Generation Of Nonequilibrium Phonons Research Articles

Superlubricity through graphene multilayers between Ni(111) surfaces

A single graphene layer placed between two parallel Ni(111) surfaces screens the strong attractive force and results in a significant reduction of adhesion and sliding friction. When two graphene layers are inserted, each graphene is attached to one of the metal surfaces with a significant binding and reduces the adhesion further. In the sliding motion of these surfaces the transition from stick-slip to continuous sliding is attained, whereby non-equilibrium phonon generation through sudden processes is suppressed. The adhesion and corrugation strength continues to decrease upon insertion of the third graphene layer and eventually saturates at a constant value with increasing number of graphene layers. In the absence of Ni surfaces, the corrugation strength of multilayered graphene is relatively higher and practically independent of the number of layers. Present first-principles calculations reveal the superlubricant feature of graphene layers placed between pseudomorphic Ni(111) surfaces, which is achieved through the coupling of Ni-3d and graphene-$\pi$ orbitals. The effect of graphene layers inserted between a pair of parallel Cu(111) and Al(111) surfaces are also discussed. The treatment of sliding friction under the constant loading force, by taking into account the deformations corresponding to any relative positions of sliding slabs, is the unique feature of our study.

Energy relaxation in IR laser excited Hg1-xCdxTe

IR laser excitation of Hgl-xCdxTe by low-fluence femtosecond and high fluence microsecond pulses was explored for the technologically important alloy fractions x ∼ 0.2 and x ∼ 0.28. We have used first principles (LAPW) electronic structure calculations and finite element modelling, supported by Monte Carlo simulation for the description of femtosecond pulse carrier relaxation and the transport parameters. Laser wavelengths considered were 6.4 – 10.6 μm for x ∼ 0.2 and 3.8 – 4.8 μm for x ∼ 0.28, with an incident 1 microsecond pulse fluence of 2 J/cm2. Many energy transfer mechanisms are invoked due to the long timescales of the microsecond pulses, and a main challenge is therefore to elucidate how these interplay in situations away from thermal equilibrium. Mechanisms studied include one- and two-photon absorption (OPA and TPA) across the band gap, inter-valence band absorption (IVA) between light- and heavy hole bands, electron-hole recombination/impact ionization, band gap renormalisation, intra-band free carrier absorption (FCA), excess carrier temperatures, non-equilibrium phonon generation, and refractive index changes. In the high fluence case, lattice temperatures evolve considerably during the laser pulse in response to the heated carriers. The chosen photon energies lie just above the band gap at the starting lattice temperature of 77 K, and nonlinear effects therefore dominate as the material heats up and the band gap begins to exceed the photon energy. Because of the low photon energy we must rely on Auger recombination, inter-valence band absorption and free carrier absorption to heat the carrier plasma. Although some Hgl-xCdxTe material parameters are now relatively well known, existing data for many of the processes are inadequate for cases far away from thermal equilibrium. Furthermore, the role of Auger recombination in relation to non-intrinsic recombination has been a matter of debate lately. In this respect, information from experiments can only be interpreted correctly if the intrinsic behaviour of the material is well known, and we have therefore assumed intrinsic Hgl-xCdxTe for the present study.

Two-dimensional electron gases: Theory of ultrafast dynamics of electron-phonon interactions in graphene, surfaces, and quantum wells

Many-particle electron-phonon interaction effects in two-dimensional electron gases are investigated within a Born–Markov approach. We calculate the electron-phonon interaction on a microscopic level to describe relaxation processes of quantum confined electrons on ultrafast time scales. Typical examples, where two-dimensional electron gases play a role, are surfaces and two-dimensional nanostructures such as graphene and quantum wells. In graphene, we find nonequilibrium phonon generation and ultrafast cooling processes after optical excitation. Electron relaxation dynamics at the silicon (001) 2×1 surface exhibits two time scales, corresponding to intrasurface and inside bulk-scattering processes. For GaAs quantum wells, we present broad emission spectra in the terahertz range assisted by LO-phonons of the barrier material.

Phonon generation and phonon energy current fluctuation in quantum dot molecules

We study the phonon dynamics in a biased molecular junction with the interplay ofelectron–phonon coupling and Coulomb interaction. These interactions are taken intoaccount within the self-consistent Born approximation and mean-field methods. It is foundthat the Coulomb interaction can enhance the nonequilibrium phonon generation. Ageneral formula for the zero-frequency power spectral density of the phonon energy currentfluctuation is presented in terms of the nonequilibrium phonon Green’s functions.

