Nonequilibrium Electron Dynamics Research Articles

Time-dependent non-equilibrium dielectric response in QM/continuum approaches.

The Polarizable Continuum Models (PCMs) are some of the most inexpensive yet successful methods for including the effects of solvation in quantum-mechanical calculations of molecular systems. However, when applied to the electronic excitation process, these methods are restricted to dichotomously assuming either that the solvent has completely equilibrated with the excited solute charge density (infinite-time limit), or that it retains the configuration that was in equilibrium with the solute prior to excitation (zero-time limit). This renders the traditional PCMs inappropriate for resolving time-dependent solvent effects on non-equilibrium solute electron dynamics like those implicated in the instants following photoexcitation of a solvated molecular species. To extend the existing methods to this non-equilibrium regime, we herein derive and apply a new formalism for a general time-dependent continuum embedding method designed to be propagated alongside the solute's electronic degrees of freedom in the time domain. Given the frequency-dependent dielectric constant of the solvent, an equation of motion for the dielectric polarization is derived within the PCM framework and numerically integrated simultaneously with the time-dependent Hartree fock/density functional theory equations. Results for small molecular systems show the anticipated dipole quenching and electronic state dephasing/relaxation resulting from out-of-phase charge fluctuations in the dielectric and embedded quantum system.

Raman Scattering in Molecular Junctions: A Pseudoparticle Formulation

We present a formulation of Raman spectroscopy in molecular junctions based on a many-body state representation of the molecule. The approach goes beyond the previous effective single orbital formalism and provides a convenient way to incorporate computational methods and tools proven for equilibrium molecular spectroscopy into the realm of current carrying junctions. The presented framework is illustrated by first principle simulations of Raman response in a three-ring oligophenylene vinylene terminating in amine functional groups (OPV3) junction. The calculated shift in Stokes lines and estimate of vibrational heating by electric current agree with available experimental data. In particular, our results suggest that participation of the OPV3 cation in Raman scattering under bias may be responsible for the observed shift, and that the direction of the shift depends on renormalization of normal modes. This work is a step toward atomistic quantum ab initio modeling of the optical response of nonequilibrium electronic dynamics in molecular junctions.

Nonequilibrium electron dynamics in materials driven by high-intensity x-ray pulses

We calculated the evolution of the electron system in solid-density matter irradiated by high-intensity x-ray pulses between 2 and 8 keV using molecular dynamics. For pulses shorter than 40 fs, the kinetic energy distribution of the electrons is highly nonthermal during and right after the pulse, and a large fraction of the absorbed x-ray energy resides with the fast photoelectrons which equilibrate on the timescale of the pulse length. The average ionization and electron temperature of the bulk of the electrons are significantly lower than their equilibrium values.

Transient quantum transport in double-dot Aharonov-Bohm interferometers

Real-time nonequilibrium quantum dynamics of electrons in double-dot Aharonov-Bohm (AB) interferometers is studied using an exact solution of the master equation. The building of the coherence between the two electronic paths shows up via the time-dependent amplitude of the AB oscillations in the transient transport current and can be enhanced by varying the applied bias on the leads, the on-site energy difference between the dots and the asymmetry of the coupling of the dots to the leads. The transient oscillations of the transport current do not obey phase rigidity. The circulating current has an antisymmetric AB oscillation in the flux. The nondegeneracy of the on-site energies and the finite bias cause the occupation in each dot to have an arbitrary flux dependence as the coupling asymmetry is varied.

Open-system electronic dynamics and thermalized electronic structure

We propose and implement a novel computational method for simulating open-system electronic dynamics and obtaining thermalized electronic structures within an open quantum system framework. The system-bath interaction equation of motion is derived and modeled from the local harmonic oscillator description for electronic density change. The nonequilibrium electronic dynamics in a thermal bath is simulated using first-order kinetics. The resultant electronic densities are temperature-dependent and can take characteristics of the ground and excited states. We present results of calculations performed on H(2) and 1,3-butadiene performed at the Hartree-Fock level of theory using a minimal Slater-type orbital basis set.

Electron relaxation in metals: Theory and exact analytical solutions

The nonequilibrium dynamics of electrons is of a great experimental and theoretical value, providing important microscopic parameters of the Coulomb and electron-phonon interactions in metals and other cold plasmas. Because of the mathematical complexity of collision integrals, theories of electron relaxation often rely on the assumption that electrons are in a ``quasiequilibrium'' (QE) with a time-dependent temperature, or on the numerical integration of the time-dependent Boltzmann equation. We transform the integral Boltzmann equation to a partial differential Schr\odinger-type equation with imaginary time in a one-dimensional ``coordinate'' space reciprocal to energy which allows for exact analytical solutions in both cases of electron-electron and electron-phonon relaxations. The exact relaxation rates are compared with the QE relaxation rates at high and low temperatures.

Nonequilibrium electronic and structural Jahn-Teller dynamics in(NbSe4)3I

Femtosecond time-resolved optical spectroscopy reveals complex nonequilibrium electronic and structural relaxation dynamics associated with an electronically driven Jahn-Teller (JT) structural phase transition at $274\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the quasi-one-dimensional semiconductor ${(\mathrm{Nb}{\mathrm{Se}}_{4})}_{3}\mathrm{I}$. The relaxation dynamics of the electronic order parameter $\ensuremath{\eta}$ (associated with the JT transition) and accompanying structural relaxation are described using the Landau-Khalatnikov equation, and equations of motion for coupled phonons, respectively.

