Effects of confinement on the electron and lattice dynamics in metal nanoparticles

Effects of confinement on the electron-electron (e-e) and electron-phonon (e-ph) thermalization dynamics in noble metal clusters are calculated using simple approaches. The model predictions are compared with femtosecond pump-probe measurements which display an acceleration of the e-e and e-ph relaxation dynamics. The size-effects on the e-e relaxation dynamics are consistent with a model involving the surface-induced reduction of the screening efficiency of the Coulomb e-e interaction. With regard to the e-ph relaxation dynamics, this model yields too large time constants, pointing out deficiencies of the standard modelling of the e-ph energy exchanges in bulk metals. Analysis of these deficiencies shows that the bare e-ion interaction has to be involved in the transition matrix element describing the non-adiabatic e-ph energy exchanges.     

Dynamical properties near the metal-insulator transition: evidence for electron-assisted variable range hopping

We report independent measurements (between 20 and 200 mK) of the electronic specific heat C(e), the electron-phonon coupling G(e-ph), and the electron-phonon relaxation time tau(e-ph) (from 10(-2) to 10(-5) s) for NbxSi1-x Anderson insulator thin films. We show that the usual equation tau(e-ph) = C(e)/G(e-ph) holds only if the resistance is solely related to the electron temperature. We conclude that at sufficiently low temperatures variable range hopping transport is assisted by electron-electron interactions alone and is independent of the phonon distribution.     

Electron-phonon renormalization in cuprate superconductors

Electron-phonon (e-ph) renormalization effects in a model cuprate system CaCuO2 are studied by employing density functional theory based methods. Whereas calculations based on the local spin-density approximation (LSDA) predicts negligible e-ph coupling effects of the half-breathing Cu-O bond stretching mode, the inclusion of a screened on-site Coulomb interaction (U) in the LSDA+U calculations greatly enhances the e-ph coupling strength of this mode. The full-breathing mode, on the other hand, shows a much weaker e-ph renormalization effect.     

Electron-phonon interaction in single-wall carbon nanotubes: A time-domain study

We investigate the electron-phonon (e-ph) interaction in single-wall carbon nanotube samples at room temperature using femtosecond time-resolved photoemission. By probing electrons from the vicinity of the Fermi level we are able to study the e-ph interaction in the metallic nanotube species only. The observed electron dynamics can be used to calculate e-ph scattering matrix elements for two likely scattering scenarios: forward scattering from twistons and backscattering by longitudinal acoustic phonons. The corresponding matrix elements reveal an intrinsically weak e-ph interaction approximately 50% smaller than predicted by tight-binding calculations.     

RAA Electron-Phonon Interaction Cannot Support High TC in YBaCuO

The e-ph interaction in YBaCuO is calculated and the nonlinear Eliashberg equation is solved by the newly introduced bifurcation theory and parameter imbedding method. The transition temperature is low from the traditional (RA.4) e-ph model. The cause of some authors' improper high T_C result is discussed.     

Chirality-induced dynamic kohn anomalies in graphene

We develop a theory for the renormalization of the phonon energy dispersion in graphene due to the combined effects of both Coulomb and electron-phonon (e-ph) interactions. We obtain the renormalized phonon energy spectrum by an exact analytic derivation of the phonon self-energy, finding three distinct Kohn anomalies (KAs) at the phonon wave vector q=omega/v, 2k_{F}+/-omega/v for LO phonons and one at q=omega/v for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA q=2k_{F} in ordinary metals, originates from the dynamical screening of e-ph interaction (with a concomitant breakdown of the Born-Oppenheimer approximation) and the peculiar chirality of the graphene e-ph coupling.     

Theoretical description of electron phonon Fock space for gapless and gapped nanowires

We study the effect of electron–phonon(e–ph) interaction on the elastic and inelastic electronic transport of a nanowire connected to two simple rigid leads within the tight-binding and harmonic approximations. The model is constructed using Green's function and multi-channel techniques, taking into account the local and nonlocal e–ph interactions. Then, we examine the model for the gapless(simple chain) and gapped(PA-like nanowire) systems. The results show that the tunneling conductance is improved by the e–ph interaction in both local and nonlocal regimes, while for the resonance conductance, the coherent part mainly decreases and the incoherent part increases. At the corresponding energies which depend on the phonon frequency, two dips in the elastic and two peaks in the inelastic conductance spectra appear. The reason is the absorption of the phonon by the electron in transition into inelastic channels.     

