Electron Time Scales Research Articles

Evolution of High-energy Electron Distribution in Pulsar Wind Nebulae

In this paper, we analyze the spectral energy distributions of 17 powerful (with a spin-down luminosity greater than 1035 erg s−1) young (with an age less than 15,000 yr) pulsar wind nebulae (PWNe) using a simple time-independent one-zone emission model. Our aim is to investigate correlations between model parameters and the ages of the corresponding PWNe, thereby revealing the evolution of high-energy electron distributions within PWNe. Our findings are as follows: (1) The electron distributions in PWNe can be characterized by a double power-law with a super-exponential cutoff. (2) As PWNe evolve, the high-energy end of the electron distribution spectrum becomes harder with the index decreasing from approximately 3.5 to 2.5, while the low-energy end spectrum index remains constant near 1.5. (3) There is no apparent correlation between the break energy or cutoff energy and the age of PWNe. (4) The average magnetic field within PWNe decreases with age, leading to a positive correlation between the energy loss timescale of electrons at the break energy or the high-energy cutoff, and the age of the PWN. (5) The total electron energy within PWNe remains constant near 2 × 1048 erg, while the total magnetic energy decreases with age.

Measurement of interfacial charge transfer rate during visible light CO2 reduction under bias voltage conditions

The study of photogenerated electron transfer pathway and time scale in photoelectrocatalysis is the key factor to improve the efficiency of light energy utilization. At present, the reduction of CO2 by visible light is one of the hotspots of research, but it is difficult to determine the reduction reaction and mechanism. Therefore, we use electrochemical in situ growth technology to prepare Bi2(MoO4)3, and then use photoelectrocatalytic method to reduce CO2. The results show that the rate of charge transfer at the interface is about 10−18 cm4/s in the reduction of CO2 by visible light under bias voltage, and the catalyst can react stably for 10 h. This provides a way to optimize the photogenerated electron transfer model and to design the catalysts for the reduction of carbon dioxide under visible light.

DC electrical conductivity measurements of warm dense matter using ultrafast THz radiation

We describe measurements of the DC electrical conductivity of warm dense matter using ultrafast terahertz (THz) pulses. THz fields are sufficiently slowly varying that they behave like DC fields on the timescale of electron–electron and electron–ion interactions and hence probe DC-like responses. Using a novel single-shot electro-optic sampling technique, the electrical conductivity of the laser-generated warm dense matter was determined with <1 ps temporal resolution. We present the details of the single-shot THz detection methodology as well as considerations for warm dense matter experiments. We, then, provide proof-of-concept studies on aluminum driven to the warm dense matter regime through isochoric heating and shock compression. Our results indicate a decrease in the conductivity when driven to warm dense matter conditions and provide a platform for future warm dense matter studies.

Stability of long-sustained oscillations induced by electron tunneling

Self-oscillations are the result of an efficient mechanism generating periodic motion from a constant power source. In quantum devices, these oscillations may arise due to the interaction between single electron dynamics and mechanical motion. We show that, due to the complexity of this mechanism, these self-oscillations may irrupt, vanish, or exhibit a bistable behavior causing hysteresis cycles. We observe these hysteresis cycles and characterize the stability of different regimes in both single- and double-quantum-dot configurations. In particular cases, we find these oscillations stable for over 20 s, many orders of magnitude above electronic and mechanical characteristic timescales, revealing the robustness of the mechanism at play. Published by the American Physical Society 2024

Fast and accurate nonadiabatic molecular dynamics enabled through variational interpolation of correlated electron wavefunctions.

