Quantum-well wave-function localization and the electron-phonon interaction in thin Ag nanofilms

The electron-phonon interaction in thin Ag nanofilms epitaxially grown on Cu(111) is investigated by temperature-dependent and angle-resolved photoemission from silver quantum-well states. Clear oscillations in the electron-phonon coupling parameter as a function of the silver film thickness are observed. Different from other thin film systems where quantum oscillations are related to the Fermi-level crossing of quantum-well states, we can identify a new mechanism behind these oscillations, based on the wave-function localization of the quantum-well states in the film.     

Electronic structure and excitations on clean and nanostructured metal surfaces

We present a brief overview of recent studies and new theoretical results for electron-phonon interaction in the $\overline{Y}$ surface states on FCC(110) noble metal surfaces as well as in surface and quantum-well states of thin films. We discuss the oscillations of electron-phonon coupling parameter λ and the respective contribution to the lifetime broadening of these states. We analyse the effect of spin-orbit splitting of surface states on an electron-electron contribution to lifetimes of excited electrons (holes). Oscillations of the electron-electron contribution and quadratic dependence of the linewidth on energy is discussed for ultrathin Pb(111) films.     

Band structure and oscillatory electron-phonon coupling of Pb thin films determined by atomic-layer-resolved quantum-well states

Using a low temperature growth method, we have prepared atomically flat Pb thin films over a wide range of film thickness on a Si-(111)-7 x 7 surface. The Pb film morphology and electronic structure are investigated in situ by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. Well-defined and atomic-layer-resolved quantum-well states of the Pb films are used to determine the band structure and the electron-phonon coupling constant (lambda) of the films. We found an oscillatory behavior of lambda with an oscillation periodicity of two atomic layers. Almost all essential features in the Pb/Si(111) system, such as the growth mode, the oscillatory film stability, and the 9 monolayer envelope beating pattern, can be explained by our results in terms of the electron confinement in Pb films.     

The effect of the electron-phonon coupling on the thermal conductivity of silicon nanowires

The thermal conductivity of free-standing silicon nanowires (SiNWs) with diameters from 1-3?nm has been studied by using the one-dimensional Boltzmann's transport equation. Our model explicitly accounts for the Umklapp scattering process and electron-phonon coupling effects in the calculation of the phonon scattering rates. The role of the electron-phonon coupling in the heat transport is relatively small for large silicon nanowires. It is found that the effect of the electron-phonon coupling on the thermal conduction is enhanced as the diameter of the silicon nanowires decreases. Electrons in the conduction band scatter low-energy phonons effectively where surface modes dominate, resulting in a smaller thermal conductivity. Neglecting the electron-phonon coupling leads to overestimation of the thermal transport for ultra-thin SiNWs. The detailed study of the phonon density of states from the surface atoms and central atoms shows a better understanding of the nontrivial size dependence of the heat transport in silicon nanowire.     

On the possibility of the direct study of local electron-phonon interaction in semiconductors

Local electron-phonon interaction in deep-level states of defects in semiconductors was studied by induced absorption spectroscopy. Using ZnS:Cu single crystals as an example, it was shown that the laser modulation of two-step impurity absorption is an efficient technique for direct investigations of phonon relaxation effects in deep-level states. It was shown that the localized states in ZnS are prone to extremely strong electron-phonon coupling.     

Electron-phonon coupling spectrum in photodoped pentacene crystals

The coupling between conduction charges and the vibrational modes of the molecular lattice plays a defining role in the transport characteristics of organic semiconductors. Using electron tunneling spectroscopy, we obtain the electron--optical-phonon coupling spectrum in photodoped pentacene crystals at energies <30 meV. Comparison of the tunneling spectrum to infrared absorption data on the optical phonon density of states yields the energy dependence of the electron-phonon scattering matrix element. The integrated spectral weight of the electron-phonon coupling shows that superconductivity in pentacene is likely of electron-phonon origin.     

铁基氟化物超导体SrFe_(1-x)Co_xAsF(x=0,0.125)声子特性的第一性原理计算研究

采用基于第一性原理的平面波赝势方法,计算了铁基氟化物及其钴掺杂超导体SrFe1-xCoxAsF(x=0,0.125)在四方非磁态与正交条纹反铁磁态下的声子谱(声子色散曲线、声子态密度)及电-声子耦合常数.计算发现:条纹反铁磁相互作用下的自旋-声子耦合效应强于电-声子耦合作用使声子谱的宽度减小;自旋效应使声子的有效质量增加导致条纹反铁磁态下Fe原子与As原子的耦合振动频率减小.另外,掺杂和自旋效应是提高电-声子耦合常数的两个有效方法,但计算所得超导转变温度远小于实验测量值,表明铁基超导电性非简单的电-声子耦合配对机理.     