Decay of electronic excitations in bulk metals and at surfaces

We present a brief survey of the contemporary state of theoretical study of quasiparticles (excited electrons and holes) dynamics in bulk metals and at metal surfaces. Quasiparticle decay mechanisms are discussed in terms of electron-electron (e-e) and electron-phonon (e-ph) interactions. The e-ph decay channel is shown to be important for all materials considered. It is especially important for systems with the thickness of one monolayer. In the e-e decay channel the quasiparticle decay can be realized via one-electron transfer processes, via creation of electron-hole pairs, and via plasmon excitation. In ferromagnetic systems the electron (hole) decay via the Stoner pair excitation or/and magnon excitation is made possible.     

IR excitation spectra of low dimensional CT crystals: Multidimensional linear response theory approach

The effects of electron-phonon (e-ph) coupling in the infrared (IR) excitation spectra of low-dimensional charge transfer crystals are investigated through multidimensional, mean-field linear response theory. This very general approach enables one to take into account simultaneously different types of e-ph coupling, showing how the vibrational modes are indirectly mixed through their common interaction with charge-transfer and/or localized electrons. The somewhat unexpected consequences of this mixing are illustrated by the spectral simulation of two simple model systems.     

Temperature dependence of photo-hole decay in 4d derived Quantum Well States in monolayer Ag(111) films on Pd(111), Ni(111), Mo(110) and Cu(100)

We present experimental data on the temperature dependence of photo-hole decay obtained by Angle Resolved Photoemission (ARPES) measurements from 4d derived Quantum Well States (QWS) on Ag(111) monolayer films deposited on Pd(111), Ni(111), Mo(110) and Cu(100). We have found a significant increase of the Ag 4d electron–phonon (e-ph) coupling strength with respect to the bulk values. The increase is attributed to different mechanisms that are associated with the interaction of the Ag film with under laying substrate. It is proposed that the main channels that contribute to the increased e-ph coupling originate from the inter-band transitions that involve bulk states of the substrates.     

Time-resolved photoexcitation of the superconducting two-gap state in MgB2 thin films

Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is governed by the sub-ps electron-phonon (e-ph) relaxation present at all temperatures, the few-ps e-ph process well pronounced below 70 K, and the sub-ns superconducting relaxation below T(c). The amplitude of the superconducting component versus temperature follows the superposition of the isotropic dirty gap and the three-dimensional pi gap dependences, closing at two different T(c) values. The time constant of the few-ps relaxation exhibits a double divergence at temperatures corresponding to the T(c)'s of the two gaps.     

Two-site two-electron generalized Hubbard-Holstein model: a perturbation study

The two-site two-electron generalized Hubbard-Holstein model is studied within a perturbation method based on a variational phonon basis obtained through the modified Lang-Firsov (MLF) transformation. The ground-state wave function and the energy are found including up to the seventh and eighth order of perturbation, respectively. The convergence of the perturbation corrections to the ground state energy, as well as to the correlation functions, are investigated. The kinetic energy and the correlation functions involving charge and lattice deformations are studied as a function of electron-phonon(e-ph) coupling and electron-electron interactions for different values of the adiabaticity parameter. The simultaneous effect of the e-ph coupling and Coulomb repulsion on the kinetic energy shows interesting features.     

Quantum dynamics of a driven correlated system coupled to phonons

Nonequilibrium interplay between charge, spin, and lattice degrees of freedom on a square lattice is studied for a single charge carrier doped in the t-J-Holstein model. In the presence of a static electric field we calculate the quasistationary state. With increasing electron-phonon (e-ph) coupling the carrier mobility decreases; however, we find increased steady state current due to e-ph coupling in the regime of negative differential resistance. We explore the distribution of absorbed energy between the spin and the phonon subsystem. For model parameters as relevant for cuprates, the majority of the gained energy flows into the spin subsystem.     

Phase separation close to the density-driven Mott transition in the Hubbard-Holstein model

The density-driven Mott transition is studied by means of dynamical mean-field theory in the Hubbard-Holstein model, where the Hubbard term leading to the Mott transition is supplemented by an electron-phonon (e-ph) term. We show that an intermediate e-ph coupling leads to a first-order transition at T=0, which is accompanied by a phase separation between a metal and an insulator. The compressibility in the metallic phase is substantially enhanced. At quite larger values of the coupling, a polaronic phase emerges coexisting with a nonpolaronic metal.     

Evidence for the importance of extended Coulomb interactions and forward scattering in cuprate superconductors

The prevalent view of the high-temperature superconducting cuprates is that their essential low-energy physics is captured by local Coulomb interactions. However, this view been challenged recently by studies indicating the importance of longer-range components. Motivated by this, we demonstrate the importance of these components by examining the electron-phonon (e-ph) interaction with acoustic phonons in connection with the recently discovered renormalization in the near-nodal low-energy (~8-15 meV) dispersion of Bi(2)Sr(2)CaCu(2)O(8+δ). By studying its nontrivial momentum and doping dependence we conclude a predominance of forward scattering arising from the direct interplay between the e-ph and extended Coulomb interactions. Our results thus demonstrate how the low-energy renormalization can provide a pathway to new insights into how these interactions interplay with one another and influence pairing and dynamics in the cuprates.     

