Showing papers on "Electric potential published in 2020"

Electret formation in transition metal oxides by electrochemical amorphization

Abstract: Transition metal oxides (TMOs) are an important class of materials that show a wide range of functionalities involving spin, charge, and lattice degrees of freedom. The strong correlation between electrons in d-orbitals and the multivalence nature give rise to a variety of exotic electronic states ranging from insulator to superconductor and cause intriguing phase competition phenomena. Despite a burst of research on the multifarious functionalities in TMOs, little attention has been paid to the formation and integration of an electret—a type of quasi-permanent electric field generator useful for nanoscale functional devices as an electric counterpart to permanent magnets. Here, we find that an electret can be created in LaMnO3 thin films by tip-induced electric fields, with a considerable surface height change, via solid-state electrochemical amorphization. The surface charge density of the formed electret area reaches ~400 nC cm−2 and persists without significant charge reduction for more than a year. The temporal evolution of the surface height, charge density, and electric potential are systematically examined by scanning probe microscopy. The underlying mechanism is theoretically analyzed based on a drift-diffusion-reaction model, suggesting that positively charged particles, which are likely protons produced by the dissociation of water, play crucial roles as trapped charges and a catalysis to trigger amorphization. Our finding opens a new horizon for multifunctional TMOs. A material that generates its own electric field has been developed by scientists in South Korea. An electret is the electrical equivalent of a magnet in that it is formed of two electric poles rather than two magnetic poles. Just as magnetic dipoles give rise to permanent magnets, electret materials create a quasi-permanent electric field. They are useful for microphones, photocopiers and many other electrical devices. Yong-Jin Kim and Chan-Ho Yang from the Korea Advanced Institute of Science and Technology, Daejeon, have created an electret using transition metal oxides. The researchers used a sharp tip of platinum-coated silicon to “write” a charged pattern with a density similar to that of commercially available electrets into a thin film of lanthanum manganite. These patterns persisted for more than a year. An electret can be created in a complex transition metal oxide LaMnO3 by tip-induced electric fields with a considerable surface height change via solid-state electrochemical amorphization. The surface charge density of the formed electret area reaches ~400 nC cm−2 and persists without significant charge reduction for more than a year. Our finding opens a new horizon for multifunctional transition metal oxides by providing an electric counterpart to permanent magnets.

Variability and origins of grain boundary electric potential detected by electron holography and atom-probe tomography

Abstract: A number of grain boundary phenomena in ionic materials, in particular, anomalous (either depressed or enhanced) charge transport, have been attributed to space charge effects. Developing effective strategies to manipulate transport behaviour requires deep knowledge of the origins of the interfacial charge, as well as its variability within a polycrystalline sample with millions of unique grain boundaries. Electron holography is a powerful technique uniquely suited for studying the electric potential profile at individual grain boundaries, whereas atom-probe tomography provides access to the chemical identify of essentially every atom at individual grain boundaries. Using these two techniques, we show here that the space charge potential at grain boundaries in lightly doped, high-purity ceria can vary by almost an order of magnitude. We further find that trace impurities (<25 ppm), rather than inherent thermodynamic factors, may be the ultimate source of grain boundary charge. These insights suggest chemical tunability of grain boundary transport properties. A number of grain boundary phenomena in ionic materials, such as anomalous charge transport, have been attributed to space charge effects. Space charge potential at grain boundaries in lightly doped, high-purity ceria is now shown to vary by almost an order of magnitude.

Any l-state solutions of the Schrodinger equation interacting with Hellmann–Kratzer potential model

Abstract: The approximate analytical solutions of the radial Schrodinger equation have been obtained with a newly proposed potential called Hellmann–Kratzer potential. The potential is a superposition of Hellmann potential and modified Kratzer potential. The Hellmann–Kratzer potential actually comprises of three different potentials which include Yukawa potential, Coulomb potential and Kratzer potential. The aim of combining these potentials is to have a wide application. The energy eigenvalue and the corresponding wave function are calculated in a closed and compact form using the Nikiforov–Uvarov method. The energy equation for some potentials such as Kratzer, Hellmann, Yukawa and Coulomb potentials has also been obtained by varying some potential parameters. Our results excellently agree with the already existing literature. Some numerical results have been computed. We have plotted the behaviour of the energy eigenvalues with different potential parameters and also reported on the numerical result.

