Quasiparticle relaxation dynamics in heavy fermion compounds

We present the first femtosecond studies of electron-phonon (e-ph) thermalization in heavy-fermion compounds. The e-ph thermalization time tau(ep) increases below the Kondo temperature by more than 2 orders of magnitude as T=0 K is approached. Analysis using the two-temperature model and numerical simulations based on Boltzmann's equations suggest that this anomalous slowing down of the e-ph thermalization derives from the large electronic specific heat and the suppression of scattering between heavy electrons and phonons.     

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.     

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.     

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.     

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.     

Optical determination of electron-phonon coupling in carbon nanotubes

We report on an optical method to directly measure electron-phonon coupling in carbon nanotubes by correlating the first and second harmonic of the resonant Raman excitation profile. The method is applicable to 1D and 0D systems and is not limited to materials that exhibit photoluminescence. Experimental results for electron-phonon coupling with the radial breathing mode in 5 different nanotubes show coupling strengths from 3-11 meV. The results are in good agreement with the chirality and diameter dependence of the e-ph coupling calculated by Goupalov et al.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Size-dependence of the effective electron-phonon energy relaxation in hollow gold nanospheres

In this paper the size-dependence of energy relaxation, which is induced by the electron-phonon (e-ph) interactions and the electron-surface (e-s) interactions in noble metal nanoparticles, is investigated. Then based on the analysis and the derivation of the e-s coupling constant in metal nanospheres, the formula of the effective electron-phonon coupling constant G_eff of hollow gold nanospheres (HGNs) is deduced. Moreover, the correctness of the formulae gets proved by contrast analyses of the calculated G_eff values and experimental values obtained from literature.     

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.     

Effect of disorder and isotope on the properties of a two orbital double exchange system

A two-site single electron double exchange model incorporating orbital degeneracy, superexchange and electron-phonon (e-ph) interaction is studied using exact diagonalization method. The core spins are treated quantum mechanically. We study the ground state phase diagram as well as the magnetic susceptibility and the kinetic energy of the system as a function of temperature. Effect of difference in site energies, which mimics the role of site-diagonal disorder, is investigated. The susceptibility shows a peak at a characteristic temperature which we have referred to as T₀. The variation of T₀ with e-ph coupling and that of the isotope-shift exponent (α) with T₀ are obtained. We also investigate the field-induced change in the kinetic energy, which is related to the colossal magnetoresistance (CMR) of the system, and find that the disorder enhances the CMR even for the two-site system.     

Dynamics of electrons and holes at surfaces

We present ab initio calculation results for electron-phonon (e-ph) contribution to hole lifetime broadening of the surface state on Al(0 0 1). We show that e-ph coupling in this state is significantly stronger than in bulk Al at the Fermi level. It makes the e-ph decay channel very important in the formation of the hole decay in the surface state at . We also present the results for e-e lifetime broadening in a quantum-well state in 1 ML K/Cu(1 1 1). We show that this contribution is not negligible and is much larger than that in a surface state on Ag(1 1 1).     

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.     

Observation of radiative leptonic decay of the tau lepton

Using 4.68 fb(-1) of e(+)e(-) annihilation data collected with the CLEO II detector at the Cornell Electron Storage Ring, we have studied tau radiative decays tau(-)-->nu(tau)&mgr;(-)nu;(&mgr;)gamma and tau(-)-->nu(tau)e(-)nu;(e)gamma. For a 10 MeV minimum photon energy in the tau rest frame, the branching fraction for radiative tau decay to a muon or electron is measured to be (3.61+/-0.16+/-0. 35)x10(-3) or (1.75+/-0.06+/-0.17)x10(-2), respectively. The branching fractions are in agreement with standard model theoretical predictions.