Bulk Regime Research Articles

Local stability of the free additive convolution

We prove that the system of subordination equations, defining the free additive convolution of two probability measures, is stable away from the edges of the support and blow-up singularities by showing that the recent smoothness condition of Kargin is always satisfied. As an application, we consider the local spectral statistics of the random matrix ensemble A+UBU⁎, where U is a Haar distributed random unitary or orthogonal matrix, and A and B are deterministic matrices. In the bulk regime, we prove that the empirical spectral distribution of A+UBU⁎ concentrates around the free additive convolution of the spectral distributions of A and B on scales down to N−2/3.

Counter-Ions Near a Charged Wall: Exact Results for Disc and Planar Geometries

Macromolecules, when immersed in a polar solvent like water, become charged by a fixed surface charge density which is compensated by ``counter-ions'' moving out of the surface. Such classical particle systems exhibit poor screening properties at any temperature and the trivial bulk regime (far away from the charged surface) with no particles, so the validity of standard Coulomb sum rules is questionable. In the present paper, we concentrate on the two-dimensional version of the model with the logarithmic interaction potential. We go from the finite disc to the semi-infinite planar geometry. The system is exactly solvable for two values of the coupling constant $\Gamma$: in the Poisson-Boltzmann mean-field limit $\Gamma\to 0$ and at the free-fermion point $\Gamma=2$. We show that the finite-size expansion of the free energy does not contain universal term as is usual for Coulomb fluids. For the coupling constant being an arbitrary positive even integer, using an anticommuting representation of the partition function and many-body densities we derive a sequence of sum rules. As a result, the contact density of counter-ions at the wall is available for the disc. The amplitude function, which characterizes the asymptotic inverse-power law behavior of the two-body density along the wall, is found to be related to the particle density profile. The dielectric susceptibility tensor, calculated exactly for an arbitrary coupling and the particle number, exhibits the anticipated disc value in the thermodynamic limit, in spite of zero contribution from the bulk region. Some of the results obtained in the Poisson-Boltzmann limit are generalized to an arbitrary Euclidean dimension.

The role of quantum confinement in the formation of Schottky barriers in Pb–Si interfaces

Abstract Schottky barriers form when semiconductors are in contact with metal overlayers establishing a common Fermi level. Few theoretical studies of these materials exist as electronic structure calculations are computationally intensive for mismatched interfaces. We explicitly model a Pb(111) film on a Si(111) substrate. For thick Pb overlayers, we find a bulk regime where the Fermi level is pinned. For thin film regimes (less than five overlayers), structural relaxations dominate the interfacial energy as charge transfer is suppressed by quantum confinement. In this case, the Schottky barrier height follows the trend of the metal work function.

Perturbation of the yield-stress rheology of polymer thin films by nonlinear shear ultrasound.

We investigate the nonlinear response of macromolecular thin films subjected to high-amplitude ultrasonic shear oscillation using a sphere-plane contact geometry. At a film thickness comparable to the radius of gyration, we observe the rheological properties intermediate between bulk and boundary nonlinear regimes. As the driving amplitude is increased, these films progressively exhibit oscillatory linear, microslip, and full slip regimes, which can be explained by the modified Coulomb friction law. At highest oscillation amplitudes, the interfacial adhesive failure takes place, being accompanied by a dewettinglike pattern. Moreover, the steady state sliding is investigated in thicker films with imposed shear stresses beyond the yield point. We find that applying high-amplitude shear ultrasound affects not only the yielding threshold but also the sliding velocity at a given shear load. A possible mechanism for the latter effect is discussed.

Revealing the surface and bulk regimes of isothermal graphene nucleation and growth on Ni with in situ kinetic measurements and modeling

In situ optical diagnostics are used to reveal the isothermal nucleation and growth kinetics of graphene on Ni across a wide temperature range (560°C<T<840°C) by chemical vapor deposition from single, sub-second pulses of acetylene. An abrupt, two-orders of magnitude change in growth times (∼100–1s) is revealed at T=680°C. Above this temperature, sigmoidal kinetics are measured and attributed to autocatalytic nucleation and growth from carbon dissolved in the bulk of the Ni film. However, for T<680°C fast surface nucleation and growth occurring during the gas pulse appears responsible for the drastic alteration of the kinetics of subsequent dissolution-mediated growth. A simple and general kinetic model for isothermal graphene growth is developed that includes the nucleation phase and the effects of carbon solubility in metals, describes delayed nucleation, and allows the interpretation of the competition between surface- and bulk-nucleation and growth. The easily-implemented optical reflectivity diagnostics and the simple kinetic model described here allow a pathway to optimize the growth of graphene on metals with arbitrary carbon solubility.