Material Properties Influencing the Charge Decay of Electret Filters and their Impact on Filtration Performance.

Abstract: Electret filters as opposed to mechanical filters display the enhanced ability to capture airborne particles with the electrostatic attraction. However, the environmental aging during shelf-life or use may cancel its benefit by dissipating the charges. This work investigates the polymeric attributes influencing the charge decay and the electrostatic filtration of electret filters, employing polymers with different dielectric constants (er) and wettability. As accelerated aging, high temperature (120 °C) or high humidity (25 °C, 90% RH) was applied to the electret filters for 48 h. For the humidity aging, wetting property of material was a critical factor affecting the charge decay and the filtration performance, as the absorbed water increases the electrical conductivity. For the thermal aging, the material with the highest er deteriorated the electric potential and the filtration performance by the largest extent, due to the lower band gap energy for charge transfer. The results of this study implicate that er and wettability are important material parameters influencing the electric conductivity and chain mobility, and they can be used as convenient predictors for charge retention capacity affecting the robust electrostatic filtration performance.

Sparse nonlinear models of chaotic electroconvection

Abstract: Convection is a fundamental fluid transport phenomenon, where the large-scale motion of a fluid is driven, for example, by a thermal gradient or an electric potential. Modeling convection has given rise to the development of chaos theory and the reduced-order modeling of multiphysics systems; however, these models have been limited to relatively simple thermal convection phenomena. In this work, we develop a reduced-order model for chaotic electroconvection at high electric Rayleigh number. The chaos in this system is related to the standard Lorenz model obtained from Rayleigh-Benard convection, although our system is driven by a more complex three-way coupling between the fluid, the charge density, and the electric field. Coherent structures are extracted from temporally and spatially resolved charge density fields via proper orthogonal decomposition (POD). A nonlinear model is then developed for the chaotic time evolution of these coherent structures using the sparse identification of nonlinear dynamics (SINDy) algorithm, constrained to preserve the symmetries observed in the original system. The resulting model exhibits the dominant chaotic dynamics of the original high-dimensional system, capturing the essential nonlinear interactions with a simple reduced-order model.

Analytical Solutions of the Schrödinger Equation with Class of Yukawa Potential for a Quarkonium System Via Series Expansion Method

Abstract: A class of Yukawa potential is adopted as the quark-antiquark interaction potential for studying the mass spectra of heavy mesons. The potential was made to be temperature-dependent by replacing the screening parameter with Debye mass. We solved the radial Schrodinger equation analytically using the series expansion method and obtained the energy eigenvalues. The present results are applied for calculating the mass spectra of heavy mesons such as charmonium and bottomonium . Two special cases were considered when some of the potential parameters were set to zero, resulting into Hellmann potential, and Coulomb potential, respectively. The present potential provides satisfying results in comparison with experimental data and the work of other researchers.

Interaction of an atmospheric pressure plasma jet with grounded and floating metallic targets: simulations and experiments

Abstract: The interaction of kHz μs-pulsed atmospheric pressure He jets with metallic targets is studied through simulations and experiments, focusing on the differences between floating and grounded targets. It is shown that the electric potential of the floating target is close to grounded in the instants after the impact of the discharge, but rises to a high voltage, potentially more than half of the applied voltage, at the end of the 1 μs pulse. As a result, a return stroke takes place after the discharge impact with both grounded and floating targets, as a redistribution between the high voltage electrode and the low voltage target. Electric field, electron temperature and electron density in the plasma plume are higher during the pulse with grounded target than with floating target, as gradients of electric potential progressively dissipate in the latter case. Finally, at the fall of the pulse, another electrical redistribution takes place, with higher intensity with the highly-charged floating target than with the grounded target. It is shown that this phenomenon can lead to an increase in electric field, electron temperature and electron density in the plume with floating target.

Analysis of a functionally graded thermopiezoelectric finite rod excited by a moving heat source

Abstract: In this paper, the thermoelastic vibrations of a functionally graded thermo-piezoelectric rod of limited length are considered using the generalized theory of thermoelasticity. Both ends of the rod are fixed at zero voltage and are exposed to a movable axial heat source. Using the Laplace transform analysis, the problem was solved. The expressions for the displacements, temperature, stress, electric field and electric potential were obtained. To consider the influence of the properties of gradient materials, the moving heat source velocity and thermo-piezoelectric response, some comparisons were constructed for different distributions of the physical quantities. The non-homogeneity effect on the field variables is also discussed in some detail.