Nonequilibrium phonon generation in coupled Wannier-Stark ladders from a semiconductor superlattice in a three-terminal device

We present an experimental and a theoretical study on the midinfrared electroluminescence associated with transitions between electric-field-induced conduction states, forming Wannier-Stark (WS) ladders, in a strongly coupled GaAs∕AlAs superlattice inserted in a heterostructure bipolar transistorlike device. Electroluminescence shows wide tunability (100–210meV) of the diagonal intra-WS radiative transition, up to the merging with the vertical inter-WS emission, at constant injected emitter current. Intra-WS linewidth analysis suggests electric field homogeneity on the whole superlattice for F⩾160kV∕cm, until intervalley electronic scattering limits the radiative efficiency above 230kV∕cm. In this range, the integral optical intensity is constant, as expected by a transistorlike optoelectronic device. Monte Carlo simulations give a good agreement between experimental and theoretical electroluminescence spectra. Theoretical investigation indicates that intersubband scattering via LO phonon is responsible for the population of the states of the excited ladder. It shows that transitions between WS ladder states are responsible for hot phonon generation at fixed momenta by inter- and intrasubband nonradiative transitions.

Hot electron cooling rates via the emission of LO-phonons in InN

We report on the energy and momentum relaxation of hot electrons in n-type epilayer InN grown on sapphire substrate using molecular beam epitaxy (MBE). Hall and pulsed I–V measurements are carried out in the temperature range between 77 and 300 K. Drift velocity versus electric field characteristics show that, at 77 K, the drift velocity saturates just above vd ∼ 8 × 106 cm s−1 at electric fields in excess of E ∼ 12 kV cm−1. The mobility comparison method together with the power balance equation is used to obtain the electron temperature as a function of applied electric field and the electron energy loss rate as a function of electron temperature. Our results show conclusively that the effective energy relaxation time constant in InN is 200 fs. This is about six times slower than the theoretical value for the e–LO phonon scattering time of 31 fs. The effects of non-equilibrium phonon generation on slowing down of the energy relaxation and increasing the momentum relaxation processes are discussed using a theoretical model first developed for GaAs and then adapted to the III-N material systems.

Generation of nonequilibrium phonons in semiconductors and dielectrics by a pulse-heated metallic film: Model and experiment

Characteristics of a phonon generator in the form of a pulse-heated metallic film, viz., the time dependences of the film temperature and the kinetics of phonon ejection from the film into a substrate, are considered. The time dependences of the film temperature are calculated for cadmium telluride, diamond, and silicon substrates. It is shown that the duration of film cooling substantially exceeds the heating pulse length and the film continues to generate phonons with lower frequencies at the end of heating pulse. The inference is drawn that the film cooling should be correctly taken into account in analysis of the propagation of nonequilibrium acoustic phonons, specifically for phonon processes occurring in nanostructures.

Generation of Nonequilibrium Phonons, Suppression of Heat Transport, and Rapid Disintegration of a Metal Lattice

A self‐similar description of electron and phonon kinetics in metals is proposed for the case of a sharp gradient of the electron temperature. A nonequilibrium distribution of the phonon number density peaked near the plasma frequency of ions and extended along the heat‐flow axis is found. It is shown that Cherenkov generation of nonequilibrium phonons causes suppression of the electron heat flow followed by rapid disintegration of the metal lattice.

Generation of nonequilibrium phonons in Bi in a quantizing magnetic field

The voltage across a Bi single crystal in the [E×H] direction (transverse voltage E⊥) is investigated in strong crossed electric E and magnetic H fields under phonon generation conditions. Information on electron acousto-emf Eea of bismuth, forming segments with a negative differential conductivity on current–voltage characteristics j vs. E⊥, is obtained. Acousto-emf is measured as the absolute value of the difference in transverse voltage before and after the transition of the sample to the phonon generation mode. It is found that the dependence of acousto-emf on quantizing magnetic field is nonmonotonic. This is associated with oscillations in the electron–phonon generation rate ∂Nq/∂t in a magnetic field, i.e., with oscillations of the phonon–electron collision frequency τpe−1.

Phonon generation by carrier recombination in a-Si:H

We present results of pump-probe anti-Stokes Raman experiments at 1.8 K on the generation of nonequilibrium phonons in a-Si:H following intense optical excitation. We obtained a decay time of ∼70 ns for the TO phonons in all samples studied. In plasma deposited samples we observed an additional slowly decaying ( ⪢100 ns ) contribution to the Raman signal, which was not seen in hot-wire deposited samples. We propose a model to explain the additional phonons as resulting from fast nonradiative recombination of free carriers with carriers localized in the tail states.

Femtosecond investigation of the hot-phonon effect in GaAs at room temperature

Slowing down of the hot electron-lattice thermalization by generation of nonequilibrium phonons (hot-phonon effect) is investigated in GaAs at room temperature using high-sensitivity femtosecond single- and double-wavelength absorption saturation techniques. Measurements were performed for different amplitudes of the hot-phonon effect by changing either the individual excess energy of the photoexcited carriers or their density in the range 7\ifmmode\times\else\texttimes\fi{}${10}^{15}$--4\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. After an initial regime dominated by nonequilibrium LO-phonon k-space redistribution, the long-time delay (\ensuremath{\ge}3 ps) electron thermalization is found to occur with a characteristic time of \ensuremath{\sim}1.9 ps, independent of the total energy initially injected into the carriers and essentially reflecting the LO-phonon energy relaxation. This is in agreement with numerical simulations of the coupled carrier-phonon relaxation dynamics, indicating that energy transfers to holes are responsible for this slight reduction of the thermalization time compared to the LO-phonon lifetime (identified with the dephasing time ${\mathit{T}}_{2}$/2\ensuremath{\sim}2.1 ps). \textcopyright{} 1996 The American Physical Society.