Deactivation of lipopolysaccharide by Ar and H2 inductively coupled low-pressure plasma

Using an inductively coupled plasma system, we study the effects of direct plasma, plasma-generated high-energy photons in the ultraviolet and vacuum ultraviolet (UV/VUV), and radical treatments on lipopolysaccharide (LPS). LPS is a biomolecule found in the outer membrane of Gram-negative bacteria and a potent stimulator of the immune system composed of polysaccharide and lipid A, which contains six aliphatic chains. LPS film thickness spun on silicon was monitored by ellipsometry while the surface chemistry was characterized before and after treatments by x-ray photoelectron spectroscopy (XPS). Additionally, biological activity was measured using an enzyme-linked immunosorbent assay under (a) a sensitive regime (sub-µM concentrations of LPS) and (b) a bulk regime (above µM concentrations of LPS) after plasma treatments. Direct plasma treatment causes rapid etching and deactivation of LPS in both Ar and H2 feed gases. To examine the effect of UV/VUV photons, a long-pass filter with a cut-off wavelength of 112 nm was placed over the sample. H2 UV/VUV treatment causes material removal and deactivation due to atomic and molecular UV/VUV emission while Ar UV/VUV treatment shows minimal effects as Ar plasma does not emit UV/VUV photons in the transmitted wavelength range explored. Interestingly, radical treatments remove negligible material but cause deactivation. Based on the amphiphilic structure of LPS, we expect a lipid A rich surface layer to form at the air–water interface during sample preparation with polysaccharide layers underneath. XPS shows that H2 plasma treatment under direct and UV/VUV conditions causes oxygen depletion through removal of C–O and O–C = O bonds in the films, which does not occur in Ar treatments. Damage to these groups can remove aliphatic chains that contribute to the pyrogenicity of LPS. Radical treatments from both Ar and H2 plasmas remove aliphatic carbon from the near-surface, demonstrating the important role of neutral species.

Microfluidic Chip With Integrated a-Si:H Photodiodes for Chemiluminescence-Based Bioassays

On-chip optical detection of chemiluminescent reactions is presented. The device is based on the integration of thin film hydrogenated amorphous silicon photosensors on a functionalized glass substrate ensuring both a good optical coupling and an optimal separation between biological or chemical reagents and the sensing elements. The sensor has been characterized and optimized using the chemiluminescent system composed by the enzyme horseradish peroxidase (HRP) and luminol/peroxide/enhancer cocktail. The detectability of HRP is at the attomole level with a sensitivity of 1.46 fA/fg. Experiments, involving the detection of immobilized bio-specific probes on the functionalized surface have been performed both in bulk and microfluidics regime, proving the ability of the system to effectively detect chemiluminescent reactions and their kinetics. In particular, results achieved using conventional polydimethylsiloxane microfluidics for samples and reagents handling confirmed the good detection capabilities of the proposed system.

Transition between direct and indirect band gap in silicon nanocrystals

Using ground-state density functional theory we study the transition from indirect to direct band gap in hydrogen-terminated silicon nanocrystals (NCs) as a function of decreasing diameter. The studied range, from 1.0 to 4.6 nm diameter of nanocrystals, with spherical and Wulff-shape NCs, covers the transition from nano- to bulk regime. A change in the symmetry of the lowest unoccupied state as a function of decreasing NC diameter is observed, gradually increasing the oscillator strength of transitions from the highest occupied to the lowest unoccupied state. Real space and Fourier space characteristics of highest occupied and lowest unoccupied states are explored in detail and linked to a smooth transition from nano- to bulk regime.

Transition from Molecule to Solid State: Reactivity of Supported Metal Clusters

The evolution of the adsorption energy of carbon monoxide (CO) molecules on palladium (Pd) clusters as function of Pd particle size from the molecular regime (less than ~100 atoms per particle) to the bulk regime has been revealed. This adsorption energy is retrieved from the residence time of CO molecules on the Pd clusters, measured by a pulsed molecular beam technique, versus temperature. Unprecedented accuracy on the determination of the particle size has been achieved here by using a regular array of metal clusters exhibiting a size dispersion down to the ultimate limit of a Poisson distribution. This allows getting rid of the convolution effects that generally occur when considering particles grown through other techniques.

The Ground State Energy of the Three Dimensional Ginzburg-Landau Functional Part I: Bulk Regime

We consider the Ginzburg-Landau functional defined over a bounded and smooth three dimensional domain. Supposing that the magnetic field is comparable with the second critical field and that the Ginzburg-Landau parameter is large, we determine a precise asymptotic formula for the minimizing energy. In particular, this shows how bulk superconductivity decreases in average as the applied magnetic field approaches the second critical field from below. Other estimates are also obtained which allow us to obtain, in a subsequent paper [19], a fine characterization of the second critical field. The approach relies on a careful analysis of several limiting energies, which is of independent interest.

