The well known nonlinear fluctuating hydrodynamics theory has grouped diffusions in anharmonic chains into two universality classes: one is the Kardar-Parisi-Zhang (KPZ) class for chains with either asymmetric potential or nonzero static pressure and the other is the Gaussian class for chains with symmetric potential at zero static pressure, such as Fermi-Pasta-Ulam-Tsingou (FPUT)-β chains. However, little is known of the nonequilibrium transient diffusion in anharmonic chains. Here, we reveal that the KPZ class is the only universality class for nonequilibrium transient diffusion, manifested as the KPZ scaling of the side peaks of momentum correlation (corresponding to the sound modes correlation), which was completely unexpected in equilibrium FPUT-β chains. The underlying mechanism is that the nonequilibrium soliton dynamics cause nonzero transient pressure so that the sound modes satisfy approximately the noisy Burgers equation, in which the collisions of solitons was proved to yield the KPZ dynamic exponent of the soliton dispersion. Therefore, the unexpected KPZ universality class is obtained in the nonequilibrium transient diffusion in FPUT-β chains and the corresponding carriers of nonequilibrium transient diffusion are attributed to solitons.

Atomic resolution secondary electron imaging (ARSEI) has become an alternative tool for material characterization; however, its application potential is still underestimated due to insufficient understanding of the imaging mechanism. In this work, a robust quantum trajectory Monte Carlo method is developed to theoretically investigate the physical processes of ARSEI and thereby establish a new imaging mechanism. The new mechanism suggests that ARSEI can even identify the atoms of the same element in different atomic layers near the surface with a depth difference <2 angstrom. Thus, ARSEI can be expected to greatly promote the development of material characterization. For example, it may help to directly observe the AA, AB, and BA stacking in few-layer two-dimensional materials and to determine in which layer of a multilayer material an atom is adsorbed.

According to the Boltzmann distribution assumption of solitons, in thermal equilibrium, there is the most probable soliton whose average kinetic energy per site equals the thermal energy k(B)T. Based on the momentum excitation method, the soliton can be numerically excited in the static Fermi-Pasta-Ulam (FPU) chains. By associating the excited soliton with the corresponding most probable soliton, the temperature dependence of the velocity of solitons in thermal equilibrium can be numerically evaluated. The results agree very well with the temperature dependence of the sound velocity of energy transfer. This confirms that solitons are promising candidates for energy carriers in FPU chains. Moreover, the validity of the Boltzmann distribution assumption of solitons in FPU chains is also confirmed. This work sheds light on how to numerically (even experimentally) investigate solitons in thermal equilibrium.

Currently, effective phonons (renormalized or interacting phonons) rather than solitary waves (for short, solitons) are regarded as the energy carriers in nonlinear lattices. In this work, by using the approximate soliton solutions of the corresponding equations of motion and adopting the Boltzmann distribution for these solitons, the average velocities of solitons are obtained and are compared with the sound velocities of energy transfer. Excellent agreements with the numerical results and the predictions of other existing theories are shown in both the symmetric Fermi-Pasta-Ulam-β lattices and the asymmetric Fermi-Pasta-Ulam-αβ lattices. These clearly indicate that solitons are suitable candidates for energy carriers in Fermi-Pasta-Ulam lattices. In addition, the root-mean-square velocity of solitons can be obtained from the effective phonons theory.

The Bohmian trajectory method is employed to study electron diffraction in crystalline materials. It provides a fresh understanding of the process of electron diffraction, including traveling channels of electrons and formation of diffraction patterns. By combining it with the Bloch wave method, the electron trajectories can be calculated more efficiently than the traditional wave-packet propagation algorithm. Meanwhile, we propose a momentum expectation approach which is a good approximation method with even higher computational efficiency. Both methods result in intuitive and accurate electron trajectories for the simulation of the electron backscatter diffraction (EBSD) pattern. Excellent agreement has been obtained between the simulated trajectory distributions and the experimental EBSD pattern from Mo (001) at 20 kV, where the Kikuchi patterns and higher order Laue zone rings are characterized.

So far, only the energy thresholds of single discrete breathers in nonlinear Hamiltonian systems have been analytically obtained. In this work, the energy thresholds of discrete breathers in thermal equilibrium and the energy thresholds of long-lived discrete breathers which can remain after a long time relaxation are analytically estimated for nonlinear chains. These energy thresholds are size dependent. The energy thresholds of discrete breathers in thermal equilibrium are the same as the previous analytical results for single discrete breathers. The energy thresholds of long-lived discrete breathers in relaxation processes are different from the previous results for single discrete breathers but agree well with the published numerical results known to us. Because real systems are either in thermal equilibrium or in relaxation processes, the obtained results could be important for experimental detection of discrete breathers.