We build on the concept of eigenvector continuation to develop an efficient multi-state method for the rigorous and smooth interpolation of a small training set of many-body wavefunctions through chemical space at mean-field cost. The inferred states are represented as variationally optimal linear combinations of the training states transferred between the many-body bases of different nuclear geometries. We show that analytic multi-state forces and nonadiabatic couplings from the model enable application to nonadiabatic molecular dynamics, developing an active learning scheme to ensure a compact and systematically improvable training set. This culminates in application to the nonadiabatic molecular dynamics of a photoexcited 28-atom hydrogen chain, with surprising complexity in the resulting nuclear motion. With just 22 DMRG calculations of training states from the low-energy correlated electronic structure at different geometries, we infer the multi-state energies, forces and nonadiabatic coupling vectors at 12 000 geometries with provable convergence to high accuracy along an ensemble of molecular trajectories, which would not be feasible with a brute force approach. This opens up a route to bridge the timescales between accurate single-point correlated electronic structure methods and timescales of relevance for photo-induced molecular dynamics.

Spin-flip dynamics in core-excited states in the basis of irreducible spherical tensor operators.

Recent experimental advances in ultrafast science have put different processes occurring on the electronic timescale below a few femtoseconds in focus. In the present theoretical work, we demonstrate how the transformation and propagation of the density matrix in the basis of irreducible spherical tensors can be conveniently used to study sub-few fs spin-flip dynamics in core-excited transition metal compounds. With the help of the Wigner-Eckart theorem, such a transformation separates the essential dynamical information from the geometric factors governed by the angular momentum algebra. We show that an additional reduction can be performed by the physically motivated truncation of the spherical tensor basis. In particular, depending on the degree of coherence, the ultrafast dynamics can be considered semi-quantitative in the notably reduced spherical basis when only the total populations of the basis states of the given spin are of interest. Such truncation should be especially beneficial when the number of high-spin basis states is vast, as it reduces computational costs.

Attosecond metrology in circular polarization

Attosecond metrology with linearly polarized light pulses is the basis of a highly successful research area. An even broader impact can be expected from a generalized metrology that covers two-dimensional polarization states, enabling notably the study of chiroptical phenomena on the electronic time scale. Here, we introduce and demonstrate a comprehensive approach to the generation and complete characterization of elliptically to circularly polarized attosecond pulses. The generation relies on a plug-in device of unprecedented simplicity. For the characterization, we introduce Stokes-parameter and attosecond-resolved reconstruction of optical waveforms (SPARROW), which encodes the attosecond-metrology information into the photoemission angle in the polarization plane and accesses all four Stokes parameters of the attosecond pulses. Our study demonstrates a physically transparent scheme for attosecond metrology with elliptical to fully circular polarizations, applicable to both table-top and accelerator-based light sources, which will unlock studies of chiral molecules, magnetic materials, and novel chiroptical phenomena on the most fundamental time scales.

Ultrafast activation of the double-exchange interaction in antiferromagnetic manganites

In doped manganite systems, strong electronic correlations result in rich phase diagrams where electron delocalization strongly affects the magnetic order. Here, we employ a femtosecond all-optical pump-probe scheme to impulsively photodope the antiferromagnetic parent manganite system CaMnO3 and unveil the formation dynamics of a long-range ferromagnetic state. We resonantly target intense charge transfer electronic transitions in CaMnO3 to photodope the system and probe the subsequent dynamics of both charges and spins using a unique combination of time-resolved terahertz spectroscopy and time-resolved magneto-optical Faraday measurements. We demonstrate that photodoping promotes a long-lived population of delocalized electrons and induces a net magnetization, effectively promoting ferromagnetism resulting from light-induced carrier-mediated short-range double-exchange interactions. The picosecond set time of the magnetization, much longer than the electron timescale, and the presence of an excitation threshold are consistent with the formation of ferromagnetic patches in an antiferromagnetic background.

Role of electron temperature at extremely low density in negative positive ion plasma

It is pointed out that the role of electrons in the dynamics of pair ion and negative positive ion plasmas cannot be neglected even at extremely low density of electrons, i.e., ne0n+0≪mem+ (where nj0 is the background density of jth species and mj is the mass of the particles of the j-species while j = e, +, −) because electron thermal velocity is almost always larger than thermal velocities of ions, i.e., vT± ≪ vTe. An analysis of electrostatic waves in unmagnetized negative positive ion electron (NPIE) and pair ion electron (PIE) plasmas is presented for the case ωpe ≪ ωp+ (where ωpj=(4πnj0e2mj)1/2 is the plasma oscillation frequency corresponding to j-species). The electron dynamics contribute to electrostatic perturbations at ion plasma oscillation time scale at longer wavelengths for λDe2k2<1 where λDe=(Te4πne0)1/2 is the electron Debye length. On the other hand, the electron plasma wave turns into thermal wave when the conditions ωpe ≪ ωp± and 1≪λDe2k2 hold simultaneously and ion acoustic wave approaches the sum of ion plasma oscillation frequencies of positive and negative ions. The only electrostatic normal mode of such a plasma is the ion plasma wave corresponding to longer wavelengths, which also includes the contribution of electrons. The electron thermal wave is separated from plasma oscillations and electron time scale disappears with respect to electrostatic perturbations. Similar situation also occurs in plasmas having negatively charged dust particles. To elaborate these points, the analytical results are applied to the two experiments with NPIE and PIE plasmas.

Radiation belt dynamics and the decay time scale of electrons at different energy and altitude

Since the discovery of the Radiation belt, scientists have had strong interests in the magnetized space that protects us from the bombardment of solar storms. Early satellite observations show that the radiation belts consist of energetic particles and exhibit a two-belt structure. The outer belt electrons are accelerated during geomagnetic storms, while the inner-belt is mostly formed by the cosmic ray. Recently, the NASA Van Allen probes discovered a temporary storage ring that locates in the middle of the two belts when solar storm event occurs, and the radiation belts may exhibit a three-belt structure. This study aims to analyze the three-belt structure from short-term variation and long-term variation. We calculated the decay time scale in the heart of the radiation belt which describes the lifetime of the high-energy electrons. The study of the decay time is of great significance in understanding the dynamics of the whole radiation belt.

Orbital character of the spin-reorientation transition in TbMn6Sn6

Ferromagnetic (FM) order in a two-dimensional kagome layer is predicted to generate a topological Chern insulator without an applied magnetic field. The Chern gap is largest when spin moments point perpendicular to the kagome layer, enabling the capability to switch topological transport properties, such as the quantum anomalous Hall effect, by controlling the spin orientation. In TbMn6Sn6, the uniaxial magnetic anisotropy of the Tb3+ ion is effective at generating the Chern state within the FM Mn kagome layers while a spin-reorientation (SR) transition to easy-plane order above TSR = 310 K provides a mechanism for switching. Here, we use inelastic neutron scattering to provide key insights into the fundamental nature of the SR transition. The observation of two Tb excitations, which are split by the magnetic anisotropy energy, indicates an effective two-state orbital character for the Tb ion, with a uniaxial ground state and an isotropic excited state. The simultaneous observation of both modes below TSR confirms that orbital fluctuations are slow on magnetic and electronic time scales < ps and act as a spatially-random orbital alloy. A thermally-driven critical concentration of isotropic Tb ions triggers the SR transition.

The wave-particle interactions between chorus waves and electrons in the Earth radiation belts

There exists a large number of high-energy electrons and various plasma waves in the Earth’s radiation belts. The chorus waves, as a kind of whistler-mode waves can cause electrons acceleration and loss by wave-particle interactions, thus playing a vital role in the dynamic evolution of the radiation belts. To improve our understandings of such mechanisms and facilitate people to anticipate the dramatic variation of space environment, we utilized the wave and field data from Van Allen Probes to statistically study the acceleration and loss timescale of electrons by chorus waves. The first part in this paper presents the temporal-spatial distribution of the chorus characteristics and background field parameters. The second part exhibits the loss and acceleration timescales of electrons caused by chorus waves. The main results in this paper includes: As geomagnetic activity and L shells increase, the pitch angle and momentum diffusion of electrons driven by chorus waves strengthens. The dramatic acceleration of electrons by chorus waves exists in the region with lower fpe /fce . Compared with keV electrons, MeV electrons are hard to be scattered by chorus waves, and electrons be precipitated into the atmosphere within several hours during highly disturbed times.

