Study Of Electron Transport Research Articles

Many-body nonequilibrium effects in all-electric electron spin resonance

Motivated by recent developments in measurements of electron spin resonances of individual atoms and molecules with a scanning tunneling microscope (ESR-STM), we study electron transport through an impurity under periodic driving as a function of the transport parameters in a model junction. The model consists of a single-orbital quantum impurity connected to two electrodes via time-dependent hopping terms. The hopping terms are treated at the lowest order in perturbation theory to recover a Lindblad-like quantum master equation with electron transport. As in the experiment, the ESR-STM signal is given by the variation of the long-time DC current with the driving frequency. The density-matrix coherences play an important role in the evaluation of the ESR-STM signal. Electron correlation is included in our impurity mode. The charging energy $U$ has significant influence on the spin dynamics depending on the sign and magnitude of the applied DC bias. Our model allows direct insight into the origin of the ESR signal from the many-body dynamics of the impurity.

Transport and noise of hot electrons in GaAs using a semianalytical model of two-phonon polar optical phonon scattering

Recent ab-initio studies of electron transport in GaAs have reported that electron-phonon (e-ph) interactions beyond the lowest order play a fundamental role in charge transport and noise phenomena. Inclusion of the next-leading-order process in which an electron scatters with two phonons was found to yield good agreement for the high-field drift velocity, but the characteristic non-monotonic trend of the power spectral density of current fluctuations (PSD) with electric field was not predicted. The high computational cost of the ab-initio approach necessitated various approximations to the two-phonon scattering term, which were suggested as possible origins of the discrepancy. Here, we report a semi-analytical transport model of two-phonon electron scattering via the Fr\"ohlich mechanism, allowing a number of the approximations in the ab-initio treatment to be lifted while retaining the accuracy to within a few percent. We compare the calculated and experimental transport and noise properties as well as scattering rates measured by photoluminescence experiments. We find quantitative agreement within 15% for the drift velocity and 25% for the $\Gamma$ valley scattering rates, and agreement with the $\Gamma-L$ intervalley scattering rates within a factor of two. Considering these results and prior studies of current noise in GaAs, we conclude that the most probable origin of the non-monotonic PSD trend versus electric field is the formation of space charge domains rather than intervalley scattering as has been assumed.

An extended Vlasov–Fokker–Planck approach for kinetic simulations of laser plasmas

Vlasov–Fokker–Planck simulation codes occupy an important niche in modeling laser-produced plasmas, since they are well suited to studying the effect of collisions on electron kinetic phenomena, especially energy transport. One of the most important elements of energy transport is the absorption of laser light by the plasma; however, simulating this in detail requires resolving oscillations of the laser light, whose characteristic timescale is orders of magnitude shorter than the simulation time needed to study transport physics. For this reason, most Vlasov–Fokker–Planck codes used to study electron transport in laser plasmas rely on simplified models of the laser–plasma coupling. Their underlying assumptions nominally preclude their use for modeling laser light having short-scale structure in space or time, such as broadband lasers. In this work, we derive a more general computational framework suitable for arbitrarily structured laser fields. Our approach is based on an extended set of Vlasov–Fokker–Planck equations that separately solve for the low- and high-frequency plasma response. We implement these extended Vlasov–Fokker–Planck equations in the spherical harmonic code K2 and demonstrate the performance of the method on several laser absorption test problems, with particular attention to the judicious selection of time steps, time integrators, and spherical harmonic truncation, according to the intensity and spectrum of the laser light under consideration. Comparison with the widely used Langdon absorption operator shows the Langdon operator performs remarkably well for predicting laser heating in the simple cases considered here, even in situations that would seem to violate its underlying assumptions.

