Electronic state calculations of spherical diamond nanocrystals

Electronic-state calculations of diamond nanocrystals simulated by ultrasmall quantum spheres of diamond passivated by hydrogen are performed by the extended Hückel-type nonorthogonal tight-binding method. Two kinds of surface configuration (ideal and dimerized ones) are studied. Special attention has been paid to surface as well as quantum-confinement effects. The calculated results have demonstrated that, while the HOMO (highest occupied molecular orbital) energies are independent of the surface configuration and depend clearly on the size of the diamond spheres, the LUMO (lowest unoccupied molecular orbital) energies of the diamond spheres with one or two dimers on the surface are rather insensitive to the size, in agreement with experiment. The latter is found to be ascribed to the occurrence of surfacelike states associated with the backbonds of the dimer. It is shown that calculated lifetimes across the energy gap are less than 100 microseconds, suggesting that the diamond nanocrystals are promising light-emitting materials.     

Spin injection and detection in silicon

Spin injection and detection in silicon is a difficult problem, in part because the weak spin-orbit coupling and indirect gap preclude using standard optical techniques. Two ways to overcome this difficulty are proposed, both based on spin-polarized transport across a heterojunction. Using a realistic transport model incorporating the relevant spin dynamics of both electrons and holes, it is argued that symmetry properties of the charge current can be exploited to detect electrical spin injection in silicon using currently available techniques.     

Spin coherence and electromagnetically induced transparency via exciton correlations

We report experimental studies on exciton spin coherence induced via Coulomb correlations between excitons with opposite spins, including correlations associated with unbound as well as bound exciton pairs. Electromagnetically induced transparency resulting from the spin coherence is demonstrated in the transient optical response in GaAs quantum wells.     

Diffraction peak profiles of surface relaxed spherical nanocrystals

Abstract

A model is proposed for surface relaxation of spherical nanocrystals. Besides reproducing the primary effect of changing the average unit cell parameter, the model accounts for the inhomogeneous atomic displacement caused by surface relaxation and its effect on the diffraction line profiles. Based on three parameters with clear physical meanings - extension of the sub-coordination effect, maximum radial displacement due to sub-coordination, and effective hydrostatic pressure - the model also considers elastic anisotropy and provides parametric expressions of the diffraction line profiles directly applicable in data analysis. The model was tested on spherical nanocrystals of several fcc metals, matching atomic positions with those provided by Molecular Dynamics (MD) simulations based on embedded atom potentials. Agreement was also verified between powder diffraction patterns generated by the Debye scattering equation, using atomic positions from MD and the proposed model.     

纳米半导体中多重激子效应研究进展

多重激子效应是指纳米半导体吸收一个高能光子后产生两个甚至多个电子-空穴对的物理过程,不仅具有重要的基础研究意义,而且在新型太阳电池及高性能光电子器件领域具有潜在应用价值.综述了多重激子效应的发展历程;总结了纳米半导体的材料组分、体系结构甚至表面质量对多重激子效应的影响;介绍了多重激子效应的实验测试分析方法以及解释多重激子效应的理论方法;概括了目前多重激子效应在器件中的应用并对其应用前景进行展望.     

Semiconductor nanocrystals in carrier-transporting polymers: Charge generation and charge transport

By incorporating semiconductor nanocrystals in carrier-transporting polymers, an interesting class of photoconductive nanocomposites is created. The presence of semiconductor nanocrystals enhances the photoinduced charge generation efficiency and extends the sensitivity range, while the polymer matrix is responsible for charge transport. A wide variety of semiconductors and polymers have been used. In this paper, we review material synthesis and discuss the effects of semiconductor nanocrystals on the charge transport and charge generation properties.     

Spin coherence during optical excitation of a single nitrogen-vacancy center in diamond

We examine the quantum spin state of a single nitrogen-vacancy (NV) center in diamond at room temperature as it makes a transition from the orbital ground state (GS) to the orbital excited state (ES) during nonresonant optical excitation. While the fluorescence readout of NV-center spins relies on conservation of the longitudinal spin projection during optical excitation, the question of quantum phase preservation has not been examined. Using Ramsey measurements and quantum process tomography of the optical excitation process, we measure a trace fidelity of F=0.87±0.03, which includes ES spin dephasing during measurement. Extrapolation to the moment of optical excitation yields F≈0.95. This result provides insight into the interaction between spin coherence and nonresonant optical absorption through a vibronic sideband.     

Spin magnetic moment current of impurity electron in a spherical quantum dot

Current of the spin magnetic moment of an electron coupled with hydrogen-like impurity, placed in the center of a spherical quantum dot, is studied. Confinement potential of the quantum dot is approximated by rectangular infinitely high walls. S-states of electron for both positive and negative values of total energy are considered. Dependences of spin magnetic moment current of electron on the radial variable for different values of dot radius are obtained. It is shown that for quantum dots with large radius, due to weakening of size-quantization effect, corresponding curves approach each other.     

Electrical detection of spin coherence in silicon

Experimental evidence is presented showing that photocurrents in silicon can be used as highly sensitive readout probes for coherent spin states of localized electrons, the prime candidates for quantum bits in various semiconductor based quantum computer concepts. Conduction electrons are subjected to fast Rabi oscillation induced by means of pulsed electron spin resonance. The collective spin motion of the charge carrier ensemble is reflected by a spin-dependent recombination rate and therefore by the sample conductivity. Because of inhomogeneities, the Rabi oscillation dephases rapidly. However, a microwave induced rephasing is possible causing an echo effect whose intensity contains information about the charge carrier spin state and the coherence decay.     

