Simple Hopping Research Articles

Exciton Transport in the Nonfullerene Acceptor O-IDTBR from Nonadiabatic Molecular Dynamics.

Theory, computation, and experiment have given strong evidence that charge carriers in organic molecular crystals form partially delocalized quantum objects that diffuse very efficiently via a mechanism termed transient delocalization. It is currently unclear how prevalent this mechanism is for exciton transport. Here we carry out simulation of singlet Frenkel excitons (FE) in a molecular organic semiconductor that belongs to the class of nonfullerene acceptors, O-IDTBR, using the recently introduced FE surface hopping nonadiabatic molecular dynamics method. We find that FE are, on average, localized on a single molecule in the crystal due to sizable reorganization energy and moderate excitonic couplings. Yet, our simulations suggest that the diffusion mechanism is more complex than simple local hopping; in addition to hopping, we observe frequent transient delocalization events where the exciton wave function expands over 10 or more molecules for a short period of time in response to thermal excitations within the excitonic band, followed by de-excitation and contraction onto a single molecule. The transient delocalization events lead to an increase in the diffusion constant by a factor of 3-4, depending on the crystallographic direction as compared to the situation where only local hopping events are considered. Intriguingly, O-IDTBR appears to be a moderately anisotropic 3D "conductor" for excitons but a highly anisotropic 2D conductor for electrons. Taken together with previous simulation results, two trends seem to emerge for molecular organic crystals: excitons tend to be more localized and slower than charge carriers due to higher internal reorganization energy, while exciton transport tends to be more isotropic than charge transport due to the weaker distance dependence of excitonic versus electronic coupling.

Acoustic plasmons and conducting carriers in hole-doped cuprate superconductors

The layered crystal structures of cuprates enable collective charge excitations fundamentally different from those of three-dimensional metals. Acoustic plasmons have been observed in electron-doped cuprates by resonant inelastic x-ray scattering (RIXS); in contrast, the characteristics of acoustic plasmons in hole-doped cuprates are under debate, despite extensive measurements. This contrast led us to investigate the charge excitations of hole-doped cuprate ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$. Here we present incident-energy-dependent RIXS measurements and calculations of collective charge response via the loss function to reconcile the issues above. Our results provide evidence for the acoustic plasmons of Zhang-Rice singlet (ZRS), which has a character of the Cu $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ strongly hybridized with the O $2p$ orbitals; the metallic behavior is implied to result from the movement of ZRS rather than the simple hopping of O $2p$ holes.

Phase transition, nonlinear optical parameters and electrical properties in 4-benzylpiperidinium tetraoxoselenate monohydrate

The thermal properties of the organic-inorganic hybrid material (C12H10N)2SeO4H2O have been investigated using X-ray diffraction, Raman spectroscopy, impedance and modulus spectroscopy technique. Single X-ray diffraction recorded at three different temperatures: T1=81 ​K, T2=298 ​K and T3=328 ​K reveals that the title compound crystallizes in the same space group (P21/c) for the different temperatures and the lattice parameters don't exhibit a remarkable variation. The nonlinear optical parameters exhibit a remarkable variation. This result indicates clearly that this material does not present a structural phase transition and confirm our previous interpretation of order disorder phase transition detected by DSC measurement at 300K. Raman spectroscopy at various temperatures confirms the presence of the phase transition at 300K and showed that both anionic and cationic motions contribute to the transition mechanisms. The character order disorder phase transition is undoubtedly related to the change in dynamical state of (SeO4)2− anion coupled with conformational change of [C6H5CH2C5H9NH2]+ cation above 300K. 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 orientation of selenate anions. Hence, in all temperatures the large γ value shows that the studied compound is in interest material for NLO applications.

Chain RTS/CTS scheme with Opportunistic Channel Allocation

Current Wi-Fi standards lack in terms of providing reliable multi-hop links for bandwidth-hungry applications. As a result, aerial networks which need reliable & high bandwidth multi-hop communication links, fail to maintain QoS levels. In this paper, we take a deeper look at our earlier work, Chain RTS/CTS, which makes channel reservations by repeating the RTS frame of the source of the e2e link towards the destination node using alternating channels. After receiving the RTS frame, destination node starts backward CTS chain where CTS frames are repeated similarly using the same route with forward RTS chain towards the source node of e2e link. Additionally, we introduce Opportunistic Channel Allocation designed to increase fairness when operating at 5 GHz band. Our simulations show 37% throughput improvement in multi-hop links with 68% increased link setup time when compared to simple channel hopping enhancement scheme for single e2e connection. When there are multiple concurrent e2e connections, our solution improves the throughput by 27% compared to Classical Channel Hopping (which alternates channels at each hop) with only 11% lower Jain’s Fairness index.

