The role of heat-conducting walls in the measurement of heat and mass transport in transient grating experiments

We present a two-dimensional model to account for the role of heat-conducting walls in the measurement of heat transport and Soret-effect-driven mass transport in transient holographic grating experiments. Heat diffusion into the walls leads to non-exponential decay of the temperature grating. Under certain experimental conditions it can be approximated by an exponential function and assigned an apparent thermal diffusivity D_{th, app} < D_{th, s}, where D_th,s is the true thermal diffusivity of the sample. The ratio D_{th, app}/D_{th, s} depends on only three dimensionless parameters, d /l_s, κ_s/κ_w, and D_{th, s}/D_{th, w}. d is the grating period, l_s the sample thickness, κ_s and κ_w the thermal conductivities of sample and wall, respectively, and D_th,w the thermal diffusivity of the wall. If at least two measurements are performed at different d /l_s, both D_th,s and κ_s can be determined. Instead of costly solving PDEs, D_th,s can be obtained by finding the zero of an analytic function. For thin samples and large grating periods, heat conduction into the walls plays a predominant role and the concentration grating in binary mixtures is no longer one-dimensional. Nevertheless, the normalized heterodyne diffraction efficiency of the concentration grating remains unaffected and the true thermal and collective diffusion coefficient and the correct Soret coefficient are still obtained from a simple one-dimensional model.     

On the role of substrate in light-harvesting experiments

An analysis of the emitted light distribution for a single emitter located at the planar interface of two optical media was performed. The interface of a varying refractive index substrate with air was considered, which is a common case in luminescence microscopy (spectroscopy) experiments. A modification of the radiative recombination rate induced by the variation of the substrate together with the emitted radiation spatial redistribution were taken into account. Simulation results show that the collection efficiency of the emitted light can vary several times depending on the substrate choice and the emitter intrinsic quantum efficiency.     

Derivation of bonding distance and vibration frequency from field evaporation experiments

This paper extends the analysis of previously reported data, concerning the field evaporation of rhodium atoms that escape as Rh⁺ and are then post-ionised to Rh²⁺. A more general formula is derived for obtaining values of critical distance x^cr from correlated measurements of temperature, field (F), activation energy (Q), and onset appearance energy. A zkT-type correction is included. If prior to evaporation the surface atom is vibrating in an effectively parabolic potential well, and a Gomer-type escape mechanism operates, then experimental plots of $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ versus $\text{1}\text{F}$ and of x^cr versus $\text{Q}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ should be linear. Such plots may be used to obtain estimates of zero-Q evaporation field F^e, electrical bonding distance a, and vibrational force-constant k and frequency v. These formulae and methods are applicable to a range of materials and evaporation situations. For rhodium this paper establishes that escape takes place via a Gomer-type escape mechanism, and deduces the following values: F^e = 61 ± 20 V/nm; a = 0.13 ± 0.035 nm; v = (1.4_+0.5^?0.3) × 10¹²Hz.     

The role of XPS in the study and understanding of rubber-to-metal bonding

XPS has been used to study the mechanism of the adhesion of rubber to well-characterized metal surfaces. By means of a special sample preparation technique the rubber-to-brass interface could be analyzed by X-ray photoelectron spectroscopy following vulcanization of rubber against a CuZn alloy surface. Combination of these XPS results with quantitative data on the adhesion levels of brass has led to the development of a new adhesion model. Reaction of brass with rubber results in the formation of both Cu_xS and ZnS. The amount of Cu_xS formed must be carefully controlled. On the one side, it improves the adhesion as a result of a catalytic effect on the rubber vulcanization. On the other hand, excessive Cu_xS formation leads to embrittlement of the interfacial Cu_xS/ZnS film and a loss of adhesion.     

Evidence of internal protein dynamics in transferrins from TDPAC experiments

The relaxation in liquid transferrin and ovotransferrin samples has been studied at different temperatures using the TDPAC method. Information about reorientation and internal dynamics has been obtained from immobilized protein samples. Characteristic differences between the two proteins will be discussed.     

Evidence for the production of new particles in cosmic ray experiments deep underground

In the cosmic ray experiments deep underground in the Kolar Gold Mines, a special class of events has been observed, at present 6 in number, characterised by several, (in general 3), charged particles arising from a vertex, either in air or in the thin material of the detectors, with large opening angles; the vertex is at a distance of around 70–100 cms from the rock wall. The most plausible interpretation of these events is that they are due to the decay of new, massive and long-lived particles produced in neutrino collisions inside rock, or through hitherto unknown processes.     

高分子凝胶体积相变中的氢键作用

我们把关联模型(association models)推广应用到高分子凝胶体系,研究高分子与溶剂分子间的氢键和溶剂分子与溶剂分子间的氢键在高分子凝胶体积相变中的作用.首先通过分析凝胶体积分数与温度的关系发现,由于两种氢键作用,随着温度变化高分子凝胶出现连续、不连续体积相变,结果表明在体积相变过程中两种氢键都起着重要作用.其次,对不同氢键分数条件下的旋节线的研究发现,对于高分子凝胶体积相变中出现的UCST和LCST(上临界共溶温度和下临界共溶温度)现象也是由于高分子与溶剂分子间氢键和溶剂分子与溶剂分子间氢键共同作用的结果.     