MXene-doped epoxy resin to suppress surface charge accumulation on insulators in a DC gas-insulated system

Abstract: In an HVDC gas-insulated system, the surface of the insulator accumulates electric charges that distort the surface electric field and reduce the flashover voltage. Therefore, it is crucial to find a material that can effectively suppress the surface charge accumulation. In this work, MXene, a two-dimensional nanomaterial, is doped into an epoxy resin at different concentrations. By doping a small amount of MXene (30ppm), the resistivity of the composite is improved by about 4 times compared to pure epoxy resin. At the same time, the trap level of deep traps increases to 1.07 from 1.04 eV of pure epoxy. Measuring the surface potential of the insulator and using an inversion algorithm to calculate the surface charge establishes that a doping level of 30 ppm of MXene effectively suppresses the accumulation of the surface charge of the insulator, which is only about 1/3 of that of the pure epoxy resin. The surface flashover voltage is also increased by 10%. However, when the doping amount of MXene is increased to 100 or 150 ppm, the insulation performance is lowered. We used a potential barrier model to explain the effect of MXene doping in the epoxy resin. This work presents a possible way to suppress the charge accumulation on the surface of insulators in HVDC gas-insulated systems and provides a link between the microstructure and the macroscopic properties in the material.

Quasi-stationary modeling of the DBD plasma flow control around airfoil

Abstract: This paper presents the results of numerical simulation of a dielectric barrier discharge (DBD) plasma actuator and shows its effectiveness to control air flow around the NACA(National Advisory Committee for Aeronautics)0015 airfoil. The actuator consists of two tape electrodes separated by a dielectric layer, and it is mounted on the suction side of the airfoil at 18% of the chord length. An alternating voltage with 20 kV magnitude and 10 kHz frequency is applied between both electrodes. The physical model of the DBD includes the drift of two ionic species, positive and negative, and the Poisson equation for the electric potential distribution. The spatio-temporal distribution of the electric field, the space charge density in the ambient air, and the surface charge density on the dielectric layer have been determined. The time average electric body force was entered into the air flow model, which was solved using the Spalart–Allmaras turbulence technique. The simulation of the air flow was performed for the free-stream velocities between 5 m/s and 20 m/s (Reynolds numbers 1.65 × 105–6.61 × 105 based on the chord length). The results of computations show the effect of the electrohydrodynamic actuation on the flow pattern, the lift and drag coefficients, the pressure coefficient, and the flow fluctuation near the airfoil. The ability of the DBD actuation to effectively control the aerodynamic airfoil characteristics has been confirmed, and its limitations for the discussed case have been determined.

Electrokinetic membrane pumping flow model in a microchannel

Abstract: A microfluidic pumping flow model driven by electro-osmosis mechanisms is developed to analyze the flow characteristics of aqueous electrolytes. The pumping model is designed based on a single propagative rhythmic membrane contraction applied on the upper wall of a microchannel. The flow lubrication theory coupled with a nonlinear Poisson–Boltzmann equation is used to model the microchannel unsteady creeping flow and to describe the distribution of the electric potential across the electric double layer. A generic solution is obtained for the Poisson–Boltzmann equation without the Debye–Huckel linearization. The effects of zeta potential, Debye length, and electric field on the potential distribution, pressure distribution, velocity profiles, shear stress, and net flow rate are computed and interpreted in detail. The results have shown that this electrokinetic membrane pumping model can be used to understand microlevel transport phenomena in various physiological systems. The proposed model can also be integrated with other microfluidic devices for moving microvolume of liquids in artificial capillaries used in modern biomedical applications.

