Simple Hopping Research Articles

The study of diffusion mechanism in network-forming liquid: Silica liquid

Molecular dynamics simulation is employed to investigate the diffusion mechanism in silica melt, a typical network-forming liquid. From the analysis of SiOx→SiOx±1 and OSiy→OSiy±1 reactions we reveal two moving modes: fast hopping and slow collective moving. Accordingly the atoms diffuse in the melt by simple hopping or through displacing of super-molecule (SM). A cluster analysis is performed for several of atom sets. It is shown that the melt exhibits non-uniform spatial distribution of reaction which causes the dynamics heterogeneity (DH). Further, the network structure of the melt consists of main subnet and large defective subnets. These subnets differ strongly in local environment, chemical composition and atomic density. This result evidences two distinct phases, the structure heterogeneity in silica melt and supports the polymorphism of network-forming liquid. We also find out that the node transformation spreads non-uniformly through the network structure. It takes place mainly in large defective subnet. The strong localization of node transformation is responsible for dynamical slowdown.

Hetero-junction of two quantum wires: Critical line and duality

Applying the Fermi-Bose equivalence and the boundary state formulation, we study the hetero-junction of two quantum wires. Two quantum wires are described by Tomonaga-Luttinger (TL) liquids with different TL parameters and electrons transport between two wires is depicted by a simple hopping interaction. We calculate the radiative corrections to the hopping interaction and obtain the renormalization (RG) exponent, making use of the perturbation theory based on the boundary state formulation. The model exhibits a phase transition at zero temperature. We discuss the critical line which defines the phase boundary on the two dimensional parameter space. The model also exhibits the particle-kink duality, which maps the strong coupling regime of the model onto the weak coupling regime of the dual model. The strong coupling regime of the model is found to match exactly the weak coupling regime of the dual model. This model is also important to study the critical behaviors of the two dimensional dissipative system with anisotropic friction coefficients.

Superexchange Charge Transport in Loaded Metal Organic Frameworks.

In the past, nanoporous metal-organic frameworks (MOFs) have been mostly studied for their huge potential with regard to gas storage and separation. More recently, the discovery that the electrical conductivity of a widely studied, highly insulating MOF, HKUST-1, improves dramatically when loaded with guest molecules has triggered a huge interest in the charge carrier transport properties of MOFs. The observed high conductivity, however, is difficult to reconcile with conventional transport mechanisms: neither simple hopping nor band transport models are consistent with the available experimental data. Here, we combine theoretical results and new experimental data to demonstrate that the observed conductivity can be explained by an extended hopping transport model including virtual hops through localized MOF states or molecular superexchange. Predictions of this model agree well with precise conductivity measurements, where experimental artifacts and the influence of defects are largely avoided by using well-defined samples and the Hg-drop junction approach.

2自由度振動系を利用した低衝撃高速階段昇降に関する研究

We have proposed a fast stair-climbing wheeled robot with a simple hopping mechanism that uses vibration of the two-degrees-of-freedom (2-DOF) system. The robot consists of two bodies connected by springs and a wire, hops by releasing energy stored in the springs, and quickly travels using wheels mounted on its lower body. The trajectories of the bodies during hopping depend on parameters such as the reduced mass of the two bodies, the mass ratio, the spring constant, the initial contraction of the spring and the horizontal velocity. This mechanism allows the robot to climb stairs quickly and economically and to land softly without the complex control. In this paper, we found a design solution creating two soft-landing points. As a result, the impact acceleration at landing was reduced by 57% for the riser of 0.03m and by 45% for the riser of 0.12m compared to that of free fall.

