Abstract:
We report first principles calculations of the phonon dispersions of PbTe both for its observed structure and under compression. At the experimental lattice parameter we find a near instability of the optic branch at the zone center, in accord with experimental observations.This hardens quickly towards the zone boundary. There is also a very strong volume dependence of this mode, which is rapidly driven away from an instability by compression. These results are discussed inrelation to the thermal conductivity of the material.

Abstract:
We demonstrate how to overcome serious problems in understanding and classification of vibration spectra in semiconductor alloys, following from traditional use of the virtual crystal approximation (VCA). We show that such different systems as InGaAs (1-bond->1-mode behavior), InGaP (modified 2-mode) and ZnTeSe (2-bond->1-mode) obey in fact the same phonon mode behavior - hence probably a universal one - of a percolation-type (1-bond->2-mode). The change of paradigm from the `VCA insight' (an averaged microscopic one) to the `percolation insight' (a mesoscopic one) offers a promising link towards the understanding of alloy disorder. The discussion is supported by ab initio simulation of the phonon density of states at the zone-center of representative supercells at intermediary composition (ZnTeSe) and at the impurity-dilute limits (all systems). In particular, we propose a simple ab initio `protocol' to estimate the basic input parameters of our semi-empirical `percolation' model for the calculation of the 1-bond->2-mode vibration spectra of zincblende alloys. With this, the model turns self-sufficient.

Abstract:
in this work, we have calculated ab initio the phonon dynamics of the ga-rich gaas (100) (1 × 1), gaas (100) (2 × 1), gan (100) (1 × 1), gaas (110) and gan (110) surfaces. our results for the (100) surfaces show that, while the (2 × 1) reconstruction is characterized by dimer vibrations, the (1 × 1) reconstruction presents interesting features which is closely related to the reduced mass of the compound. all the studied cases show the presence of a rayleigh mode. the calculated properties for the (110) surfaces agree very well with the available experimental data from heels and inelastic he-atom scattering as well as with other theoretical calculations.

Abstract:
in this work, we presented our preliminary ab initio results for the vibrational modes and the phonon frequencies of the sic (100) surfaces. our results are in good agreement with the available experimental data whenever this comparison is possible. for the accepted models of the c-terminated surfaces in the c(2×2) reconstruction, while in the bridge-dimer model there is an acetylene-like vibrational a1 mode at 2031 cm-1, which is infrared active, in the staggered-dimer model, there is a füchs-kliewer (fk) mode at 1328 cm-1, which is experimentally detected. for si-terminated surfaces in the p(2×1) reconstruction, instead, no fk was obtained, in contradiction with the hreels experimental results for the si-terminated surfaces, but they are in consonance with the fact that this surface should be described by a (3×2) or more complex models.

Abstract:
We present a comprehensive ab initio study of structural, electronic, lattice dynamical and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. As calculations are carried out in a periodic slab geometry, special attention is given to the convergence with respect to the slab thickness. Although the electronic structure of Bi(111) thin films varies significantly with thickness, we found that the lattice dynamics of Bi(111) is quite robust and appears converged already for slabs as thin as 6 bilayers. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in super-bulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. Electronic states of the surface band related to the outer part of the hole Fermi surfaces exhibit a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. States at the inner part of the hole surface as well as those forming the electron pocket close to the zone center show much increased couplings due to transitions into bulk projected states near Gamma_bar. For these cases, the state dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-like spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail.

Abstract:
The relation between ab initio molecular dynamics formalism and the electron-phonon interaction formalism [P.B. Allen and V. Heine, J. Phys. C 9, 2305 (1976)] is explored. The fundamental quantity obtained in the AIMD formalism - total energy for any configuration - is also obtained from the formalism (ES-DWF) that incorporates the role of Debye-Waller Factor in electronic structure calculations. The two formalisms are exactly equivalent and represent the direct and perturbation theory approaches to determine total energy. This equivalence allows either formalism to be used depending on the requirement - ES-DWF for a priori theoretical analysis and AIMD for ab initio modeling of the effect of thermal vibrations. Combining the two formalisms makes the ES-DWF formalism into an ab initio method and increases the range of problems that can be modeled ab initio. It is also theoretically possible to obtain self-consistent band structures from AIMD calculations. This study clarifies the incorrect assumptions regarding the two formalisms that exist in published literature. By combining the two formalisms and including self-energy effects, more accurate results can be obtained, ab initio, within the adiabatic approximation, than by using AIMD alone.

Abstract:
With a correlation of nonequilibrium carriers relaxation and coherent phonons displacive excitation, the coherent optical phonon oscillations in YBa2Cu3O7-δ thin films excited by femtosecond laser pulse are simulated theoretically. It is revealed that as the oxygen concentration decreases, the coherent phonon oscillations become easier to be observed due to the decrease of the local coupling between the carriers and the lattice vibrations in the CuO2 plane.

Abstract:
Ab initio calculations of optical-phonon deformation potentials (ODP's), i.e., d0, d30, d10 (val) and d10 (con) for sixteen semiconductors were carried out systematically. The calculations are based on the LMTO-ASA band-structure method within the framework of the frozen-phonon approximation model, in which the displacement of empty sphere is considered to match its atomic sphere partners, We have compared the d0 values obtained by several different theoretical calculation methods and studied the main factors affecting them. It is pointed out that the two different models (rhombohedral strain model and frozen-phonon model) for calculations will lead to different results of d0.

Abstract:
We formulate an ab initio downfolding scheme for electron-phonon coupled systems. In this scheme, we calculate partially renormalized phonon frequencies and electron-phonon coupling, which include the screening effects of high-energy electrons, to construct a realistic Hamiltonian consisting of low-energy electron and phonon degrees of freedom. We show that our scheme, which we call constrained density-functional perturbation theory (cDFPT), can be implemented by slightly modifying the conventional DFPT, which is one of the standard methods to calculate phonon properties from first principles. Our scheme can be applied to various phonon-related problems, such as superconductivity, electron and thermal transport, thermoelectricity, piezoelectricity, dielectricity and multiferroicity. We believe that the cDFPT provides a firm basis for the understanding of the role of phonons in strongly correlated materials. Here, we apply the scheme to the fullerene superconductors and discuss how the realistic low-energy Hamiltonian is constructed.

Abstract:
We propose an electron-phonon parameterization which reliably reproduces the geometry and harmonic frequencies of a real system. With respect to standard electron-phonon models, it adds a "double-counting" correction, which takes into account the lattice deformation as the system is dressed by low-energy electron-phonon processes. We show the importance of this correction by studying potassium-doped picene (K$_3$Picene), recently claimed to be a superconductor with a $T_c$ of up to 18 K. The Hamiltonian parameters are derived from ab-initio density functional theory, and the lattice model is solved by dynamical mean-field theory. Our calculations include the effects of electron-electron interactions and local electron-phonon couplings. Even with the inclusion of a strongly coupled molecular phonon, the Hubbard repulsion prevails and the system is an insulator with a small Mott gap of $\approx$ 0.2 eV.