Size-dependent melting behavior of indium nanowires

Indium nanowires with different diameters (20-80 nm) and preferred orientations were electrodeposited in the pores of the anodic aluminum oxide (AAO) templates. Differential scanning calorimetry (DSC) studies showed an approximate linear relationship between melting temperature and the reciprocal of diameters. Comparing the experimental data with several theoretical models, we found the effect of crystalline structure should be taken into account for better explaining the size-dependent melting behavior. The influences of surface configuration and nanosolids size on the melting were also discussed.     

Size-dependent melting of self-assembled indium nanostructures

We have measured the melting temperature of nanoscale indium islands on a WSe(2) substrate using perturbed angular correlations combined with scanning tunneling microscopy. The indium islands are self-assembled nanostructures whose diameter can vary between about 5 and 100 nm, depending on deposition conditions. The melting point decreases due to surface energies as the islands get smaller. This decrease depends on the faceting of the crystalline nanostructures and interactions between the islands and the substrate.     

Size-dependent spintronic properties of dilute magnetic semiconductor nanocrystals

The electronic structure and magnetic properties of Mn-doped Ge, GaAs, and ZnSe nanocrystals are investigated using real space ab initio pseudopotentials constructed within the local spin-density approximation. The ferromagnetic and half-metallicity trends found in the bulk are preserved in the nanocrystals. However, the Mn-related impurity states become much deeper in energy with decreasing nanocrystalline size, causing the ferromagnetic stabilization to be dominated by double exchange via localized holes rather than by a Zener-like mechanism.     

Size-dependent melting of nanoparticles: Hundred years of thermodynamic model

Thermodynamic model first published in 1909, is being used extensively to understand the size-dependent melting of nanoparticles. Pawlow deduced an expression for the size-dependent melting temperature of small particles based on the thermodynamic model which was then modified and applied to different nanostructures such as nanowires, prism-shaped nanoparticles, etc. The model has also been modified to understand the melting of supported nanoparticles and superheating of embedded nanoparticles. In this article, we have reviewed the melting behaviour of nanostructures reported in the literature since 1909. This article is dedicated to Indian Institute of Science which is also celebrating its centenary this year.     

On the self-consistent statistical theory of vacancies

We report here a new method of describing the effect of the presence of vacancies on the structure, dynamics and thermodynamics of a crystal. The unsymmetrized self-consistent approximation is used to determine the potential energy in which the defective structure and the anharmonicity appear from the beginning. In order to stress the power of this method, we calculate the free energy of formation of a vacancy and the concentration of vacancy in a two-dimensional triangular Lennard-Jones crystal. A comparison with Monte Carlo simulation is given and a possible application to experimentally accessible systems is discussed.     

Statistical mechanics of a system of interacting dislocations: a self-consistent field approach

Summary The equilibrium statistical mechanics of a population ofN massless dislocations is tackled in the frame of the multivariate Fokker-Planck equation associated with the corresponding system ofN nonlinear stochastic differential equations of motion. First the diffusion coefficient is found as a function of the dislocation drag parameter and absolute temperature. Then the initial many-body problem is reduced to an effective single-particle problem by means of a mean (self-consistent) field strategy leading to a nonlinear singular integral equation for the density and the “thermodynamic potential” at temperatureT. This last equation is solved numberically and the results are shown with comparison to exact continuum deterministic distributions.

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Riassunto Il lavoro affronta la meccanica statistica di equilibrio di un sistema diN dislocazioni interagenti in un campo esterno. Le dislocazioni sono trattate come singolarità prive di massa il cui moto è governato daN equazioni differenziali stocastiche di cui si studia l'associata equazione di Fokker-Planck multivariata. Si calcola anzitutto il coefficiente di diffusione in funzione del coefficiente di dissipazione e della temperatura. Quindi il problema a molti corpi viene ridotto ad un problema efficace di particella singola per mezzo di una tecnica di campo medio autoconsistente che si esprime in un'equazione integrale singolare non lineare per la densità delle dislocazioni e l'appropriato potenziale termodinamico alla temperaturaT. Quest'ultima equazione è risolta numericamente mediante un metodo di approssimazioni successive e i risultati sono illustrati confrontandoli con quelli ottenibili da una teoria esatta ma deterministica ed affetta dall'approssimazione del continuo.

