Quantum optical studies on individual acceptor bound excitons in a semiconductor

We demonstrate the generation of triggered single photons at a predetermined and well defined energy using the radiative recombination of single nitrogen-bound excitons in a semiconductor. The nitrogen atoms are embedded in a ZnSe quantum well structure and were excited by nonresonant optical pumping (82 MHz) at low temperature (4 K). We find resolution-limited photoluminescence lines (280 micro eV) which display photon antibunching under continuous optical pumping. Our results also suggest that single nitrogen-bound excitons are well suited for cavity quantum electrodynamics experiments.     

Quantum interfaces using nanoscale surface plasmons

The strong coupling between individual optical emitters and propagating surface plasmons confined to a conducting nanotip make this system act as an ideal interface for quantum networks, through which a stationary qubit and a flying photon (surface plasmon) qubit can be interconverted via a Raman process. This quantum interface paves the way for many essential functions of a quantum network, including sending, receiving, transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single-photon. Numerical simulation shows that this scheme is robust against experimental imperfections and has high fidelity. Furthermore, being smaller this interface would significantly facilitate the scalability of quantum computers.     

Quantum chemical and NMR investigations on structure of surface complexes

The semiempirical quantum chemical CNDO/2 method is used to calculate models of specific interaction between benzene, toluene, and butene molecules, respectively, and ions or hydroxyls representing active sites of adsorption on zeolitic surfaces. From energy minima of full potential curves the stabilization energies of the surface complexes have been obtained. On the basis of proposed complexes theoretical carbon-13 NMR chemical shifts of adsorbed molecules are calculated. The theoretical results are in rather good agreement with the experimental ones, confirming the conception of surface complexes. Moreover, experimental paramagnetic shifts of surface complexes containing Co²⁺ ions are tried to interprete in a quite similar way.     

Quantum chemical studies of the solvation of the hydroxide ion

Abstract

To understand and model the solvation of the hydroxide ion, OH(H₂O)^? _n clusters, n = 1?5, are studied using ab initio quantum chemical techniques, largely at the MP2 level of theory using a double zeta plus polarization functions basis extended by diffuse functions. Energies and vibrational frequencies, together with thermodynamic quantities such as enthalpies, entropies and Gibbs free energies, are computed. This permits comparison with experimental estimates of the successive thermodynamic changes associated with the reaction OH(H₂O)^? _n + H₂O → OH(H₂O)^? _n+1. The theoretical values are in good agreement with experiment. The free energy of hydration of OH^? is modelled by a composite discrete-continuum method where the effects of the first hydration shell (n = 3) are obtained from the gas phase cluster calculation, while the long-range effects are modelled using self consistent reaction field theory, namely by calculating the solvation energy of OH(H₂O)^? _n in a dielectric continuum. The best estimate of the solvation (free) energy at 298 K is ?84·5 kcal mol^?1, compared to the experimental value of ?102·8 kcal mol^?1.     

利用五粒子簇态实现四粒子态的量子隐形传态

主要研究了运用五粒子簇态作为量子通道来实现特殊形式的四粒子态的量子隐形传态方案.该方案运用了量子力学中量子纠缠的理论.在这个方案中,发送者只需要进行五粒子冯·诺依曼投影测量,接收者根据发送者的测量结果,通过在其量子位上执行一些适当的幺正变换得出原始的四量子比特状态.提出的这个方案可以很好地应对一般的窃听方式.     

Diagnostic studies of molecular plasmas using mid-infrared semiconductor lasers

Within the last decade mid-infrared absorption spectroscopy between 3 and 20 μm, known as infrared laser absorption spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organo-silicon and boron compounds has lead to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes. The aim of the present article is twofold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas using TDLs and (ii) to report on selected new applications of QCLs in the mid-infrared. PACS  07.57.Ty; 52.70.Kz; 52.80.-s     

Theoretical study of oxygen chemisorption on zinc surface by cluster models

Theoretical calculations by DV-Xα-MO have been performed for clusters representing a Zn(0001) surface with oxygen. The calculated DOS is in good agreement with the experimental UPS spectra found by Briggs or by Abbati et al. A large amount of electron flow from Zn atoms into the oxygen atom occurs and this causes (1) a remarkable upward shift of the oxygen 2p level and (2) formation of an unoccupied level near the highest occupied level. The bonds between the oxygen 2p and the zinc 4s or 4p orbitals have a strong bonding character, and the contribution of the zinc 3d orbital to the Zn-O bond is not important.     

STM studies of the characteristics of the surface fabrication process using chemical and electrical methods

2 surface using STM yielded appreciably different results. It is observed that the threshold pulse voltage is lower than that for graphite at comparable tunneling conditions. The minimum dimension of the as-produced features is appreciably larger than those on graphite. Both observations indicate higher surface instability for 1TTaS₂ as compared with graphite. Received: 27 March 1998     

Quantum confinement and surface chemistry of 0.8–1.6 nm hydrosilylated silicon nanocrystals

In the framework of density functional theory(DFT), we have studied the electronic properties of alkene/alkynehydrosilylated silicon nanocrystals(Si NCs) in the size range from 0.8 nm to 1.6 nm. Among the alkenes with all kinds of functional groups considered in this work, only those containing –NH2and –C4H3S lead to significant hydrosilylationinduced changes in the gap between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO) of an Si NC at the ground state. The quantum confinement effect is dominant for all of the alkenehydrosilylated Si NCs at the ground state. At the excited state, the prevailing effect of surface chemistry only occurs at the smallest(0.8 nm) Si NCs hydrosilylated with alkenes containing –NH2and –C4H3S. Although the alkyne hydrosilylation gives rise to a more significant surface chemistry effect than alkene hydrosilylation, the quantum confinement effect remains dominant for alkyne-hydrosilylated Si NCs at the ground state. However, at the excited state, the effect of surface chemistry induced by the hydrosilylation with conjugated alkynes is strong enough to prevail over that of quantum confinement.     

