Barrier Fluctuations Research Articles

Mechanistic study of transition metal loaded/doped Ni − MgO catalyzed dry reforming of methane: DFT calculations

In the context of carbon peaking and carbon neutrality goals, dry reforming of methane (DRM) offers both environmental and economic benefits and holds great promise for development. In this work, based on Ni loaded MgO, two structural models constructed by transition metal loading (TM+Ni − MgO, TM=Fe, Zr, Mo) and doping (Ni − TM/MgO) were used for DRM by density functional theory (DFT). And the adsorption of CO2 and CH4 on these structures was analyzed. The partial density of states (PDOS) and charge density difference (CDD) indicate that due to the presence of unfilled electron d − orbitals in the TM atoms, electron transfer occurs between them and the p − orbitals of the C and O atoms. It facilitates the interactions and enhances the adsorption between CO2 and MgO. On this basis, the DRM reactions on Fe and Ni loaded MgO (FeNi − MgO) and Ni − loaded, Fe − doped MgO (Ni − Fe/MgO) were screened for focused research. It was revealed that the addition of Fe lowered the energy barrier values for some of the elementary reaction steps, allowing the reaction to proceed more easily. And Ni − Fe/MgO showed greater advantages by possessing more stable energy barrier fluctuation intervals (ER1 − ER3) and the smallest CO2 dissociation energy barrier.

Publisher’s Note: “Stochastic model for barrier crossings and fluctuations in local timescale” [J. Math. Phys. 65, 023302 (2024)

Publisher’s Note: “Stochastic model for barrier crossings and fluctuations in local timescale” [J. Math. Phys. 65, 023302 (2024)

Stochastic model for barrier crossings and fluctuations in local timescale

The problem of computing the rate of diffusion-aided activated barrier crossings between metastable states is one of broad relevance in physical sciences. The transition path formalism aims to compute the rate of these events by analysing the statistical properties of the transition path between the two metastable regions concerned. In this paper, we show that the transition path process is a unique solution to an associated stochastic differential equation, with a discontinuous and singular drift term. The singularity arises from a local time contribution, which accounts for the fluctuations at the boundaries of the metastable regions. The presence of fluctuations at the local time scale calls for an excursion theoretic consideration of barrier crossing events. We show that the rate of such events, as computed from excursion theory, factorizes into a local time term and an excursion measure term, which bears empirical similarity to the transition state theory rate expression. Since excursion theory makes no assumption about the presence of a transition state in the potential energy landscape, the mathematical structure underlying this factorization ought to be general. We hence expect excursion theory (and local times) to provide some physical and mathematical insights in generic barrier crossing problems.

Effect of spontaneous polarization field on diffusion thermopower in AlGaN/GaN heterostructures

Polarization field on diffusion thermopower Sd, of a two-dimensional electron gas (2DEG) formed at a wurtzite AlGaN/GaN heterojunction is investigated. The investigations of macroscopic polarization effect on diffusion thermopower over the wide range of temperature and barrier fluctuations bring out the role of spontaneous and piezoelectric polarization fields present in the heterostructure. The induced 2DEG charge density formed at the interface of AlGaN/GaN heterojunction due to in-built spontaneous and piezoelectric fields are assumed to be scattered by acoustic phonons via deformation and piezoelectric couplings, background and remote impurities via Coulomb interaction, interface roughness and interface charges. We find, the dominant contribution is from interface roughness scattering mechanism and that from acoustic phonons via deformation potential is negligible. Theoretical investigations of Sd show that, the effect of the spontaneous polarization is to reduce the diffusion thermopower significantly at practically relevant temperatures.

Addressing the Conflict between Mobility and Stability in Oxide Thin‐film Transistors

Amorphous oxide semiconductor thin-film transistors (AOS TFTs) are ever-increasingly utilized in displays. However, to bring high mobility and excellent stability together is a daunting challenge. Here, the carrier transport/relaxation bilayer stacked AOS TFTs are investigated to solve the mobility-stability conflict. The charge transport layer (CTL) is made of amorphous In-rich InSnZnO, which favors big average effective coordination number for all cations and more edge-shared structures for better charge transport. Praseodymium-doped InSnZnO is used as the charge relaxation layer (CRL), which substantially shortens the photoelectron lifetime as revealed by femtosecond transient absorption spectroscopy. The CTL and CRL with the thickness suitable for industrial production respectively afford minute potential barrier fluctuation for charge transport and fast relaxation for photo-generated carriers, resulting in transistors with an ultrahigh mobility (75.5 cm2 V-1 s-1 ) and small negative-bias-illumination-stress/positive-bias-temperature-stress voltage shifts (-1.64/0.76V). The design concept provides a promising route to address the mobility-stability conflict for high-end displays.

