Simple Electronic Model Research Articles

Spacetime geometry of spin, polarization, and wavefunction collapse

To incorporate quantum nonlocality into general relativity, we propose that the preparation and measurement of a quantum system are simultaneous events. To make progress in realizing this proposal, we introduce a spacetime geometry that is endowed with particles which have no distinct points in their worldlines; we call these particles ‘pointons’. This new geometry recently arose in nonnoetherian algebraic geometry. We show that on such a spacetime, metrics are degenerate and tangent spaces have variable dimension. This variability then implies that pointons are spin-12 fermions that satisfy the Born rule, where a projective measurement of spin corresponds to an actual projection of tangent spaces of different dimensions. Furthermore, the 4-velocities of pointons are necessarily replaced by their Hodge duals, and this transfer from vector to pseudo-tensor introduces a free choice of orientation that we identify with electric charge. Finally, a simple composite model of electrons and photons results from the metric degeneracy, and from this we obtain a new ontological model of photon polarization.

Effects of the transversely nonuniform plasma density in a blowout regime of a plasma wakefield accelerator.

We present an analytical study on the effects of the transverse plasma gradient in the blowout regime of a plasma wakefield accelerator. The analysis departs from a simple ballistic model of plasma electrons and allows us to derive a complete analytic solution for the pseudopotential and, consequently, for the wakefield. We demonstrate that the transverse plasma gradient modifies the bubble shape and affects the wakefield; namely, the dipole plasma gradient results in a dipole component of the wakefield. Analysis suggests that, despite the asymmetry, the instability due to the fixed transverse plasma gradient is unlikely, as the total wakefield has a single stable point inside the bubble. The only effect that occurs is the shift of the electromagnetic center. We point out that random fluctuation of the transverse plasma gradient could become an issue.

Hückeloid model for planar boranes

Diborane has long been realized as analogous to ethylene in terms of frontier molecular orbitals (MOs), their symmetries and splitting patterns. But might other conjugated hydrocarbons manifest a similar boron-substituted and H2-supplemented borane? That is, for a planar conjugated hydrocarbon with a neighbor-paired resonance pattern, we formulate corresponding boranes with each carbon atom replaced by a boron atom, and a pair of H-atoms added to each double bond of the resonance structure—one H above the molecular plane and one below. These polyboranes differ from previously known stable higher boranes. Following up on ab initio computations on selected such boranated species, we propose a simple Huckel-like electron model to describe the π-networks of these novel polyboranes. Remarkably for the even annulenic case we find a set of low-lying MO levels resembling those of the corresponding conjugated hydrocarbons. Some correlations are found beyond the annulenic case.

Phase Diagram and Mechanism of Superconductivity in Strongly Correlated Electrons

We investigate the phase diagram of two-dimensional (2D) Hubbard model by employing the optimization variational Monte Carlo method. The 2D Hubbard model is the most simple electronic model for cuprate high-temperature superconductors. The phase diagram consists of three regions; they are antiferromagnetic insulator region, superconducting region, and the coexistent region of superconductivity and antiferromagnetism. The phase diagram obtained by numerical calculations well agrees with the experimental phase diagram for high-temperature cuprates.

Simple electronic model of enzymatic reactions

Electron transfers between active sites of the enzyme and the substrate (product) have been taken into account in kinetic equations for multistep enzymatic reactions with some reversible steps by introducing weak and strong chemisorption states for the intermediates based on Wolkenstein s electronic theory of catalysis originally developed for semiconductors. As a result, the expression for the reaction rate as a function of energy difference between the highest occupied molecular orbital (HOMO) of the active site of the enzyme and the bonding state of the substrate (product) adsorbed onto the surface of the enzyme has been obtained. The case of interaction between the LUMO of the enzyme and the HOMO of the substrate, as well as different factors impacting on the rate of enzymatic reactions have been also discussed.

Transport in Bilayer Graphene near Charge Neutrality: Which Scattering Mechanisms Are Important?

Using the semiclassical quantum Boltzmann equation (QBE), we numerically calculate the dc transport properties of bilayer graphene near charge neutrality. We find, in contrast to prior discussions, that phonon scattering is crucial even at temperatures below 40K. Nonetheless, electron-electron scattering still dominates over phonon collisions allowing a hydrodynamic approach. We introduce a simple two-fluid hydrodynamic model of electrons and holes interacting via Coulomb drag and compare our results to the full QBE calculation. We show that the two-fluid model produces quantitatively accurate results for conductivity, thermopower, and thermal conductivity.