Calculation of Carrier Mobility in Copper Phthalocyanine by Simple Hopping Model

The Monte Carlo approach is used to calculate carrier mobility in molecular copper phthalocyanine (CuPc) with applied electric field in the range of 0.5 to 20 × 103 kV/cm. Density functional theory (DFT) is employed to derive the molecular interaction between neighboring molecules with various applied electric fields. The result of DFT calculation to evaluate transfer integral that used to calculate hopping rate in the range of applied electric fields. The charge transfer rate between adjacent molecules can be estimated by using the Marcus–Levich–Jortner (MLJ) formalism. The charge is assumed to be localized on the donor and then transferred to the acceptor. Tunneling is modeled by including selected vibration modes at the quantum mechanical level. The result of hopping rate is in ordered of 1015 s-1 for hole hopping in direction of applied electric field on the contrary hopping rate in ordered of 1014 s-1. The result of mobility can be calculated in range of 0.44 - 10.0 cm2/Vs decrease as a function of applied electric field that calculated by simple hopping model.

Impedance, AC conductivity and electric modulus analysis of l-leucine l-leucinium picrate

In this paper, we report the measurements of impedance spectroscopy for the l-leucine l-leucinium picrate compound synthesized by slow evaporation technique at room temperature in the frequency and temperature ranges of 209 Hz – 1 MHz and 393–433 K, respectively. The Nyquist plots exhibited single semi-circular arcs which were well fitted to an equivalent circuit. The frequency dependence of the AC conductivity has been investigated using the Jonscher universal power law: σac(ω)=σdc+A ωs. The exponent s remains constant in the investigated temperature range and is almost equal to 0.62. Single relaxation peak is observed in the imaginary part of the electrical modulus, suggesting the response of grain. The close values of activation energies obtained from the analysis of hopping frequency, electric modulus and dc conductivity indicate that the transport in the title compound can be described through a simple hopping mechanism, dominated probably by the motion of L-leucinium cations.

Influence of Interparticle Structure on the Steady-State and Transient Current within Arrays of Thiocyanate-Treated PbS Nanocubes

This paper describes the dependence of the DC conductivity, film charging dynamics, and transient photocurrent dynamics of quasi-two-dimensional arrays of thiocyanate-capped PbS nanocubes (NCs) on the edge length of the NC. Arrays were prepared monolayer-by-monolayer using self-assembly at a liquid–air interface. Across-film conductivity increases with NC size with a dependence consistent with a simple diffusional hopping model. Upon application of a constant source-drain bias, the measured dark current decays exponentially to a nonzero steady-state value as immobile hole traps fill. Illumination with 532-nm light produces a repeatable photoresponse, which also fits to an exponential function. The lifetimes associated with decay of the dark current and growth of the photocurrent both increase with increasing NC size. Comparison of the electrical data with electron microscopy images reveals that this trend is related to the connectivity of the percolation networks within the film, which depends on the inte...

Electrical conduction mechanism and transport properties of LiCrP2O7 compound

Ac electrical conductivity of LiCrP2O7 compound is been investigated by means of impedance spectroscopy measurements over the frequency and temperature ranges of 40 Hz–7 MHz and 460 to 700 K, respectively. The real and imaginary parts of complex impedance are well fitted to two equivalent circuit models. Besides, the nature of frequency dependence of ac conductivity follows the Jonscher's law, while calculated dc conductivity follows Arrhenius behavior with two different activation energies Edc(I) = 0.49 eV for T < 550 K and Edc(II) = 0.91 eV for T > 550 K. Actually, the values of activation energies obtained from the impedance Ea (I and II), dc conductivity Edc (I and II) and the relaxation time Eτ (I and II) are in good agreement, and hence the transport in the titled compound can be described through a simple hopping mechanism, dominated by the motion of Li+ ions. The temperature dependence of the power s-factor shows a decrease below 550 ± (5) K with increasing temperature (region I) whereas above 550 ± (5) K (region II), it increases with increasing temperature. An agreement between the experimental and theoretical results are discussed and it is suggested that the ac conductivity can be explained by the correlated barrier hopping (CBH) and non-overlapping small polaron tunneling model (NSPT) in region I and II, respectively.

Surface hopping with a manifold of electronic states. II. Application to the many-body Anderson-Holstein model.