Unsteady solute dispersion by electrokinetic flow in a polyelectrolyte layer-grafted rectangular microchannel with wall absorption

Abstract: The dispersion of a neutral solute band by electrokinetic flow in polyelectrolyte layer (PEL)-grafted rectangular/slit microchannels is theoretically studied. The flow is assumed to be both steady and fully developed and a first-order irreversible reaction is considered at the wall to account for probable surface adsorption of solutes. Considering low electric potentials, analytical solutions are obtained for electric potential, fluid velocity and solute concentration. Special solutions are also obtained for the case without wall adsorption. To track the dispersion properties of the solute band, the generalized dispersion model is adopted by considering the exchange, the convection and the dispersion coefficients. The solutions developed are validated by comparing the results with the predictions of finite-element-based numerical simulations. Even though the solutions can take any form of initial solute concentration into account, the results are presented by considering a solute band of rectangular shape. The results reveal that, while the short-term transport coefficients are strongly affected by the initial concentration profile, the long-term values are not dependent upon the initial conditions. In addition, it is shown that the mass transport coefficients are strong functions of the channel aspect ratio; hence, approximating a rectangular geometry by the space between two parallel plates may lead to considerable errors in the estimation of mass transport characteristics. This is particularly important for the dispersion coefficient for which the long-term values for a slit microchannel are quite different from those for a rectangular channel of very high aspect ratio. It is also illustrated that the exchange and convection coefficients increase on increasing the Damkohler number, whereas the opposite is true for the dispersion coefficient. The convection and dispersion coefficients are generally increasing functions of the PEL fixed charge density and the PEL thickness and decreasing functions of the PEL friction coefficient. Last but not least, a thicker electric double layer is found to provide a larger degree of solute dispersion, which is the opposite of that observed in a microchannel with bare walls.

Aharonov-Bohm effect for bound states from the interaction of the magnetic quadrupole moment of a neutral particle with axial fields

Abstract: The behavior of a neutral particle with magnetic quadrupole moment that interacts with axial magnetic and electric fields is analyzed. From the interaction of the magnetic quadrupole moment with the axial magnetic field, a spectrum of energy analogous to a Coulomb potential in two dimensions is obtained. Furthermore, the presence of an axial electric field is also considered. From the interaction of the magnetic quadrupole moment with this electric field, an analog of the Aharonov-Bohm effect is obtained. Finally, the Aharonov-Bohm effect for bound states is analyzed when this neutral particle system is subject to the two-dimensional harmonic oscillator.

How Life Works—A Continuous Seebeck-Peltier Transition in Cell Membrane?

Abstract: This paper develops a non-equilibrium thermodynamic approach to life, with particular regards to the membrane role. The Onsager phenomenological coefficients are introduced in order to point out the thermophysical properties of the cell systems. The fundamental role of the cell membrane electric potential is highlighted, in relation to ions and heat fluxes, pointing out the strictly relation between heat exchange and the membrane electric potential. A Seebeck-like and Peltier-like effects emerge in order to simplify the description of the heat and the ions fluxes. Life is described as a continuos transition between the Peltier-like effect to the Seebeck-like one, and viceversa.

Electric potential barriers in the magnetic nozzle

Abstract: Magnetic nozzles are convergent-divergent applied magnetic fields which are commonly used in electric propulsion, manufacturing, and material processing industries. This paper studies the previously overlooked physics in confining the thermalized ions injected from a near-uniform inlet in the magnetic nozzle. Through fully kinetic planar-3V particle-in-cell (PIC) modeling and simulation, an electric potential barrier is found on the periphery of the nozzle throat, which serves to confine the thermalized ions by the electric force. With the initial thermal energy as driving force and insufficient magnetic confinement, the ions overshoot the most divergent magnetic line, which results in the accumulation of positive space charges around the throat. The accumulated charges would create an ion-confining potential barrier with limited extent. Apart from the finite-electron Larmor radius (FELR) effect, two more factors are put forward to account for the limited extent of the potential barrier: the depletion of ion thermal energy and the short-circuiting effect. The influences of inlet temperature ratio of ions to electrons and magnetic inductive strength B_{0} are quantitively investigated using the PIC code. The results indicate that the potential barrier serves as a medium to transfer the gas dynamic thrust to the magnetic nozzle while providing constrain to the ions, like the solid wall in a de Laval nozzle. In high-B_{0} regime, the finite-ion Larmor radius (FILR) effect becomes dominant rather than the FELR effect in the plasma confinement of magnetic nozzles.

Partial Data Inverse Problems for Nonlinear Magnetic Schr\"odinger Equations

Abstract: We prove that the knowledge of the Dirichlet-to-Neumann map, measured on a part of the boundary of a bounded domain in $\mathbb{R}^n, n\geq2$, can uniquely determine, in a nonlinear magnetic Schrodinger equation, the vector-valued magnetic potential and the scalar electric potential, both being nonlinear in the solution.