Can Electrostatic Discharge Sensitive electronic devices be damaged by electrostatic fields?

In recent years the susceptibility of ElectroStatic Discharge Sensitive devices (ESDS) to electrostatic fields has been questioned. This paper proposes that very high impedance voltage sensitive ESDS such as MOSFETs or MOS capacitors can be damaged due external field changes without making contact with other conductors in the presence of the field. A simple electronic model is proposed. In a practical evaluation of this risk, discharges are demonstrated to occur due changing external fields, as a result of breakdown of a voltage sensitive structure in a high impedance circuit with one terminal continuously grounded.

On the lack of intrinsic bistability of photon emission in a double quantum dot micromaser

We study a micromaser model based on a solid-state setup with a double quantum dot coupled to a microwave cavity. This model relevant for description of existing experimental setups was used previously and predicted bistable photon emission. Here we revisit the model with the focus on the bistability of the emission process and using a recently developed theory of quantum metastability we come to the conclusion that in fact it does not feature any bistable behavior. We analyze in detail the method of full counting statistics used for the prediction of the bistability applying it to a simple electronic model and identify a likely cause behind its failure, namely, the large ambiguity of the fitting procedure.

5f states with spin-orbit and crystal field splittings

Within a localized 5f framework, a simple single electron model is developed to include a treatment of both spin-orbit splitting and crystal field splitting in the actinide 5f states. A further picture is proposed, including total angular momentum coupling.

Influence of proton bunch parameters on a proton-driven plasma wakefield acceleration experiment

We use particle-in-cell (PIC) simulations to study the effects of variations of the incoming 400 GeV proton bunch parameters on the amplitude and phase of the wakefields resulting from a seeded self-modulation (SSM) process. We find that these effects are largest during the growth of the SSM, i.e. over the first five to six meters of plasma with an electron density of $7 \times10^{14}$ cm$^{-3}$. However, for variations of any single parameter by $\pm$5%, effects after the SSM saturation point are small. In particular, the phase variations correspond to much less than a quarter wakefield period, making deterministic injection of electrons (or positrons) into the accelerating and focusing phase of the wakefields in principle possible. We use the wakefields from the simulations and a simple test electron model to estimate the same effects on the maximum final energies of electrons injected along the plasma, which are found to be below the initial variations of $\pm$5%. This analysis includes the dephasing of the electrons with respect to the wakefields that is expected during the growth of the SSM. Based on a PIC simulation, we also determine the injection position along the bunch and along the plasma leading to the largest energy gain. For the parameters taken here (ratio of peak beam density to plasma density $n_{b0}/n_0 \approx 0.003$), we find that the optimum position along the proton bunch is at $\xi \approx -1.5 \; \sigma_{zb}$, and that the optimal range for injection along the plasma (for a highest final energy of $\sim$1.6 GeV after 10 m) is 5-6 m.

Directly photoexcited Dirac and Weyl fermions in ZrSiS and NbAs

We report ultrafast optical measurements of the Dirac line-node semimetal ZrSiS and the Weyl semimetal NbAs, using mid-infrared pump photons from 86 meV to 500 meV to directly excite Dirac and Weyl fermions within the linearly dispersing bands. In NbAs, the photoexcited Weyl fermions initially form a non-thermal distribution, signified by a brief spike in the differential reflectivity whose sign is controlled by the relative energy of the pump and probe photons. In ZrSiS, electron-electron scattering rapidly thermalizes the electrons, and the spike is not observed. Subsequently, hot carriers in both materials cool within a few picoseconds. This cooling, as seen in the two materials' differential reflectivity, differs in sign, shape, and timescale. Nonetheless, we find that it may be described in a simple model of thermal electrons, without free parameters. The electronic cooling in ZrSiS is particularly fast, which may make the material useful for optoelectronic applications.