We investigate a simple surface hopping (SH) approach for modeling a single impurity level coupled to a single phonon and an electronic (metal) bath (i.e., the Anderson-Holstein model). The phonon degree of freedom is treated classically with motion along--and hops between--diabatic potential energy surfaces. The hopping rate is determined by the dynamics of the electronic bath (which are treated implicitly). For the case of one electronic bath, in the limit of small coupling to the bath, SH recovers phonon relaxation to thermal equilibrium and yields the correct impurity electron population (as compared with numerical renormalization group). For the case of out of equilibrium dynamics, SH current-voltage (I-V) curve is compared with the quantum master equation (QME) over a range of parameters, spanning the quantum region to the classical region. In the limit of large temperature, SH and QME agree. Furthermore, we can show that, in the limit of low temperature, the QME agrees with real-time path integral calculations. As such, the simple procedure described here should be useful in many other contexts.

ANISOTROPY DIFFUSION DYNAMICS BEHAVIORS ON Pd(110) SURFACES: A MOLECULAR DYNAMICS STUDY

Using modified analytic embedded atom method and molecular dynamics (MDs) simulation, the self-diffusion dynamics behaviors of Pd adatom on perfect Pd (110) and reconstruction Pd (110)-(1 × 2) surfaces have been studied. Our simulations show the diffusion of Pd adatom is 1D motion along the [Formula: see text] direction of the channel on Pd (110)-(1 × 2) surface between 650 and 900 K. On perfect Pd (110) surface, the adatom diffuses along the [Formula: see text] direction of the channel in the low temperature range from 450 to 550 K by the simple hopping mechanism, and the diffusion is 2D between 600 and 800 K, the diffusion across the [Formula: see text] direction of the channel wall is by the exchange mechanism with an atom of channel wall. The diffusion dynamics behaviors have been derived from the Arrhenius law. On perfect Pd (110) surface, the diffusion dynamics behaviors along the [Formula: see text] direction of the channel may be divided into two parts in different temperature ranges. Our results show that the diffusion mobility D of Pd adatom along the [Formula: see text] direction on perfect Pd (110) surface is quicker than that on Pd (110)-(1 × 2) surface.

Studies of electric, dielectric, and conduction mechanism by OLPT model of Li4P2O7

The polycrystalline samples of Li4P2O7 were prepared by solid-state reaction technique. The formation of the compounds was checked by X-ray diffraction technique (XRD). Detailed dielectric and electrical properties of the compounds were analyzed as a function of frequency (200 Hz–5 MHz) and temperature (611–671 K). The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes suggesting the presence of grains and grain boundaries in the sample. Analysis of the dielectric constants e″ and loss tangent tan (δ) with frequency shows a distribution of relaxation times. The activation energy found from the Arrhenius plot confirms that the conduction process in the material is not due to simple hopping mechanism. The temperature dependence of frequency exponent s was investigated to understand the conduction mechanism in Li4P2O7. The overlapping large polaron tunneling (OLPT) model can explain the temperature dependence of the frequency exponent.

Two-dimensional NMR measurement and point dipole model prediction of paramagnetic shift tensors in solids.

A new two-dimensional Nuclear Magnetic Resonance (NMR) experiment to separate and correlate the first-order quadrupolar and chemical/paramagnetic shift interactions is described. This experiment, which we call the shifting-d echo experiment, allows a more precise determination of tensor principal components values and their relative orientation. It is designed using the recently introduced symmetry pathway concept. A comparison of the shifting-d experiment with earlier proposed methods is presented and experimentally illustrated in the case of (2)H (I = 1) paramagnetic shift and quadrupolar tensors of CuCl2⋅2D2O. The benefits of the shifting-d echo experiment over other methods are a factor of two improvement in sensitivity and the suppression of major artifacts. From the 2D lineshape analysis of the shifting-d spectrum, the (2)H quadrupolar coupling parameters are 〈Cq〉 = 118.1 kHz and 〈ηq〉 = 0.88, and the (2)H paramagnetic shift tensor anisotropy parameters are 〈ζP〉 = - 152.5 ppm and 〈ηP〉 = 0.91. The orientation of the quadrupolar coupling principal axis system (PAS) relative to the paramagnetic shift anisotropy principal axis system is given by (α,β,γ)=(π2,π2,0). Using a simple ligand hopping model, the tensor parameters in the absence of exchange are estimated. On the basis of this analysis, the instantaneous principal components and orientation of the quadrupolar coupling are found to be in excellent agreement with previous measurements. A new point dipole model for predicting the paramagnetic shift tensor is proposed yielding significantly better agreement than previously used models. In the new model, the dipoles are displaced from nuclei at positions associated with high electron density in the singly occupied molecular orbital predicted from ligand field theory.