Structural, photoluminescent and electrochemical properties of self-assembled Co3[Co(CN)6]2/ZnO nanocomposite

Advanced functional nanocomposites designed for multiple applications can be developed using supramolecular chemistry. In this work, we report the synthesis and characterization of nanosized composite material (Co3[Co(CN)6]2/ZnO) formed by cobalt Prussian blue analogue (Co3[Co(CN)6]2) and zinc oxide nanoparticles (ZnO NPs). Structural, morphological and optical characterization, supramolecular arrangement, and electrochemical properties of the materials were investigated. Based on TEM results, Co3[Co(CN)6]2/ZnO predominantly presented a size-polydisperse cube-shaped morphology. A decrease of the nanocomposite mean particle diameter relative to Co3[Co(CN)6]2 was observed. Similar behavior was confirmed by dynamic light scattering with decrease of hydrodynamic diameter value after nanocomposite formation. The Co3[Co(CN)6]2 and Co3[Co(CN)6]2/ZnO exhibited potential zeta values at −69.5 and −59.6 mV respectively suggesting superficial negative character and excellent colloidal stability, even after ZnO NPs incorporation. The electronic spectral of UV–Vis region exhibited semiconductor properties of the nanocomposite despite the presence of the cobalt complex. Interesting that Co3[Co(CN)6]2/ZnO showed a voltammetric profile with two redox pair with E1/2 values of 0.46 and 0.54 V attributed to the Co+/Co2+ and Co2+/Co3+, respectively, and an irreversible reduction process at −0.74 V attributed to Zn2+/Zn0 species, which differ from the electrochemical processes observed for individual componentes, Co3[Co(CN)6]2 (E1/2 = 0.40 V) and ZnO species (Epc = −0.74 V). The electron transport study of the nanocomposite indicated that the both electrochemical redox processes were controlled by diffusion and adsorption.

Electronic Conductivity of Lithium Solid Electrolytes

AbstractWhile significant efforts are being devoted to improving the ionic conductivity of lithium solid electrolytes (SEs), electronic transport, which has an important role in the calendar life, energy density, and cycling stability of solid‐state batteries (SSBs), is rarely studied. Here, the electronic conductivities of three representative SEs, including Li3PS4, Li7La3Zr2O12, and Li3YCl6, are reported. It is reported that the electronic conductivities of SEs are overestimated from the conventional measurements. By revisiting direct current polarizations using two‐blocking‐electrode cells and the Hebb‐Wagner approach, their sources of inaccuracy are provided and the anodic decomposition of SE is highlighted as the key source for the overestimated result. Modifications in the electrode selection and data interpretation are also proposed to approach the intrinsic electronic conductivity of SEs. A two‐step polarization method is also proposed to estimate the electronic conductivity of sulfides that decompose during measurement. Measured by the modified approach, the electronic conductivities of all SEs are one or two orders of magnitude lower than the reported value. Despite that, the electronic conductivity of sulfides seems to be still quite high to enable SSBs with a long calendar life of >10 years, highlighting the critical need for a more careful study of electronic transport in lithium SEs.

Simulation studies of secondary electron yield with electron transport from Cu (110) surfaces containing C2, N2, CO2, or NO2 adsorbates

Secondary electron yields of (110) copper surfaces, covered with either carbon, nitrogen, or their dioxides, have been studied by employing combined first principles methods for the material properties and Monte Carlo simulations for electron transport. Furthermore, by studying electron transport inside the Cu system and modeling the power loss taking account of the inelastic electron scattering within the material, changes in the thermal energy of the system have been modeled. The physical reasons behind the increase and decrease of the yield for each system from an electronic perspective are discussed. In agreement with results observed in studies of secondary electron emission, it is shown that the formation of C2 and N2 monolayers reduce the secondary electron yields, while CO2 and NO2 increase the yield significantly. It is demonstrated that in the case of C2 and N2 formation, changes in the surface electronic barrier reduce the probability of electron escape from the Cu surface, resulting in lower secondary electron emission. Formation of CO2 and NO2, on the other hand, reduce the electronic barrier effects. In addition, due to weak bonding of the CO2 layer with the Cu host, the surface provides an additional source of secondary electrons resulting in higher electronic emission yield. Moreover, the NO2 adsorbate creates a surface electric field that changes the surface electron energy and increases the electron escape probability. Additionally, it is verified that thermal change in the system is negligible and so during secondary electron emission measurements, negligible (if any) surface adsorption or desorption could occur.

