Velocity renormalization and carrier lifetime in graphene from the electron-phonon interaction

We present a first-principles investigation of the phonon-induced electron self-energy in graphene. The energy dependence of the self-energy reflects the peculiar linear band structure of graphene and deviates substantially from the usual metallic behavior. The effective band velocity of the Dirac fermions is found to be reduced by 4%-8%, depending on doping, by the interaction with lattice vibrations. Our results are consistent with the observed linear dependence of the electronic linewidth on the binding energy in photoemission spectra.     

Abnormal neutron star matter at ultrahigh densities

We consider three models, based on the mean field and sigma models, all of which fit the saturation properties and the symmetry energy of nuclear matter. None of the models yields an abnormal state of neutron star matter at supemuclear density even when pion condensation is taken into account.     

Intranuclear cascading at ultrahigh energy in heavy-ion interactions

Intranuclear cascading mechanism one of the important non-linear effects in high energy nucleusnucleus collisions is investigated. The data on multiplicity (n _s ) and pseudorapidity (η) distributions of shower particles produced by³²S and¹⁶O at 200A GeV,¹⁶O at 60A GeV,²⁸Si at 14.5A GeV and He at ≈140A GeV are presented and compared with the string model VENUS, which takes into account the cascade interactions of secondary particles. The effect of the intranuclear collisions on the distributions of <η> versus <n _s > is discussed for all the beams.     

Phonons and electron-phonon interactions in rare-gas crystals at high pressures

The lattice dynamics of compressed rare-gas crystals is theoretically investigated within the ab initio approach in the framework of the Tolpygo model, which explicitly allows for the deformation of electron shells. The deformation of the electron shells is associated with the retardation of the electron response and treated as a nonadiabaticity (the electron-phonon interaction). This approach and the ab initio short-range repulsive potentials are used to construct the dynamic matrix, which makes it possible to calculate the phonon frequencies and the electron-phonon interaction of crystals in the series Ne-Xe at any point of the Brillouin zone. The contributions of the long-range Coulomb and van der Waals forces to the dynamic matrix are the structure sums that depend only on the lattice type. The structure sums for the face-centered cubic lattice are calculated using the Ewald and Emersleben methods, as well as the direct summation over the vectors of the face-centered cubic lattice. The use of 20 spheres in the last case provides an accuracy of no less than four significant figures. An analysis of the role played by the phonon-electron interaction at five points of high symmetry in the Brillouin zone (X, L, U, K, W) at high pressures demonstrates that not only the longitudinal phonon modes (at the points X and L) but also the transverse phonon modes (at the points U, K, and W) are softened. The inclusion of the electron-phonon interaction at the point X improves agreement between the theoretical and experimental phonon frequencies for the argon crystal.     

Carrier dynamics in GaN at extremely low excited carrier densities

Carrier dynamics in GaN was studied using fluorescence lifetime measurement in the frequency domain technique in the temperature range from 8 to 300 K at very low and very high excitation levels. The study was performed in a high-quality GaN epilayer exhibiting a room-temperature nonequilibrium carrier lifetime of 2 ns, which was determined by a light-induced transient grating (four-wave mixing) technique. The results reveal the roles of donor–acceptor pair recombination and conduction band–acceptor recombination in yellow luminescence band formation.     

Effect of electron-phonon interactions on Raman line at ferromagnetic ordering

The theory of Raman scattering in half-metals by optical phonons interacting with conduction electrons is developed. We evaluate the effect of electron-phonon interactions at ferromagnetic ordering in terms of the Boltzmann equation for carriers. The chemical potential is found to decrease as the temperature decreases. Both the linewidth and frequency shift exhibit a dependence on temperature.     

Flow equations for electron-phonon interactions

A recently proposed method of continuous unitary transformations is used to decouple the interaction between electrons and phonons. The differential equations for the couplings represent an infinitesimal formulation of a sequence of Fröhlich transformations. The two approaches are compared. Our result will turn out to be less singular than Fröhlich's. Furthermore the interaction between electrons belonging to a Cooper pair will always be attractive in our approach. Even in the case where Fröhlich's transformation is not defined (Fröhlich actually excluded these regions from the transformation), we obtain an elimination of the electron-phonon interaction. This is due to a sufficiently slow change of the phonon energies as a function of the flow parameter.     