Kinetic Competition Model and Size-Dependent Phase Selection in 1-D Nanostructures

The first phase selection and the phase formation sequence between metal and silicon (Si) couples are indispensably significant to microelectronics. With increasing scaling of device dimension to nano regime, established thermodynamic and kinetic models in bulk and thin film fail to apply in 1-D nanostructures. Herein, we present an unique size-dependent first phase formation sequence in 1-D nanostructures, with Ni-Si as the model system. Interfacial-limited phase which forms the last in thin film, NiSi(2), appears as the dominant first phase at 300-800 °C due to the elimination of continuous grain boundaries in 1-D silicides. On the other hand, θ-Ni(2)Si, the most competitive diffusion-limited phase takes over NiSi(2) and wins out as the first phase in small diameter nanowires at 800 °C. Kinetic parameters extracted from in situ transmission electron microscope studies and a modified kinetic growth competition model quantitatively explain this observation. An estimated critical diameter from the model agrees reasonably well with observations.

Pool Boiling Characteristics of Metallic Nanofluids

Nanofluids, solid-liquid suspensions with solid particles of size of the order of few nanometers, have created interest in many researchers because of their enhancement in thermal conductivity and convective heat transfer characteristics. Many studies have been done on the pool boiling characteristics of nanofluids, most of which have been with nanofluids containing oxide nanoparticles owing to the ease in their preparation. Deterioration in boiling heat transfer was observed in some studies. Metallic nanofluids having metal nanoparticles, which are known for their good heat transfer characteristics in bulk regime, reported drastic enhancement in thermal conductivity. The present paper investigates into the pool boiling characteristics of metallic nanofluids, in particular of Cu-H2O nanofluids, on flat copper heater surface. The results indicate that at comparatively low heat fluxes, there is deterioration in boiling heat transfer with very low particle volume fraction of 0.01%, and it increases with volume fraction and shows enhancement with 0.1%. However, the behavior is the other way around at high heat fluxes. The enhancement at low heat fluxes is due to the fact that the effect of formation of thin sorption layer of nanoparticles on heater surface, which causes deterioration by trapping the nucleation sites, is overshadowed by the increase in microlayer evaporation, which is due to enhancement in thermal conductivity. Same trend has been observed with variation in the surface roughness of the heater as well.

A kinetic model of the structural transformation of a molecular crystal initiated by the escape of mobile defects

A diffusion model of the polymorphic transformation of a molecular crystal accompanied by the escape of mobile defects is suggested. The model describes both bulk and frontal regimes of the process. The frontal regime is possible when the concentration of mobile defects exceeds some threshold value, the existence of which explains spread of times of the beginning of the polymorphic transition in a crystal. The temperature dependence of the rate of a polymorphic process wave was obtained. Internal crystal microstresses that appeared in a phase transition wave were theoretically estimated.

Clusters in intense FLASH pulses: ultrafast ionization dynamics and electron emission studied with spectroscopic and scattering techniques

FLASH, the first FEL operating at short wavelength, has paved the way for novel types of experiments in many different scientific disciplines. Key questions for the first experiments with this new type of light source are linked to light–matter interaction and ionization processes. This paper gives an overview of the ultrafast ionization dynamics and electron emission of pure and doped rare gas clusters illuminated with intense short-wavelength pulses by summarizing the findings of recent years' work at FLASH. Atomic clusters are ideal for investigating the light–matter interaction because their size can be tuned from the molecular to the bulk regime, thus allowing us to distinguish between intra and interatomic processes. The ionization processes turned out to be strongly wavelength dependent. Plasma absorption, while dominant at 13 eV, becomes insignificant at photon energies above 40 eV. The cluster ionization and disintegration proceed in several steps on a time scale from fs to ps. Insight into the involved processes can be obtained with ion and electron spectroscopy. The high intensity of FLASH pulses opens the door for a new imaging approach to study nanoparticles. Scattering patterns of single and few clusters can be recorded in a single shot. Initial results of scattering experiments and their comparison to Mie calculations show that two- and three-dimensional structural information of gas phase particles can be obtained this way.

Evolutionary structure prediction and electronic properties of indium oxide nanoclusters

Indium sesquioxide is widely used as a transparent conducting oxide in modern optoelectronic devices; the rising cost of indium has generated interest in the nanoscale properties of In(2)O(3), and questions arise as to the nature of its physicochemical properties below the bulk regime. We report the stable and metastable stoichiometric clusters of (In(2)O(3))(n), where n = 1-10, as predicted from an evolutionary search within the classical interatomic potential and quantum density functional energy landscapes. In contrast to the paradigm set by ZnO, which favours high symmetry bubble-like structures, the In(2)O(3) nanoclusters are found to tend towards dense, low symmetry structures approaching the bulk system at remarkably small molecular masses. Electronic characterisation is performed at the hybrid density functional and many-body GW levels to obtain accurate predictions of the spectroscopic properties, with mean values of the ionisation potentials and electron affinities calculated as 7.7 and 1.7 eV, respectively.

