Size dependence of biexciton binding energy in single InAs/GaAs quantum dots

This paper studies the size dependence of biexciton binding energy in single quantum dots (QDs) by using atomic force microscopy and micro-photoluminescence measurements. It finds that the biexciton binding energies in the QDs show "binding" and "antibinding" properties which correspond to the large and small sizes of QDs, respectively. The experimental results can be well interpreted by the biexciton potential curve, calculated from the exciton molecular model and the Heitler-London method.

System size dependence of associated yields in hadron-triggered jets STAR Collaboration

We present results on the system size dependence of high transverse momentum di-hadron correlations at root s(NN) = 200 GeV as measured by STAR at RHIC. Measurements in d + Au, Cu + Cu and Au + Au collisions reveal similar jet-like near-side correlation yields (correlations at small angular separation Delta phi similar to 0, Delta eta similar to 0) for all systems and centralities. Previous measurements have shown Chat the away-side (Delta phi similar to pi) yield is suppressed in heavy-ion collisions. We present measurements of the away-side Suppression as a function of transverse momentum and centrality in Cu + Cu and Au + Au collisions. The suppression is found to be similar in Cu + Cu and An + An collisions at a similar number of participants. The results are compared to theoretical calculations based on the patron quenching model and the modified fragmentation model. The observed differences between data and theory indicate that the correlated yields presented here will further constrain dynamic energy loss models and provide information about the dynamic density profile in heavy-ion collisions. (C) 2009 Elsevier B.V. All rights reserved.

Center of mass energy and system-size dependence of photon production at forward rapidity at RHIC

We present the multiplicity and pseudorapidity distributions of photons produced in Au + Au and Cu + Cu collisions at root(NN)-N-s = 62.4 and 200 GeV. The photons are measured in the region -3.7 < eta < -2.3 using the photon Multiplicity detector in the STAR experiment at RHIC. The number of photons produced per average number of participating nucleon pairs increases with the beam energy and is independent of (lie collision centrality. For collisions with similar average numbers of participating nucleons the photon multiplicities are observed to be similar for An + Au and Cu + Cu collisions at a given beam energy. The ratios of the number of charged particles to photons in the measured pseudorapidity range are found to be 1.4 +/- 0.1 and 1.2 +/- 0.1 for root(NN)-N-s = 62.4 and 200 GeV, respectively. The energy dependence of this ratio could reflect varying contributions from baryons to charged particles, while mesons are the dominant contributors to photon production in the given kinematic region. The photon pseudorapidity distributions normalized by average number of participating nucleon pairs, when plotted as a function of eta-Y-beam, are found to follow a longitudinal scaling independent of centrality and colliding ion species at both beam energies. (C) 2009 Elsevier B.V. All rights reserved.

Size dependence of biomass spectra and abundance spectra: the optimal distributions

Based on the hypothesis of self-optimization, we derive four models of biomass spectra and abundance spectra in communities with size-dependent metabolic rates. In Models 1 and 2, the maximum diversity of population abundance in different size classes subject to the constraints of constant mean body mass and constant mean respiration rate is assumed to be the strategy for ecosystems to organize their size structure. In Models 3 and 4, the organizing strategy is defined as the maximum diversity of biomass in different size classes without constraints on mean body mass and subject to the constant mean specific respiration rate of all individuals, i.e. the average specific respiration rate over all individuals of a community or group, which characterizes the mean rate of energy consumption in a community. Models 1 and 2 generate peaked distributions of biomass spectral density whereas Model 3 generates a fiat distribution. In Model 4, the distributions of biomass spectral density and of abundance spectral density depend on the Lagrangian multipler (lambda (2)). When lambda (2) tends to zero or equals zero, the distributions of biomass spectral density and of abundance spectral density correspond to those from Model 3. When lambda (2) has a large negative value, the biomass spectrum is similar to the empirical fiat biomass spectrum organized in logarithmic size intervals. When lambda (2) > 0, the biomass spectral density increases with body mass and the distribution of abundance spectral density is an unimodal curve. (C) 2001 Elsevier Science B.V. All rights reserved.

A Study of Size-Dependent Microindentation

We recently proposed a strain gradient theory to account for the size dependence of plastic deformation at micron and submicron length scales. The strain gradient theory includes the effects of both rotation gradient and stretch gradient such that the rotation gradient influences the material character through the interaction between the Cauchy stresses and the couple stresses; the stretch gradient measures explicitly enter the constitutive relations through the instantaneous tangent modulus. Indentation tests at scales on the order of one micron have shown that measured hardness increases significantly with decreasing indent size. In the present paper, the strain gradient theory is used to model materials undergoing small-scale indentations. A strong effect of including strain gradients in the constitutive description is found with hardness increasing by a factor of two or more over the relevant range behavior. Comparisons with the experimental data for polycrystalline copper and single crystal copper indeed show an approximately linear dependence of the square of the hardness, H 2, on the inverse of the indentation depth, 1/h, I.e., H-2 proportional to 1/h, which provides an important self-consistent check of the strain gradient theory proposed by the authors earlier.

