Coulomb Field Research Articles

Impact of electron velocity modulation on microwave power performance for AlGaN/GaN HFETs

In this study, we demonstrate the effects of electron velocity modulation (Δve/ΔVgs) on the microwave power performance for AlGaN/GaN HFETs. In order to conduct the experiments, AlGaN/GaN HFETs with gate lengths ranging from 500 to 80 nm were fabricated. Electron transport was investigated by coupling a drift-diffusion solver with the Monte Carlo method. As gate lengths (Lg) varied from 500 to 200 nm, the increased polarization Coulomb field scattering led to an increase in Δve/ΔVgs and the stronger electric field (E) increased ve and enhanced the transconductance (gm), which in turn led to a greater power gain (Gp) in the HFETs. The higher power output (Pout) was also due to the increased ve that boosted the saturated output current (Ids,sat). The unique phenomenon that occurs from electron velocity modulation of AlGaN/GaN HFETs at electron densities (ns) < 3.42 × 1012cm−2 can be used as an effective mechanism to enhance the power gain of AlGaN/GaN HFETs.

On the Internal Bremsstrahlung accompanying β-decay and its potential relevance in the application of radioactive sources.

An in-depth analysis of the decay process for β-emitting radionuclides highlights, for some of them, the existence of high-order effects usually not taken into account in literature as considered negligible in terms of energy and yield, and referred to as Internal Bremsstrahlung (IB). This set of β-radionuclides presents, besides their β spectrum, a continuous γ emission due to the Coulomb field braking action on the emitted electron following the decaying nucleus. 
In this work, we review the theoretical and experimental studies on the IB process focusing on its actual importance for the pure β emitters. It emerges that there is no satisfactory model able to reproduce the experimental IB distribution for most of the investigated beta emitters and the several measurements are sometimes at odds with each other. Moreover, as recently demonstrated, the IB process can give a relevant contribution to the physics of beta emitters thus requiring its inclusion in the physics of the beta decay.
A discussion on the importance of considering the IB process in both applicative fields such as nuclear medicine, industrial applications, and research or calibration laboratories, and in other relevant fields of particle physics or astrophysics, such as the research on dark matter or neutrino mass, is presented.

Observation of Attosecond Time Delays in Above-Threshold Ionization.

Attosecond-scale temporal characterization of photoionization is essential in understanding how light and matter interact on the most fundamental level. However, characterizing the temporal property of strong-field above-threshold ionization has remained unreached. Here, we propose a novel photoelectron interferometric method to disentangle the contribution of Coulomb effect from an attoclock, allowing us to clock energy-resolved time delays of strong-field above-threshold ionization. We disentangle two types of Coulomb effects for the attoclock, i.e., one arising from the Coulomb disturbance of a single electron trajectory and the second effect arising from the photoelectron phase space distortion due to the Coulomb field. We find that the second Coulomb effect manifests itself as an energy-resolved attosecond time delay in the electron emission, which is relevant to the effect of nonadiabatic initial longitudinal momentum at the tunnel exit. Our study further indicates a sensitivity of the time delay to the temporal profile of the released electron wave packet within one half laser cycle. The temporal width of the released electron wave packet is found to increase with energy, which contradicts the common assumption in the adiabatic picture.

A polar diatomic molecule under a high-frequency laser field: classical analytical solution

AbstractDynamics of a polar diatomic molecule (represented as the oscillating rotator model) under external fields is the most fundamental problem and the testbed in molecular physics—analogously to the dynamics of hydrogenic atom (represented as the model of an electron in the Coulomb field) under external fields being the most fundamental problem and the testbed in atomic physics. We present the classical analytical study of the dynamical Stark effect in the oscillating rotator under a high-frequency laser field. We obtain the analytical results by using the method of effective potentials. We demonstrate that under the laser field, the rotation frequency increases, while the oscillation frequency decreases, and the amplitude of the oscillations increases. More significantly, the laser field causes the generation of the second harmonic in the oscillations of this system. This is an important, counterintuitive result in studying this fundamental physical system. We also drew attention to some flaws in the literature on this subject.

Superheavy magic nuclei: Ground-state properties, bubble structure and α-decay chains

A systematic investigation of superheavy nuclei in the isotopic chains of proton numbers Z=106, 114, 120, and 126 together with isotonic chains of neutron numbers N=162, 172, and 184 is presented in the theoretical framework of relativistic mean-field density functionals based on density-dependent meson-nucleon couplings. Ground-state properties, including binding energy, shape, deformation, density profile, and radius, are estimated to provide compelling evidence of magicity in these even-even nuclei, aligning with the concept of the ‘island of stability.’ The analysis reveals central depletion in the charge density, indicating a bubble-like structure, primarily attributed to the substantial repulsive Coulomb field and the influence of higher l-states. A thorough examination of potential decay modes, employing various semi-empirical formulas, is presented. The probable α-decay chains are evaluated, demonstrating excellent agreement with available experimental data.

