Electron-phonon Coupling Properties Research Articles

Temperature dependence of the dielectric function in the spectral range (0.5–8.5) eV of an In2O3 thin film

We present the dielectric function of a bcc-In2O3 thin film in the wide spectral range from near-infrared to vacuum-ultraviolet and for temperatures 10 K–300 K, determined by spectroscopic ellipsometry. From the temperature dependence of electronic transition energies, we derive electron-phonon coupling properties and found hints that the direct parabolic band-band transitions involve In-d states. Further we discuss possible excitonic contributions to the dielectric function.

Comparing electron-phonon coupling strength in diamond, silicon, and silicon carbide: First-principles study

Renormalization of the electronic band gap due to electron-phonon coupling in the tetrahedral semiconductors diamond, silicon and cubic silicon carbide is studied from first principles. There is a marked difference between the coupling of the vibrational state to the valence band maximum and to the conduction band minimum. The strength of phonon coupling to the valence band maximum is similar between the three systems and is dominated by vibrations that change the bond length. The coupling strength to the conduction band minimum differs significantly in diamond, silicon carbide and silicon. In diamond, the coupling is dominated by six small pockets of vibrational states in the phonon Brillouin zone, that are ultimately responsible for the stronger electron-phonon coupling in this material. Our results represent a first step towards the development of an a priori understanding of electron-phonon coupling in semiconductors and insulators, that should aid the design of materials with tailored electron-phonon coupling properties.

Energy transfer and electron–phonon coupling properties in Gd2(WO4)3:Eu phosphor

In this paper, Gd2(WO4)3:Eu phosphors was synthesized through co-precipitation which is a simple and low-costing method. The crystal structure and morphology of the phosphors were characterized by means of XRD and field emission scanning electron microscope. The emission spectra and excitation spectra (phonon sideband spectra) of every sample were measured. The Huang–Rhys factor was derived from the phonon sideband spectra. The energy transfer type that gives rise to concentration quenching was measured, and critical energy transfer distance (Rc) was calculated. The intrinsic lifetime for 5D0 level of Eu3+ and the phonon number generated in the self-quenching process was obtained based on Auzel’s model. Finally, the energy transfer rate between Eu3+ ions was derived on the basis of the fluorescence lifetime.

Electron-phonon coupling and electronic properties of electron-doped graphene

In graphene, a kink was observed at energies of 200 meV below the Fermi level ${\ensuremath{\epsilon}}_{F}$ by angle-resolved photoemission spectroscopy (ARPES). It was claimed that the kink structure can be explained by the electron-phonon interaction [Phys. Rev. Lett. 99, 236802 (2007)], but their theoretical results were not consistent with the ARPES data as well as the first-principles results. We revisit this approach and find a different quasiparticle dispersion which agrees with the ARPES data and the first-principle calculations.

Aluminum Conducts Better than Copper at the Atomic Scale: A First-Principles Study of Metallic Atomic Wires

Using a first-principles density functional method, we have studied the electronic structure, electron-phonon coupling, and quantum transport properties of atomic wires of Ag, Al, Au, and Cu. Non-equilibrium Green's function-based transport studies of finite atomic wires suggest that the conductivity of Al atomic wires is higher than that of Ag, Au, and Cu in contrast to the bulk where Al has the lowest conductivity among these systems. This is attributed to the higher number of eigenchannels in Al wires, which becomes the determining factor in the ballistic limit. On the basis of density functional perturbation theory, we find that the electron-phonon coupling constant of the Al atomic wire is lowest among the four metals studied, and more importantly, that the value is reduced by a factor of 50 compared to the bulk.

Spin-resolved electron-phonon coupling in FeSe and KFe2Se2

The effect of the static magnetic moments of iron on electron-phonon interactions in layered FeSe and KFe${}_{2}$Se${}_{2}$ is studied. First-principles techniques based on the pseudopotential density functional approach and the local spin density approximation are utilized to calculate the band structures, phonon dispersions, and electron-phonon coupling properties. Our results indicate that the introduction of iron magnetic moments leads to significant changes in electronic structure induced by Fe 3$d$ states near the Fermi level, to phonon frequency softening for several vibrational modes, and to a dramatic increase in electron-phonon coupling for specific modes. The increase in Brillouin-zone-averaged coupling is about twofold. Our estimates of superconducting transition temperatures based on the McMillan equation yield values closer to experimental results for the spin-resolved case. However, these values are not large enough to explain the observed transition temperature.