Anisotropy of electron-phonon interactions in alkali metals

Electron-phonon scattering in the solid alkalis is distinguished from that in most other metals by a combination of three circumstances: The phonon spectra and structure factors are very anisotropic, the Fermi surface in the reduced zone is simply connected and virtually spherical and important large momentum transfers (0.7<(q/2k _F)<1.0) fall within the first large peaks of the phonon structure factors. Anisotropy of microscopic contributions to the macroscopic coefficients is controlled by and is quite sensitive to values of electron-ion matrix elements at large momentum transfer, and can be explored by a realistic yet relatively simple theoretical calculation. A brief summary is presented of such calculations, for the all alkalies, of mean free paths, thermoelectric powers, and electron-phonon mass enhancements. The results show marked anisotropy only for lithium, are consonant with experimental low field Hall coefficients and in addition indicate strong anisotropy in the mass enhancement for lithium.     

Absorption edge shift in ZnO thin films at high carrier densities

The optical absorption edge has been measured as a function of carrier concentration for thin films of zinc oxide prepared by organometallic chemical vapour deposition and reactive R.F. magnetron sputtering. Large shifts of the absorption edge have been observed which are only a function of the carrier concentration. Below n = 3 × 10¹⁹ cm^-3 the shifts are well described by the Burstein-Moss model. For carrier concentrations between 3–5 ×10¹⁹cm^-3, the shift decreases very rapidly, finally increasing again with further increases in the carrier density. These effects are consistent with a merging of the donor band with the conduction band following a semiconductor-metal transition.     

Electric field effect tuning of electron-phonon coupling in graphene

Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.     

Infrared reflectivity spectroscopy of electron-phonon interactions

The crucial role of electron-phonon interactions and their signatures in optical conductivity deduced from the analysis of infrared-visible reflection spectra is reviewed. The usefulness of a general phenomenological model able to describe the couplings of phonons, polarons, plasmon, superconducting condensate, is discussed. Among other examples of applications, the problem of the signature of Cooper pair condensation in the superconducting phase of high-T _c cuprates is discussed in the framework of London and Mattis-Bardeen models.     

Magneto-transport properties of silicon inversion layers at low carrier densities

Conductivity σ and magneto-resistance of n- and p-channel MOSFET's with low oxide-charge density were investigated between 1.2 K and 4.2 K and carrier concentrations between 10¹¹/cm² and 10¹²/cm². Data were taken at different source drain electric field strengths. At small source drain fields and low carrier concentrations σ showed an activated behaviour with the activation energy decreasing with increasing carrier concentration. At source drain fields of the order 1 V/cm pronounced deviations from the activated behaviour were observed. In addition, the channel current-voltage characteristics of the devices studied were nonlinear. Shubnikov-De Haas oscillations were recorded at surface carrier concentrations as low as 10¹¹/cm². The data cannot be explained with a hopping model or by the existence of a mobility edge.     

Kohn anomalies and electron-phonon interactions in graphite

We demonstrate that graphite phonon dispersions have two Kohn anomalies at the Gamma-E(2g) and K-A'1 modes. The anomalies are revealed by two sharp kinks. By an exact analytic derivation, we show that the slope of these kinks is proportional to the square of the electron-phonon coupling (EPC). Thus, we can directly measure the EPC from the experimental dispersions. The Gamma-E(2g) and K-A'1 EPCs are particularly large, while they are negligible for all the other modes at Gamma and K.     

One-electron energies in the presence of electron-phonon interactions

Decoupling of an electron-ion interacting system is considered and the pseudopotential method of Phillips and Kleinman is extended to include the effect of electron-phonon interactions. It is shown that the effective potential experienced by a decoupled electron is the instantaneous pseudopotential averaged over all possible phonon states. The averaged pseudopotential is not only periodic but also temperature-dependent. After decoupling, a simple procedure for calculating single-electron excitation energies resulting from the coupling term is given and the result is formally in agreement with that obtained by Green's function method.     

Interplay of electron-electron and electron-phonon interactions in molecular junctions

In this work we consider a current carrying molecular junction with both electron-phonon and electron-electron interactions taken into account. After performing Lang-Firsov transformation and considering Markov approximations in accordance to weak coupling to the electronic leads, we obtain the master equation governing the time evolution of the reduced density matrix of the junction. The steady state of the density matrix can be used to obtain I-V characteristic of the junction in several regimes of strengths of the interactions. Our results indicate that the system can show negative differential conductance (that is, the current decreases by increasing the applied bias voltage) in some regimes as an interplay between the electron-phonon and Coulomb interactions.