Thermal rectification and negative differential thermal conductance were realized in harmonic chains in this work. We used the generalized Caldeira-Leggett model to study the heat flow. In contrast to most previous studies considering only the linear system-bath coupling, we considered the nonlinear system-bath coupling based on recent experiment [Eichler et al., Nat. Nanotech. 6, 339 (2011)]. When the linear coupling constant is weak, the multiphonon processes induced by the nonlinear coupling allow more phonons transport across the system-bath interface and hence the heat current is enhanced. Consequently, thermal rectification and negative differential thermal conductance are achieved when the nonlinear couplings are asymmetric. However, when the linear coupling constant is strong, the umklapp processes dominate the multiphonon processes. Nonlinear coupling suppresses the heat current. Thermal rectification is also achieved. But the direction of rectification is reversed compared to the results of weak linear coupling constant.

In this paper, a novel quantum-trajectory Monte Carlo simulation method is developed to study electron beam interaction with a crystalline solid for application to electron microscopy and spectroscopy. The method combines the Bohmian quantum trajectory method, which treats electron elastic scattering and diffraction in a crystal, with a Monte Carlo sampling of electron inelastic scattering events along quantum trajectory paths. We study in this work the electron scattering and secondary electron generation process in crystals for a focused incident electron beam, leading to understanding of the imaging mechanism behind the atomic resolution secondary electron image that has been recently achieved in experiment with a scanning transmission electron microscope. According to this method, the Bohmian quantum trajectories have been calculated at first through a wave function obtained via a numerical solution of the time-dependent Schrödinger equation with a multislice method. The impact parameter-dependent inner-shell excitation cross section then enables the Monte Carlo sampling of ionization events produced by incident electron trajectories travelling along atom columns for excitation of high energy knock-on secondary electrons. Following cascade production, transportation and emission processes of true secondary electrons of very low energies are traced by a conventional Monte Carlo simulation method to present image signals. Comparison of the simulated image for a Si(110) crystal with the experimental image indicates that the dominant mechanism of atomic resolution of secondary electron image is the inner-shell ionization events generated by a high-energy electron beam.

We present in this work the calculation of Bohmian quantum trajectories representing the wave function propagation in a crystal for a focused electron probe in a scanning transmission electron microscope (STEM). The wave function and quantum trajectories are obtained from the calculation of time-dependent Schrödinger equation by fast Fourier transformation multislice algorithm. In our work, the Bohmian quantum trajectories of a scanning probe penetrating a Cu crystal are studied as an example of this calculation scheme. The results help us to better understand the electron diffraction process in a microscopic imaging from a trajectory-based point of view. This Bohmian quantum trajectory method can be used to extend the application of classical Monte Carlo method from the study of electron interaction with amorphous solid to crystalline structure.

The quantum heat transfer in a harmonic chain with a dephasing reservoir is investigated based on the master equation method. The incoming phonons from the thermal reservoir will be scattered back partially into the reservoir at the contact site, leading to the contact resistance. Dephasing in the harmonic chain suppresses the transmitting of scattered phonons into the harmonic chain. Then the heat flux in the chain is reduced by dephasing. However, the heat flux is symmetric to interchanging the reservoirs temperatures even with the dephasing. Therefore, the thermal rectification cannot be realized with pure dephasing only.

It has been experimentally achieved atomic resolution imaging by using secondary electron (SE) signals in a scanning transmission electron microscope with aberration correction. The underlying physical mechanism needs to be understood and has attracted considerable theoretical interest. Several recent calculations taking account of the inner‐shell ionization for high‐energy SE production have not included the cascade production of low‐energy SE signals which was believed to destroy the local information in SE imaging. In this work, we have developed a new theoretical method, a quantum Monte Carlo simulation, to calculate atomic resolution SE image by including every physical factor in SE generation, transportation and emission for a crystalline solid. This quantum Monte Carlo simulation method combines the Bohmian quantum trajectory method for treating electron elastic scattering and diffraction in a crystal with a conventional Monte Carlo sampling of inelastic scattering events along quantum trajectory paths. Simulation of atomic resolution SE image for atom columns in a copper crystal is performed. The contribution of the inner‐shell excitation to atomic resolution SE imaging is studied. Copyright © 2014 John Wiley & Sons, Ltd.