A theoretical investigation of adsorbate-induced surface relaxation effects using cluster models: Al on Si(111)

Differently sized cluster calculations are used to investigate theoretically the preferred adsorption site for an Al adatom on the Si(111) surface. By performing partial geometry optimizations at the Hartree-Fock level on AlSi_n subclusters around the site of interest we find significant Al adatom-induced surface relaxation effects distorting the Si atoms from their bulk lattice positions. The largest relaxation effects take place at the T₄ site resulting in Al adsorption at the T₄ site to be 5 kcal/mol more stable than at the H₃ site and considerably more stable than adsorption at the T₁ site. However, we only have confidence in this result after performing for the T₄ site a partial geometry optimization on the AlSi₅ subcluster in the AlSi₂₆H₂₄ cluster and by including appropriate correlation corrections.     

Combined photoreflectance/photoluminescence studies of the stability of semiconductor surface passivation

Combined photoreflectance/photoluminescence measurements are proposed for the study of the stability of semiconductor surface passivation. With such an approach, laser action on both charged and recombination-active electron states can be investigated. The efficiency of the method proposed is demonstrated with GaAs substrates passivated by selenium.     

两量子比特非线性相互作用模型中的量子关联

运用量子态变换的方法论证了两个qubit纯态中量子关联与纠缠的等价性.并利用三种带有横场的非线性相互作用模型研究了两个qubit体系中的量子关联.发现合适的横场对于最大量子关联态的获得、平均量子关联的提高都有着积极的作用.两个qubit体系获得最大量子关联时,不同模型,不同的横场,对应的量子态却各不相同.     

Quantum chemical calculations on Al-CVD using DMEAA: surface reaction mechanism of AlH3 on Al(111)

An investigation has been made of the elementary surface reactions of alane and its surface dissociation products using density functional theory and a cluster model to elucidate the reaction mechanism of the deposition of aluminium in aluminium chemical vapour deposition (Al-CVD) using dimethylethylamine alane (DMEAA). Molecular structures, potential energies, and normal mode frequencies of the reactants, the products, and the transition states were calculated for each of the surface elementary reactions. Based on transition state theory, rate constants were estimated from the calculation results. The proposed reaction mechanism is the following. DMEAA is adsorbed onto an aluminium surface without any dissociation in the gas phase; DMEAA dissociates into dimethylethylamine (DMEA) and alane on the surface; DMEA is desorbed and alane remains adsorbed; the adsorbed alane molecules dissociate to AlH₂, AlH, and H on the surface; hydrogen molecules to be desorbed are produced from the hydrogen atoms of AlH whose aluminium atoms form a quasi-aluminium surface on the original surface. The cluster size and structure dependence in quantum chemical calculations has also been considered using model clusters.     

CO chemisorption on two-dimensional cobalt clusters: A surface science approach to cluster chemistry

We have studied the CO chemisorption properties of small two-dimensional clusters (islands) and of monolayers and multilayers of cobalt deposited on Cu(100). The behaviour of one monolayer of cobalt is indistinguishable from the multilayers indicating weak influence of the copper support twoards the adsorption of CO. Below one monolayer of cobalt we observed the appearance of new peaks in the thermal desorption spectrum of CO related to the increasingly large number of edge atoms and small size of the cobalt islands. Of particular interest is the number of CO molecules bound per cobalt atom as a function of island size. This number was found to vary from 0.65, with the compression structure formed on the monolayer and multilayer cobalt coverages, up to approximately 3 when the cobalt coverage decreased to 0.002 monolayers.     

Gas chemical studies using corona discharge reactors

Corona discharges with voltages up to 60 kV (DC) were studied with the aim to induce chemical reactions in flue gases at atmospheric pressure. Various plasma reactors with different geometries of multi-needle arrays were tested. The power input was optimised by studying the electrical parameters of the set-up systematically. Both, solid and liquid electrodes were used in combination with the needle arrays. A precise positioning of the corona needles allowed operation without a ballast resistor. Formation rates for CO and the sum of NO₂ and O₃ are reported and discussed. Three catalytic anode-coatings were tested for their potential to decompose carbon dioxide.     

Topographical studies of semiconductor surfaces by using a combined photo-acoustoelectric method

A simple and rapid nondestructive method for a qualitative analysis of the electronic topography of semiconductor surfaces is presented, which is based on the transverse acoustoelectric effect. The method allows to follow the spatial distribution of the density and the relaxation time of energy states at the surface of the semiconductor and can be used to check its homogeneity as well as to visualize surface electronic structures.     

Quantum physics and chemistry of surfaces

Within the last 15 years, exceptional progress has been made in the experimental study of solid surfaces. Detailed surface-spectroscopic informa. tion is now available on a wide range of surfaces, with semiconductor and transition metal surfaces studied most.¹ In response to this experimental effort a later-starting—but at present equally vigorous—theoretical effort has been undertaken.²