Anomalous magnetothermoelectric behavior in massive Dirac materials

Extensive studies of electron transport in Dirac materials have shown positive magneto-resistance (MR) and positive magneto-thermopower (MTP) in a magnetic field perpendicular to the excitation current or thermal gradient. In contrast, measurements of electron transport often show a negative longitudinal MR and negative MTP for a magnetic field oriented along the excitation current or thermal gradient; this is attributed to the chiral anomaly in Dirac materials. Here, we report a very different magneto-thermoelectric transport behavior in the massive Dirac material ZrTe5. Although thin flakes show a commonly observed positive MR in a perpendicular magnetic field, distinct from other Dirac materials, we observe a sharp negative MTP. In a parallel magnetic field, we still observe a negative longitudinal MR, however, a remarkable positive MTP is observed for the fields parallel to the thermal gradients. Our theoretical calculations suggest that this anomalous magneto-thermoelectric behavior can be attributed to the screened Coulomb scattering. This work demonstrates the significance of impurity scattering in the electron transport of topological materials and provides deep insight into the novel magneto-transport phenomena in Dirac materials.

Spin Reorientation Transition and Negative Magnetoresistance in Ferromagnetic NdCrSb3 Single Crystals.

High-quality NdCrSb3 single crystals are grown using a Sn-flux method, for electronic transport and magnetic structure study. Ferromagnetic ordering of the Nd3+ and Cr3+ magnetic sublattices are observed at different temperatures and along different crystallographic axes. Due to the Dzyaloshinskii-Moriya interaction between the two magnetic sublattices, the Cr moments rotate from the b axis to the a axis upon cooling, resulting in a spin reorientation (SR) transition. The SR transition is reflected by the temperature-dependent magnetization curves, e.g., the Cr moments rotate from the b axis to the a axis with cooling from 20 to 9 K, leading to a decrease in the b-axis magnetization f and an increase in the a-axis magnetization. Our elastic neutron scattering along the a axis shows decreasing intensity of magnetic (300) peak upon cooling from 20 K, supporting the SR transition. Although the magnetization of two magnetic sublattices favours different crystallographic axes and shows significant anisotropy in magnetic and transport behaviours, their moments are all aligned to the field direction at sufficiently large fields (30 T). Moreover, the magnetic structure within the SR transition region is relatively fragile, which results in negative magnetoresistance by applying magnetic fields along either a or b axis. The metallic NdCrSb3 single crystal with two ferromagnetic sublattices is an ideal system to study the magnetic interactions, as well as their influences on the electronic transport properties.

Theoretical study of phonon and electron transport in low band gap Janus MXene monolayer MoWCO2 for thermoelectric application

A theoretical study is performed on narrow bandgap Janus monolayer MoWCO2 using the density functional theory and the Boltzmann transport equation. The scattering rate is calculated for electron–phonon, phonon–phonon, phonon-boundary, and electron-boundary scattering. It has a power factor (6.5 × 103 μW/mK2) for p-type and (1.5 × 103 μW/mK2) for n-type at T = 700 K. A strong effect of surface scattering is observed in phonon transport, and lattice thermal conductivity is reduced to 65 W/m K from 308 W/m K at T = 300K for 1 μm width (L) of ribbon. In contrast, there is no change observed in electrical conductivity. This reduction in thermal conductivity improves the thermoelectric figure of merit to 0.33 (p-type) and 0.08 (n-type) at T = 700 K for L = 10 nm from 0.04 (p-type) and 0.01 (n-type). The obtained Young's modulus and Poisson's ratio are 244 N/m and 0.55, respectively, indicating that the material can be deformed under small strain. The obtained in-plane piezoelectric coefficients are e11 = 268 pC/m and d11 = 1.6 pm/V. This indicates the material will be suitable for wearable thermoelectric devices and sensor applications.