Method and Experimental Study of Voltage Measurement Based on Electric Field Integral With Gauss–Legendre Algorithm

Abstract: At present, the method of obtaining the voltage of transmission lines by using the inverse problem of the electric field is generally faced with the difficulties of solving data equations. Aiming at the above problems, the method that solves transmission wire voltage through spatial electric field integration based on D-dot sensor is proposed. In this paper, with the ground as the reference potential and the vertical line of the transmission line to the ground as the integral path, D-dot electric field sensor is applied to measure the instant electric field values of several nodes on the integral path and the transmission wire voltage is directly solved combined with Gauss–Legendre numerical integration algorithm. The Gauss–Legendre algorithm is verified by the electric field distribution data on the integration path obtained by simulation. Furthermore, the improvement of the Gauss–Legendre algorithm is realized through optimizing the integral interval so that the location of integral nodes is more reasonable. Finally, the measurement system based on LabVIEW is designed and the experiment platform of the analog transmission line is constructed. Experiment results show that the measurement method of transmission wire voltage through the improved Gauss–Legendre algorithm based on D-dot sensor owns high accuracy with an error of less than 1%.

Study of the radial Schrödinger equation with external magnetic and AB flux fields by the extended Nikiforov–Uvarov method

Abstract: Applicability of the extended Nikiforov–Uvarov method in order to obtain eigenstate solutions of the radial Schrodinger equation in the presence of external magnetic field and Aharonov–Bohm (AB) flux fields is investigated. This study includes two analytical solutions of two different physical problems: solution of the non-relativistic wave equation for radial scalar power potential and solution for a charged particle with position-dependent mass interacted via Morse plus Coulomb potential under the influence of external magnetic field and AB flux fields. Wave function solutions for each case are achieved in terms of biconfluent Heun polynomials.

Electro-mechanical vibration characteristics of piezoelectric nano shells

Abstract: This paper presents the application of higher-order shear deformation theory and nonlocal elasticity theory to electro-mechanical vibration analysis of a doubly curved piezoelectric nano shell resting on Pasternak's foundation. The piezoelectric doubly curved nanoshell is subjected to initial electro-mechanical loads. Effect of initial electro-mechanical loads is contributed in external works. Size effects are captured by nonlocal elasticity theory and Hamilton's principle is employed to derive the equation of motion in terms of three displacements of the middle surface, two rotational components and one electric potential. The main novelty of this paper is investigating concurrent effect of initial electro-mechanical loads, higher-order shear deformation theory and size dependent theory on the free vibration responses of doubly curved piezoelectric nano shell with various boundary conditions. The electro-mechanical vibration response of the doubly curved piezoelectric nano shell is investigated using an analytical method in terms of various parameters such as two opening angles, small scale parameter, spring and shear parameters of foundation and initial electric potential. It is concluded that increasing the nonlocal parameter leads to decrease of the natural frequencies of shell while increasing the applied electric potential increases the natural frequencies.

Sampling mobility profiles of confined fluids with equilibrium molecular dynamics simulations.

Abstract: We show how to evaluate mobility profiles, characterizing the transport of confined fluids under a perturbation, from equilibrium molecular dynamics simulations. The correlation functions derived with the Green–Kubo formalism are difficult to sample accurately, and we consider two complementary strategies: improving the spatial sampling, thanks to a new estimator of the local fluxes involving the forces acting on the particles in addition to their positions and velocities, and improving the temporal sampling, thanks to the Einstein–Helfand approach instead of the Green–Kubo one. We illustrate this method in the case of a binary mixture confined between parallel walls, under a pressure or chemical potential gradient. All equilibrium methods are compared to standard non-equilibrium molecular dynamics (NEMD) and provide the correct mobility profiles. We recover quantitatively fluid viscosity and diffusio-osmotic mobility in the bulk part of the pore. Interestingly, the matrix of mobility profiles for local fluxes is not symmetric, unlike the Onsager matrix for the total fluxes. Even the most computationally efficient equilibrium method (the Einstein–Helfand approach combined with the force-based estimator) remains less efficient than NEMD to determine a specific mobility profile. However, the equilibrium approach provides all responses to all perturbations simultaneously, whereas NEMD requires the simulation of several types of perturbations to determine the various responses, each with different magnitudes to check the validity of the linear regime. While NEMD seems more competitive for the present example, the balance should be different for more complex systems, in particular for electrolyte solutions for the responses to pressure, salt concentration, and electric potential gradients.

