Electron-lattice Energy Exchange Research Articles

Електрон-ґратковий енергообмін і гарячі електрони в острівкових металевих плівках

В роботi розглядаються завершальнi тези теорiї автора, яка описує гарячi електрони в острiвкових металевих плiвках. Детально дослiджено, як розмiрнi залежностi електрон-ґраткового енергообмiну впливають на електроннi температури гарячих електронiв при наближеннi системи до критичних розмiрiв. Виявлено високу чутливiсть електронної температури до розмiрiв металевої наночастинки в околi критичних значень. Отримано результати обчислювальних експериментiв, якi пiдтверджують основнi положення теорiї.

Dynamics of the MnAs α/β-Striped Microstructure and of the Fe Magnetization Reversal in Fe/MnAs/GaAs(001): An Optical-Laser Pump–Free-Electron-Laser Probe Scattering Experiment

It was shown recently that the Fe magnetization reversal in the Fe/MnAs/GaAs(001) epitaxial system, attained by temperature control of the regular stripe pattern of the MnAs α- and β-phases, can also be driven by an ultrashort optical laser pulse. In the present time-resolved scattering experiment, we address the dynamics of the MnAs α-β self-organized stripe pattern induced by a 100 fs optical laser pulse, using as a probe the XUV radiation from the FERMI free-electron laser. We observe a loss in the diffraction intensity from the ordered α-β stripes that occurs at two characteristic timescales in the range of ~10−12 and ~10−10 s. We associate the first intensity drop with ultrafast electron-lattice energy exchange processes within the laser-MnAs interaction volume and the second with thermal diffusion towards the MnAs/GaAs interface. With the support of model calculations, the observed dynamics are interpreted in terms of the formation of a laterally homogeneous MnAs overlayer, the thickness of which evolves in time, correlating the MnAs microstructure dynamics with the Fe magnetization response.

Nonlocal two-temperature model: Application to heat transport in metals irradiated by ultrashort laser pulses

Extended two-temperature models with allowance for time (relaxation) and space nonlocal effects have been developed to describe, in particular, heat conduction in metals under ultrashort laser irradiation, which overheats the electron gas in comparison with the lattice leading to energy exchange between them. The time nonlocal effects arise when the electron gas is driven out of local (thermal) equilibrium, whereas the space nonlocal effects begin to be significant when the spatial extent of the temperature gradient is comparable with the mean-free path of the heat carrier. The electron-lattice energy exchange as well as the space–time nonlocal effects in the electron gas leads to a hierarchy of space–time nonlocal heat conduction equations for the electron and lattice temperatures, which are partial differential equation of higher order in comparison with the classical heat conduction equation. The models presented here may be used in combination with molecular dynamic and phase-field approaches.

New peculiarity in the temperature and size dependence of electron-lattice energy exchange in metal nanoparticles

The work is dedicated to the development of the electron-lattice energy exchange theory in metal particles, including dependence on particle size and electron temperature. We compared, for infinite metal, the expressions for the constants of the electron-lattice energy exchange by using the quantum-kinetic approach, with the classical kinetic approach. Both methods give the same result. But as we show, a quantum-kinetic approach in its standard form, for the metal particles smaller than the mean free path of electrons, cannot be applied, while the classical approach can be easily adjusted for such particle. We found a new peculiarity of the electron-lattice energy exchange. This exchange essentially depends on temperature if a particle size is in the range near a certain "magic size". The change of the electron temperature can alter (around the "magic size") the number of the acoustic modes, which are involved in energy transfer. As a result, the constant of the electron–phonon interaction in nanoparticles can be either much higher or much lower than the corresponding constant for infinite metal.

Excitation and relaxation of olivine after swift heavy ion impact

A multiscale model was developed to describe excitation and initial relaxation of an insulator after an impact of a swift heavy ion (SHI) decelerated in the electronic stopping regime. This model consists of a combination of three methods: (a) Monte Carlo simulations of the nonequilibrium kinetics of the electron subsystem of a solid at the femtosecond scale after the projectile passage. The complex dielectric function (CDF) is used to construct the cross sections for the MC model taking into account a collective response of the electron ensemble to excitation. (b) A molecular-kinetic approach describing further spatial spreading of electrons after finishing of ionization cascades up to hundred femtoseconds. And (c) molecular dynamics simulations of a reaction of the lattice on the excess energy transferred from the relaxing electron subsystem at the picosecond time scale. The dynamic structure factor (DSF) formalism is used to calculate the electron-lattice energy exchange in a general way which is valid for sub-picosecond timescales, beyond the phononic approximation of the lattice dynamics. The calculations were performed for 2GeV Au ion in olivine, demonstrating a heating of the lattice up to 700K in the nanometric scale picoseconds after the projectile passage.

