Results 1 - 10 of 25079
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[en] In this work we report the results of theoretical analysis of the effect of the thermal environment on the thermoelectric efficiency of molecular junctions. The environment is represented by two thermal phonon baths associated with the electrodes, which are kept at different temperatures. The analysis is carried out using the Buttiker model within the scattering matrix formalism to compute electron transmission through the system. This approach is further developed so that the dephasing parameters are expressed in terms of relevant energies, including the thermal energy, strengths of coupling between the molecular bridge and the electrodes and characteristic energies of electron–phonon interactions. It is shown that the latter significantly affect thermoelectric efficiency by destroying the coherency of electron transport through the considered system. (paper)
[en] Full text: Conned optical phonons are studied in the case of a spherical Quantum-Dot/Quantum-Well (Q D/Q W) heterostructures in the framework of a phenomenological approach where the mechanical and the electrostatic matching boundary conditions are fulfilled at the Q D/Q W interfaces. The prototypical case used as example is a Q D/Q W formed by CdS/Hg S where the spherical shell of Hg S is sandwiched by an exterior spherical core of CdS. Over this structure we have used a host material considered as an infinite dielectric medium which will not participate of the polar optical vibrations. We are studying the normal modes for conned and for interface phonons, their frequencies dependence on the geometrical and on the material parameters. In this work, we are also showing the electron-phonon interaction Hamiltonian and discussing possible applications for this new capped Q D-Q W semiconductor heterostructure. (author)
[en] The quasiparticle-phonon model is adopted to investigate the microscopic structure of some low-lying states (known as mixed-symmetry states) recently discovered in nuclei around closed shells. The study determines quantitatively the phonon content of these states and shows that their main properties are determined by a subtle competition between particle-particle and particle-hole quadrupole interactions and by the interplay between orbital and spin-flip motion.
[en] Using the Eliashberg theory of superconductivity we have examined several properties of a model in which electrons are coupled only to rattling phonon modes represented by a sharp peak in the electron-phonon coupling function. Our choice of parameters was guided by experiments on β-pyrochlore oxide superconductor KOs2Os6. We have calculated the temperature dependence of the superconducting gap edge; the quasi-particle decay rate; the NMR relaxation rate assuming that the coupling between the nuclear spins and the conduction electrons is via a contact hyperfine interaction, which would be appropriate for the O-site in KOs2Os6; and the microwave conductivity. We examined the limit of very strong coupling by considering three values of the electron-phonon coupling parameter λ = 2.38, 3, and 5 and did not assume that the rattler frequency Ω0 is temperature dependent in the superconducting state. We obtained a very unusual temperature dependence of the superconducting gap edge Δ(T), very much like the one extracted from photoemission experiments on KOs2O6. (author)
[en] The coupling of elementary excitation modes in hot nuclei is studied. For this aim the quasiparticle-phonon nuclear model (QPM) is extended to a finite temperature by using the formalism of the thermo field dynamics. First the energies and structures of one-phonon states are calculated in the thermal random phase approximation and then the thermal QPM Hamiltonian HQPM is expressed in terms of thermal quasiparticles and thermal RPA-phonons. The equation for the energies taking into account mixing of one-and two-thermal phonon states is derived. The expression of the coupling matrix element between thermal phonons is given. 15 refs. (author)
[en] This paper reports on the spectroscopic study of YbVO4 in the range 10-300 K. All the Raman phonons were properly assigned. Temperature-dependent Raman spectra showed no anomalous changes, which indicate that neither structural phase transition nor strong electron-phonon coupling occur, at least within this temperature range. This result is sharp contrast to those of YbPO4 where strong mixing of the upper crystal-field states Γ6 and Γ7 with an Eg3 phonon at room-temperature occurs. A possible explanation for the absence of electron-phonon coupling in YbVO4 is the occurrence of the Γ6 state at energy higher than the Eg3 phonon, unlike the coincidence of Γ6 and Eg3 energies in YbPO4.
[en] The Green's-function method has been used to obtain a general equation for the effective field in a nucleus, taking into account both 1p1h and 2p2h configurations. This equation has been used as the starting point for derivation of a previously developed microscopic model of taking 1p1h+phonon configurations into account in magic nuclei. The equation for the density matrix is analyzed in this model. It is shown that the number of quasiparticles is conserved. An equation is obtained for the effective field in the coordinate representation, which provides a formulation of the problem in the 1p1h+2p2h+continuum approximation. The equation is derived and quantitatively analyzed in the space of one-phonon states
[en] Full text: Recently, many publications have appeared where the main aim of theoretical investigations into optical phonon confinement has been to reduce the scattering rate with electrons in a quantum well. It is mainly optical phonon scattering that limits the electron mobility at room temperature and hence any structure that can change the form of the phonon potential to give a reduced rate is advantageous. One of the main suggestions to produce this reduction is the inclusion one or more monolayers of a different material inside the quantum well. Neglecting the effect to the electron wavefunctions, two models of optical phonon confinement have been used to suggest that the phonon modes are changed considerably and hence a reduction should be expected. The model most employed for phonon confinement is the dielectric continuum (DC) model. This model neglects the mechanical nature connecting the phonons of the different materials across the interface and can be expressed with just the electrostatic potential in the non-retarded limit. The boundary conditions used are those of electromagnetic theory, i.e. normal D and tangential E fields continuous. The DC model has been used by to obtain a reduction in the ground state intra subband scattering rate by including a monolayer in the quantum well. This reduction is quite significant and is against a sum rule which states that the results should be between the scattering rates via the bulk phonons of each material. Another model, the hybrid model, takes into account the mechanical nature of the phonons at the interfaces and includes a condition on the phonon displacement as well as the electromagnetic boundary conditions. The hybrid model was used to suggest that a reduction in the inter subband scattering rate should occur for a quantum well with a monolayer. This is because the antisymmetric modes of the quantum well are changed drastically when imposing the additional mechanical boundary conditions at the interfaces between the monolayer and the well material. In this report we have extended the calculation for a barrier of finite thickness in the centre of the well. We have obtained the intra subband and inter subband scattering rates and have shown that the effect on the total scattering rates is not marginal. (author)
[en] We report first principles calculations of the electronic structure, phonon dispersions and electron phonon coupling of LaNiPO. These calculations show that this material can be explained as a conventional electron phonon superconductor in contrast to the FeAs based high temperature superconductors.