Loss of ergodicity in a quantum hopping model of a dense many body system with repulsive interactions

In this work we report on a loss of ergodicity in a simple hopping model, motivated by the Hubbard Hamiltonian, of a many body quantum system at zero temperature, quantized in Euclidean time. We show that this quantum system may lose ergodicity at high densities on a large lattice, as a result of both Pauli exclusion and strong Coulomb repulsion. In particular we study particle hopping susceptibilities and the tendency towards particle localization. It is found that the appearance and existence of quantum phase transitions in this model, in the case of high density and strong Coulomb repulsion, depends on the starting configuration of particle trajectories in the numerical simulation. We argue that this breakdown may be the Euclidean time version of a breakdown of the eigenstate thermalization hypothesis in real time quantization.

Using Energy Shaping and Regulation for Limit Cycle Stabilization, Generation, and Transition in Simple Locomotive Systems

Abstract This paper explores new ways to use energy shaping and regulation methods in walking systems to generate new passive-like gaits and dynamically transition between them. We recapitulate a control framework for Lagrangian hybrid systems, and show that regulating a state varying energy function is equivalent to applying energy shaping and regulating the system to a constant energy value. We then consider a simple one-dimensional hopping robot and show how energy shaping and regulation control can be used to generate and transition between nearly globally stable hopping limit cycles. The principles from this example are then applied on two canonical walking models, the spring loaded inverted pendulum (SLIP) and compass gait biped, to generate and transition between locomotive gaits. These examples show that piecewise jumps in control parameters can be used to achieve stable changes in desired gait characteristics dynamically/online.

Synthesis and biological evaluation of platensic alcohol as an adamantane surrogate in antitumor drug lead adaphostin

Adamantane has been widely used as a “lipophilic bullet” in drug discovery and development, due to its unique diamond-like architecture with benign pharmacological/ pharmaceutical properties. Platensimycin is a natural product isolated from a soil streptomycete, which contains an adamantane-like moiety extensively modified from a diterpenoid precursor. In the current study, platensic alcohol was semisynthesized from platensimycin and used as an adamantane surrogate in anticancer drug lead adaphostin. The resulting hybrid platensic alcohol/adaphostin compounds, eg. 4a and 4b, exhibited similar cytotoxic activity with adaphostin against the tested cancer cell lines. In particular, 4b generates significantly more reactive oxygen species (ROS) and shows stronger synergy with the clinically used histone deacetylase inhibitor vorinostat than adaphostin, probably due to the presence of two hydroquinone groups. Density functional theory calculation supports that there could be certain π-π stacking interaction in 4b in aqueous solution, which might explain that 4b has similar serum stability with adaphostin. Our study not only leads to the identification of 4b as a potent ROS generating agent, but showcases a simple scaffold hopping strategy to harvest lipophilic scaffolds from natural products.

Dielectric Relaxation and Non‐Overlapping Small Polaron Tunneling Model Conduction Studies of Ag2‐xNaxZnP2O7 (x = 0, 1, and 2) Materials

AbstractThe Ag2−xNaxZnP2O7 (x = 0, 1, and 2) materials are prepared using the solid–solid method followed by heat treatment. The investigated samples are characterized by combining X‐ray powder diffraction and electrical impedance spectroscopy techniques. The Rietveld refinement of the X‐ray diffraction (XRD) pattern revealed that these Ag2ZnP2O7, AgNaZnP2O7, and Na2ZnP2O7 compounds are indexed in the tetragonal system with the P42/n space group. Their dielectric properties are measured in the frequency and temperature ranges of 200–107 Hz and 419–518 K, respectively. In these compounds, the dielectric relaxation can be described by a Cole–Cole model. The frequency dependence of the conductivity is interpreted in terms of Jonscher's law. For the three compounds, the activation energy for conduction has a different value compared to the activation energy for the relaxation process. This indicates that the conduction in the compound is not predicted by the simple hopping model. Then, the temperature dependence of the power‐law exponent(s) suggests that the non‐overlapping small polaron tunneling model is the dominant transport process in these materials. Moreover, the values of defect states N(EF) and the tunnel distance RW show the electrical performance of the Ag2ZnP2O7 compound.