Bose-Einstein condensation in semiconductors: the key role of dark excitons

Bose-Einstein condensation in semiconductors is controlled by the nonelementary-boson nature of excitons. Pauli exclusion between the fermionic components of composite excitons produces dramatic exchange couplings between bright and dark states. In microcavities, where bright excitons and photons form polaritons, they force the condensate to be linearly polarized, as observed. In bulk, they also force linear polarization, but of dark states, due to interband Coulomb scatterings. To evidence this dark condensate, indirect processes are thus needed.     

Evidence on single-molecule transport in electrostatically-gated molecular transistors

We show that, if adequately formulated for molecular electronics, the barrier picture can quantitatively reproduce the currents and describe the orbital gating in the molecular transistors fabricated by Song et al. [H. Song, Y. Kim, Y.H. Jang, H. Jeong, M.A. Reed, T. Lee, Nature 462 (2009) 1039]. Based on our results, we demonstrate (i) that the measured current represents the contribution of a single   molecule, and (ii) the linear dependence of the molecular orbital energy offset _εg${\epsilon }_g$ on the voltage _Vt$V_t$ at the Fowler–Nordheim minimum, validating thereby the transition voltage spectroscopy for the gated single molecule devices of Song et al.     

Evidence for spontaneous spin-polarized transport in magnetic nanowires

The exploitation of the spin in charge-based systems is opening revolutionary opportunities for device architecture. Surprisingly, room temperature electrical transport through magnetic nanowires is still an unresolved issue. Here, we show that ferromagnetic (Co) suspended atom chains spontaneously display an electron transport of half a conductance quantum, as expected for a fully polarized conduction channel. Similar behavior has been observed for Pd (a quasimagnetic 4d metal) and Pt (a nonmagnetic 5d metal). These results suggest that the nanowire low dimensionality reinforces or induces magnetic behavior, lifting off spin degeneracy even at room temperature and zero external magnetic field.     

Chemical bonding differences evidenced from J-coupling in solid state NMR experiments involving quadrupolar nuclei

Small scalar J-coupling between quadrupolar nuclei and spin 1/2 can be measured in inorganic solids using J-Resolved experiments and further used to acquire 2D J-HQMC heteronuclear correlation, giving detailed insight into the chemical bonding scheme.     

Evidence for multiple parton interactions from the observation of multi-muon events in Drell-Yan experiments

We calculate the production of two lepton pairs via the Drell-Yan mechanism. To establish the existence of multiple parton interactions in a single hadron collision, lepton pair production is superior to jet production from the theoretical, phenomenological and experimental points of view. The calculated rate agrees with the previously unexplained four-muon events observed in πp and pp interactions. Our results lend support to a recent analysis of ISR four-jet events and we discuss some implications of this study.     

On the possible role of the bootstrap effect in the RF current drive experiments

Simple analytical relations are derived for the radial profile of the bootstrap current in a tokamak. Estimates are given for the case of both neoclassical and anomalous diffusion. The dependence of the plasma resistivity on the radial profile of the electron temperature is taken into account. The possibility of experimental revealing the bootstrap current is discussed.     

On the possible role of the bootstrap effect in the RF current drive experiments

Conditions of existence of the bootstrap effect at anomalous diffusion in tokamaks are discussed. Assuming that these conditions are fulfilled at lower hybrid current drive, various models of anomalous diffusion and of temperature profiles are considered. Toroidal current density profiles and related quantities are calculated, demonstrating strong dependences on the electron temperature profiles and on the effective ion charge Z_eff. Recommendations for experimental revealing the bootstrap current are presented.     

Interferometric and noise signatures of Majorana fermion edge states in transport experiments

Domain walls between superconducting and magnetic regions placed on top of a topological insulator support transport channels for Majorana fermions. We propose to study noise correlations in a Hanbury Brown-Twiss type interferometer and find three signatures of the Majorana nature of the channels. First, the average charge current in the outgoing leads vanishes. Furthermore, we predict an anomalously large shot noise in the output ports for a vanishing average current signal. Adding a quantum point contact to the setup, we find a surprising absence of partition noise which can be traced back to the Majorana nature of the carriers.     

Calculation of transport coefficient profiles in modulation experiments as an inverse problem

The calculation of transport profiles from experimental measurements belongs in the category of inverse problems which are known to come with issues of ill-conditioning or singularity. A reformulation of the calculation, the matricial approach, is proposed for periodically modulated experiments, within the context of the standard advection-diffusion model where these issues are related to the vanishing of the determinant of a 2×2 matrix. This sheds light on the accuracy of calculations with transport codes, and provides a path for a more precise assessment of the profiles and of the related uncertainty.     

The role of surface science experiments in understanding heterogeneous catalysis

Heterogeneous catalysis is of enormous industrial importance for the large scale production of chemicals. A continuous research effort is devoted to improving the activity and selectivity of catalysts. Research is also expected to replace gradually empiricism by a more fundamental understanding of the governing factors of catalysis. Surface science in particular is capable of providing this fundamental information because of the advent of a multitude of novel surface characterization techniques. These new surface analytical tools operative under vacuum conditions are used to characterize catalytic surfaces with respect to composition, crystallographic and electronic structure. At the same time catalytic reactions are studied on these well characterized surfaces. A comparison of results from these studies with those obtained by more traditional catalysis research, e.g. at high pressure and on supported catalysts, turns out to be most interesting and informative.