Electron-phonon interaction in the quantum well state of the 1 ML Na/Cu(111) system

The electron-phonon interaction in the quantum well state formed by a Na monolayer coating on Cu(111) is investigated theoretically. The calculations show that the electron-phonon coupling constant γ in this state decreases insignificantly (≈1%) compared to the value of γ for a clean copper surface. The corresponding electron-phonon contribution to the lifetime τ of the quantum well state increases by a factor of 1.5 compared to τ for the clean Cu(111) surface.     

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.     

Superconductivity in monolayer Pb on Si(111) from first principles

Observation of superconductivity in a single layer of Pb on the (111) surface of bulk silicon has renewed interest in a longstanding question; can superconductivity persist to the ultimate atomic limit? Using first-principles techniques, we investigate the total electron-phonon coupling in monolayer Pb supported by a Si(111) substrate. Our ultra-fine sampling of the electronic structure, lattice dynamics and electron-phonon matrix elements in the nearly two-dimensional Brillouin zone yields a total electron-phonon coupling parameter which explains the experimentally observed superconducting transition temperature of 1.83 K [T. Zhang, et al., Nat. Phys. 6 (2010) 104]. The observed suppression of the superconducting transition temperature from the bulk value of 7.2 K is found to arise from the interplay of reduced electron-phonon matrix elements and a modification of the lattice dynamics resulting from the Pb-Si bonding.     

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.     

Surface-enhanced electron-phonon coupling and irreversible reconstruction of MgO(001)

A possible mechanism for the thermally irreversible reconstruction of MgO(001) is presented. The theory is based on the electronic structure and the electron-phonon interaction at the surface. It is suggested that a narrower band gap at the surface enhances the electronphonon coupling and that this leads to an adiabatic potential curve with a metastable and a stable state which can result in the irreversible structural transformation.     

Electronic states and electron-phonon coupling at the 2×1 reconstructed Si(111) surface

The electronic structure of Si(111)-2x1 is studied selfconsistently for several values of the reconstruction parameters within Haneman's model. The derivatives of the dangling-bond bands with respect to the surface atom displacements are calculated and the surface electron-phonon coupling is estimated.     

Novel superconductivity in metallic SnH(4) under high pressure

From first-principles calculations, a high-pressure metallic phase of SnH(4) with a novel layered structure intercalated by "H(2)" units is revealed. This structure is stable at pressure between 70 and 160 GPa. A remarkable feature of this structure is the presence of soft modes in the phonon band structure induced by Fermi surface nesting and Kohn anomalies that lead to very strong electron-phonon coupling. The application of the Allen-Dynes modified McMillan equation with the calculated electron-phonon coupling parameter lambda shows that a superconducting critical temperature close to 80 K can be achieved at 120 GPa.     

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 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.     

On a rudimentary model of instability and (2 x 1) reconstruction of Si (001) and (111) surfaces

A recently proposed simple qualitative theory of instability and reconstruction of surfaces of solids exhibiting a certain covalent component of bonding is applied to (2 x 1) Si (001) and (111) surfaces with a possible extension to analogous surfaces of germanium and GaAs (001)., Shockley surface states from the vicinity of the Fermi energy are supposed to play a crucial role in the electron-phonon coupling. For the Si (001) surface the interaction between bridge and dangling bonds causes the dimerization (primary effect) and the buckling distortions to couple. If the latter effect is pronounced, the asymmetric dimer might exist as a stable or metastable surface phase. For the Si (111) surface the Pandey -bonded chain model seems to be a natural candidate for reconstruction from the present simple point of view.     

Electron-beam-induced disordering of the Si(001)-c(4 x 2) surface structure

An electron beam (EB) irradiation effect on the Si(001)-c(4 x 2) surface was investigated by using low-energy electron diffraction. Quarter-order spots become dim and streaky by EB irradiation below approximately 40 K, indicating a disordering in the c(4 x 2) arrangement of buckled dimers. A quantitative analysis of decreasing rates of the spot intensity at various conditions of beam current, beam energy, and substrate temperature leads to a proposal for a mechanism of the disordering in the buckled-dimer arrangement in terms of electronic excitation, electron-phonon coupling, and carrier concentration.     