Dipolar Noise in Fluorinated Molecular Wires.

We demonstrate a strategy to directly map and quantify the effects of dipole formation on electrical transports and noises in the self-assembled monolayers (SAMs) of molecular wires. In this method, the SAM patterns of fluorinated molecules with dipole moments were prepared on conducting substrates, and a conducting probe in contact-mode atomic force microscopy was utilized to map currents and noises through the probe on the molecular patterns. The maps were analyzed to extract the characteristic parameters of dipolar noises in SAMs, and the results were compared with those of hydrogenated molecular patterns without dipole moments. At rather low bias conditions, the fluorinated molecular junctions exhibited a tunneling conduction and a resistance value comparable to that of the hydrogenated molecules with a six-times-longer length, which was attributed to stronger dipoles formation in fluorinated molecules. Interestingly, conductance (G) in different regions of fluorinated molecular patterns exhibited a strong correlation with a noise power spectral density of SI/I2 like SI/I2 ∝ G−2, which can be explained by enhanced barrier fluctuations produced by the dipoles of fluorinated molecules. Furthermore, we observed that the noise power spectral density of fluorinated molecules showed an anomalous frequency (f) dependence like SI/I2 ∝ 1/f1.7, possibly due to the slowing down of the tunneling of carriers from increased barrier fluctuations. In rather high bias conditions, conductions in both hydrogenated and fluorinated molecules showed a transition from tunneling to thermionic charge transports. Our results provide important insights into the effects of dipoles on mesoscopic transport and resistance-fluctuation in molecules and could have a significant impact on the fundamental understanding and applications in this area.

Microcanonical coarse-graining of the kinetic Ising model.

We propose a scheme for coarse-graining the dynamics of the 2-D kinetic Ising model onto the microcanonical ensemble. At subcritical temperatures, 2-D and higher-dimensional Ising lattices possess two basins of attraction separated by a free energy barrier. Projecting onto the microcanonical ensemble has the advantage that the dependence of the crossing rate constant on environmental conditions can be obtained from a single Monte Carlo trajectory. Using various numerical methods, we computed the forward rate constants of coarse-grained representations of the Ising model and compared them with the true value obtained from brute force simulation. While coarse-graining preserves detailed balance, the computed rate constants for barrier heights between 5 kT and 9 kT were consistently 50% larger than the true value. Markovianity testing revealed loss of dynamical memory, which we propose accounts for coarse-graining error. Committor analysis did not support the alternative hypothesis that microcanonical projection is incompatible with an optimal reaction coordinate. The correct crossing rate constant was obtained by spectrally decomposing the diffusion coefficient near the free energy barrier and selecting the slowest (reactive) component. The spectral method also yielded the correct rate constant in the 3-D Ising lattice, where coarse-graining error was 6% and memory effects were diminished. We conclude that microcanonical coarse-graining supplemented by spectral analysis of short-term barrier fluctuations provides a comprehensive kinetic description of barrier crossing in a non-inertial continuous-time jump process.

A collective elastic fluctuation mechanism for decoupling and stretched relaxation in glassy colloidal and molecular liquids.

We propose a microscopic theory for the decoupling of self-diffusion and structural relaxation in glass-forming liquids within the Elastically Collective Nonlinear Langevin Equation (ECNLE) activated dynamics framework. Our central hypothesis is that the heterogeneity relevant to this problem is static fluctuations of local density on the scale of 3-4 particle diameters and how this changes local packing correlations. These fluctuations modify the degree of dynamical cage expansion that mechanistically couples intracage large amplitude hopping and longer range collective elasticity in ECNLE theory. Decoupling only emerges in the deeply supercooled regime where the strongly temperature dependent elastic barrier becomes non-negligible relative to its noncooperative local analog. The theory makes predictions for various aspects of the decoupling phenomenon, including apparent fractional power law Stokes-Einstein behavior, that appear to be consistent with experiments and simulations on hard sphere fluids and molecular liquids. Of central importance is a microscopic connection between the barrier fluctuation variance and most probable barrier height. Sensible results are also obtained for the nonexponential stretching of a generic relaxation time correlation function and its temperature evolution. Nonuniversality can arise from the relative importance of the local and collective barriers (related to fragility) and the precise magnitude of the length scale that defines the transition from local cage to elastic physics. Comparison is made with a traplike model based on a Gaussian distribution of barriers.

Effects of intergranular capacitance and resistance dispersion on polycrystalline semiconductor impedance

In the present work we focused on the effects, often disregarded, of the intergranular barrier fluctuations on the total impedance of polycrystalline structures. These fluctuations come from the discreteness nature and random distribution of ionized donors at the polycrystal, which are responsible of forming the intergranular barriers. Firstly, we used a numerical model that takes into account the point character of the involved donors and found that resistances and capacitances present distributions that become narrower with the grain size. Secondly, we built a bricklayer model with capacitances and resistances taken from the found distributions and calculated the total impedance, focusing on tin oxide as an example. We found the total polycrystal capacitance and resistance dependence on the specific dispersions of intergranular capacitances and resistances.

The role of spin-phonon coupling in enhanced desorption kinetics of antioxidant flavonols from magnetic nanoparticles aggregates

Abstract An important criterion in the evaluation of drug delivery systems based on nanoparticles is the drug release kinetics. From numerous published works, it is obvious that the fraction of the released drug as a function of time shows qualitatively the same behaviour, even when the release is assisted by an external force. In this study a series of experiments has been performed on a system that can be considered as the simplest drug delivery system. It consists only of uncoated/unprotected superparamagnetic magnetite nanoparticles in the form of mesoporous aggregates and adsorbed flavonols (quercetin, myricetin, or myricitrin). The simultaneous utilization of external permanent and oscillating magnetic fields remarkably accelerated the flavonol release. There is no a specific pulling force that eventually causes the rupture of the bonds between the molecules and the particle surface, i.e. the detachment of an individual molecule is thermally activated. The adsorption of molecules onto nanoparticles follows almost without delay quick changes in the field-induced motion of the nanoparticles, and they are subject to more frequent collisions with the surrounding solvent molecules. The barrier fluctuations occur due to the magnetic field dependence of the nanoparticle phonon spectra, to which the adsorbed flavonol molecule vibrations are strongly coupled. The phonon frequencies and phonon damping of a ferromagnetic nanoparticle are actually affected by the magnetic field through the spin-phonon coupling. The rate of release (spontaneous or forced) is invariably a first-order process, which can be considered a basic property of a broad class of drug nanodelivery systems.

Effects of intergranular barrier fluctuations on the electrical conductivity of polycrystalline semiconductors

We studied the influence of intergranular barrier fluctuations on the electrical response of 3D semiconductor polycrystals. We first computed with a numerical simulation model the dispersion in the intergranular barrier height on polycrystalline tin oxide due to the punctual character of the donors. Then, in order to quantify the effects of the barrier fluctuation in the overall conductivity of the semiconductor, we added the dispersion to the well known brick-layer model and determined the connection between impedance measurements and grain boundary resistivity. We found that, the brick-layer model gives lower values for the real intergrain resistivity. However, the error can be quantified indicating that the brick-layer model is not a bad approximation to determine electrical properties of intergrains of a polycrystal, specially for relatively large grains.