A Single-legged Robot Inspired by the Jumping Mechanism of Click Beetles and Its Hopping Dynamics Analysis

The click beetle can jump up with a hinge when it is on the dorsal side. This jumping mechanism is simple and suitable as an inspiration for designing a simple, small, and reliable hopping robot. We report a single-legged robot inspired by the jumping mechanism of click beetles. It is 85 mm high, 60 mm long, and 41 mm wide, and weighs about 49 g. The robot has good hopping performance that the hopping height is about 4 times – 4.3 times of its body height. It is capable for rescue missions that require to enter enclosed spaces through cracks and narrow channels. In addition, hopping dynamics of the robot is important to understand its jumping mechanism and improve the robot’s hopping performance. But existing dynamic study does not complete the analysis including all stages in the hopping which are pre-hopping, take-off, and air-flying. We propose the decomposition method to study dynamics of the three stages separately, and synthesize them with related parameters. The dynamic synthesis of multi-motion states in a hopping cycle of the single-legged hopping robot is implemented. The hopping performance and dynamic synthesis theory of the robot are verified by simulations and experiments. Our study helps lay the foundation for design and hopping control of simple hopping robot systems.

Na/Li substitution effect on the structural, electrical and magnetic properties of LiCr(MoO4)2 and β─Li0.87Na0.13Cr(MoO4)2

The novel bi-molybdate β─Li0.87Na0.13Cr(MoO4)2 was prepared by solid state reaction route. Single crystal X-ray diffraction experiment revealed that the compound crystallizes in the triclinic system, in P −1 space group with a = 6.715 (2), b = 7.160 (3), c = 7.237 (1) Å, α = 91.16° (3), β = 110.59° (2), γ = 105.54° (3). Its crystal structure is isotypic to LiCr(MoO4)2 which has interesting magnetic and electrochemical properties [1–3]. Bond valence sum (BVS) and charge distribution (CHARDI) validation tools supported the structural model.The electrical properties were systematically studied by impedance spectroscopy. The ionic conductivity measurements are performed on pellets of 82% and 87% relative density for LiCr(MoO4)2 and β─Li0.87Na0.13Cr(MoO4)2 respectively. AC impedance spectroscopy studies show that the highest overall conductivity is σ326°C = 7.86 × 10−7 S cm−1Probable diffusion pathways of Li+ ions in the both structures were simulated using the bond valence sum BVS maps method. This analysis shows that the ionic transport in these materials is essentially due to simple hopping of Li+ ions parallel to (101) plane.For β─Li0.87Na0.13Cr(MoO4)2 compound, the in-situ High Temperature X-Ray Diffraction (HTXRD), in the temperature range from 25 to 650 °C, were also performed and Unit-cell thermal expansion has been discussed.The magnetic study show that these compounds present an antiferromagnetic order below the temperatures TN = 16 and 30 K for LiCr(MoO4)2 and β─Li0.87Na0.13Cr(MoO4)2 respectively.

Nonadiabatic Molecular Dynamics at Metal Surfaces.

Dynamics at molecule-metal interfaces are a subject of intense current interest and come in many different flavors of experiments: gas-phase scattering, chemisorption, electrochemistry, nanojunction transport, and heterogeneous catalysis, to name a few. These dynamics involve nuclear degrees of freedom entangled with many electronic degrees of freedom (in the metal), and as such there is always the possibility for nonadiabatic phenomena to appear: the nuclei do not necessarily need to move slower than the electrons to break the Born-Oppenheimer (BO) approximation. In this Feature Article, we review a set of dynamical methods developed recently to deal with such nonadiabatic phenomena at a metal surface, methods that serve as alternatives to Tully's independent electron surface hopping (IESH) model. In the weak molecule-metal coupling regime, a classical master equation (CME) can be derived and a simple surface hopping approach is proposed to propagate nuclear and electronic dynamics stochastically. In the strong molecule-metal interaction regime, a Fokker-Planck equation can be derived for the nuclear dynamics, with electronic DoFs incorporated into the overall friction and random force. Lastly, a broadened classical master equation (BCME) can interpolate between the weak and strong molecule-metal interactions. Here, we briefly review these methods and the relevant benchmarking data, showing in particular how the methods can be used to calculate nonequilibrium transport properties. We highlight several open questions and pose several avenues for future study.

Eigenvalue repulsion and eigenvector localization in sparse non-Hermitian random matrices.