Wurtzite-to-tetragonal structure phase transformation and size effect in ZnO nanorods

The deformation of [0001]-oriented ZnO nanorods with hexagonal cross sections under uniaxial tensile loading is analyzed through molecular statistical thermodynamics (MST) simulations. The focus is on the size dependence of mechanical behavior in ZnO nanorods with diameters ranging from 1.95 to 17.5 nm. An irreversible phase transformation from the wurtzite (P6(3)mc space group) structure to a tetragonal structure (P4(2)/mnm space group) occurs during the tensile loading process. Young's modulus before the transformation demonstrates a size dependence consistent with what is observed in experiments. A stronger size dependence of response is seen after the transformation and is attributed to the polycrystalline nature of the transformed structure. A comparison of the MST and molecular dynamics (MD) methods shows that MST is 60 times faster than MD and yields results consistent with the results of MD.

Solvent size effect on the static and dynamic properties of polymer chains in athermal solvents

The static and dynamic properties of polymer chains in athermal solvents with different sizes are studied by molecular dynamics method. With increasing solvent size, the radius of gyration and the diffusion coefficient of the polymer decay fast until a critical solvent size is reached. For the polymer diffusion coefficients, this decay only depends on the solvent size; while for the radius of gyration of polymers, this decay depends on both solvent size and the length of the polymers. The increase of solvent size also makes the polymer tend to be thicker ellipsoid until a critical solvent size is reached. The static scaling exponent of the polymer also shows the solvent size dependence. Moreover, four regions are identified where the polymers show different dynamic behaviors according to the dynamic structure factors of the polymer.

Finite Element Solutions for Plane Strain Mode I Crack with Strain Grading Effect

In this paper, a new phenomenological theory with strain gradient effects is proposed to account for the size dependence of plastic deformation at micro- and submicro-length scales. The theory fits within the framework of general couple stress theory and three rotational degrees of freedom omega(i) are introduced in addition to the conventional three translational degrees of freedom mu(i). omega(i) is called micro-rotation and is the sum of material rotation plus the particles' relative rotation. While the new theory is used to analyze the crack tip field or the indentation problems, the stretch gradient is considered through a new hardening law. The key features of the theory are that the rotation gradient influences the material character through the interaction between the Cauchy stresses and the couple stresses; the term of stretch gradient is represented as an internal variable to increase the tangent modulus. In fact the present new strain gradient theory is the combination of the strain gradient theory proposed by Chen and Wang (Int. J. Plast., in press) and the hardening law given by Chen and Wang (Acta Mater. 48 (2000a) 3997). In this paper we focus on the finite element method to investigate material fracture for an elastic-power law hardening solid. With remotely imposed classical K fields, the full field solutions are obtained numerically. It is found that the size of the strain gradient dominance zone is characterized by the intrinsic material length l(1). Outside the strain gradient dominance zone, the computed stress field tends to be a classical plasticity field and then K field. The singularity of stresses ahead of the crack tip is higher than that of the classical field and tends to the square root singularity, which has important consequences for crack growth in materials by decohesion at the atomic scale. (C) 2002 Elsevier Science Ltd. All rights reserved.

Prevalence Of Shear Banding In Compression Of Zr41Ti14Cu12.5Ni10Be22.5 Pillars As Small As 150 Nm In Diameter

Recently, the size dependence of mechanical behaviors, particularly the yield strength and plastic deformation mode, of bulk metallic glasses (BMG) has created a great deal of interest. Contradicting conclusions have been drawn by different research groups, based on various experiments on different BMG systems. Based on in situ compression transmission electron microscopy (TEM) experiments on Zr41Ti14Cu12.5Ni10Be22.5 (Vit 1) nanopillars, this paper provides strong evidence that shear banding still prevails at specimen length scales as small as 150 nm in diameter. This is supported by in situ and ex situ images of shear bands, and by the carefully recorded displacement bursts under load control its well as load drops under displacement control. Finite element modeling of the stress state within the pillar shows that the unavoidable geometry constraints accompanying such experiments impart a strong effect on the experimental results, including non-uniform stress distributions and high level hydrostatic pressures. The seemingly improved compressive ductility is believed to be due to such geometry constraints. Observations underscore the notion that the mechanical behavior of metallic glasses, including strength and plastic deformation mode, is size independent at least in Vit 1. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Polaronic correction to the first excited electronic energy level in an anisotropic semiconductor quantum dot

Within the framework of second-order Rayleigh-Schrodinger perturbation theory, the polaronic correction to the first excited state energy of an electron in an quantum dot with anisotropic parabolic confinements is presented. Compared with isotropic confinements, anisotropic confinements will make the degeneracy of the excited states to be totally or partly lifted. On the basis of a three-dimensional Frohlich's Hamiltonian with anisotropic confinements, the first excited state properties in two-dimensional quantum dots as well as quantum wells and wires can also be easily obtained by taking special limits. Calculations show that the first excited polaronic effect can be considerable in small quantum dots.