The effect of stochastic barrier fluctuation on semiclassical transmission probability and Shannon entropy of a symmetric double well potential

AbstractStochastic fluctuation of barrier height and width of a symmetric double well plays a very significant role in the corresponding dynamics by increasing the semiclassical transmission probability and Shannon entropy of the system. The population of the system has been observed to be spread into several under barrier states starting from the or [, where and are the wave functions describing the two lowest degenerate states] in presence of the stochastic fluctuation. This distribution over several states is manifested by steady increase in Shannon entropy. However, any arbitrary value of the stochastic fluctuation cannot increase the populations of the upper energy states and consequently no gain in the net value of Shannon entropy results. There is an optimum frequency for which the Shannon entropy passes through a maximum, which is also found out in this work. We have also calculated the semiclassical WKB like transmission probability as a function of time and it is clear that the random fluctuation of barrier causes the transmission probability to increase to a significant amount. As the total energy of the system remains below the potential barrier, this transmission probability is equivalent to tunneling probability. It has been clearly shown that if the fluctuation is made to be periodic (without changing the frequency and magnitude of the fluctuation) it cannot effect any significant change in the overall dynamics.

Thickness Effect on Fluctuation of Electron States in Thin Film and Implications for Lattice Constant Change Due to Size Reduction

We propose a model for predicting the fluctuation of electron states in thin films as a function of film thickness. The model is derived based on the assumption of the existence of potential barrier fluctuation on the film surface. Since the wave functions of electrons in the film are determined by the boundary conditions of the potential on the film surface, the potential fluctuation on the film surface implies fluctuation of the electron states in the film. The model was extended to predict the effect of size on the lattice constant of thin films or nanoparticles.

Escape of coupled Brownian particles across a fluctuating barrier.

The escape of two harmonically coupled Brownian particles across the fluctuating barrier of a bistable potential is investigated with correlated additive and multiplicative fluctuations. Positive correlations enhance the rate of escape across the barrier when the coupling is effective, whereas for weakly coupled particles, escape becomes difficult. It is found that the system exhibits the phenomenon of resonant activation when the rate of barrier fluctuations is comparable to the relaxation time in the bistable potential. Using a decoupling ansatz, we derive the Markovian limit of the problem in the steady state, under the constraint that the barriers fluctuate on a time scale faster than the relative oscillation of the two particles. Adiabatic elimination of the fast variable of the dynamical system is discussed in appropriate limits.

Red River barrier and Pleistocene climatic fluctuations shaped the genetic structure of Microhyla fissipes complex (Anura: Microhylidae) in southern China and Indochina.

South China and Indochina host striking species diversity and endemism. Complex tectonic and climatic evolutions appear to be the main drivers of the biogeographic patterns. In this study, based on the geologic history of this region, we test 2 hypotheses using the evolutionary history of Microhyla fissipes species complex. Using DNA sequence data from both mitochondrial and nuclear genes, we first test the hypothesis that the Red River is a barrier to gene flow and dispersal. Second, we test the hypothesis that Pleistocene climatic cycling affected the genetic structure and population history of these frogs. We detect 2 major genetic splits that associate with the Red River. Time estimation suggests that late Miocene tectonic movement associated with the Red River drove their diversification. Species distribution modeling (SDM) resolves significant ecological differences between sides of the Red River. Thus, ecological divergence also probably promoted and maintained the diversification. Genogeography, historical demography, and SDM associate patterns in southern China with climate changes of the last glacial maximum (LGM), but not Indochina. Differences in geography and climate between the 2 areas best explain the discovery. Responses to the Pleistocene glacial–interglacial cycling vary among species and regions.

Enzymatic Flexibility and Reaction Rate: A QM/MM Study of HIV-1 Protease

The relevance of conformational fluctuations on enzyme rates has been a matter of debate for decades. Single molecule experiments have detected variations on the catalytic rates between different enzyme molecules, and within the same enzyme molecule, in a time scale larger than turnover. Computational methods can detect different energy barriers, induced by thermal conformational fluctuations, at a microscopic time scale, several orders of magnitude faster than the turnover rate of the fastest enzyme. Others have observed these barrier fluctuations, but few computational studies have dissected them in detail and tried to understand their origins and consequences. For this purpose, we studied the first step of the reaction catalyzed by HIV-1 Protease, starting from 40 different conformations. We found activation free energies ranging from 14.5 to 51.3 kcal·mol–1. The calculated apparent barrier is 16.5 kcal·mol–1, which is very close to the experimental value of 15.9 kcal·mol–1 for product release. These f...

Optimal persistent currents for interacting bosons on a ring with a gauge field.