Enhancement of Heating Performance of Carbon Nanotube Sheet with Granular Metal

A strategy for enhancing the heating performance of freestanding carbon nanotube (CNT) sheet is presented that involves decorating the sheet with granular-type palladium (Pd) particles. When Pd is added to the sheet, the heating efficiency of CNT sheet is increased by a factor of 3.6 (99.9 °C cm(2)/W vs 27.3 °C cm(2)/W with no Pd). Suppression of convective heat transfer loss attributes to the enhanced heat generation efficiency. However, higher heating response of CNT/Pd sheet was observed compared to CNT sheet, hence suggesting that the electron-lattice energy exchange could be additional heating mechanism in the presence of granular-type particles of Pd having a diameter of 10 nm or less. CNT sheet/Pd is quite stable, retaining its initial characteristics even after 300 cycles of on-off voltage pulses and shows fast thermal responses of the heating and cooling rates being 154 and -248 °C/s, respectively.

Acoustic Vibration Modes and Electron–Lattice Coupling in Self-Assembled Silver Nanocolumns

Using ultrafast spectroscopy, we investigated electron-lattice coupling and acoustic vibrations in self-assembled silver nanocolumns embedded in an amorphous Al2O3 matrix. The measured electron-lattice energy exchange time is smaller in the nanocolumns than in bulk silver, with a value very close to that of isolated nanospheres with comparable surface to volume ratio. Two vibration modes were detected and ascribed to the breathing and extensional mode of the nanocolumns, in agreement with numerical simulations.

Peculiarity of electron–phonon energy exchange in metal nanoparticles and thin films

The size dependence of electron-lattice energy exchange in nanoparticles has been studied. Both surface and bulk energy exchange parameters are examined and it is demonstrated that the bulk energy exchange has non-monotonic oscillations versus size of the particles. It has been found that the amplitude of such oscillations increases with decreasing a particle size until the critical size reaches L c. These bulk interaction related oscillations disappear for the particles less than L c, and only the surface energy exchange remains as the energy flow between electrons and phonons subsystems. It has been shown that there exists an interval of particles sizes with total energy exchange of few orders less than in massive bulk metals. This condition is crucial for existence of hot electrons in stationary conditions in metal nanoparticles, metal island films and thin films as have been observed experimentally.

Size effect in electron–lattice energy exchange in small metal particles

The paper examines electron–lattice energy exchange in confined metal systems (metal islands). An expression is derived for the energy, which an electron loses per unit of time to initiate acoustic oscillations in the lattice in accordance to the Cherenkov mechanism. In confined metal systems an electron moves, oscillating, from one potential wall to the other. The expression obtained for energy exchange converts to the generally known expression for bulk metals when the distance between the potential walls is increasing. The intensity of the bulk electron–lattice energy exchange oscillates and tends to zero at reaching a certain size with decrease of the distance between the walls.

Femtosecond optical investigation of electron–lattice interactions in an ensemble and a single metal nanoparticle

Electron–lattice energy exchange is investigated in an ensemble of silver nanoparticles of mean diameter 9 nm and in a single 30-nm particle using a femtosecond pump–probe technique. The dependences of the measured transient transmission change and of the electron energy loss kinetics on the excitation amplitude are compared to the results of numerical simulations of nonequilibrium electron relaxation and of the two-temperature model. The good agreement between the theoretical and experimental data indicates that, for the studied low particle density samples, hot-electron cooling is dominated by electron–lattice coupling in a nanoparticle both for weak and large electron heating with a minor influence of their surrounding environment (glass or polymer matrix).

Energy and charge trapping by localized vibrations: Electron-vibrational coupling in anharmonic lattices.

We study the time dependence of the energy redistribution between an initially localized electron and the vibrations of a one-dimensional anharmonic lattice. Numerical solutions show the periodic electron-lattice energy exchange, in contrast to the usual irreversible gradual decrease of the electronic energy. This energy trapping is shown to occur due to the appearance of localized vibrations, induced by strong enough electron-vibrational coupling in the anharmonic lattice. For these strong-coupling situations, we observed both localized (trapped) and conductive electronic charge.