Magnetotransport in a Model of a Disordered Strange Metal

Despite much theoretical effort, there is no complete theory of the 'strange' metal state of the high temperature superconductors, and its linear-in-temperature, $T$, resistivity. Recent experiments showing an unexpected linear-in-field, $B$, magnetoresistivity have deepened the puzzle. We propose a simple model of itinerant electrons, interacting via random couplings with electrons localized on a lattice of quantum 'dots' or 'islands'. This model is solvable in a large-$N$ limit, and can reproduce observed behavior. The key feature of our model is that the electrons in each quantum dot are described by a Sachdev-Ye-Kitaev model describing electrons without quasiparticle excitations. For a particular choice of the interaction between the itinerant and localized electrons, this model realizes a controlled description of a diffusive marginal-Fermi liquid (MFL) without momentum conservation, which has a linear-in-$T$ resistivity and a $T \ln T$ specific heat as $T\rightarrow 0$. By tuning the strength of this interaction relative to the bandwidth of the itinerant electrons, we can additionally obtain a finite-$T$ crossover to a fully incoherent regime that also has a linear-in-$T$ resistivity. We show that the MFL regime has conductivities which scale as a function of $B/T$; however, its magnetoresistance saturates at large $B$. We then consider a macroscopically disordered sample with domains of MFLs with varying densities of electrons. Using an effective-medium approximation, we obtain a macroscopic electrical resistance that scales linearly in the magnetic field $B$ applied perpendicular to the plane of the sample, at large $B$. The resistance also scales linearly in $T$ at small $B$, and as $T f(B/T)$ at intermediate $B$. We consider implications for recent experiments reporting linear transverse magnetoresistance in the strange metal phases of the pnictides and cuprates.

Spin resonance and spin fluctuations in a quantum wire

This is a review of theoretical works on spin resonance in a quantum wire associated with the spin-orbit interaction. We demonstrate that the spin-orbit induced internal “magnetic field” leads to a narrow spin-flip resonance at low temperatures in the absence of an applied magnetic field. An applied dc magnetic field perpendicular to and small compared with the spin-orbit field enhances the resonance absorption by several orders of magnitude. The component of applied field parallel to the spin-orbit field separates the resonance frequencies of right and left movers and enables a linearly polarized ac electric field to produce a dynamic magnetization as well as electric and spin currents. We start with a simple model of noninteracting electrons and then consider the interaction that is not weak in 1d electron system. We show that electron spin resonance in the spin-orbit field persists in the Luttinger liquid. The interaction produces an additional singularity (cusp) in the spin-flip channel associated with the plasma oscillation. As it was shown earlier by Starykh and his coworkers, the interacting 1d electron system in the external field with sufficiently large parallel component becomes unstable with respect to the appearance of a spin-density wave. This instability suppresses the spin resonance. The observation of the electron spin resonance in a thin wires requires low temperature and high intensity of electromagnetic field in the terahertz diapason. The experiment satisfying these two requirements is possible but rather difficult. An alternative approach that does not require strong ac field is to study two-time correlations of the total spin of the wire with an optical method developed by Crooker and coworkers. We developed theory of such correlations. We prove that the correlation of the total spin component parallel to the internal magnetic field is dominant in systems with the developed spin-density waves but it vanishes in Luttinger liquid. Thus, the measurement of spin correlations is a diagnostic tool to distinguish between the two states of electronic liquid in the quantum wire.

Contact resistance and phase slips in mesoscopic superfluid atom transport.

We have experimentally measured transport of superfluid, bosonic atoms in a mesoscopic system: a small channel connecting two large reservoirs. Starting far from equilibrium (superfluid in a single reservoir), we observe first resistive flow transitioning at a critical current into superflow, characterized by oscillations. We reproduce this full evolution with a simple electronic circuit model. We compare our fitted conductance to two different microscopic phenomenological models. We also show that the oscillations are consistent with LC oscillations as estimated by the kinetic inductance and effective capacitance in our system. Our experiment provides an attractive platform to begin to probe the mesoscopic transport properties of a dilute, superfluid, Bose gas.

Comparative study of gyrokinetic, hybrid-kinetic and fully kinetic wave physics for space plasmas

A set of numerical solvers for the linear dispersion relations of the gyrokinetic (GK), the hybrid-kinetic (HK), and the fully kinetic (FK) model is employed to study the physics of the KAW and the fast magnetosonic mode in these models. In particular, we focus on parameters that are relevant for solar wind oriented applications (using a homogeneous, isotropic background), which are characterized by wave propagation angles averaging close to 90°. It is found that the GK model, while lacking high-frequency solutions and cyclotron effects, faithfully reproduces the FK wave physics close to, and sometimes significantly beyond, the boundaries of its range of validity. The HK model, on the other hand, is much more complete in terms of high-frequency waves, but owing to its simple electron model it is found to severely underpredict wave damping rates even on ion spatial scales across a large range of parameters, despite containing full kinetic ion physics.