Semiclassical Monte Carlo: a first principles approach to non-adiabatic molecular dynamics.

Modeling the dynamics of photophysical and (photo)chemical reactions in extended molecular systems is a new frontier for quantum chemistry. Many dynamical phenomena, such as intersystem crossing, non-radiative relaxation, and charge and energy transfer, require a non-adiabatic description which incorporate transitions between electronic states. Additionally, these dynamics are often highly sensitive to quantum coherences and interference effects. Several methods exist to simulate non-adiabatic dynamics; however, they are typically either too expensive to be applied to large molecular systems (10's-100's of atoms), or they are based on ad hoc schemes which may include severe approximations due to inconsistencies in classical and quantum mechanics. We present, in detail, an algorithm based on Monte Carlo sampling of the semiclassical time-dependent wavefunction that involves running simple surface hopping dynamics, followed by a post-processing step which adds little cost. The method requires only a few quantities from quantum chemistry calculations, can systematically be improved, and provides excellent agreement with exact quantum mechanical results. Here we show excellent agreement with exact solutions for scattering results of standard test problems. Additionally, we find that convergence of the wavefunction is controlled by complex valued phase factors, the size of the non-adiabatic coupling region, and the choice of sampling function. These results help in determining the range of applicability of the method, and provide a starting point for further improvement.

Ac conductivity and NSPT model conduction of KAlP2O7 compound

The KAlP2O7 compound was prepared by conventional solid-state reaction technique. The structure is confirmed by X-ray powder diffraction. The frequency-dependent electrical properties of the sample were investigated in the temperature range of 528–668 K and in the frequency range of 209 Hz–5 MHz by impedance spectroscopy. The ac conductivity has been fitted and studied using Jonscher’s equation, whose exponent s varied with the temperature, showing that the non-overlapping small polaron tunneling (NSPT) is the suitable model to describe the electrical conduction mechanism. The variation of dc conductivity suggests Arrhenius type. Besides, the modulus data were analyzed using the Kohlrausch–Williams–Watts (KWW) stretched exponential function. The peak positions ωm of the above spectra shifted towards higher frequencies with the increase in temperature. Moreover, the value of activation energy for the bulk obtained from the analysis of equivalent circuit (1.11 eV) and modulus relaxation (0.91 eV) confirms that the transport is not due to a simple hopping mechanism and that the ion transport is probably due to the hopping of K+ ions along [101] tunnels direction. The thermodynamic parameters were also calculated.

AC conductivity and mechanism of conduction study of lithium barium pyrophosphate Li2BaP2O7 using impedance spectroscopy

The Li2BaP2O7 compound has been obtained by the conventional solid-state reaction and characterized by X-ray powder diffraction. The title material crystallizes in the monoclinic system with C2/c space group. Electrical properties of the compound have been studied using complex impedance spectroscopy in the frequency range 200 Hz–5 MHz and temperature range 589–724 K. Temperature dependence of the DC conductivity and modulus was found to obey the Arrhenius law. The obtained values of activation energy are different which confirms that transport in the titled compound is not due to a simple hopping mechanism. AC conductivity measured follows the power-law dependence σAC ∼ ωs typical for charge transport. Therefore, the experimental results are analyzed with various theoretical models. Temperature dependence of the power law exponent s strongly suggests that tunneling of large polarons is the dominant transport process.