Dunkl algebra and vacuum pair creation: Exact analytical results via Bogoliubov method

This paper examines the generation of scalar particle pairs resulting from the application of an external electric field's Coulomb potential in both three- and two-dimensional scenarios, employing Dunkl's formalism. In doing this, we present two distinct sets of exact solutions to the Dunkl-Klein-Gordon equation. Subsequently, we employ the canonical approach based on the Bogoliubov transformation to link the “in” and “out” states in order to compute both the probability of pair creation and the density of particle creation. It is shown that the conditions necessary for the creation of pairs of spinless particles depend on several factors: nucleus charge Z, quantum numbers, and Dunkl parameter. It is also shown that the Dunkl formalism enhances particle pair production.

Influence of the Bias Voltage on Effective Electron Velocity in AlGaN/GaN High Electron Mobility Transistors.

The small-signal S parameters of the fabricated double-finger gate AlGaN/GaN high electron mobility transistors (HEMTs) were measured at various direct current quiescent operating points (DCQOPs). Under active bias conditions, small-signal equivalent circuit (SSEC) parameters such as Rs and Rd, and intrinsic parameters were extracted. Utilizing fT and the SSEC parameters, the effective electron velocity (νe-eff) and intrinsic electron velocity (νe-int) corresponding to each gate bias (VGS) were obtained. Under active bias conditions, the influence mechanism of VGS on νe-eff was systematically studied, and an expression was established that correlates νe-eff, νe-int, and bias-dependent parasitic resistances. Through the analysis of the main scattering mechanisms in AlGaN/GaN HEMTs, it has been discovered that the impact of VGS on νe-eff should be comprehensively analyzed from the aspects of νe-int and parasitic resistances. On the one hand, changes in VGS influence the intensity of polar optical phonon (POP) scattering and polarization Coulomb field (PCF) scattering, which lead to changes in νe-int dependent on VGS. The trend of νe-int with changes in VGS plays a dominant role in determining the trend of νe-eff with changes in VGS. On the other hand, both POP scattering and PCF scattering affect νe-eff through their impact on parasitic resistance. Since there is a difference in the additional scattering potential corresponding to the additional polarization charges (APC) between the gate-source/drain regions and the region under the gate, the mutual effects of PCF scattering on the under-gate electron system and the gate-source/drain electron system should be considered when adjusting the PCF scattering intensity through device structure optimization to improve linearity. This study contributes to a new understanding of the electron transport mechanisms in AlGaN/GaN HEMTs and provides a novel theoretical basis for improving device performance.

Laser-assisted radiative recombination in a cold hydrogen plasma

Abstract We study the process of laser-assisted radiative recombination of an electron with a proton in a cold hydrogen plasma employing the semiclassical Kramers’ approach which involves calculation of classical trajectories in combined laser and Coulomb fields and the use of the correspondence principle. Due to the Coulomb focusing effect, recombination is the most effective when the initial electron momentum is parallel to the laser polarization. Orders of magnitude enhancement of the cross section, as compared to the laser-free case, is observed in this case. With increasing angle between the electron momentum and polarization, the recombination cross section drops. However, even after averaging over Maxwellian velocity distribution we obtain a substantial enhancement of the recombination rate constant, as compared to the zero-field case. For the field intensities in the range 30–350 MW cm−2, the enhancement occurs in the region of the radiation wavelength from 5 to 20 µm and for the plasma temperature from 20 to 300 K.

Subcycle dynamics of excitons under strong laser fields.

Excitons play a key role in the linear optical response of two-dimensional (2D) materials. However, their role in the nonlinear response to intense, nonresonant, low-frequency light is often overlooked as strong fields are expected to tear the electron-hole pair apart. Using high-harmonic generation as a spectroscopic tool, we theoretically study their formation and role in the nonlinear optical response. We show that the excitonic contribution is prominent and that excitons remain stable even when the driving laser field surpasses the strength of the Coulomb field binding the electron-hole pair. We demonstrate a parallel between the behavior of strongly laser-driven excitons in 2D solids and strongly driven Rydberg states in atoms, including the mechanisms of their formation and stability. Last, we show how the excitonic contribution can be singled out by encapsulating the 2D material in a dielectric, tuning the excitonic energy and its contribution to the high-harmonic spectrum.