Quantum coherence and entangled states in intracavity third subharmonic generation process

The dynamics of correlation of photon number fluctuations of interacting modes for the process of intracavity third subharmonic generation is investigated. It is shown that the entangled field states by the variables of photon number can be obtained in this system. The quantum dynamics of the photons number, the quantum entropy and the Wigner function of the stationary states of the fundamental mode and the third subharmonic mode have also been studied. It is found that the dynamics of these quantities depends highly on the value of the coupling coefficient of the interacting modes. It is shown that at long interaction times and for the large values of the coupling coefficient of the modes, the mode of the third subharmonic is localized in the three-component state with the same probability of detection of the mode in each component of the state. The quantum entropy of the state is less than the maximal entropy of the three-component state ln3, which points out the presence of quantum mechanical interference between the components of the state of third subharmonic mode.     

Spin detection and manipulation with scanning tunneling microscopy

Over the past few decades, spin detection and manipulation at the atomic scale using scanning tunneling microcopy has matured, which has opened the possibility of realizing spin-based functional devices with single atoms and molecules.This article reviews the principle of spin polarized scanning tunneling microscopy and inelastic tunneling spectroscopy,which are used to measure the static spin structure and dynamic spin excitation, respectively. Recent progress will be presented, including complex spin structure, magnetization of single atoms and molecules, as well as spin excitation of single atoms, clusters, and molecules. Finally, progress in the use of spin polarized tunneling current to manipulate an atomic magnet is discussed.     

Spin reorientation in nickel. surface impedance and electromagnetic generation of sound

The surface resistance features of Ni in magnetic fields when the domain structure vanishes have been detected experimentally and investigated theoretically. The anomaly of impedance smooth part is accompanied by the change in the acoustic resonance line shapes.     

Sideband generation using strongly driven raman coherence in solid hydrogen

Parametric Raman sideband generation is investigated using strongly driven Raman coherence in solid hydrogen. We show that the Raman coherence prepared with two coaxial single-mode lasers beats with multimode laser radiation with very broad bandwidth and efficiently replicates the broadband nature to the Raman sidebands without the restriction of phase matching. We demonstrate that this efficient replication occurs mainly on the negative side of Raman detuning, where the medium adiabatically follows the antiphased state.     

Second-harmonic generation from a thin spherical layer and No-generation conditions

The dispersion of the electro-optical effect in polydisperse hydrosols of diamond is studied in the external field frequency ranging from 10 Hz to 2.5 MHz. The conservative dichroism, which is determined by the difference in the turbidity constants for light polarized along and perpendicular to the particle-orienting field, is chosen as the electro-optic effect. The dispersion dependences of the dichroism are determined and used to calculate the dispersion of the average values of the polarizability anisotropy of the diamond particles. The influence of the concentration of monovalent ions in an aqueous KCl solution on these dispersion dependences is studied. The experimental dependences of the dispersion are compared with the theoretical ones within the model of the polarizability of the colloidal particles which takes into account the polarizability of the diffuse part of their double electric layer.     

Spin coherence of holes in GaAs/(Al,Ga)As quantum wells

Carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a diluted hole gas is studied by picosecond pump-probe Kerr rotation. For resonant optical excitation of the positively charged exciton the spin precession shows two types of oscillations: Electron spin beats decaying with the charged exciton radiative lifetime of 50 ps, and long-lived hole spin beats with dephasing times up to 650 ps, which decrease with increasing temperature, underlining the importance of hole localization. The mechanism of hole spin coherence generation is discussed.     

Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

Within the range of photon energies illuminating the Earth's surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron‐hole pair; energy of a photon in excess of the semiconductor's bandgap is efficiently converted to heat through interactions between the electron and hole with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron‐hole pair generation from a single photon with energy greater than twice the effective band gap. This multiple carrier pair process, referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the near‐term future for both fundamental research and the development of working devices which exploit MEG.     

Enhanced nonlinear generation in a three-level medium with spatially dependent coherence

We consider a method for efficient parametric generation of a laser pulse. A single laser field is injected into a three-level medium that has two lower states and one excited state. The lower states are initially prepared in a position-dependent coherent superposition state. It is shown that, by proper choice of the position dependence of the superposition along the direction of propagation, the incoming field can be completely converted to a new field.     

Simultaneous optical coherence tomography imaging and beta particle detection

A prototype hybrid catheter device designed for imaging and detection of vascular diseases is introduced. The prototype device integrates a high-resolution optical coherent tomography probe and a high-sensitivity beta detector into a single unit. With this prototype device we demonstrate the feasibility of simultaneous optical coherence tomography imaging and detection of beta particles.     

Optical second harmonic generation and ordering effects in the system of ferroelectric barium titanate nanocrystals

The temperature dependence of the optical second harmonic generation in nanocrystalline BaTiO₃:Eu³⁺ revealed a strong hysteresis in the C_4v-O_h ferroelectric phase transition region. The explanation of the effect by the ordering processes in the system of ferroelectric nanocrystals is confirmed by the experiments in an external electric field.     

Degree of coherence and dimensions of the generation region of powder lasers

A degree of coherence of powder-laser radiation of 10–20% is estimated by the magnitude of the image contrast for a speckle, although cases are recorded where it attains 40–50%. It is found that as the average speckle intensity increases, the contrast decreases. It is established that the minimum, dimensions of the generation region in powders are 20–30 μm. Cases of existence of several generation centers simultaneously are noted. An attempt to interpret the results obtained is made. Institute of Radio Engineering and Electronics, Russian Academy of Sciences, 11, Mokhovaya St., Moscow, 103907. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 780–786, September–October, 1998.