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Резюме Рассматривается равновесная статистическая механика совокупностиN безмассовых дислокаций в рамках уравнения фоккера-Планка связанного с соответствующей системойN нелинейных стохастических дифференциальных уравнений движения. Сначала получается коэффициент диффузии, как функция параметра, характеризующего сопротивление при движении дислокаций, и абсолютной температуры. Эатем исходная проблема многих тел сводится к эффективной проблеме одной частицы, используя подход самосогласованного, поля, который приводит к нелинейному сингулярному интегральному уравнению для плотности и ?термодинамического потенциала? при температуреT. Полученное уравнение решается численно. Результаты сравниваются с точными с точными непрерывными детерминистическими распределениями.

     

Nuclear collective motions in a self-consistent Landau-Vlasov approach

A semiclassical theory of nuclear collective motions based on the Vlasov equation is extended to take into account self-consistency effects provided by the residual interaction. Using separable forces a correlated response function is derived which has several striking similarities with fully quantum results. Explicit spin-orbit terms are also included through a clear WKB correspondence with the quantum response. Finite temperature effects are analysed showing little change in the strength distribution. Through a relaxation time approach two-body collision terms are introduced. This is essential to account for the damping widths of isoscalar quadrupole and octupole giant resonances. However a quite important interplay between self-consistent (Landau damping) and collisional damping is also revealed.     

Size-dependent intrinsic radiative decay rates of silicon nanocrystals at large confinement energies

We study ultrafast photoluminescence (PL) dynamics of Si nanocrystals (NCs). The early-time PL spectra (<1 ns), which show strong dependence on NC size, are attributed to emission involving NC quantized states. The PL spectra recorded for long delays (>10 ns) are almost independent of NC size and are likely due to surface-related recombination. Based on instantaneous PL intensities measured 2 ps after excitation, we determine intrinsic radiative rate constants for NCs of different sizes. These constants sharply increase for confinement energies greater than approximately 1 eV indicating a fast, exponential growth of the oscillator strength of zero-phonon, pseudodirect transitions.     

Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach

Transition-metal centers are the active sites for a broad variety of biological and inorganic chemical reactions. Notwithstanding this central importance, density-functional theory calculations based on generalized-gradient approximations often fail to describe energetics, multiplet structures, reaction barriers, and geometries around the active sites. We suggest here an alternative approach, derived from the Hubbard U correction to solid-state problems, that provides an excellent agreement with correlated-electron quantum chemistry calculations in test cases that range from the ground state of Fe2 and Fe2- to the addition elimination of molecular hydrogen on FeO+. The Hubbard U is determined with a novel self-consistent procedure based on a linear-response approach.     

Modeling cohesive energy and melting temperature of nanocrystals

A model has been developed to account for the size dependent cohesive energy and melting temperature of nanocrystals. This model can deal with the thermodynamic properties of nanoparticles (spherical and non-spherical), nanowires and nanofilms with free surface or non-free surface (embedded in a matrix). The cohesive energy depression of nanocrystals has been predicted, and the conditions of superheating are obtained. It is found that the present theoretical results are consistent with the available experimental values.     

A self-consistent approach to localization transition

The Anderson transition is investigated using a self-consistent approximation in analogy with mean-field approximations in classical spin systems. Mobility edge trajectories in a three-dimensional disordered system with various distributions of the site energies are obtained. The present results are qualitatively in good agreement with the results obtained by using the finite-size scaling method.     

Gaussian self-consistent approach to multichain polymer systems

Here we extend the Gaussian self-consistent method that has been successfully applied to the single polymer collapse problem to a polymer solution of many identical chains earlier. This method permits a complete study of the equilibrium thermodynamic properties as well as of the kinetic transformations. We discuss various aspects of the aggregation and collapse phenomena paying particular attention to the interplay between the intra- and inter-molecular degrees of freedom. We show that, at equilibrium, in part of the parameter space our equations may be reduced to those of the Flory-Huggins mean field theory. In kinetics a new phenomenon, notably the existence of a long-lived metastable state of “mesoglobules”, is found.     

The melting of silicon nanocrystals: Submicron thin-film structures derived from nanocrystal precursors

The size-dependent melting temperature has been shown for the group IV semiconductor silicon. A cursory comparison is made between silicon nanocrystal melting and that observed in the group II–VI material Cds and the group III–V material GaAs. Particles dispersed at a number density such that there are interactions between nanocrystals are observed to sinter before size-dependent melting occurs. Using an electron beam to selectively remove an organic mask from a substrate, this phenomenon is exploited to produce thin-film structures. This work has implications in the production of nanoelectronics, nonlinear optics and solar conversion technologies.     