Scanning drift tube measurements and kinetic studies of electron transport in CO

We present scanning drift tube measurements of electron swarm transport coefficients in CO as a function of the reduced electric field at room temperature under time-of-flight conditions. The measurements are compared to modeling results and other available experimental data on swarm transport over the broad range of from 2 Td to 1603 Td. The modeling results are obtained in Monte Carlo simulations and by solving the electron Boltzmann equation using a multi-term approach and the density gradient expansion procedure. We find generally good agreement between the measured and calculated transport coefficients. We propose a strategy to improve the cross-section set used to explain certain discrepancies at lower values. Measurements and calculations of electron transport coefficients under hydrodynamic conditions are complemented by Monte Carlo simulations of electron transport in an idealized steady-state Townsend (SST) setup. The ionization coefficient is calculated as a function of from the spatial density profiles of the electrons and compared to the corresponding values evaluated from the knowledge of the effective ionization frequency, drift velocity and longitudinal diffusion coefficient. Contrary to the traditional views, according to which the spatial relaxation of the mean energy and other transport properties for electrons in molecular gases is most commonly monotonic or quasi-monotonic, we find a ‘window’ of where the SST transport properties of the electrons exhibit oscillatory behavior as they relax towards the equilibrium state far downstream from the electron emitting boundary.

Two-phonon scattering in nonpolar semiconductors: A first-principles study of warm electron transport in Si

The ab-initio theory of charge transport in semiconductors typically employs the lowest-order perturbation theory in which electrons interact with one phonon (1ph). This theory is accepted to be adequate to explain the low-field mobility of non-polar semiconductors but has not been tested extensively beyond the low-field regime. Here, we report first-principles calculations of the electric field-dependence of the electron mobility of Si as described by the warm electron coefficient, $\beta$. Although the 1ph theory overestimates the low-field mobility by only around 20%, it overestimates $\beta$ by over a factor of two over a range of temperatures and crystallographic axes. We show that the discrepancy in $\beta$ is reconciled by inclusion of on-shell iterated 2-phonon (2ph) scattering processes, indicating that scattering from higher-order electron-phonon interactions is non-negligible even in non-polar semiconductors. Further, a ~20% underestimate of the low-field mobility with 2ph scattering suggests that non-trivial cancellations may occur in the perturbative expansion of the electron-phonon interaction.

Direct Magnetic Evidence, Functionalization, and Low-Temperature Magneto-Electron Transport in Liquid-Phase Exfoliated FePS3.

Magnetism and the existence of magnetic order in a material is determined by its dimensionality. In this regard, the recent emergence of magnetic layered van der Waals (vdW) materials provides a wide playground to explore the exotic magnetism arising in the two-dimensional (2D) limit. The magnetism of 2D flakes, especially antiferromagnetic ones, however, cannot be easily probed by conventional magnetometry techniques, being often replaced by indirect methods like Raman spectroscopy. Here, we make use of an alternative approach to provide direct magnetic evidence of few-layer vdW materials, including antiferromagnets. We take advantage of a surfactant-free, liquid-phase exfoliation (LPE) method to obtain thousands of few-layer FePS3 flakes that can be quenched in a solvent and measured in a conventional SQUID magnetometer. We show a direct magnetic evidence of the antiferromagnetic transition in FePS3 few-layer flakes, concomitant with a clear reduction of the Néel temperature with the flake thickness, in contrast with previous Raman reports. The quality of the LPE FePS3 flakes allows the study of electron transport down to cryogenic temperatures. The significant through-flake conductance is sensitive to the antiferromagnetic order transition. Besides, an additional rich spectra of electron transport excitations, including secondary magnetic transitions and potentially magnon-phonon hybrid states, appear at low temperatures. Finally, we show that the LPE is additionally a good starting point for the mass covalent functionalization of 2D magnetic materials with functional molecules. This technique is extensible to any vdW magnetic family.

Enhanced electron transport and self-similarity in quasiperiodic borophene nanoribbons with line defects.