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.     

Dispersion of incoherent spectral features in systems with strong electron-phonon coupling

We study (inverse) photoemission from systems with strong coupling of doped carriers to phonons. Using an adiabatic approximation, we develop a method for calculating spectra. This method is particularly simple for systems where the electron-phonon coupling can be neglected in the initial state, e.g., the undoped t-J model. The theory then naturally explains why the electron-phonon coupling just leads to a broadening of spectra calculated without electron-phonon coupling, without changing the dispersion. This is in agreement with recent angle-resolved photoemission spectroscopy (ARPES) on undoped cuprates, and it supports the interpretation in terms of strong electron-phonon interaction. The theory also shows that for systems with strong electron-phonon coupling in the initial state, the result cannot in general be related to the spectrum obtained without electron-phonon coupling.     

Fermi surface and anisotropic spin-orbit coupling of Sb(111) studied by angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterizing the surface electronic states of group-V semimetals. The universality or dissimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge or spin density wave.     

Physical properties of the noncentrosymmetric superconductor Mg10Ir19B16

Specific heat, electrical resistivity, and magnetic susceptibility measurements on a high quality sample of Mg10Ir19B16 provide a self-consistent determination of its superconducting properties. They indicate that Mg10Ir19B16 is a type-II superconductor [T{C}=4.45 K, kappa(0) approximately 20], with an electron-phonon coupling constant lambda{ep}=0.66. An analysis of the T-dependent specific heat shows that superconducting properties are dominated by an s-wave gap (Delta=0.7 meV). Point contact tunneling data provide evidence for multiple superconducting gaps, as expected from strong asymmetric spin-orbit coupling.     

Search for E(2g) phonon modes in MgB2 single crystals by point-contact spectroscopy

The electron-phonon interaction in magnesium diboride MgB2 single crystals is investigated by point-contact (PC) spectroscopy. For the first time the electron coupling with E(2g) phonon modes is resolved in the PC spectra. The correlation between intensity of the extremely broad E(2g) modes in the PC spectra and value of the superconducting gap is established. Our observations favor current theoretical models for electron-phonon mediated superconductivity in MgB2, and they better match the harmonic phonon model.     

On a rudimentary model of the W(001) surface instability and reconstruction

A simple qualitative theory of the reconstruction of the W (001) surface based on the assumption of an important role of Shockley surface states from the vicinity of the Fermi energy in the electron-phonon interaction is presented. While translational symmetry chooses state coupling via the pseudo-Jahn-Teller effect, the local symmetry determines the magnitude of the corresponding matrix elements. It appears that the in-plane reconstruction modes M?₅ and X?₃ are more promising whereas the buckled modes seem unfavourable since they lead to an ionic superstructure on the surface.     

Quantum dephasing in carbon nanotubes due to electron-phonon coupling

We report on the effect of electron-phonon coupling on quantum transport in carbon nanotubes. The vibrational atomic displacements as well as the electron-phonon coupling strength are introduced through a time-dependent perturbation of the pi-electron Hamiltonian. The effect of dephasing on the Kubo conductance is studied for metallic and semiconducting nanotubes, and from a phenomenological law, coherence length (time) scales are found to fluctuate within the range 10 to 150 nm (0.01 to 4 ps) depending on the energy of charge carriers and phonon amplitude.     

Anomalous dispersion of longitudinal optical phonons in Nd(1.86)Ce(0.14)CuO(4+delta) determined by inelastic x-ray scattering

The phonon dispersions of Nd(1.86)Ce(0.14)CuO(4+delta) along the [xi,0,0] direction have been determined by inelastic x-ray scattering. Compared to the undoped parent compound, the two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are shifted to lower energies. Moreover, an anomalous softening of the bond-stretching band is observed at about q = (0.2,0,0). These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor.