Electronic Relaxation Research Articles

Microscopic calculation of spin torques in textured antiferromagnets

A microscopic calculation is presented for the spin-transfer torques (STT) and damping torques in metallic antiferromagnets (AF). It is found that the sign of the STT is opposite to that in ferromagnets because of the AF transport character, and the current-to-STT conversion factor is enhanced near the AF gap edge. The dissipative torque parameter $\beta_n$ and the damping parameter $\alpha_n$ for the N\'eel vector arise from spin relaxation of electrons. Physical consequences are demonstrated for the AF domain wall motion using collective coordinates, and some similarities to the ferromagnetic case are pointed out such as intrinsic pinning and the specialty of $\alpha_n = \beta_n$. A recent experiment on a ferrimagnetic GdFeCo near its angular-momentum compensation temperature is discussed.

Ultrafast Transfer and Transient Entrapment of Photoexcited Mg Electron in Mg@C_{60}.

Electron relaxation is studied in endofullerene Mg@C_{60} after an initial localized photoexcitation in Mg by nonadiabatic molecular dynamics simulations. Two approaches to the electronic structure of the excited electronic states are used: (i)an independent particle approximation based on a density-functional theory description of molecular orbitals and (ii)a configuration-interaction description of the many-body effects. Both methods exhibit similar relaxation times, leading to an ultrafast decay and charge transfer from Mg to C_{60} within tens of femtoseconds. Method (i)further elicits a transient trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption or photoelectron spectroscopy in gas phase, in solution, or as thin films.

Reaction–diffusion dynamics in an attractive stepwise-linear potential energy curve under the Gaussian-sink Action

In the present report, we have introduced the Fredholm integral method to solve the Smoluchowski equation in the Laplace domain. We get an exact semi-analytical solution for the linear potential energy curve in the dynamic diffusion process, and the survival probability is calculated by the numerical inverse Laplace transform method. We apply our method in two different physical contexts for finding different observable like average rate constant in electronic relaxation in solution and quantum yields in a photosynthetic system or doped molecular crystal.

Ultrafast time-resolved x-ray absorption spectroscopy of ionized urea and its dimer through ab initio nonadiabatic dynamics.

Investigating the early dynamics of chemical systems following ionization is essential for our understanding of radiation damage. However, experimental as well as theoretical investigations are very challenging due to the complex nature of these processes. Time-resolved x-ray absorption spectroscopy on a femtosecond timescale, in combination with appropriate simulations, is able to provide crucial insights into the ultrafast processes that occur upon ionization due to its element-specific probing nature. In this theoretical study, we investigate the ultrafast dynamics of valence-ionized states of urea and its dimer employing Tully's fewest switches surface hopping approach using Koopmans' theorem to describe the ionized system. We demonstrate that following valence ionization through a pump pulse, the time-resolved x-ray absorption spectra at the carbon, nitrogen, and oxygen K-edges reveal rich insights into the dynamics. Excited states of the ionized system give rise to time-delayed blueshifts in the x-ray absorption spectra as a result of electronic relaxation dynamics through nonadiabatic transitions. Moreover, our statistical analysis reveals specific structural dynamics in the molecule that induce time-dependent changes in the spectra. For the urea monomer, we elucidate the possibility to trace effects of specific molecular vibrations in the time-resolved x-ray absorption spectra. For the urea dimer, where ionization triggers a proton transfer reaction, we show how the x-ray absorption spectra can reveal specific details on the progress of proton transfer.

Molecular Dynamics in Ionic Liquid/Radical Systems.

Molecular dynamics of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Emim-Tf2N) with either of the four organic stable radicals, TEMPO, 4-benzoyloxy-TEMPO, BDPA, and DPPH, is studied by using Nuclear Magnetic Resonance (NMR) and Dynamic Nuclear Polarization (DNP). In complex fluids at ambient temperature, NMR signal enhancement by DNP is frequently obtained by a combination of several mechanisms, where the Overhauser effect and solid effect are the most common. Understanding the interactions of free radicals with ionic liquid molecules is of particular significance due to their complex dynamics in these systems, influencing the properties of the ion-radical interaction. A combined analysis of EPR, DNP, and NMR relaxation dispersion is carried out for cations and anions containing, respectively, the NMR active nuclei 1H or 19F. Depending on the size and the chemical properties of the radical, different interaction processes are distinguished, namely, the Overhauser effect and solid effect, driven by dominating dipolar or scalar interactions. The resulting NMR relaxation dispersion is decomposed into rotational and translational contributions, allowing the identification of the corresponding correlation times of motion and interactions. The influence of electron relaxation time and electron-nuclear spin hyperfine coupling is discussed.

Kinetics analysis of the electron transfer from nano-TiO2 to O2 through on-line absorptions and theoretical modeling

On-line optical absorptions were monitored under steady light illuminations to study the electron relaxations happening through the transfer from nano-TiO2 to O2, which are found to be slow and dispersive. A quasi-equilibrium (QE) theory and Monte Carlo simulations are developed to model the electron transfer, and they give good fittings to the early stage electron relaxations (over 70%). It is shown that the electron QE population at traps is kept during the whole electron relaxations. The slow kinetics is attributed to both the low probability (ptr) for an electron transferring to an O2 from a trap and the multi-trapping transport. The dispersive feature is ascribed to the dynamic decrease in the quasi-Fermi level (EF). The electron transfer rate constants just after the termination of light illuminations are taken out from the QE model fittings to analyze the relaxation kinetics. It is found that O2 amounts mainly affect the electron transfer by changing ptr; light intensities and temperatures mainly affect the electron transfer by changing the multi-trapping transport. The difference between the conduction band edge and the EF is the thermal barrier of the electron transfer from TiO2 to O2. The apparent activation energy (Eapp) of the electron transfer, determined from the absorption decays measured at different temperatures, is smaller than the real thermal barrier because of the decrease of EF with temperatures. The disagreement between the simulations and the later stage relaxations is not caused by the none-QE electron distribution at deep traps, and additional deep traps with a different distribution should also contribute to the electron relaxations.

A multilayer graphene-based absorber for the sub-THz regime

This work presents a four-layer absorber based on graphene patterns. Periodic arrays of biased graphene nanodisks and ribbons are exploited, while a graphene sheet top layer enhances the overall absorption. The presented structure is modeled using the circuit model method and simulated to verify its validity and accuracy. A sufficient number of simulations are performed to investigate the robustness of the proposed structure versus the variation of physical parameters such as the thicknesses of the layers, the chemical potentials, and the electron relaxation time. Wideband absorption in the frequency range of 8–9.5 THz is obtained for the proposed device, which can thus be considered for use in several optical applications including optical sensing and detection.

Diffusion–reaction approach to electronic relaxation in solution: Exact solution of Smoluchowski equation for parabolic potential in presence of a rectangular sink

We give an analytical method for finding the exact solution for the problem of electronic relaxation in solution, modelled by a particle diffusing under the influence of parabolic potential in the presence of a sink of rectangular shape. The Smoluchowski equation is used to model the diffusive motion. We have derived an analytical formula for the average rate constant in closed form. Solving Smoluchowski equation analytically with a sink of arbitrary width has been an unsolved problem and is important as a realistic model for non-radiative electronic relaxation of a molecule in solution.

Ultrafast Extreme Ultraviolet Photoelectron Spectroscopy of Nonadiabatic Photodissociation of CS2 from 1B2 (1Σu+) State: Product Formation via an Intermediate Electronic State.

We studied nonadiabatic dissociation of CS2 from the 1B2 (1Σu+) state using ultrafast extreme ultraviolet photoelectron spectroscopy. A deep UV (200 nm) laser using the filamentation four-wave mixing method and an extreme UV (21.7 eV) laser using the high-order harmonic generation method were employed to achieve the pump-probe laser cross-correlation time of 48 fs. Spectra measured with a high signal-to-noise ratio revealed clear dynamical features of vibrational wave packet motion in the 1B2 state; its electronic decay to lower electronic state(s) within 630 fs; and dissociation into S(1D2), S(3PJ), and CS fragments within 300 fs. The results suggest that both singlet and triplet dissociation occur via intermediate electronic state(s) produced by electronic relaxation from the 1B2 (1Σu+) state.

Clarifying Ultrafast Carrier Dynamics in Ultrathin Films of the Topological Insulator Bi2Se3 Using Transient Absorption Spectroscopy

Ultrafast carrier dynamics in the topological insulator Bi2Se3 have recently been intensively studied using a variety of techniques. However, we are not aware of any successful experiments exploiting transient absorption (TA) spectroscopy for these purposes. Here we demonstrate that if the ~730 nm wavelength pumping (~1.7 eV photon energy) is applied to ultrathin Bi2Se3 films, TA spectra cover the entire visible region, thus unambiguously pointing to two-photon excitation (~3.4 eV). The carrier relaxation dynamics is found to be governed by the polar optical phonon cascade emission occurring in both the bulk states and the Dirac surface states (SS), including SS-bulk-SS vertical electron transport and being also exclusively influenced by whether the Dirac point is presented between the Dirac cones of the higher energy (~1.5 eV) Dirac SS (known as SS2). We have recognized that SS2 act as a valve substantially slowing down the relaxation of electrons when the gap between Dirac cones exceeds the polar optical phonon and resonant defects energies. The resulting progressive accumulation of electrons in the gapped SS2 becomes detectable through the inverse bremsstrahlung type free carrier absorption.

Valley relaxation of resident electrons and holes in a monolayer semiconductor: Dependence on carrier density and the role of substrate-induced disorder

Using time-resolved optical Kerr rotation, we measure the low temperature valley dynamics of resident electrons and holes in exfoliated WSe$_2$ monolayers as a systematic function of carrier density. In an effort to reconcile the many disparate timescales of carrier valley dynamics in monolayer semiconductors reported to date, we directly compare the doping-dependent valley relaxation in two electrostatically-gated WSe$_2$ monolayers having different dielectric environments. In a fully-encapsulated structure (hBN/WSe$_2$/hBN, where hBN is hexagonal boron nitride), valley relaxation is found to be monoexponential. The valley relaxation time $\tau_v$ is quite long ($\sim$10~$\mu$s) at low carrier densities, but decreases rapidly to less than 100~ns at high electron or hole densities $\gtrsim$2 $\times 10^{12}$~cm$^{-2}$. In contrast, in a partially-encapsulated WSe$_2$ monolayer placed directly on silicon dioxide (hBN/WSe$_2$/SiO$_2$), carrier valley relaxation is multi-exponential at low carrier densities. The difference is attributed to environmental disorder from the SiO$_2$ substrate. Unexpectedly, very small out-of-plane magnetic fields can increase $\tau_v$, especially in the hBN/WSe$_2$/SiO$_2$ structure, suggesting that localized states induced by disorder can play an important role in depolarizing spins and mediating the valley relaxation of resident carriers in monolayer transition metal-dichalcogenide semiconductors.

Electronic states and relaxation dynamics of Au─ centers in RbCl crystals

We investigated the electronic states and nonradiative transitions among the relaxed excited states of Au− centers in RbCl crystals. To this end, we measured the temperature dependences of the luminescence intensities and decay time constants of the A′, B′, and C′ luminescence bands, which are attributed to the radiative transitions from the relaxed excited states of Au− centers. The intensities and decay time constants of these luminescence bands change with temperature, through the thermally activated nonradiative transitions among the relaxed excited states. The temperature dependences of the luminescence intensities and decay time constants were analyzed utilizing rate equations, except the decay time constant of the C′ luminescence. From the best fitting results, the activation energies of the relaxed excited states are estimated. The C′ luminescence has a longer decay time constant than that expected from the dipole-allowed transition. The temperature dependence of the decay time of the C’ luminescence is discussed by considering a trapped level.

Spin orbit coupling induced enhancement of thermoelectric performance of HfX2 (X = S, Se) and its Janus monolayer

Rashba spin-orbit coupling (SOC) plays a vital role in transporting carriers under external electric field or strain in two-dimensional transition metal di-chalcogenides in the absence of inversion symmetry. Here, the effect of SOC on the electronic and thermoelectric properties of two-dimensional monolayer HfS2, HfSe2 and their Janus monolayer HfSSe has been investigated using density functional theory (DFT) and Boltzmann transport equation (BTE). A considerable enhancement in thermoelectric power factor (PF) for n-type carriers has been observed due to the SOC in all the monolayers. This enhanced power factor for n-type carriers is mainly because of increased valley degeneracy at the conduction band minima (CBM), as there is no enhancement in electron relaxation time (τ) with SOC. The lattice thermal conductivity (κph) at room temperature is very low for these monolayers compared to extensively used TMDCs such as MoS2 and WS2. This is due to the strong coupling between acoustic and optical modes, which results in low group velocity and a short lifetime of phonons in HfS2, HfSe2 and HfSSe Janus monolayers. The ultralow value of κph results a very high thermoelectric figure of merit (ZT) at room temperature for n-type carriers and which increases significantly with SOC. The highest ZT values of 0.90 (HfSe2), 0.84 (HfS2) and 0.81 (HfSSe) for n-type at 600 K has been achieved with SOC. Our theoretical investigation predicts that there is a significant effect of SOC on the thermoelectric performance of HfX2 (X = S, Se) and its Janus monolayer HfSSe and they can be a revolutionary candidate for the fabrication of a highly efficient thermoelectric power generator.

DEER and RIDME Measurements of the Nitroxide-Spin Labelled Copper-Bound Amine Oxidase Homodimer from Arthrobacter Globiformis

In the study of biological structures, pulse dipolar spectroscopy (PDS) is used to elucidate spin–spin distances at nanometre-scale by measuring dipole–dipole interactions between paramagnetic centres. The PDS methods of Double Electron Electron Resonance (DEER) and Relaxation Induced Dipolar Modulation Enhancement (RIDME) are employed, and their results compared, for the measurement of the dipolar coupling between nitroxide spin labels and copper-II (Cu(II)) paramagnetic centres within the copper amine oxidase from Arthrobacter globiformis (AGAO). The distance distribution results obtained indicate that two distinct distances can be measured, with the longer of these at c.a. 5 nm. Conditions for optimising the RIDME experiment such that it may outperform DEER for these long distances are discussed. Modelling methods are used to show that the distances obtained after data analysis are consistent with the structure of AGAO.

Pressure effects on the electrical transport and anharmonic lattice dynamics of r-GeTe: A first-principles study

Various strategies for thermoelectric material optimization have been widely studied and used for promoting electrical transport and suppressing thermal transport. As a nontraditional method, pressure has shown great potential, as it has been applied to obtain a high thermoelectric figure of merit, but the microscopic mechanisms involved have yet to be fully explored. In this study, we focus on r-GeTe, a low-temperature phase of GeTe, and investigate the pressure effects on the electronic structure, electrical transport properties and anharmonic lattice dynamics based on density functional theory (DFT), the Boltzmann transport equations (BTEs) and perturbation theory. Electronic relaxation times are obtained based on the electron-phonon interaction and the constant relaxation time approximation. The corresponding electrical transport properties are compared with those obtained from previous experiments. Hydrostatic pressure is shown to increase valley degeneracy, decrease the band effective mass and enhance the electrical transport property. At the same time, the increase in the low-frequency phonon lifetime and phonon group velocity leads to an increase in lattice thermal conductivity under pressure. This study provides insight into r-GeTe under hydrostatic pressure and paves the way for a high-pressure strategy to optimize transport properties.

Linking Electronic Relaxation Dynamics and Ionic Photofragmentation Patterns for the Deprotonated UV Filter Benzophenone-4.

Understanding how deprotonation impacts the photophysics of UV filters is critical to better characterize how they behave in key alkaline environments including surface waters and coral reefs. Using anion photodissociation spectroscopy, we have measured the intrinsic absorption electronic spectroscopy (400–214 nm) and numerous accompanying ionic photofragmentation pathways of the benzophenone-4 anion ([BP4–H]−). Relative ion yield plots reveal the locations of the bright S1 and S3 excited states. For the first time for an ionic UV filter, ab initio potential energy surfaces are presented to provide new insight into how the photofragment identity maps the relaxation pathways. These calculations reveal that [BP4–H]− undergoes excited-state decay consistent with a statistical fragmentation process where the anion breaks down on the ground state after nonradiative relaxation. The broader relevance of the results in providing a basis for interpreting the relaxation dynamics of a wide range of gas-phase ionic systems is discussed.

Role of Electronic Relaxation Rates in the Casimir Force between High-Tc Superconductors

We revisit the problem of the Casimir force between high-Tc superconductors below and above the critical temperature for the superconducting transition. Ceramic superconductors exhibit a different temperature dependence of the reflectivity when switching from the normal to the superconducting state. We leverage this unique characteristic with respect to ordinary metals to claim that these kind of materials can prove useful as an alternative system where the long-standing discussion on the role of electronic relaxation can be addressed. Furthermore, we show that the two main damping mechanisms associated with free and mid-infrared electrons dominate at very distinct scales, meaning that they can be considered separately when the Casimir force is measured as a function of slab distance. This facilitates the experimental identification of the role of the two electronic relaxation contributions to the force.

Formative period in the x-ray-induced photodissociation of organic molecules

The authors combine x-ray free electron laser pulses and infrared laser pulses to characterize the femtosecond-scale electronic relaxation, ring opening and fragmentation processes on a thiophene molecule.

Dynamics and chemical mode analysis of plasma thermal-chemical instability

The stability of the weakly ionized plasma and the transition from a stable homogeneous discharge to unstable filaments play an important role in gas laser physics, plasma-assisted combustion, chemical reforming, and material synthesis. Theoretical stability analysis and thermal-chemical mode analysis were performed to understand the mechanism of plasma thermal-chemical instability by using a zero-dimensional plasma system with both simplified and detailed chemical kinetics of H2/O2/N2 mixtures. The plasma dynamic and kinetic models accounted for multiple physical mechanisms in the chemically-reactive weakly ionized plasma, including ionization, attachment/detachment, recombination, vibrational and electronic energy relaxation, convective and diffusive species/heat removal, Joule heating, and detailed chemical kinetics. An analytical criterion and the explosive mode species/temperature pointers were formulated while the representative active species were identified for different thermal-chemical modes. The results showed that in addition to the classical thermal-ionization mechanism, various chemical modes from chemical heat imbalance and elementary kinetics significantly modified the time dynamics and the stability of the weakly ionized plasma. The present analysis provides insights and guidance to control plasma instability using chemical kinetics.

Femtosecond dynamics of diphenylpropynylidene in ethanol and dichloromethane

Carbon chains with an odd number of C atoms are reactive intermediates with a high biradical character. Here we report a joint experimental and computational investigation of the dynamics of diphenylpropynylidene, C6H5-C3-C6H5, in dichloromethane and ethanol. The biradical is generated by ultraviolet light from 1,3-diphenyldiazopropyne. Electron paramagnetic resonance spectra are recorded to elucidate the spin multiplicity and geometry of the biradical. In both solvents a triplet ground state at 4 K is verified. Transient absorption spectra provide insight into the fate of the biradical. A study in deaerated dichloromethane permits us to follow the photophysics of diphenylpropynylidene and to extract time constants for its vibrational as well as electronic relaxation. In the presence of oxygen, a more complex photochemistry is observed that permits us to derive a model for the reaction of the biradical with O2. In ethanol, the spectra recorded in the presence and absence of O2 are very similar, which can be explained by the similarity of the chromophores of the reaction products.