Transverse asymmetry in molecular strong-field photoelectron holography induced by initial phase structure and Coulomb field

Transverse asymmetry in molecular strong-field photoelectron holography induced by initial phase structure and Coulomb field

Lorentz contraction of electric field lines for a point charge in uniform motion

We examine a logical foundation of depicting a Lorentz contraction of a Coulomb field (an electric field of a point charge in uniform motion) by means of the ‘Lorentz contracted’ field lines. Two existing arguments for a contraction of field lines sound appealing and lead to very simple calculations yielding the correct results. However, one of them is a victim to subtle logical weaknesses, as it relies on ascribing a degree of physical reality to the electric field lines. The other one correctly proves what it sets out to prove. But it does not provide a proof, or even a suggestion, of an additional result that can be obtained by a new poof that we present here. Though our idea is very simple, the calculations used to prove it—based on a little known, half a century old result by Tsien—are somewhat more involved than those from past arguments.

The Role of Coulomb Fields in the Formation of Emittance in Photoguns

The Role of Coulomb Fields in the Formation of Emittance in Photoguns

Lepton pair production in muon-nucleus scattering

The MUonE experiment aims at providing a novel determination of the leading hadronic contribution to the muon anomalous magnetic moment through the study of elastic muon-electron scattering. Since the initial-state electrons are bound in a low-Z atomic target, the interaction between the incoming muons and the nuclei is expected to be the main source of experimental background. In this article, we study the production of a real lepton pair from the muon-nucleus scattering, discussing its numerical impact in the MUonE kinematic configuration. The process is described as a scattering of a muon in an external Coulomb field with the addition of a form factor to describe the nuclear charge distribution. The calculation is implemented in the fully differential Monte Carlo event generator Mesmer, without introducing any approximation on the angular variables.

Traveling wave phenomena of inhomogeneous half-wave equation

In this paper, we are concerned with traveling wave phenomena of the inhomogeneous half-wave equation, which models the energy of a spin zero particle in the Coulomb field. We study the Gagliardo-Nirenberg and critical Hardy-Sobolev inequalities with velocity 0<|v|<1 and obtain the estimates for the best constants and optimizers of inequalities. Moreover, we establish the non-scattering results with small traveling wave for energy subcritical and critical cases.

Bias-dependent electron velocity and short-channel effect in scaling sub-100 nm InAlN/GaN HFETs

This work reports on the extraction and simulation of the electron velocity–gate voltages relationship for sub-100 nm InAlN/GaN heterojunction field-effect transistors (HFETs). A peak electron velocity (ve) at an electron density (ns) of 0.41 × 1013 cm−2 was observed at 1.06 × 107 cm/s in an InAlN/GaN HFET with 60 nm gate length (Lg) by delay time analysis. The ve at a high ns of 1.5 × 1013 cm−2 was observed at 0.6 × 107 cm/s. This peak ve behavior is explained by polarization Coulomb field (PCF) scattering and optical phonon scattering based on a Monte Carlo method. As Lg scaled from 350 to 60 nm, the current gain cutoff frequency (fT) and transconductance (gm) were improved. However, the thermal performance was degraded with a bad figure of merit P150 °C. Although a weakening of the control capability of Vgs on ns (Δns/ΔVgs) was observed in the shorter Lg device, which leads to a decrease in device gm, the larger electron velocity by the increased lateral electric field (E) and the larger Δve/ΔVgs by the increased PCF scattering still enhance the peak gm. Results indicate that the enhancement of Δve/ΔVgs is a vital method to strengthen the modulation of the gate on current and to suppress the short channel effect in GaN HFET. Our work supports a deeper understanding and analysis of sub-100 nm InAlN/GaN HFET device performance and physical mechanisms.

Decoherence from horizons: General formulation and rotating black holes

Recent work by Danielson, Satishchandran, and Wald (DSW) has shown that black holes—and, in fact, Killing horizons more generally—impart a fundamental rate of decoherence on all nearby quantum superpositions. The effect can be understood from measurement and causality: An observer (Bob) in the black hole should be able to disturb outside quantum superpositions by measuring their superposed gravitational fields, but since his actions cannot (by causality) have this effect, the superpositions must automatically disturb themselves. DSW calculated the rate of decoherence up to an unknown numerical factor for distant observers in Schwarzschild spacetime, Rindler observers in flat spacetime, and static observers in de Sitter spacetime. Working in electromagnetic and Klein-Gordon analogs, we flesh out and generalize their calculation to derive a general formula for the precise decoherence rate for Killing observers near bifurcate Killing horizons. We evaluate the rate in closed form for an observer at an arbitrary location on the symmetry axis of a Kerr black hole. This fixes the numerical factor in the distant-observer Schwarzschild result, while allowing new exploration of near-horizon and/or near-extremal behavior. In the electromagnetic case we find that the decoherence vanishes entirely in the extremal limit, due to the “black hole Meissner effect” screening the Coulomb field from entering the black hole. This supports the causality picture: Since Bob is unable to measure the field of the outside superposition, no decoherence is necessary—and indeed none occurs. Published by the American Physical Society 2024

