population of metastable high-spin isomeric states are studied in relativistic projectile fragmentation reactions. The initial angular momentum distribution are generated from hole excitations. We find that the angular momentum distribution of the excited prefragments are considerably broadened due to light particle evaporation. The model reproduces the experimentally measured population of relatively low-lying states and underpredicts states with high angular momentum I?17?$I?17?$. We propose that coupling the spin of the excited and hole states in the prefragment will give a better understanding of the data.     

Abnormal temperature dependence of photoluminescence from self-assembled InAs quantum dots covered by an InAlAs/InGaAs combination layer

In this report we have investigated the temperature dependence of photoluminescence (PL) from self-assembled InAs quantum dots (QDs) covered by an InAlAs/InGaAs combination layer. The ground state experiences an abnormal variation of PL linewidth from 15 K up to room temperature. Meanwhile, the PL integrated intensity ratio of the first excited state to the ground state for InAs QDs unexpectedly decreases with increasing temperature, which we attribute to the phonon bottleneck effect. We believe that these experimental results are closely related to the partially coupled quantum dots system and the large energy separation between the ground and the first excited states.     

NLIE of Dirichlet sine-Gordon model for boundary bound states

We investigate boundary bound states of sine-Gordon model on the finite-size strip with Dirichlet boundary conditions. For the purpose we derive the nonlinear integral equation (NLIE) for the boundary excited states from the Bethe ansatz equation of the inhomogeneous XXZ spin 1/2 chain with boundary imaginary roots discovered by Saleur and Skorik. Taking a large volume (IR) limit we calculate boundary energies, boundary reflection factors and boundary Lüscher corrections and compare with the excited boundary states of the Dirichlet sine-Gordon model first considered by Dorey and Mattsson. We also consider the short distance limit and relate the IR scattering data with that of the UV conformal field theory.     

Graphene wormholes: A condensed matter illustration of Dirac fermions in curved space

We study the properties of graphene wormholes in which a short nanotube acts as a bridge between two graphene sheets, where the honeycomb carbon lattice is curved from the presence of 12 heptagonal defects. By taking the nanotube bridge with very small length compared to the radius, we develop an effective theory of Dirac fermions to account for the low-energy electronic properties of the wormholes in the continuum limit, where the frustration induced by the heptagonal defects is mimicked by a line of fictitious gauge flux attached to each of them. We find in particular that, when the effective gauge flux from the topological defects becomes maximal, the zero-energy modes of the Dirac equation can be arranged into two triplets, that can be thought as the counterpart of the two triplets of zero modes that arise in the dual instance of the continuum limit of large spherical fullerenes. We further investigate the graphene wormhole spectra by performing a numerical diagonalization of tight-binding Hamiltonians for very large lattices realizing the wormhole geometry. The correspondence between the number of localized electronic states observed in the numerical approach and the effective gauge flux predicted in the continuum limit shows that graphene wormholes can be consistently described by an effective theory of two Dirac fermion fields in the curved geometry of the wormhole, opening the possibility of using real samples of the carbon material as a playground to experiment with the interaction between the background curvature and the Dirac fields.