Magnetic field-dependent hot carrier relaxation in GaAs quantum wells

Energy loss rates are calculated for electrons, confined in a GaAs quantum well and by a magnetic field perpendicular to its plane, interacting with unconfined LO phonons. Phonon emission rates calculated are compared with the temperature decay of carriers photoexcited by a picosecond laser pulse, measured by time-resolved luminescence spectroscopy. Measured energy loss rates are much less than those calculated assuming equilibrium phonon populations. A numerical calculation of the evolution of the coupled electron and phonon systems demonstrates that this is due to the generation of non-equilibrium phonons. After a few tens of picoseconds carrier cooling is governed by phonon decay; the carrier temperature decay curve depends on the volume of phonon phase space to which the carriers are coupled, the phonon lifetime and the carrier specific heat, and is relatively insensitive to the field-dependence of the carrier-phonon scattering matrix elements and carrier density of states.

Raman scattering in the presence of photoexcited nonequilibrium carriers in semiconductors: Theoretical considerations.

A theory of Raman scattering from a semiconductor in the presence of nonequilibrium phonons and carriers, both generated and probed by very short subpicosecond laser pulses, is presented. Because of (1) the short duration of the probe pulse, (2) the generation of nonequilibrium phonons by the optically injected carriers, and (3) the presence of nonequilibrium (``hot'') carriers, the spontaneous Raman scattering signal from both electronic and ionic excitations can show a distinct temporal evolution. We explicitly derive this time-varying signal, and contrast it with the usual case of long (i.e., cw) probe pulses and equilibrium carriers in polar semiconductors like GaAs. We find that the use of a short probe pulse simply broadens the observed spectral features. For hot carriers, the concept of an average time-dependent longitudinal dielectric function \ensuremath{\epsilon}(q,\ensuremath{\omega},t) is introduced to determine the coupled LO-phonon--plasmon modes at a given time t, while carriers are relaxing towards the band extrema. When the carriers are very hot, a plasmalike collective excitation does not exist because of heavy Landau damping, and the electronic Raman scattering is only due to the single-particle excitations. The phononlike collective mode can shift in frequency as the carriers cool.

X-ray evidence of direct generation of non-equilibrium phonons in quartz by infrared radiation

X-ray evidence of direct generation of non-equilibrium phonons in quartz by infrared radiation

X-ray evidence of direct generation of nonequilibrium phonons in quartz by infrared radiation

Direct electromagnetic stimulation of transverse-optical phonons in quartz has been accomplished by irradiation with a ${\mathrm{CO}}_{2}$ laser, and experimentally observed through enhancement of x-ray diffuse scattering due to zone-center and zone-boundary phonons.

Hot electrons and phonons in quantum-well AlxGa1-xAs-GaAs heterostructures

Abstract Formulas are presented to calculate the heating of charge carriers and the generation of nonequilibrium phonons by intense photoexcitation of quantum-well heterostructures. It is shown that the deviations from thermal equilibrium are more pronounced for quasi-two-dimensional structures than for bulk material. Supporting experimental data are given on an MO-CVD Al x Ga 1- x As-GaAs heterostructure consisting of a large GaAs quantum well ( L z ∼ 200 A) coupled to a phonon-generating array of seven small GaAs quantum wells ( L z ∼ 50 A).

Hot electrons and phonons under high intensity photoexcitation of semiconductors

It has become well established during the last few years that intense photoexcitation of a semiconductor leads to the heating of the carriers and the generation of nonequilibrium phonons. These phenomena which result from the relaxation of photoexcited carriers to the band extrema by interaction with other carriers and by emission of phonons, are reviewed in this paper. At relatively low intensities (<10 5W/cm 2 for GaAs) the photoexcited carrier distribution is Maxwellian with a carrier temperature T e different from the lattice temperature. T e as high as 150K and effective phonon temperatures as high as 3700K have been observed in GaAs. The observed variation of T e with excitation intensity leads to the conclusion that in semiconductors like GaAs the polar optical mode scattering is the dominant energy loss mechanism from the electron gas to the lattice. Similar results are obtained in CdSe and CdS. At higher intensities (>10 5W/cm 2 for GaAs), the carrier dist0ribution becomes non-Maxwellian for reasons not well understood at present. We will also discuss some recent measurements of variation of T e with excitation wavelength and of the transmission spectra of photoexcited GaAs.