The mechanism causing the ballistic thermal rectification in the harmonic system is analytically studied. Using the conceptual model of a dephasing probe, we obtain that phase breaking is unnecessary to realize the ballistic thermal rectification. However, it can change the strength of rectification. Asymmetric phonon-phonon interaction inducing the energy exchange of the normal mode of vibration is necessary to realize the ballistic thermal rectification. Moreover, the quantum statistics of phonons in at least one thermal reservoir is crucial to guarantee the asymmetric phonon-phonon interaction.

Journal Article Quantum Monte Carlo Simulation for Atomic Resolution SEM/STEM Image Get access Z Ruan, Z Ruan Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China Search for other works by this author on: Oxford Academic Google Scholar M Zhang, M Zhang Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China Search for other works by this author on: Oxford Academic Google Scholar RG Zeng, RG Zeng Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China Search for other works by this author on: Oxford Academic Google Scholar B Da, B Da Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China Search for other works by this author on: Oxford Academic Google Scholar Y Ming, Y Ming School of Physics and Material Science, Anhui University, Hefei, Anhui 230601, P.R. China Search for other works by this author on: Oxford Academic Google Scholar SF Mao, SF Mao School of Nuclear Science and Technology, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China Search for other works by this author on: Oxford Academic Google Scholar ZJ Ding ZJ Ding Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 20, Issue S3, 1 August 2014, Pages 30–31, https://doi.org/10.1017/S1431927614001871 Published: 27 August 2014

A Monte Carlo simulation including surface excitation, Auger electron- and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true-secondary-electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up-to-date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam-sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500eV to 3000eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright (c) 2014 John Wiley & Sons, Ltd.

By coupling a temperature probe to the asymmetric three-terminal mesoscopic dielectric system, ballistic thermal rectification at low temperature is analytically studied based on the Landauer formulation of transport theory. It is seen that thermal rectification is a purely quantum effect and the quantum statistics of phonons in thermal reservoirs is necessary. Moreover, when the phonon re-emits into the system from the temperature probe, energy changing is necessary to realize thermal rectification. Another necessary condition is the different asymmetries for phonons with different frequencies, which is reflected by the dependence of the ratio tau(RC)(omega)/tau(RL)(omega) on omega, the phonon's frequency, where tau(RC)(omega) and tau(RL)(omega) are respectively the transmission coefficients from two asymmetric terminals to the temperature probe. The analytical results are confirmed by extensive numerical simulations.

This paper studies the thermal properties of three-terminal mesoscopic dielectric systems in the nonlinear response regime at low temperature. For a symmetric three-terminal system, when the temperature is finitely different between the left and right thermal reservoirs, the temperature of the central thermal reservoir is always higher than the averaging temperature of the others. This nonlinear thermal phenomenon is also observed for asymmetric three-terminal systems. At the end, a model of thermal rectification is presented.

The eigenvalue problem in a non-concentric circular quantum corral was studied with a conformal mapping method. The energy spectra of electrons show the properties of anti-crossing between the nearby energy levels. This property of energy spectra is caused by the Jacobian of the conformal transformation. We also give the probability density of the electronic states that confined in the non-concentric circular quantum corral. Quantum corral geometry has a stronger effect on electronic states of higher energies than that of lower energies.

This paper computes the conductance of an open ellipse cavity and discusses the effect of finite leads on conductance. The lead introduces mode coupling with bound states in the cavity which contributes to Fano resonant line shapes in conductance. By examining the resonant states in the cavity, the effects of state mixing and annular probability distribution of wave function due to the presence of leads are found. The results have been compared with the transport properties of other systems. The finite leads result in two effects, i.e. the evanescent mode contribution and additional oscillations, to the conductance.

In this work, using the scattering matrix method, we have investigated the transmission coefficients and the thermal conductivity in a double-bend waveguide structure. The transmission coefficients show strong resonances due to the scattering in the midsection of a double-bend structure; the positions and the widths of the resonance peaks are determined by the dimensions of the midsection of the structure. And the scattering in the double-bend structure makes the thermal conductivity decreases with the increasing of the temperature first, then increases after reaches a minimum. Furthermore, the investigations of the multiple double-bend structures indicate that the first additional double-bend structure suppresses the transmission coefficient and the frequency gap formed; and the additional double-bend structures determine the numbers of the resonance peaks at the frequency just above the gap region. These results could be useful for the design of phonon devices.