Polar optical phonon states and their electron-phonon coupling properties in a wurtzite nitride quantum dot

Within the framework of the macroscopic dielectric continuum model, the interface-optical-propagating (IO-PR) mixing phonon modes of a quasi-zero-dimensional (Q0D) wurtzite cylindrical quantum dot (QD) structure are derived and studied. The approximative analytical-phonon-states of IO-PR mixing modes are given. It is found that there are two types of IO-PR mixing phonon modes, i.e. ρ-IO/z-PR mixing modes and the z-IO/ρ-PR mixing modes existing in Q0D wurtzite QDs. And each IO-PR mixing modes also have symmetrical and antisymmetrical forms. Via a standard procedure of field quantization, the Frohlich Hamiltonians of electron-(IO-PR) mixing phonons interaction are obtained. And the orthogonal relations of polarization eigenvectors for these IO-PR mixing modes are also deduced. Numerical calculations of dispersive relation and electron-phonon coupling properties on a wurtzite GaN cylindrical QD are carried out. The behaviors that the IO-PR mixing phonon modes in wurtzite QDs reduce to the IO modes and PR modes in wurtzite QW and QWR systems are analyzed deeply from both of the viewpoints of physics and mathematics. The result shows that the present theories of polar mixing phonon modes in wurtzite cylindrical QDs are consistent with the phonon modes theories in wurtzite QWs and QWR systems. The coupling properties of electron-(IO-PR) mixing modes interactions are studied and analyzed in detail. An abnormal increase of electron-phonon coupling strength are observed as the azimuthal quantum numbers and order of phonon modes increase, which is ascribed to the modulation effect of different dielectric functions of wurtzite crystals in radius- and axial-directions. The analytical electron-phonon interaction Hamiltonians obtained here are useful for further investigating phonon influence on optoelectronics properties of wurtzite Q0D QD structures.

Electron–phonon coupling properties inMgB2 observed by Raman scattering

The influence of sintering temperature on the critical transition temperature,Tc,for MgB2 superconductor was investigated systematically with the aid of room temperatureRaman scattering measurements and Raman spectral fit analysis. The Ramanspectra for all samples can be fitted with one phonon peak coming from theE2g modeat the Γ point of the Brillouin zone and two peaks coming from sampling ofthe phonon density of states (PDOS) due to disorder. The enhancedE2g mode in the Raman spectra with increasing sintering temperatureshows gradual strengthening of the electron–phonon coupling (EPC) inMgB2, which is thereason why the Tc of samples increases with increasing sintering temperature. The strength ofelectron–E2g coupling isestimated to be about 2.0 ± 0.5, which is larger than the average strength of the coupling of electrons with all the phonon modes,∼1.23.The Tc dependence on the profiles of the PDOS peaks is described using the variation of the peaks’frequencies and linewidths in different samples.

Polaronic self-trapped energy and effective mass of a cylindrical wurtzite GaN nanowire

The ground-state self-trapped energy and effective mass of the polaron in a freestanding wurtzite GaN nanowire (NW) are studied by employing the perturbation approach. Based on the dielectric continuum model and the Loudon uniaxial crystal model, the full polar optical phonon modes, i.e. quasi-confined (QC) phonon modes and surface optical (SO) phonon modes, in the one-dimensional (1D) wurtzite systems have been analyzed. The vibrating spectra of the two types of phonon modes and their electron–phonon coupling properties have been discussed and analyzed. The numerical results on the ground-state polaron self-trapped energy and correction of effective mass in the 1D GaN NWs reveal that these are far larger than those in 1D GaAs NW systems. The reasons resulting in this obvious difference in the two 1D structures can be attributed to the different electron–phonon coupling constants and electron effective masses of bulk materials constituted of the two types of 1D confined systems. Moreover, our calculated results regarding the polaronic self-trapped energy are consistent with the recent experimental observation in GaN NW systems.

Vibration properties of propagating and half-space optical phonons in a wurtzite Q1D GaN-based quantum wire

Abstract By employing the dielectric continuum and Loundon’s uniaxial crystal models, the propagating (PR) and half-space (HS) optical phonon modes and the corresponding Frohlich electron–phonon interaction Hamiltonians in a quasi-one-dimensionality (Q1D) wurtzite quantum wire (QWR) are derived and studied. Numerical calculations on a wurtzite GaN/Al 0.15 Ga 0.85 N QWR are performed, and the discussions are mainly focused on the dependent relationships of the dispersion and electron–phonon coupling properties of PR and HS modes on the free wave-number k z in z -direction and the azimuthal quantum number m . The calculated results show that there usually exist infinite branches of PR modes in the wurtzite QWR systems for given k z and m . The total branch number of the PR modes will decrease as k z increases. The dispersive frequencies of the HS modes fall into two frequency ranges, i.e. a high-frequency range and a low-frequency range. For certain k z and m , there also are infinite branches of HS modes in the high-frequency range, and only finite branches of HS modes could exist in the low-frequency range. The behavior that the 0H HS mode reduces to the IO modes for a relatively large k z has been obviously observed. Once the frequency of the low-frequency HS modes is lower than the characteristic frequency ω t T , GaN , all the low-frequency HS phonon branches i L will respectively reduce to the corresponding PR i optical phonon branches. Moreover, the dispersive properties of the PR and HS modes in Q1D QWR and Q2D quantum well structures have been compared in detail. The profound physical reasons resulting in these features have also been analyzed in depth. At last, the coupling properties between an electron with PR and HS phonon modes are discussed in detail.