Slowing Hot Electron Cooling in CdSe Quantum Dots Using Electron‐Rich Exciton‐Delocalizing Ligands

AbstractUnderstanding hot carrier dynamics in semiconductor nanocrystals is an important research focus due to their applications in photonics and photovoltaic devices. In this report, we investigated the effects of surface‐bound exciton‐delocalizing ligands (EDLs) on the lifetimes of hot electrons in CdSe quantum dots (QDs). After treatment of CdSe with two different phenylithiocarbamates (PTCs), a class of EDLs, the depletion times of the band‐edge exciton bleach were roughly equivalent as observed through ultrafast transient absorption spectroscopy. However, following the initial ultrafast depletion, the PTC‐treated samples continued to deplete while the untreated CdSe began recovering. Inspection of other transient features – such as the 3rd exciton and hot biexciton – reveal a general trend in which the PTC‐treated samples relax more slowly at short times (&lt;10 ps) when compared with the untreated CdSe. At longer delay times, in the range of nanoseconds, the CdSe+CF3OPTC loses nearly 80 % of its excited state populations, while the CdSe+MeOPTC loses only 20–40 %. We discuss the role that exciton delocalization plays in determining these observed rates as well as how they compare to previous studies. Kinetic differences between the two ligands are attributed to their electron donating/withdrawing abilities and coupling to the CdSe QD. Coherent vibrational wavepacket analysis supports this line of reasoning, showing increased coupling between the exciton and the longitudinal optical (LO) phonon due to increased Coulombic field strength around the hole and electron‐donating MeOPTC. These results indicate that electron‐rich PTCs are especially good candidates for use in QD devices that would make use of hot carriers.

Influence of post fabrication annealing on device performance of InAlN/GaN high electron mobility transistors

In this study, effect of post-fabrication annealing (PFA) on the electrical performance of the InAlN/GaN high electron mobility transistors (HEMTs) has been investigated. With PFA, the gate Schottky barrier height increases by 94 % (from 1.03 eV to 2.00 eV) and the interface trap state density (Dit) is reduced by 27 %. Compared with the device without PFA, a higher on-current (11 %), a larger on/off current ratio (∼1 order), a higher transconductance (3 %), a reduced gate leakage current (84 %) and a lower subthreshold swing (10 %) are recorded in device with PFA. In addition, the improved electron mobility is observed and the reduction of polarization Coulomb field (PCF) scattering is proven in device with PFA due to the enhancement of polarization electrical field in InAlN/GaN heterostructure.

Polarized Coulomb field scattering in LaAlO3/SrTiO3 heterojunction field-effect transistors

LaAlO3/SrTiO3 heterojunction field-effect transistors (HFETs) were fabricated. Using a combination of measured current–voltage (I–V) output curves and gate-source capacitance–voltage (C–V) characteristic curves for the fabricated LaAlO3/SrTiO3 HFETs, and considering scattering mechanisms for longitudinal optical phonon, interface rough, electron-electron (E-E), acoustic phonon, and polarized Coulomb field (PCF), the channel electron mobility of LaAlO3/SrTiO3 HFETs has been calculated and analyzed. The results showed that PCF scattering is a significant carrier scattering mechanism in LaAlO3/SrTiO3 HFETs. The heterostructure of the LaAlO3/SrTiO3 system has strong polarization characteristics. This paper is the first to demonstrate that PCF scattering is a significant carrier scattering mechanism in LaAlO3/SrTiO3 HFETs, following GaN HFETs, thus demonstrating that LaAlO3/SrTiO3 HFETs also have PCF scattering effect.

Optimizing Pb beam losses at the LHCb for maximum luminosity

In addition to the physics program with proton beams, the Large Hadron Collider (LHC) at CERN also provides collisions of fully-stripped Pb beams for about one month per year. When colliding Pb nuclei, electromagnetic interactions are the dominating processes because of the intense Coulomb field produced by the ions. These ’ultra-peripheral’ interactions give rise to ions with a changed magnetic rigidity. This causes losses in the machine that can impose limits on the luminosity. Among them, the bound-free pair production (BFPP) causes a localised power deposition downstream of each collision point, which could induce superconducting magnet quenches if not well controlled. These losses were studied and successfully mitigated for most LHC experiments, however the recent request by LHCb to increase the Pb-Pb luminosity requires a revision of BFPP collisional loss limitations. In this paper, the simulation of BFPP losses from Pb-Pb collisions around LHCb is presented. The loss patterns are discussed for different beam parameters. Finally, a mitigation strategy by means of an orbit bump is studied.