飞秒强激光脉冲与氢团簇相互作用产生的最大质子能量与团簇尺寸的相关性

报道了飞秒强激光脉冲（60fs，790nm，2×10^16W／cm^2与纳米尺寸的氢团簇（半径rc约为1～3nm）相互作用，产生的最大质子能量Emax对于团簇半径rc相关性的实验研究结果。从激光-氢团簇相互作用产生的质子的飞行时间谱测量以及团簇尺寸的瑞利散射诊断结果，得到Emax与rc^2成线性正比关系，比例系数为0．75，与报道的理论模拟结果一致，表明氢团簇发生纯库仑爆炸。实验结果同时提示，进一步的理论模拟应考虑气体喷流中团簇的尺寸分布。

Controllable electron g-factors in HgMnTe quantum spheres

The electronic structure and Lande electron g-factors of manganese-doped HgTe quantum spheres are investigated, in the framework of the eight-band effective-mass model and the mean-field approximation. It is found that the electronic structure evolves continuously from the zero-gap configuration to an open-gap configuration with decreasing radius. The size dependence of electron g-factors is calculated with different Mn-doped effective concentration, magnetic field, and temperature values, respectively. It is found that the variations of electron g-factors are quite different for small and large quantum spheres, due to the strong exchange-induced interaction and spin-orbit coupling in the narrow-gap DMS nanocrystals. The electron g-factors are zero at a critical point of spherical radius R-c; however, by modulating the nanocrystal size their absolute values can be turned to be even 400 times larger than those in undoped cases. Copyright (c) EPLA, 2008.

Investigation of phtoluminesecence under hydrostatic pressure in different sized ZnS : Mn nanoparticles

The PL spectra for the 10, 4. 5, 3. 5, 3, 1 nm sized ZnS:Mn2+ nanoparticles and corresponding bulk material under different pressures were investigated. The orange emission band originated from the T-4(1)-(6)A(1) transition of Mn2+ ions showed obvious red shift with the increasing of pressures. The pressure coefficients of Mn-related emissions measured from bulk, 10, 4. 5, 3.5 and 3 nm samples are -29.4 +/- 0.3, -30.1 +/- 0.3, -33.3 +/- 0.6, -34.6 +/- 0.8 and -39 +/- 1 meV/GPa, respectively. The absolute value of the pressure coefficient increases with the decrease of the size of particles. The size dependence of crystal field strength Dq and Racah parameter B accounts for the size behavior of the Mn-related emission in ZnS:Mn nanoparticles. The pressure behavior of Mn-related emission in the 1 nm sized sample is somewhat different from that of other nanoparticles. It may be due to smaller size of 1 nm sample and the special surface condition since ZnS nanoparticles are formed in the cavities of ziolite-Y for the 1 nm sample.

Photoluminescence of doped ZnS nanoparticles under hydrostatic pressure

The pressure dependence of the photoluminescence from ZnS : Mn2+, ZnS : Cu2+, and ZnS : Eu2+ nanoparticles were investigated under hydrostatic pressure up to 6 GPa at room temperature. Both the orange emission from the T-4(1) - (6)A(1) transition of Mn2+ ions and the blue emission from the DA pair transition in the ZnS host were observed in the Mn-doped samples. The measured pressure coefficients are -34.3(8) meV/GPa for the Mn-related emission and -3(3) meV/GPa for the DA band, respectively. The emission corresponding to the 4f(6)5d(1) - 4f(7) transition of Eu2+ ions and the emission related to the transition from the conduction band of ZnS to the t(2) level of Cu2+ ions were observed in the Eu- and Cu-doped samples, respectively. The pressure coefficient of the Eu-related emission was found to be 24.1(5) meV/GPa, while that of the Cu-related emission is 63.2(9) meV/GPa. The size dependence of the pressure coefficients for the Mn-related emission was also investigated. The Mn emission shifts to lower energies with increasing pressure and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS : Mn2+ nanoparticles than in bulk. Moreover, the absolute pressure coefficient increases with the decrease of the particle size. The pressure coefficients calculated based on the crystal field theory are in agreement with the experimental results. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.