Frequency and temperature dependence of the dielectric properties and AC electrical conductivity in mixed pyrophosphate ceramic

Pyrophosphate LiKZnP2O7 has been prepared by a solid state reaction method and characterized by X-ray powder diffraction at room temperature. This compound crystallizes in the monoclinic system with Pc space group. The AC electrical conductivity has been investigated in the frequency range 200 Hz–5 MHz and temperature range 561–696 K. It can be observed from the experimental data that the AC conductivity of this compound is proportional to ωs, (s < 1), the value of s has a tendency to decrease with temperature. The values of the activation energy deduced from the DC conductivity (0.88 eV), modulus (0.84 eV) and hopping frequency (0.81 eV) confirm that the transport in the sample is due to a simple hopping of Li+/K+ ions.

AC Conductivity and Mechanism of Conduction Study of ?-Sr2P2O7 Using Impedance Spectroscopy

The ceramic sample Sr2P2O7 was prepared by a solid-state reaction technique at high temperature. Electrical properties and modulus analysis were studied using complex impedance spectroscopy in the frequency range 200 Hz–5 MHz and temperature range 602-714 K. The difference of the value of activation energy for the bulk obtained from analysis of equivalent circuit (0.81 eV) and modulus relaxation (0.69 eV) confirms that the transport is not due from a simple hopping mechanism. The average of the power law exponent s is reasonably interpreted by the overlapping large polaron tunneling (OLPT) model. The mechanism of conduction is probably due from the displacements of the Sr2+ ion in the tunnel-type cavities along the b axis.

2A2-D06 2自由度振動系を利用した2輪型高速階段昇降における必要踏面距離の短縮(車輪型/クローラ型移動ロボット(2))

In this paper, we propose a fast stair-climbing robot with a simple hopping mechanism using vibration of the two-degrees-of-freedom (2-DOF) system. This robot consists of upper and lower bodies connected by a spring and a wire, and hops by releasing stored energy in the spring and travels quickly using wheels mounted in the lower body. These mechanisms allow the robot to climb stairs quickly and economically and land softly without a complex control. Here, we developed a two-wheeled inverted pendulum mechanism to reduce the required tread length. As a result, the proposed mechanism reduced impact acceleration by 66% during the first landing and 60% during the second landing for the climbing height of 200 mm and reduced the required tread length by 92%.

Dynamics of Cu monomer, dimer and trimer on Ag (110) (1 × 2) missing‐row reconstructed surface

The authors aimed to investigate the diffusion of Cu monomer, dimer and trimer on Ag (110) (1×2) missing‐row surface. This problem is addressed through molecular dynamic simulation based on semi‐empirical many‐body potential, derived from the embedded atom method. Within this approach, we have identified and calculated the activation energy of each microscopic mechanism. Thus, for Cu monomer, the diffusion process occurs essentially by simple hopping between nearest‐neighbor sites. While for the Cu dimer, three processes have been identified such as dissociation–reassociation (DR), concerted jump (CJ) and leapfrog mechanisms (LF) with a slight predominance of DR process and a dual competition between CJ and LF processes. But, in the case of small one‐dimensional cluster such as trimer on (110)(1×2) missing‐row reconstructed surface, the main diffusion mechanism is the LF process. These results shed light on the diffusion processes on missing‐row reconstructed surfaces, especially for heterogeneous systems. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Realising Haldane's vision for a Chern insulator in buckled lattices

The Chern insulator displays a quantum Hall effect with no net magnetic field. Proposed by Haldane over 20 years ago, it laid the foundation for the fields of topological order, unconventional quantum Hall effects, and topological insulators. Despite enormous impact over two decades, Haldane's original vision of a staggered magnetic field within a crystal lattice has been prohibitively difficult to realise. In fact, in the original paper Haldane stresses his idea is probably merely a toy model. I show that buckled lattices with only simple hopping terms, within in-plane magnetic fields, can realise these models, requiring no exotic interactions or experimental parameters. As a concrete example of this very broad, and remarkably simple principle, I consider silicene, a honeycomb lattice with out-of-plane sublattice anisotropy, in an in-plane magnetic field, and show that it is a Chern insulator, even at negligibly small magnetic fields, which is analogous to Haldane's original model.