Recent experiments have revealed multiple borophene phases of distinct lattice structures, suggesting that the unit cells of ν1/6 and ν1/5 boron sheets, namely α and β chains, serve as building blocks to assemble into novel borophene phases. Motivated by these experiments, we present a theoretical study of electron transport along two-terminal quasiperiodic borophene nanoribbons (BNRs), with the arrangement of the α and β chains following the generalized Fibonacci sequence. Our results indicate that the energy spectrum of these quasiperiodic BNRs is multifractal and characterized by numerous transmission peaks. In contrast to the Fibonacci model that all the electronic states should be critical, both delocalized and critical states appear in the quasiperiodic BNRs, where the averaged resistance saturates at the inverse of one conductance quantum for the delocalized states in the large length limit and contrarily exhibits a power-law dependence on the nanoribbon length for the critical states. Besides, the self-similarity is observed from the transmission spectrum, where the conductance curves overlap at different energy regions of two quasiperiodic BNRs of different Fibonacci indices and the resistance curves are analogous to each other at different energy scales of a single quasiperiodic BNR. These results complement previous studies on quasiperiodic systems where the multifractal energy spectrum and the self-similarity are observed by generating quasiperiodic potential energies, suggesting that borophene may provide an intriguing platform for understanding the structure-property relationships and exploring the physical properties of quasiperiodic systems.

Tunnel barrier to spin filter: Electronic-transport characteristics of transition metal atom encapsulated in a small Cadmium Telluride cage.

First-principles theory-based comparative electronic-transport studies were performed for an atomic chain of Au, a bare Cd9Te9 cage-like cluster, and a single transition metal (TM) (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd) atom encapsulated within the Cd9Te9 using Au(111) as the electrodes. The bare cluster was semiconducting and acted as a tunnel barrier up to a particular applied bias and then beyond that the device displayed a linear current-voltage relationship. Several TMs (Ti, V, Cr, Mn, Fe) encapsulated in the cage showed a half-metallic behavior and spin-filtering effect in the I-V characteristics of the device. Detailed qualitative and quantitative analyses of the I-V characteristics for metallic, semiconducting, and half-metallic nanostructures were carried out for quantifying the use of these TMs in spintronic device applications.

Electronic properties of lithium-ion battery cathodes studied in ion-gated transistor configuration

Electronic and ionic transport governs lithium-ion battery (LIB) operation. The in operando study of electronic transport in lithium-ion transition metal oxide (LMOx) cathodes at different states of charge enables the evaluation of the state of health of LIBs and the optimization of their performance. We report on electronic transport in LIB cathode materials at different states of charge controlled in operando in ion-gated transistor (IGT) configuration. We considered LiNi0.5Mn0.3Co0.2O2 (NMC532)- and LiMn1.5Ni0.5O4 (LNMO)-based composite materials formulated like in conventional LIB cathodes and operated in the organic electrolyte LP30 (1M LiPF6 in ethylene carbonate:dimethyl carbonate 1:1 v/v). NMC532- and LNMO-based cathode materials were used as the transistor channel materials and LP30 as the ion gating medium. Beyond its impact on the field of LIBs, our work advances the design of novel devices based on mixed ionic and electronic transport, including neuromorphic computing.

Determination of hot-electron drift velocity in (Be)ZnMgO/ZnO 2DEG channels

Recent experimental study of electron transport in ZnO/ZnMgO and BeZnMgO/ZnO heterostructures containing two-dimensional electron gas (2DEG) channels of two polarities is reported where electrons are accelerated and become hot by a pulsed electric field. The measurements with electrical pulses ranging from 2 ns to 10 ns in duration ensure the control of self-heating effect. Electron transport in the ZnO 2DEG channels located in ZnO layers at the ZnMgO or BeZnMgO barrier or in ZnO 3DEG channels is treated mainly in terms of drift velocity. The highest values of 1.3 × 107 cm/s at 360 kV/cm, 2.0 × 107 cm/s at 270 kV/cm, and 2.5 × 107 cm/s and 320 kV/cm, respectively, are attained and explained by emphasizing the effect of hot phonons.

All-electrical valley filtering in graphene systems (II): Numerical study of electron transport in valley valves

This work performs a numerical study of electron transport through the fundamental logic gate in valleytronics—a valley valve consisting of two or increasing number of valley filters. Various typical effects on the transport are investigated, such as those due to interface scattering, long- and short-range impurity scattering, edge roughness, strain, inter-filter spacing, or increasing number of valley filters. For illustration, we consider the class of specific valves built from graphene quantum wire valley filters in single layer or bilayer graphene, with the filters subject to separate control of in-plane, transverse electric fields. The nearest-neighbor tight-binding model of graphene is used to formulate the corresponding transport problem, and the algorithm of the recursive Green's function method is applied to solve for the corresponding transmission coefficient. In the case of two-filter valves, the result explicitly demonstrates the existence of a pronounced on-off contrast in electron transmission between the two configurations of valves, namely, one with identical and the other with opposite valley polarities in the two constituent filters. The contrast is shown to be enhanced when increasing the number of filters in valves. Signatures of Fano–Fabry–Pérot type resonances in association with interface scattering and inter-filter spacing are illustrated. Electron backscattering due to impurities is found to be sizably suppressed, with the valve performance showing considerable robustness against edge roughness scattering. On the other hand, the presence of a uniaxial strain modifies the electron transmission and results in an interesting quasi-periodic modulation of transmission as we vary the strain strength.

Aharonov–Bohm-like effects and Fano resonances in circular DNA molecular junctions

DNA electronics has reattracted great interest in recent years and showed a number of fascinating phenomena. Here, we present a theoretical study of electron transport through a two-terminal circular DNA junction under a perpendicular magnetic field. Our results indicate that this circular DNA exhibits Aharonov–Bohm-like effects and a semiconductor–insulator–semiconductor transition by tuning this magnetic field, with the transmission spectrum nearly oscillating periodically. Interestingly, Fano resonances appear around integer multiples of the magnetic flux quantum accompanied by several vortices of local currents. Moreover, the circular DNA behaves as a nanoscale switch, and this switching effect is robust and could be improved for longer molecular length and weaker molecule–electrode coupling, with the on/off ratio exceeding 104 for long circular DNA. These results may help for designing novel circular DNA-based molecular devices.

Electron Transport in n-Type InSe van der Waals Crystals with Co Impurities

Intercalation and doping are promising routes to tune properties of van der Waals (vdW) semiconductors and pave the way for their applications in digital electronics beyond Moore’s law, sensors and spintronics. The indium selenide (InSe) vdW crystal shows great promise for use in next-generation semiconductor technologies. For these applications to be realized, the effects of impurities on properties of InSe must be understood. Here, we present a comparative experimental study of electron transport in n-type InSe semiconductor doped and electrochemically intercalated with magnetic cobalt (Co) impurities. It is shown that the presence of Co decreases the free electron density, the Hall mobility along layers and the conductivity anisotropy σ⊥C/σ‖C. Furthermore, this leads to a change of the behavior of σ⊥C(T) dependence from a metallic one in pristine samples to a semiconducting one in samples with Co. We also demonstrate that the interaction of electrons with space-charge regions is an effective scattering mechanism, which should be taken into account in doped and intercalated crystals. The present work is important for the basic physics knowledge of the effect of Co impurities on physical properties of InSe, which is needed to tailor the parameters of this semiconductor for applications in electronics and spintronics.

Photon and magnetic field controlled electron transport of a multiply-resonant photon-cavity double quantum dot system

We study electron transport through double quantum dots (DQD) coupled to a cavity with a single photon mode. The DQD is connected to two electron reservoirs, and the total system is under an external perpendicular magnetic field. The DQD system exhibits a complex multi-level energy spectrum. By varying the photon energy, several anti-crossings between photon dressed electron states of the DQD-cavity system are found at low strength of the magnetic field. The anti-crossings are identified as multiple Rabi resonances arising from the photon exchange between these states. As the results, a dip in the current is seen caused by the multiple Rabi resonances. By increasing the strength of the external magnetic field, a dislocation of the current dip to a lower photon energy is found and the current dip can be diminished. The interplay of the strength of the magnetic field and the geometry of the states the DQD system can weaken the multiple Rabi resonances in which the exchange of photon between the anti-crossings is decreased. We can therefore confirm that the electron transport behavior in the DQD-cavity system can be controlled by manipulating the external magnetic field and the photon cavity parameters.