Chaotic polaronic and bipolaronic states in the adiabatic Holstein model

A rigorous proof for the existence of bipolaronic states is given for the adiabatic Holstein model for any lattice at any dimension, periodic or not, and for an arbitrary band filling, provided that the electron-phonon coupling (in dimensionless units) is large enough. The existence of mixed polaronic-bipolaronic states is also proven, but for larger electron-phonon coupling. These states consist of arbitrary distributions of bipolarons (or of bipolarons and polarons) localized in real space which can be simply labeled by pseudospin configurations as for a lattice gas model. The theory not only applies to periodic crystals, but also to quasicrystals, amorphous structures, polymer network, etc.When these bipolaronic and mixed polaronic-bipolaronic states exist, it is proven that: (1) These bipolaronic (and mixed polaronic-bipolaronic) states exhibit a nonzero phonon gap with a nonvanishing lower bound and an electronic gap at the Fermi energy. (2) These structures are insulating. The perturbation generated by any local change in the bipolaronic or polaronic distribution or by any charged impurity or defect decays exponentially at long distance. (3) These bipolaronic (and mixed polaronic-bipolaronic) states persist for any uniform magnetic field. (4) For large enough electron-phonon coupling, the ground state of the extended adiabatic Holstein model is a bipolaronic state when there is no uniform magnetic field or when it is small enough. It becomes a mixed polaronic-bipolaronic state for large enough magnetic field (note that the mixed polaronic-bipolaronic states are magnetic).In one-dimensional models, the ground state is an incommensurate (or commensurate) charge density wave (CDW) as predicted by Peierls (this result is not rigorous, but has been confirmed numerically). It is proven that the ground state becomes a bipolaronic charge density wave (BCDW) at large enough electron-phonon coupling. The existence of a transition by breaking of analyticity (TBA), which was numerically observed as a function of the electron-phonon coupling, is then confirmed. In that case, the shape of the effective bipolaron can be numerically calculated. It is observed that its size diverges at the TBA. The physical properties of BCDWs are rather different from those predicted by standard charge density wave theory. Bipolaronic charge density waves can also exist in models which are not only low-dimensional, but purely two- or three-dimensional.The technique for proving these theorems is an application of the concept of anti-integrability initially developed for Hamiltonian dynamical systems. It consists in proving that the eigenstates of the (trivial) Hamiltonian (called antiintegrable) obtained by canceling all electronic and lattice kinetic terms survive as a uniformly continuous function of the electronic kinetic energy terms in the Hamiltonian up to a certain threshold.     

Quenching of stimulated phonon emission in AlxGa1−xAs-GaAs quantum-well heterostructures

Data are presented on MO-CVD Al_xGa_1?xAs-GaAs multiple quantum-well heterostructures (QWH's) consisting of a large GaAs well (500 Å) coupled to one or two arrays of smaller wells (50 Å) that act, when photopumped, as phonon sources and resonators. These data show that the induced phonon-sideband laser operation of the larger well is quenched when the phonon-generating array, but not the larger well, is destroyed by thermal annealing. An estimate of the electron-phonon coupling coefficient for indirect optical transitions is presented, indicating enhanced coupling with increased phonon density and establishing a basis for stimulated phonon emission in QWH's.     

Effect of the Electron—Phonon Coupling on Barrier D^— Quantum Dots in Magnetic Fields

The influence of the electron-phonon coupling of the energy of low-lying states of the barrier D^- center,which consists of a positive ion located on the z-azis at a distance from the two-dimensional quantum dot plane and two electrons in the dot plane bound by the ion,is investigated at arbitrary strength of maguetic field by mading use of the method of few-body physics.Discontinuous ground-state energy transitions induced by the magnetic field are reported.The dependence of the binding energy of the D^- ground state on the quantum dot radius is obtained.A considerable enhancement of the binding is found for the D^- ground state,which results from the confinement of electrons and electron-phonon coupling.     

Inelastic light scattering in the Eu and Yb monochalcogenides

Multiphonon inelastic light scattering has been investigated in the magnetically ordering Eu and in the diamagnetic Yb monochalocogenides. All aspects of this scattering suggest that the multi-phonon lines results from recombination during time resolved relaxation ot the excited “hot” electron. The multiphonon scattering in these compounds is therefore interpreted by a two-step process of absorption followed by emission (hot luminescence). The observation of zone-center and zone-boundary multiphonon scattering is related to the electron-phonon coupling which is dependent on the kinetic energy of the excited photo-electron. Hence, with excitation into the bottom of the conduction band the coupling to the phonon system is dominated by the Fröhlich polaron concept, leading to zone-center LO scattering, whereas at higher excitation energies the electron-phonon coupling is no longer selective and is dependent primarily on the LO phonon density of states. The relaxation process and the electron-phonon coupling are strongly dependent on magnetic order. However, by comparison of the Eu monochalcogenides with the corresponding Yb compounds it is evident that the general phenomenon of multiphonon scattering in these compounds is independent of a spin system and is determined alone by the unique band structure.     