Diffusion and coupled fluxes in concentrated alloys under irradiation: a self-consistent mean-field approach

When an alloy is irradiated, atomic transport can occur through the two types of defects which are created: vacancies and interstitials. Recent developments of the self-consistent mean field (SCMF) kinetic theory could treat within the same formalism diffusion due to vacancies and interstitials in a multi-component alloy. It starts from a microscopic model of the atomic transport via vacancies and interstitials and yields the fluxes with a complete Onsager matrix of the phenomenological coefficients. The jump frequencies depend on the local environment through a ‘broken bond model’ such that the large range of frequencies involved in concentrated alloys is produced by a small number of thermodynamic and kinetic parameters. Kinetic correlations are accounted for through a set of time-dependent effective interactions within a non-equilibrium distribution function of the system. The different approximations of the SCMF theory recover most of the previous diffusion models. Recent improvements of the theory were to extend the multi-frequency approach usually restricted to dilute alloys to diffusion in concentrated alloys with jump frequencies depending on local concentrations and to generalize the formalism first developed for the vacancy diffusion mechanism to the more complex diffusion mechanism of the interstitial in the dumbbell configuration. To cite this article: M. Nastar, C. R. Physique 9 (2008).     

A collapsing bubble in a fluid: a statistical mechanical approach

The first non-equilibrium statistical mechanical theory is presented for the mechanical and thermal behaviour of the collapse of a microsropically small bubble in a liquid. First the number density and temperature space-time profiles for the special case of weakly interacting particles, the perfect gas model, are obtained. This is then generalized to a model in which the motion of the molecules is characterized by a single finite diffusion constant. The results for the collapse of a small bubble in a typical fluid are compared with those recently obtained through computer simulation. The agreement with the simulation is remarkably good for the perfect gas model; very high temperatures, sufficient for sonoluminescence, appear in a simple and natural way. An unexpected conclusion is that the perfect gas model agrees better with computer simulation than the model characterized by a single bulk diffusion constant. This may be because the collapse of the bubble is controlled by the leading shell of the fluid where the fluid density is low.     

Spectral description of fluctuating electromagnetic fields of solids using a self-consistent approach

A new way to evaluate the spectral-correlation properties of thermal fields of solids is suggested. The principal element here is the surface linear response function of an inhomogeneous electron subsystem of solids. Along with straightforward calculations using the known response functions, the suggested method allows calculating the response functions self-consistently based on the time dependent density functional theory. The self-consistent calculation of the linear response function followed by an application of the fluctuation–dissipation theorem yields spectral power densities of the fluctuating electromagnetic fields.     

The band structure of several zinc-blende semiconductors from a self-consistent pseudopotential approach

Using a nearly self-consistent pseudopotential, band structures are calculated for seven III-V compounds. The core-valence interaction is treated in the empty-core approximation using a Pauling-Slater scaling procedure. All adjustable constants have been eliminated from the theory. The results are extremely encouraging.     

Robustness of spatial average equalization: a statistical reverberation model approach

Traditionally, multiple listener room equalization is performed to improve sound quality at all listeners, during audio playback, in a multiple listener environment (e.g., movie theaters, automobiles, etc.). A typical way of doing multiple listener equalization is through spatial averaging, where the room responses are averaged spatially between positions and an inverse equalization filter is found from the spatially averaged result. However, the equalization performance, will be affected if there is a mismatch between the position of the microphones (which are used for measuring the room responses for designing the equalization filter) and the actual center of listener head position (during playback). In this paper, we will present results on the effects of microphone-listener mismatch on spatial average equalization performance. The results indicate that, for the analyzed rectangular configuration, the region of effective equalization depends on (i) the distance of a listener from the source, (ii) the amount of mismatch between the responses, and (iii) the frequency of the audio signal. We also present some convergence analysis to interpret the results.     

Andreev reflection between a normal metal and the FFLO superconductor II: A self-consistent approach

We consider Andreev reflection in a two dimensional junction between a normal metal and a heavy fermion superconductor in the Fulde–Ferrell (FF) type of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. We assume s-wave symmetry of the superconducting gap. The parameters of the superconductor: the gap magnitude, the chemical potential, and the Cooper pair center-of-mass-momentum Q, are all determined self-consistently within a mean-field (BCS) scheme. The Cooper pair momentum Q is chosen as perpendicular to the junction interface. We calculate the junction conductance for a series of barrier strengths. In the case of incoming electron with spin σ = ↑ only for magnetic fields close to the upper critical field H_c2, we obtain the so-called Andreev window, i.e. the energy interval in which the reflection probability is maximal, which in turn is indicated by a peak in the conductance. The last result differs with other non-self-consistent calculations existing in the literature.