Complex networks with directed, local interactions are ubiquitous in nature and often occur with probabilistic connections due to both intrinsic stochasticity and disordered environments. Sparse non-Hermitian random matrices arise naturally in this context and are key to describing statistical properties of the nonequilibrium dynamics that emerges from interactions within the network structure. Here we study one-dimensional (1D) spatial structures and focus on sparse non-Hermitian random matrices in the spirit of tight-binding models in solid state physics. We first investigate two-point eigenvalue correlations in the complex plane for sparse non-Hermitian random matrices using methods developed for the statistical mechanics of inhomogeneous two-dimensional interacting particles. We find that eigenvalue repulsion in the complex plane directly correlates with eigenvector delocalization. In addition, for 1D chains and rings with both disordered nearest-neighbor connections and self-interactions, the self-interaction disorder tends to decorrelate eigenvalues and localize eigenvectors more than simple hopping disorder. However, remarkable resistance to eigenvector localization by disorder is provided by large cycles, such as those embodied in 1D periodic boundary conditions under strong directional bias. The directional bias also spatially separates the left and right eigenvectors, leading to interesting dynamics in excitation and response. These phenomena have important implications for asymmetric random networks and highlight a need for mathematical tools to describe and understand them analytically.

Na3Bi(AsO4)2: Synthesis, crystal structure and ionic conductivity

Single crystals of the new Bi(III) arsenate, Na3Bi(AsO4)2 have been isolated and their structure has been determined by X-ray diffraction. This compound crystallizes in the monoclinic space group P21/n with a = 5.5275 (2) Å; b = 19.271 (7) Å; c = 7.156 (3) Å, β = 92.104 (32)° and Z = 4. Na3Bi(AsO4)2 features a three-dimensional open anionic framework made up of corner-sharing BiO6 and AsO4 polyhedra. Its structure can be described as buckled infinite anionic layers parallel to (001) planes, connected via As–O–Bi bridges to form a three-dimensional open framework. This architecture gives rise to wide tunnels running along the [100] direction where the sodium ions are located. The ionic conductivity measurements are performed on pellets of 91% relative density. The highest overall conductivity of 1.3 × 10−5 S cm−1 was found at 500 °C with Ea = 0.76 eV. The value of the activation energy obtained from the analysis of the impedance spectra suggests the ionic transport in Na3Bi(AsO4)2 is essentially due to simple hopping of Na+ cations. Furthermore, the frequency dependence of AC conductivity was shown to obey the Jonscher universal power law.

Concerted Control of Stance and Balance Locomotor Subfunctions—Leg Force as a Conductor

In human locomotion, the complex structure of the human body is controlled such that conceptual models (e.g., the spring-loaded-inverted-pendulum model) can describe the significant features. This suggests that the interplay of the complex control and musculoskeletal systems projects into a low-dimensional space to perform different movements. Such simplification can involve splitting the task into different modular control subproblems (locomotor subfunctions) that can be solved individually. Here, we asked how two locomotor subfunctions, namely stance and balance, could be coordinated to generate repeatable and robust motor commands. We developed a simple neuromechanical hopping model, based on decoupling axial and perpendicular leg forces. For this, bouncing behaviors and trunk posture control can be addressed by a knee extensor muscle and biarticular thigh muscles, respectively. We suggest utilizing the leg force feedback as interplay among environment, body mechanics, and sensory control to synchronize the decoupled subfunctions. We evaluated this approach in push recovery, attenuating ground drop perturbations and by investigating its sensitivity to the reflex gain as the control parameter. Leg force feedback can improve robustness of hopping by generating rhythmic hopping patterns. Such a parsimony model-based control concept could simplify controlling assistive devices, such as exoskeletons and prostheses.

Optical and electrical properties of the yavapaiite-like molybdate NaAl(MoO4)2

Polycrystalline sample of the sodium aluminum molybdate, NaAl(MoO4)2, was prepared by conventional solid-state reaction method. Ultraviolet-visible-near-infrared (UV-Vis-NIR) diffuse reflectance spectroscopy revealed that the optical band gap values of this material at ambient conditions are 3.77 and 3.35 eV for direct and indirect transition, respectively. We also report the electrical properties of the sample using alternating current (AC) complex impedance spectroscopy (CIS) technique over a frequency range of 40 Hz to 5 MHz at several temperatures in the range 693–883 K. These studies showed that temperature dependence of the direct current (DC) conductivity (σdc) and the relaxation frequency (fr) obey the Arrhenius law. However, the obtained values of the activation energy are different, confirming that ionic transport in the material is not due to a simple hopping mechanism. Based on DC conductivity data, NaAl(MoO4)2 can be classified as low ionic conductor. The differential thermal analysis (DTA) shows the presence of a structural phase transition at 580 °C, which is confirmed by the variation of fr and σdc as a function of temperature. The bond valence sum map (BVSM) analysis indicates that the sodium ions seem to be trapped in their own sites. Consequently, the Na+ ion transport in the interlayer spaces is very difficult.