Apparatus for attosecond transient-absorption spectroscopy in the water-window soft-X-ray region

We present an apparatus for attosecond transient-absorption spectroscopy (ATAS) featuring soft-X-ray (SXR) supercontinua that extend beyond 450 eV. This instrument combines an attosecond table-top high-harmonic light source with mid-infrared (mid-IR) pulses, both driven by 1.7–1.9 mJ, sub-11 fs pulses centered at 1.76 upmum. A remarkably low timing jitter of sim 20 as is achieved through active stabilization of the pump and probe arms of the instrument. A temporal resolution of better than 400 as is demonstrated through ATAS measurements at the argon L_{2,3}-edges. A spectral resolving power of 1490 is demonstrated through simultaneous absorption measurements at the sulfur L_{2,3}- and carbon K-edges of OCS. Coupled with its high SXR photon flux, this instrument paves the way to attosecond time-resolved spectroscopy of organic molecules in the gas phase or in aqueous solutions, as well as thin films of advanced materials. Such measurements will advance the studies of complex systems to the electronic time scale.

Fast electron and slow hole spin relaxation in CsPbI3 nanocrystals

Spin-dependent properties of lead halide perovskites (LHPs) have recently gained significant attention paving their way toward spin-optoelectronic applications. However, separate measurements of the electron and hole spin relaxation rates are so far missing in LHPs. The knowledge of the electron and hole spin relaxation timescales is necessary to understand the spin-dependent properties of LHPs. Here, we report on the spin polarization dynamics in CsPbI3 nanocrystals (NCs). We employ polarization dependent ultrafast differential transmission spectroscopy (DTS) at room temperature to study the spin polarization dynamics in this system. In the case of pure CsPbI3 NCs, it is not possible to measure separately electron and hole spin relaxation rates from the polarization dependent DTS. Here, we introduce the soluble fullerene derivative PC60BM as an electron acceptor along with CsPbI3 to create an imbalance between the photoexcited electrons and holes in the NCs and, thus, affecting their spin-dependent carrier distribution. CsPbI3:PC60BM blend sample shows a distinct difference in the spin dependent kinetics of the DTS spectra as compared to the NCs-only sample. With the help of a kinetic model for the spin-dependent charge carrier distributions, we separately determine the electron and hole spin relaxation times in CsPbI3 NCs. We find that the room temperature hole spin lifetime (τh ∼ 5 ps) is ∼13 times longer than the electron spin lifetime (τe ∼ 0.4 ps). We ascribe the fast electron spin relaxation to the presence of strong spin–orbit coupling in the conduction band, which is ineffective for holes in the s-type valence band.

Kinetic modeling and experiments of a pulsed-bias plasma in a multipole plasma chamber

A pulse of electron timescale applied to a planar electrode immersed in a homogeneous plasma in a multipole plasma chamber (MPC) is modeled using a fully kinetic particle-in-cell (PIC) approach. In the time-explicit PIC simulations, we observed that the ion-sheath expansion is accompanied by electron timescale harmonic plasma oscillations at the sheath edge that decay after applying the pulse. First, we validate our PIC approach by comparing it with previous analytical and semi-empirical sheath expansion studies. Then, we compare our PIC results with experiments conducted in the MPC where similar electron frequency oscillations were excited when an electron timescale pulse was applied to a flat-conductor plate. In both PIC simulations and experiments, we find that the shape of the applied pulse dictates the amplitude of the sheath edge oscillations. In the PIC simulations, we observe that Landau damping has no discernible effect on these oscillations. However, in the experiments, the presence of a hot electron population results in a higher damping of electron oscillations. In both PIC simulations and experiments, the amplitude of the electron frequency oscillations decreases with the applied pulse width and these oscillations disappear for a linear pulse of a longer timescale of tpulse=1 μs (ωpetpulse=178), in the PIC simulations.