Continuum versus discrete flux behaviour in large mesoscopic Bi2Sr2CaCu2O8+δ disks

We have used scanning Hall probe and ‘local’ Hall magnetometry measurements to map flux profiles in superconducting Bi2Sr2CaCu2O8+δ disks whose diameters span the crossover between the bulk and mesoscopic vortex regimes. The behaviour of large disks (⩾20μm diameter) is well described by analytic models that assume a continuous distribution of flux in the sample. Small disks (⩽10μm diameter), on the other hand, exhibit clear signatures of the underlying discrete vortex structure as well as competition between triangular ‘Abrikosov’ ordering and the formation of shell structures driven by interactions with circulating edge currents. At low fields we are able to directly observe the characteristic mesoscopic compression of vortex clusters which is linked to oscillations in the diameter of the vortex “dome” in increasing magnetic fields. At higher fields, where single vortex resolution is lost, we are still able to track configurational changes in the vortex patterns, since competing vortex orders impose unmistakable signatures on ‘local’ magnetisation curves. Our observations are in excellent agreement with molecular-dynamics numerical simulations which lead us to a natural definition of the lengthscale for the crossover between discrete and continuum behaviour in our system.

Pattern Formation during Deformation of a Confined Viscoelastic Layer: From a Viscous Liquid to a Soft Elastic Solid

We study pattern formation during tensile deformation of confined viscoelastic layers. The use of a model system [poly(dimethylsiloxane) with different degrees of cross-linking] allows us to go continuously from a viscous liquid to an elastic solid. We observe two distinct regimes of fingering instabilities: a regime called "elastic" with interfacial crack propagation, where the fingering wavelength scales only with the film thickness, and a bulk regime called "viscoelastic," where the fingering instability shows a Saffman-Taylor-like behavior. We find good quantitative agreement with theory in both cases and present a reduced parameter describing the transition between the two regimes and allowing us to predict the observed patterns over the whole range of viscoelastic properties.

Organochalcogenolates of Palladium(II) and Platinum(II): Versatile Molecular Precursors for Metal Chalcogenides

This presentation intends to discuss the design and synthesis of low temperature single source molecular precursors for the synthesis of palladium chalcogenides, both in bulk and nano-size regime. Thus a variety of organochalcogenolates of palladium and platinum differing in nuclearity (mono-, bi-, tri- and hexa- nuclear) have been isolated and characterized by uv-vis, NMR ( 1 H, 13 C, 31 P, 77 Se, 125 Te, 195 Pt) spectroscopy and X-ray crystallography. Thermal behavior of several complexes, such as [Pd(SeCOAr) 2 (PR 3 ) 2 ], [Pd(OAc)(ECH 2 CH 2 CH 2 NMe 2 )] 2 , etc. has been studied by TG analyses. Solvothermal decomposition yields palladium chalcogenides nano-particles (e.g., Pd 7 Te 3 , Pd 17 Se 15 ) which were characterized by XRD, EDAX and SEM method.

Boundary Green’s function for spin-incoherent interacting electrons in one dimension

The spin-incoherent regime of one-dimensional electrons has recently been explored using the Bethe ansatz and a bosonized path integral approach, revealing that the spin incoherence dramatically influences the correlations of charge excitations. We here introduce a bosonization scheme for strongly interacting electrons, allowing us to generalize the description to account for the presence of an open boundary. By calculating the single-electron Green's function we find that the charge sector power-law scaling is highly sensitive to the boundary. Our result allows for a detailed description of the crossover between boundary and bulk regimes. We predict that scanning tunneling microscopy on a spin-incoherent system will pick up oscillations in the differential tunneling conductance as a function of the applied voltage V at intermediate distances x from a real or a dynamically generated boundary. The wavelength of the oscillations, pi v(c)/x, probes the speed v(c) of the charge excitations, and therefore the strength of the electron-electron interaction.

Numerical modeling of the flow of a granular material in a shaft reactor

The 3D flow of monodisperse spherical viscoelastic particles with Coulomb friction in a cylindrical hopper with a conical discharger has been simulated using the discrete element method. According to the simulated data, the flow is pulsed, the flow velocity decreases as the cone half-angle is widened, flow arrests take place at θ > 30°, and the flow changes from the bulk regime to the funnel-type regime at θ > 70°.