Electrical, dielectric properties and study of AC electrical conduction mechanism of Li0.9□0.1NiV0.5P0.5O4.

In this paper, we report the measurements of impedance spectroscopy for a new olivine-type lithium deficiency Li0.9□0.1NiV0.5P0.5O4 compound. It was synthesized by the conventional solid-state technique. All the X-ray diffraction peaks of the compound are indexed, and it is found that the sample is well crystallized in orthorhombic olivine structure belonging to the space group Pnma. Conductivity and dielectric analyses of the sample are carried out at different temperatures and frequencies using the complex impedance spectroscopy technique. The electrical conductivity of Li0.9□0.1NiV0.5P0.5O4 is higher than that of parent compound LiNiV0.5P0.5O4. 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 title compound is not due to a simple hopping mechanism. To determine the conduction mechanism, the AC conductivity and its frequency exponent have been analysed in this work by a theoretical model based on quantum mechanical tunnelling: the non-overlapping small polaron tunnelling model.

Impedance spectroscopy study and phase transition in phospho-vanadium mixed oxide LiZnV0.5P0.5O4

An X-ray crystallographic study has allowed us to identify a powder of the type LiZnV0.5P0.5O4, which contains 50% of vanadium and 50% of phosphore, inside the binary system LiZnVO4–LiZnPO4. The structure is isotypic with the phenacite like LiZnP04. X-ray diffraction patterns are indexed according to the lattice parameters of the rhombohedral system and the R3 space group. IR spectra show the presence of VO4 and PO4 groups in the network of this material. The experimental results indicate that $$\sigma_{\text{AC}}$$ ( $$\omega$$ ) is proportional to $$\left( {\omega^{n} } \right)$$ . The activation energy found from the Arrhenius plot confirms that the conduction processing of the material is not due to simple hopping mechanism. The temperature dependence of frequency exponent n was investigated to understanding the conduction mechanism in LiZnV0.5P0.5O4. The non-overlapping small Polaron tunneling (NSPT) model can explain the temperature dependence of the frequency exponent. A phase transition at T = 623 K has been evidenced by Differential scanning calorimetry (DSC) and subsequently confirmed by the analysis of dielectric and electric properties.

Repetition and pair-interaction of string-like hopping motions in glassy polymers

The dynamics of many glassy systems are known to exhibit string-like hopping motions each consisting of a line of particles displacing one another. By using the molecular dynamics simulations of glassy polymers, we show that these motions become highly repetitive back-and-forth motions as temperature decreases and do not necessarily contribute to net displacements. Particle hops which constitute string-like motions are reversed with a high probability, reaching 73% and beyond at low temperature. The structural relaxation rate is then dictated not by a simple particle hopping rate but instead by the rate at which particles break away from hopping repetitions. We propose that disruption of string repetitions and hence also structural relaxations are brought about by pair-interactions between strings.

Simple diffusion hopping model with convection.

We present results from a new variant of a diffusion hopping model, the convective diffusive lattice model, to describe the behavior of a particulate flux around bluff obstacles. Particle interactions are constrained to an underlying square lattice where particles are subject to excluded volume conditions. In an extension to previous models, we impose a real continuous velocity field upon the lattice such that particles have an associated velocity vector. We use this velocity field to mediate the position update of the particles through the use of a convective update after which particles also undergo diffusion. We demonstrate the emergence of an expected wake behind a square obstacle which increases in size with increasing object size. For larger objects we observe the presence of recirculation zones marked by the presence of symmetric vortices in qualitative agreement with experiment and previous simulations.

Low Temperature Conductivity inn-Type Noncompensated Silicon below Insulator-Metal Transition

We investigate the transport properties ofn-type noncompensated silicon below the insulator-metal transition by measuring the electrical and magnetoresistances as a function of temperatureTfor the interval 2–300 K. Experimental data are analyzed taking into account possible simple activation and hopping mechanisms of the conductivity in the presence of two impurity bands, the upper and lower Hubbard bands (UHB and LHB, resp.). We demonstrate that the charge transport develops with decreasing temperature from the band edge activation (110–300 K) to the simple activation with much less energy associated with the activation motion in the UHB (28–90 K). Then, the Mott-type variable range hopping (VRH) with spin dependent hops occurs (5–20 K). Finally, the VRH in the presence of the hard gap (HG) between LHB and UHB (2–4 K) takes place. We propose the empiric expression for the lowTdensity of states which involves both the UHB and LHB and takes into account the crossover from the HG regime to the Mott-type VRH with increasing temperature. This allows us to fit the lowTexperimental data with high accuracy.