Electron-lattice coupling effects in nonadiabatic polarization switching of charge-order-induced ferroelectrics

We carry out first-principles density-functional-theory calculations to elucidate the polarization switching mechanism in charge-ordering-induced ferroelectrics based on the prototypical case of the (SrVO$_3$)$_1$(LaVO$_3$)$_1$ superlattice. We find that lattice relaxation for a specific charge ordering state can "lock" that state in, making non-adiabatic switching to a different CO variant energetically prohibitive, and in some cases, even making the energy barrier for adiabatic switching prohibitively large. We classify charge-ordering materials into two types, polyhedral breathing and off-centering displacement, based on the type of lattice mode most strongly coupled to the charge ordering. We demonstrate that the non-adiabatic electron hopping induced by an external electric field is expected only in off-centering-displacement-type charge-ordering-induced ferroelectrics. This successfully explains the different observed switching behaviors of LuFe$_2$O$_4$ and Fe$_3$O$_4$. These results offer a new understanding of the polarization switching mechanism in charge-ordering-induced ferroelectrics that provides guidance for the design and discovery of charge-ordering-induced ferroelectric materials and suggests a strategy for realizing "electronic ferroelectricity" with polarization switching on electronic rather than lattice time scales.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin β components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

Generation of intense phase-stable femtosecond hard X-ray pulse pairs

Coherent nonlinear spectroscopies and imaging in the X-ray domain provide direct insight into the coupled motions of electrons and nuclei with resolution on the electronic length scale and timescale. The experimental realization of such techniques will strongly benefit from access to intense, coherent pairs of femtosecond X-ray pulses. We have observed phase-stable X-ray pulse pairs containing more than 3 × 107 photons at 5.9 keV (2.1 Å) with ∼1 fs duration and 2 to 5 fs separation. The highly directional pulse pairs are manifested by interference fringes in the superfluorescent and seeded stimulated manganese Kα emission induced by an X-ray free-electron laser. The fringes constitute the time-frequency X-ray analog of Young’s double-slit interference, allowing for frequency domain X-ray measurements with attosecond time resolution.

Adiabatic theory of strong-field ionization of molecules including nuclear motion: Rescattering

We extend the adiabatic theory of strong-field ionization of molecules including nuclear motion developed in our previous paper [J. Svensmark et al., Phys. Rev. A 101, 053422 (2020)] to describe rescattering processes. The adiabatic regime in which the electronic timescale is much smaller than that of the nuclei and laser field is considered. The asymptotics of rescattering parts of the solution to the time-dependent Schr\"odinger equation (TDSE) and ionization amplitude are obtained, and thus vibrationally resolved photoelectron momentum distributions (PEMDs) in the whole range of the photoelectron momentum are found. The ionization dynamics is described in terms of a nuclear wave packet in the molecular ion created as a result of ionization of the molecule, its evolution until rescattering, and a nuclear wave packet after rescattering. This complements the three-step model by accounting for what happens with the nuclear subsystem between ionization and rescattering events. A uniform asymptotics defining the PEMDs near a backward rescattering caustic is obtained, which enables one to extract the nuclear wave packet after rescattering from the PEMDs. The theory is illustrated by comparing its predictions with accurate numerical results obtained by solving the TDSE for a one-dimensional molecule with one electronic and one internuclear degree of freedom modeling ${\mathrm{H}}_{2}$.

High accuracy analysis of Galerkin finite element method for Klein–Gordon–Zakharov equations

The main aim of this paper is to propose a Galerkin finite element method (FEM) for solving the Klein–Gordon–Zakharov (KGZ) equations with power law nonlinearity, and to give the error estimations of approximate solutions about the electronic fast time scale component q and the ion density deviation r. In which, the bilinear element is used for spatial discretization, and a second order difference scheme is implemented for temporal discretization. Moreover, by use of the combination of the interpolation and Ritz projection technique, and the interpolated postprocessing approach, for weaker regularity requirements of the exact solution, the superclose and global superconvergence estimations of q in H1−norm are deduced. At the same time, the superconvergence of the auxiliary variable φ (−Δφ=rt) in H1−norm and optimal error estimation of r in L2−norm are derived. Meanwhile, we also discuss the extensions of our scheme to more general finite elements. Finally, the numerical experiments are provided to confirm the validity of the theoretical analysis.