H2 formation from Polycyclic Aromatic Hydrocarbon molecules

SynopsisIn this work we study statistical fragmentation of Polycyclic Aromatic Hydrocarbon (PAH) molecules following collisions with keV ions. Dissociation and transition state energies for H- and H2-emissions from PAHs have been calculated and a simple electronic stopping model has been used to calculate collision-induced internal PAH temperatures. We find that H2 may be formed efficiently from pristine PAHs for internal ion temperatures above 2200 K.

Quantifying charge carrier concentration in ZnO thin films by Scanning Kelvin Probe Microscopy

In the last years there has been a renewed interest for zinc oxide semiconductor, mainly triggered by its prospects in optoelectronic applications. In particular, zinc oxide thin films are being widely used for photovoltaic applications, in which the determination of the electrical conductivity is of great importance. Being an intrinsically doped material, the quantification of its doping concentration has always been challenging. Here we show how to probe the charge carrier density of zinc oxide thin films by Scanning Kelvin Probe Microscopy, a technique that allows measuring the contact potential difference between the tip and the sample surface with high spatial resolution. A simple electronic energy model is used for correlating the contact potential difference with the doping concentration in the material. Limitations of this technique are discussed in details and some experimental solutions are proposed. Two-dimensional doping concentration images acquired on radio frequency-sputtered intrinsic zinc oxide thin films with different thickness and deposited under different conditions are reported. We show that results inferred with this technique are in accordance with carrier concentration expected for zinc oxide thin films deposited under different conditions and obtained from resistivity and mobility measurements.

Multistate Treatments of the Electronic Coupling in Donor–Bridge–Acceptor Systems: Insights and Caveats from a Simple Model

We use a simple one-dimensional delta function electronic structure model (dfm) to investigate the results of a pair of multistate diabatization techniques (i.e., based on n states, with n ≥ 2) for linear DBA and DBBA (donor-bridge-acceptor) electron-transfer systems. In particular, we focus on the physical meaning of the couplings obtained from multistate methods and their relationship to two-state (n = 2) coupling elements. On the basis of the simple dfm approach, which allows exact as well as finite basis set treatment and has no many-electron effects, we conclude that for orthogonal diabatic states, it is difficult to assign clear physical significance to multistate matrix elements for coupling beyond nearest-neighbor contacts. The implications of these results for more complex multistate many-electron treatments are discussed. It is emphasized that physically meaningful coupling elements must involve states that are orthogonal, either explicitly or implicitly.

Variation of the Seebeck coefficient with hydrogen content in carbon microfilaments

The Seebeck coefficient of vapour grown carbon fibres and carbon fibres made from polyacryolytrile precursor has been studied as a possible parameter to control hydrogen storage inside. The sign of the Seebeck coefficient gives the sign of the dominant charge carriers in the fibres, and when hydrogen is absorbed by the carbonaceous material, mainly as H+, it acts as a positive charge carrier. A simple two-band electronic model has been considered to explain the influence of hydrogenation on the Seebeck’s coefficient of these carbon microfibres. The most favourable condition for hydrogen adsorption is a moderately low pressure of hydrogen. Furthermore, it was observed that outgassing is more pronounced than expected in some types of fibres, thereby supporting the proposed presence of hydrogen generated during the manufacturing process.

Crossover from non-Fermi liquid to Fermi liquid behavior in heavy fermion systems

Many heavy fermion systems display a crossover from Fermi liquid to non-Fermi liquid behavior due to a nearby quantum critical point (QCP). Some of the features are universal and do not depend on the nature of the QCP and the tuning mechanism. A simple one-band model of heavy electrons is considered to describe the crossover. The nesting of the spherical Fermi surfaces of an electron and a hole pocket separated by a nesting vector Q and the interaction between electrons gives rise to itinerant antiferromagnetism. The order can be gradually suppressed by mismatching the nesting and a QCP is obtained as TN tends to zero. We review our results on the specific heat, the quasi-particle linewidth, the electrical resistivity, the amplitudes of de Haas-van Alphen oscillations and the dynamical spin susceptibility.