Flexoelectric effect on electric potential in piezoelectric graphene-based composite nanowire: Analytical and numerical modelling

Abstract: In this work, an analytical model was developed to study the distribution of electric potential in a graphene reinforced nanocomposite (GRNC) nanowire. The electromechanical responses such as electric potential and deflection of cylindrical GRNC cantilevered nanowire were investigated. Moreover, the conservative fully coupled finite element (FE) models were developed to validate the analytical predictions. Analytical solution shows that the piezoelectric potential in the GRNC nanowire depends on the transverse force, but it is not a function of the force acting along its axial direction. The electric potential in the tensile and compressive sections of nanowire is antisymmetric along its cross-section, making it as a “parallel plate capacitor” for nanopiezotronics applications such as nanogenerator and piezoelectric field effect transistor due to potential drop across the nanowire which assists as the gate voltage. The predictions of potential distributions across the GRNC nanowire considering piezoelectricity show better agreement with FE predictions. Outcomes of the current work reveal that the flexoelectric effect on the electromechanical behavior of GRNC nanowire is noteworthy and cannot be ignored.

Electric potential and surface oxygen ion density for planar, spherical and cylindrical metal oxide grains

Abstract: In this article, the mathematical model presented by Barami and Ghafarinia (Sensors and Actuators B: Chemical 293 (2019) 31–40) for metal oxide grains is discussed. The nonlinear governing model, which is a Poisson-Boltzmann-type equation, is solved by a simple and efficient method. Analytical expressions for the electrical potential and oxygen ion density within the planar, cylindrical and spherical metal oxide grains are expressed in terms of the absolute value of the electric potential and grain thickness. In addition, an investigation of the effects of parameters such as grain diameter, grain thickness, and uniform distribution of single donors on the performance. The validity of the derived analytical expression is established by direct comparison with computer-generated numerical simulations in addition to previously available limiting cases’ results (low and high oxygen adsorption on the grain).

In situ electrochemistry inside a TEM with controlled mass transport.

Abstract: The field of electrochemistry promises solutions for the future energy crisis and environmental deterioration by developing optimized batteries, fuel-cells and catalysts. Combined with in situ transmission electron microscopy (TEM), it can reveal functional and structural changes. A drawback of this relatively young field is lack of reproducibility in controlling the liquid environment while retaining the imaging and analytical capabilities. Here, a platform for in situ electrochemical studies inside a TEM with a pressure-driven flow is presented, with the capability to control the flow direction and to ensure the liquid will always pass through the region of interest. As a result, the system offers the opportunity to define the mass transport and control the electric potential, giving access to the full kinetics of the redox reaction. In order to show the benefits of the system, copper dendrites are electrodeposited and show reliable electric potential control. Next, their morphology is changed by tuning the mass transport conditions. Finally, at a liquid thickness of approximately 100 nm, the diffraction pattern revealed the 〈1,1,1〉 planes of the copper crystals, indicating an atomic resolution down to 2.15 A. Such control of the liquid thickness enabled elemental mapping, allowing us to distinguish the spatial distribution of different elements in liquid.

Vanishing influence of the band gap on the charge exchange of slow highly charged ions in freestanding single-layer MoS2

Abstract: Charge exchange and kinetic energy loss of slow highly charged xenon ions transmitted through freestanding monolayer ${\mathrm{MoS}}_{2}$ are studied. Two distinct exit charge state distributions, characterized by high and low charge states, are observed. They are accompanied by smaller and larger kinetic energy losses, as well as scattering angles, respectively. High charge exchange is attributed to two-center neutralization processes, which take place in close impact collisions with the target atoms. Experimental findings are compared to graphene as a target material and simulations based on a time-dependent scattering potential model. Independent of the target material, experimentally observed charge exchange can be modeled by the same electron capture and de-excitation rates for ${\mathrm{MoS}}_{2}$ and graphene. A common dependence of the kinetic energy loss on the charge exchange for ${\mathrm{MoS}}_{2}$ as well as graphene is also observed. Considering the similarities of the zero band-gap material graphene and the 1.9 eV band-gap material ${\mathrm{MoS}}_{2}$, we suggest that electron transport on the femtosecond timescale is dominated by the strong influence of the ion's Coulomb potential in contrast to the dispersion defined by the material's band structure.