Preferred states of open electronic systems

System-environment interaction may introduce dynamic destruction of quantum coherence, resulting in a special representation named as pointer states. In this work, pointer states of an open electronic system are studied. The decoherence effect is taken into account through two different ways which are Büttiker's virtual probe model and strong electron-phonon interaction in the polaron picture. The pointer states of the system with different coupling strength are investigated. The pointer states are identified by tracking the eigenstates of the density matrix in real-time propagation. It is found that the pointer states can emerge for arbitrary coupling strength. And the pointer states deform to the eigenstates of the system in the strong coupling limit, which indicates the vanish of quantumness in the strong coupling limit.     

Local and pseudolocal Jahn-Teller states of a crystal with impurity Γ8 term

The multimode Jahn-Teller effect for an impurity in the Γ₈ ground state of a double point group is considered. The vibronic coupling with the trigonal (t₂) crystal vibrations is taken into account. It is shown that the vibronic coupling leads to the appearance of local and pseudolocal electron-phonon states. The spectral and symmetry characteristics of these states are obtained.     

Singlet and triplet bipolarons on the triangular lattice

We study the Coulomb-Fröhlich model on a triangular lattice, looking in particular at states with angular momentum. We examine a simplified model of crab bipolarons with angular momentum by projecting onto the low energy subspace of the Coulomb-Fröhlich model with large phonon frequency. Such a projection is consistent with large long-range electron-phonon coupling and large repulsive Hubbard U. Significant differences are found between the band structure of singlet and triplet states: The triplet state (which has a flat band) is found to be significantly heavier than the singlet state (which has mass similar to the polaron). We test whether the heavier triplet states persist to lower electron-phonon coupling using continuous time quantum Monte Carlo (QMC) simulation. The triplet state is both heavier and larger, demonstrating that the heavier mass is due to quantum interference effects on the motion. We also find that retardation effects reduce the differences between singlet and triplet states, since they reintroduce second order terms in the hopping into the inverse effective mass.     

Strong energy dependence of the electron-phonon coupling strength on Bi(100)

We have studied the energy dependence of the electron-phonon coupling strength on Bi(100). A fit of the temperature-dependent surface state linewidth results in a change of the coupling parameter lambda from 0.20+/-0.02 to 0.72+/-0.05 as the binding energy of the surface state increases from 70 to 330 meV. This result cannot be reconciled with the usual interpretation of lambda as the electron-phonon mass enhancement parameter at the Fermi energy. We suggest that this behavior is mainly caused by the strong energy dependence of the bulk density of states in this region.     

Electron-phonon interaction effect on optical absorption in cylindrical quantum wires

Electron-phonon interaction effects on linear and nonlinear optical absorption in cylindrical quantum wires are investigated. The linear and nonlinear optical absorption coefficients are obtained by using compact-density-matrix approach and iterative method, and the numerical results are presented for GaAs/AlAs cylindrical quantum-well wires. The results show that electron-phonon interaction not only influences the relaxation rate but also distinctly influences the wave functions and energies of the electron. The correction of electron-phonon interaction effect on the wave functions of the electron dominates the values of absorption coefficients. Moreover, the correction of electron-phonon interaction effect on the energies of the electron makes the absorption peaks blue shift and become wider.     

On different mechanisms of electron-phonon scattering of electron and hole excitations on an Ag(110) surface

We present the results of a detailed theoretical study of the electron-phonon scattering of electron and hole excitations in the unoccupied and occupied surface states on an Ag(110) surface. We show that the electron-phonon coupling parameter λ in the unoccupied surface state is approximately three times smaller than that in the occupied one, because the scattering of these states is determined by different phonon modes. The difference in the phonon-induced decay mechanisms of electron and hole excitations is determined by different spatial localizations of the unoccupied and occupied surface states at the $ \overline Y $ \overline Y point of the two-dimensional Brillouin zone.     

Electron-phonon interaction at the Si(111)-7 x 7 surface

It is shown that electron-phonon interaction provides a natural explanation for the unusual band dispersion of the metallic surface states at the Si(111)-(7 x 7) surface. Angle-resolved photoemission reveals a discontinuity of the adatom band at a binding energy close to the dominant surface phonon mode at h(omega0) = 70 meV. This mode has been assigned to adatom vibrations by molecular dynamics calculations. A calculation of the spectral function for electron-phonon interaction with this well-defined Einstein mode matches the data. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and from the temperature-dependent phonon broadening yield similar values of lambda = 1.09 and lambda = 1.06.     

Transport properties and electronic structure of nonmagnetic REAg compounds

The temperature dependence of the electrical and the thermal resistivity of the three nonmagnetic compounds REAg (RE=Y, La, Lu) is investigated. As with other physical properties LaAg shows a remarkable different behaviour in its transport properties compared to the YAg and LuAg compounds. To understand this anomalous behaviour of LaAg we have performed selfconsistent ASW bandstructure calculations for all three compounds. A comparison of these results shows a much higher density of states for LaAg at the Fermi energy. Flat bands near the Fermi level in LaAg together with a stronger electron-phonon coupling can explain the mentioned differences. A stronger electron-phonon coupling in LaAg is in coincidence with other physical properties.