SIZE AND DIELECTRIC DEPENDENCE OF INTERFACE AND SURFACE OPTICAL PHONONS SPECTRA IN WURTZITE GaN/AlN QUANTUM WELL WIRES

By employing the method of electrostatic potential expansion, the interface optical (IO) and surface optical (SO) phonon modes and the corresponding Fröhlich-like electron-phonon interaction Hamiltonian in a Q1D wurtzite cylindrical quantum well wire (QWW) embedded in nonpolar dielectric matrix are derived and studied based on the dielectric continuum model and Loudon's uniaxial crystal model. Numerical calculations for a wurtzite GaN/AlN QWW are mainly focused on the size- and dielectric-dependent IO and SO phonon spectra and electron-IO (SO) phonons coupling functions. Results reveal that, in general, there are two branches of IO phonon modes and one branch of SO mode in the system. The dispersions of the IO and SO modes are obvious only when the radii ratio β and the dielectric constant of nonpolar matrix ∊d is small. The limiting frequencies of IO and SO modes for very large β have been analyzed in depth from both physical and mathematical viewpoints. The reducing behaviors of some modes have been clearly observed. Via the discussion of electrostatic potential spacial distributions of the IO and SO modes, we find that the QWW structures and dielectric constants of nonpolar matrix have little influence on the low-frequency IO mode, but they can greatly affect the potential distributions of high-frequency IO and SO modes. Detailed comparison of the dispersion behaviors of the modes and electron-phonon coupling properties in the Q1D wurtzite QWWs with those in wurtzite QWs and cubic quantum dots has also been made. Furthermore, part of the theoretical results derived in the present paper is consistent with the relatively experimental conclusion.

Impurity dimers in superconducting B-doped diamond: Experiment and first-principles calculations

On the basis of first-principles calculations and experimental secondary ion mass spectroscopy, electrical transport, and Raman measurements performed on homoepitaxial diamond, we find that at high concentrations boron atoms tend to segregate in dimers. The study of the electronic, vibrational, and electron-phonon coupling properties, corroborated by Raman measurements, shows that boron dimers may be associated to the broad Raman peak around $500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ as well as to some of the gap states which have been reported in the literature, and that they are both electrically and vibrationally inactive with respect to the electron-phonon coupling driving the superconducting transition in metallic diamond. These results bear important consequences on the evolution of the critical temperature with the impurity concentration in B-doped diamond.

Theoretical investigations of material modification using temporally shaped femtosecond laser pulses

We present a two-dimensional model, based on a drift–diffusion approach, developed to describe the dynamics of electronic excitation and lattice heating in several dielectric materials with different electron–phonon coupling properties (e.g. fused silica and sapphire) under the action of femtosecond near-infrared laser pulse trains with variable separation time between pulses. The modeling approach was aimed to describe the mechanisms that enable the spatial modulation of the structures induced by temporally modulated laser excitation and ablation of wide-band-gap dielectric materials. The possible geometric contours of the laser-induced craters on the target surfaces are discussed on the basis of the lattice-temperature profiles obtained by modeling. It was found that the observed difference in the crater shapes generated in fused silica and sapphire is conditioned by the difference in dynamics of electron excitation and recombination channels characteristic of these two materials. This effect can be used to convert a given temporal pulse modulation into spatial modulation, opening up new perspectives for material processing in order to obtain desired structure profiles.

Effect of nonadiabaticity on the Fermi-edge photoemission and tunneling spectra of superconductors

Temperature dependence of the experimental photoemission spectra of the high-Tc superconductors have been interpreted and simulated by the recently formulated molecular nonadiabatic theory of the electron-vibrational interaction. The change of the spectral feature below Tc, particularly formation of the narrow, near Fermi surface (FS) edge, peak separated by the dip from a broad spectral band at higher binding energies has been shown to be the consequence of the nonadiabatic electron-phonon coupling and corresponding properties of the nonadiabatic polarons. Simulation yields very good fit to the experimental temperature-dependent spectra, with the calculated value of the gap 30.5 meV. © 1996 John Wiley & Sons, Inc.