Optoacoustical spectrum of the metal–insulator transition compound NdNiO 3: Sub-picosecond pump–probe study

A pump–probe optical spectroscopy technique was used to study the time-resolved reflectivity Δ R ( t ) / R in the range 0–160 ps of the metal–insulator transition compound NdNiO 3. It enables, for the first time, the direct real-time observation of the non-equilibrium electron dynamics in the insulating and in the metallic state of that compound and the acoustical response. Strong change of the transient reflectivity is observed as a function of temperature. First of all, the magnitude of the fast component of transient reflectivity ( { Δ R ( t ) / R } max , 0–3 ps) increases drastically when T < T MI . Moreover, the relaxation time of the fast-component increases when the insulating state is reached. Vanishing of the acoustic echo magnitude with decreasing temperature is also reported. All these singular properties could be connected to the gap opening mechanism below MIT.

Picosecond acoustic phonon pulse generation in nickel and chromium

The generation mechanisms for picosecond acoustic phonon pulses excited by ultrashort optical pulses in metals with strong electron-phonon coupling are elucidated. By means of an interferometric probing technique to extract changes in optical phase and amplitude, the acoustic pulse shapes in the transition metals Ni and Cr are shown to be governed by both thermal and nonequilibrium electron diffusion. Quantitative comparison of the results with a model that takes into account the nonequilibrium electron dynamics and ultrasonic attenuation gives excellent agreement.

Theoretical description of ultrafast phenomena in clusters

A theoretical study of different ultrafast nonequilibrium processes taking place during and after ultrashort excitation of clusters is presented. We discuss similarities and differences for several processes involving nonequilibrium ultrafast motion of atoms and electrons. We study ultrashort relaxation of clusters in response to excitations produced by femtosecond laser pulses of different intensities. We show how different relaxation processes, such as bond breaking, melting, fragmentation, emission of atoms, or Coulomb explosion, can be induced, depending on the laser intensity and laser pulse duration. We also discuss processes involving nonequilibrium electron dynamics, such as intraband Auger decay in clusters and ultrafast electronic motion during collisions between clusters and surfaces. We show that this electron dynamics leads to Stuckelberg-like oscillations of measurable quantities, such as the electron emission yield.

Ultrafast electron dynamics at metal surfaces: Competition between electron-phonon coupling and hot-electron transport

An experimental scheme (double pump/reflectivity probe using femtosecond laser pulses) enables the investigation of nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature. The competition between electron-phonon coupling and hot-electron transport gives rise to a reduced equilibrated temperature when the two pump pulses overlap in time, and provides a way of accurately determining the electron-phonon coupling constant. These observations have important consequences for femtosecond photochemical investigations.

Non-equilibrium Electron Dynamics Phenomena in Scaled Sub-100 nm Gate Length Metal Semiconductor Field Effect Transistors: Gate-fringing, Velocity Overshoot, and Short-channel Tunneling

Ultrashort channel GaAs metal semiconductor field effect transistors were fabricated with gate lengths ranging from 30 nm to 105 nm, by electron beam lithography, in order to examine the scaling characteristics of transconductance. For gate lengths in sub-100 nanometer range, where gradual channel approximation is no longer valid, it was observed that the transconductance varies with a variety of small-dimension-related, nonequilibrium electron dynamics phenomena such as gate-fringing effect, electron velocity overshoot, and short-channel tunneling. Short-channel tunneling was suggested experimentally for the first time to explain the degradation of transistor performance, overriding an enhancement due to electron velocity overshoot for a gate length smaller than the average inelastic mean free path of an electron.

Calculations of femtosecond differential optical transmission in germanium

Nonequilibrium electron and hole carrier dynamics are calculated for femtosecond photoexcitation in Ge using an ensemble Monte Carlo method. From the carrier distributions and k⋅p band-structure calculations, the time-dependent differential optical transmission that corresponds to pump-probe experiments is determined. It is found that electrons quickly scatter out of the optically coupled region and that the primary electron relaxation channel is from the Γ valley to the X valleys, then to the L valleys. Holes dominate the nonlinear absorption spectra in Ge. It is also found that light holes make a substantial contribution to the initial transient of the differential transmission, particularly for high-energy photoexcitation.

A novel approach to a transport model for ultra-small-scale compound semiconductor devices

The concepts of the previously developed macrokinetic distribution function and the ensemble average are used to develop a transport model that can account for effects of intervalley transfer, anisotropic scattering, and nonequilibrium dynamics of electrons in compound semiconductors. The developed transport model is rather simple and can be used for modeling of ultra-small-scale transport behavior of electrons in compound semiconductor devices. The developed model is applied to various multivalley semiconductor models. The study shows that the energy-dependent macroscopic effective mass of electrons in nonparabolic bandstructure, that is usually assumed for modeling of devices, is inadequate in situations where strong anisotropic scattering is involved and velocity overshoot is pronounced. It is found that the momentum dependence of the macroscopic effective mass is also significant during the interval of pronounced velocity overshoot.