We study persistent currents for interacting one-dimensional bosons on a tight ring trap, subjected to a rotating barrier potential, which induces an artificial U(1) gauge field. We show that, at intermediate interactions, the persistent current response is maximal, due to a subtle interplay of effects due to the barrier, the interaction, and quantum fluctuations. These results are relevant for ongoing experiments with ultracold atomic gases on mesoscopic rings.

Temperature-dependent current–voltage characteristics of Er-silicide Schottky contacts to strained Si-on-insulator

We fabricated Er-silicide (ErSi1.7) Schottky contacts to strained Si-on-insulator (sSOI) with a strain level of 0.77% and investigated their electrical properties in the temperature range of 225–400K. The Schottky parameters such as the barrier height, ideality factor, and series resistance were found to strongly depend on temperature. Barrier height and ideality factor were found to decrease and increase, respectively, with decreasing temperature. The series resistance gradually increased with decreasing temperature. A discrepancy between the Schottky barrier heights calculated from the forward current–voltage (I–V) characteristics and Norde’s method indicated a deviation from the ideal thermionic emission of ErSi1.7/sSOI Schottky diode. The lateral inhomogeneity of the Schottky barrier and potential fluctuations at the interface between ErSi1.7 and sSOI could be a main cause of the difference between the calculated and theoretical values of the Richardson constant. On the basis of a thermionic emission mechanism with a Gaussian distribution of the barrier heights, temperature dependency of ErSi1.7 Schottky contact to sSOI was explained in terms of the barrier height inhomogeneities at the interface.

NONEQUILIBRIUM RATE THEORY FOR CONDUCTION IN OPEN ION CHANNELS

We present a nonequilibrium reaction rate model of the ionic transition through an open ion channel, taking account of the interaction between an ion at the entrance of the channel and an ion at the binding site in a self-consistent way. The electrostatic potential is calculated by solution of the Poisson equation for a channel modeled as a cylindrical tube. The transition rate, and the binding site occupancy as a function of the left bulk concentration are compared to 1D Brownian dynamics simulations. The analysis is performed for a single binding site of high-affinity, with the exit rate influenced by barrier fluctuations at the channel exit. The results are compared with experimental data for the permeation of the Na+ ion through the Gramicidin A channel, with which they are shown to be in good agreement.

Dynamic effects of dipolar interactions on the magnetic behavior of magnetite nanoparticles

Isothermal magnetization and initial dc susceptibility of spheroidal, nearly monodisperse magnetite nanoparticles (typical diameter: 8 nm) prepared by a standard thermo-chemical route have been measured between 10 and 300 K. The samples contained magnetite nanoparticles in the form of either a dried powder (each nanoparticle being surrounded by a stable oleic acid shell as a result of the preparation procedure) or a solid dispersion in PEGDA-600 polymer; different nanoparticle (NP) concentrations in the polymer were studied. In all samples the NPs were not tightly agglomerated nor their ferromagnetic cores were directly touching. The high-temperature inverse magnetic susceptibility is always found to follow a linear law as a function of T, crossing the horizontal axis at negative temperatures ranging from 175 to about 1,000 K. The deviation from the standard superparamagnetic behavior is related to dipolar interaction among NPs; however, a careful analysis makes it hard to conclude that such a behavior originates from a dominant antiferromagnetic character of the interaction. The results are well explained considering that the studied samples are in the interacting superparamagnetic (ISP) regime. The ISP model is basically a mean field theory which allows one to straightforwardly account for the role of magnetic dipolar interaction in a NP system. The model predicts the existence of specific scaling laws for the reduced magnetization which have been confirmed in all studied samples. The interaction of each magnetic dipole moment with the local, random dipolar field produced by the other dipoles results in the presence of a large fluctuating energy term whose magnitude is comparable to the static barrier for magnetization reversal/rotation related to magnetic anisotropy. On the basis of the existing theories on thermal crossing of a barrier whose height randomly fluctuates in time it is predicted that the rate of barrier crossing is substantially driven by the rate of barrier fluctuations, which is fast (108–109 Hz) and almost independent of temperature. As a consequence, the standard picture of superparamagnetic NPs which undergo single-particle blocking by a static barrier below the blocking temperature should be substantially revised, at least in the present materials. The ISP model is perfectly matching with the view of activated magnetization rotation whose kinetics is significantly modified by barrier height fluctuations.