Phonon-limited transport coefficients in extrinsic graphene

The effect of electron-phonon scattering processes on the thermoelectric properties of extrinsic graphene was studied. Electrical and thermal resistivity, as well as the thermopower, were calculated within the Bloch theory approximations. Analytical expressions for the different transport coefficients were obtained from a variational solution of the Boltzmann equation. The phonon-limited electrical resistivity ρ(e-ph) shows a linear dependence at high temperatures and follows ρ(e-ph) ~T(4) at low temperatures, in agreement with experiments and theory previously reported in the literature. The phonon-limited thermal resistivity at low temperatures exhibits a ~T dependence and achieves a nearly constant value at high temperatures. The predicted Seebeck coefficient at very low temperatures is Q(T) ~ Π(2)k(2)_(B)/T(3eE_(F), which shows a n(-1/2) dependence with the density of carriers, in agreement with experimental evidence. Our results suggest that thermoelectric properties can be controlled by adjusting the Bloch-Grüneisen temperature through its dependence on the extrinsic carrier density in graphene.     

Phonon-assisted Kondo effect in single-molecule quantum dots coupled to ferromagnetic leads

Based on the infinite-U Anderson model spin-polarized transport through the tunnel magnetoresistance (TMR) system of single-molecule quantum dot is investigated under the interplay of strong electron correlation and electron-phonon (e-ph) coupling. The spectral density and the nonlinear differential conductance are studied using the extended non-equilibrium Green's function method through calculating the dot-level splitting self-consistently. The results exhibit that a serial of peaks emerge on the two sides of the main Kondo peak for the antiparallel magnetic configuration of electrodes, while for the parallel case both the main and phonon-assisted satellite Kondo peaks all split up into two asymmetric peaks even at zero-bias. Correspondingly, the nonlinear differential conductance displays a set of satellite-peaks around the Kondo-peak in the presence of the e-ph interaction. Furthermore, extra maxima and minima appear in the TMR curve. The TMR alternates between the positive and the negative values along with the variation of bias voltage.     

Hierarchic Natures and Dynamics of Photoinduced Structural Phase Transition in Non-Degenerate Charge Density Wave States Via Successive Photoexcitations

We investigate the adiabatic and dynamical natures of the lattice relaxation of excitons in strongly coupled electron-phonon (e-ph) systems using the extended Peierls-Hubbard model, so as to clarify the possible mechanisms of the photoinduced structural phase transition (PISPT) via multi-photon. Focusing on the growth process of relaxed domains that is induced by multi-photoexcitation, we calculate the adiabatic potential energy surfaces relevant to the nonlinear lattice relaxations of excitons in this process. Calculated potentials lead to an essential model of a multi-stepwise potential-crossing (MSPC) system that is composed of many displaced harmonic oscillators as an elementary process of the domain growth in the strongly coupled e-ph systems. We also investigate the dynamical natures in such MSPC systems calculating the time-developments the excited wave packet in this system using the density operator. It is concluded from calculated results that the system possibly develops from the lowest-energy potential state to the higher ones by the effect of the photoexcitations followed by the lattice relaxations.     

Thermalization time of thin metal film heated by short pulse laser

Based on the hyperbolic two-step heat conduction model, using the Laplace transform and numerical inverse transform method (Riemann-sum approximation method), the thermal behaviour of thin metal films has been studied during femtosecond pulse laser heating. Also the thermalization time, which is the time for the electron gas and solid lattice to reach thermal balance, has been studied in detail. The values of thermalization time for silver (Ag), gold (Au), copper (Cu) and lead (Pb) are obtained. The effects of material parameters of the thin metal film on the thermalization time are considered for the four kinds of metals by changing one of the parameters and regarding the other parameters as constant. For a typical metal material, the order of the thermalization time is of the order of hundreds of picoseconds. The thermalization time decays exponentially with the increase of phonon－electron coupling factor or electron gas thermal conductivity, and it increases linearly with the increase of the ratio of lattice heat capacity to electron gas heat capacity. However, the relaxation time of the electron gas has very little effect on the thermalization time.     

聚对苯乙炔的基态、极化子与双极化子激发态及其稳定性

提出了描述聚对苯乙炔(PPV)晶格及电子结构的物理模型,并在该模型的基础上给出了PPV的基态及可能的极化子、双极化子激发态,并对影响激发态稳定性的因素进行了研究. 关键词：