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1T−TiSe2 is one of the most studied charge density wave (CDW) systems, not only because of its peculiar properties related to the CDW transition, but also due to its status as a promising candidate of exciton insulator signaled by the proposed plasmon softening at the CDW wave vector. Using high-resolution electron energy loss spectroscopy, we report a systematic study of the temperature-dependent plasmon behaviors of 1T−TiSe2. We unambiguously resolve the plasmon from phonon modes, revealing the existence of Landau damping to the plasmon at finite momentums, which does not support the plasmon softening picture for exciton condensation. Moreover, we discover that the plasmon lifetime at zero momentum responds dramatically to the band gap evolution associated with the CDW transition. The interband transitions near the Fermi energy in the normal phase are demonstrated to serve as a strong damping channel of plasmons, while such a channel in the CDW phase is suppressed due to the CDW gap opening, which results in the dramatic tunability of the plasmon in semimetals or small-gap semiconductors. more...

The presence of several kinds of carriers at the Fermi surface results in interesting complex dielectric properties of the bulk Pd in the low-energy excitation range. A most spectacular manifestation of this is the presence of a collective electronic excitation characterized by a soundlike dispersion, termed acoustic plasmon (AP). Due to the characteristic dispersion reaching zero energy in the long-wavelength limit, the question of the thermal stability of the excitation spectrum arises. In this work we explore this problem investigating the thermal effect on the electronic excitation spectrum in this material, tracing how the AP properties vary with the temperature increase. The main effect consists in the gradual destruction of AP in the energy range corresponding to the temperature. more...

5 p., 4 figuras, We present ab initio time-dependent density-functional theory calculation results for low-energy collective electron excitations in MgB2. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron sigma and pi bands (sigma pi mode) is demonstrated. This mode has a sine-like oscillating dispersion for energies below 0.5 eV. At even lower energy we find another collective mode (sigma sigma mode). We show the strong impact of local-field effects on dielectric functions in MgB2. These effects account for the long q-range behavior of the modes. We discuss the physics that these collective excitations bring to the energy region typical for lattice vibrations. more...

5 p., 4 figuras, We present ab initio time-dependent density-functional theory calculation results for low-energy collective electron excitations in MgB2. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron sigma and pi bands (sigma pi mode) is demonstrated. This mode has a sine-like oscillating dispersion for energies below 0.5 eV. At even lower energy we find another collective mode (sigma sigma mode). We show the strong impact of local-field effects on dielectric functions in MgB2. These effects account for the long q-range behavior of the modes. We discuss the physics that these collective excitations bring to the energy region typical for lattice vibrations. more...

5 p., 4 figuras, We present ab initio time-dependent density-functional theory calculation results for low-energy collective electron excitations in MgB2. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron sigma and pi bands (sigma pi mode) is demonstrated. This mode has a sine-like oscillating dispersion for energies below 0.5 eV. At even lower energy we find another collective mode (sigma sigma mode). We show the strong impact of local-field effects on dielectric functions in MgB2. These effects account for the long q-range behavior of the modes. We discuss the physics that these collective excitations bring to the energy region typical for lattice vibrations. more...

An experimental and theoretical study of electronic excitations in MgB2 covering the domain of large energy and momentum transfers is reported. Energy-loss spectra for several values of momentum transfers were measured in a polycrystalline sample by means of inelastic X-ray scattering spectroscopy. Ab initio calculations of the dielectric function as well as the energy-loss function were performed in the frame of the time-dependent local density approximation with inclusion of crystal local-field effects. We obtained very good agreement between the experimental and the theoretical energy dispersion of the peak maximum of the loss function. We found that crystal local-field effects are responsible for this agreement at large momenta. Fine structure observed in the measured spectra was interpreted in terms of strong interband transitions predicted by the calculations in the and directions. The theoretical dispersion of these features is in good accordance with the experimental data. Further spectral features in the measured spectra due to Mg 2s and 2p core electron excitations are also discussed. more...

We report on the results of calculation of low-energy dielectric response and optical properties of MgB2. The calculations have been performed with full inclusion of the ab initio electron band structure making use of random-phase and time-dependent local-density approximations. The role of local-field and exchange-correlation effects in MgB2 dielectric function is thoroughly examined. Index of refraction, extinction coefficient, and normal-incidence reflectivity exhibiting strong anisotropy were calculated for electromagnetic waves polarized along the a∗ and c∗ axes by using the evaluated electron-density-response functions. Our results are in good agreement with recent x-ray and optical measurements performed on MgB2 single crystals. more...

Recent advances in the theoretical description of electron and hole dynamics in several metallic systems are discussed. The role of initial and final states in the decay rates together with the dynamical screening is studied for the case of three different non-homogeneous systems, Na/Cu(111), MgB2, and Mg(0001). more...

Νέας Μεθόδου, New Method, Nouvelle Méthode, Σε ξεχωριστό φύλλο χαρτιού που περιέχεται στον υποφάκελο, αναγράφεται: "Σημείωσις. "Τας κακωθείσας ψυχάς ημών ταις αμαρτίαις αγάθυνον κ.λπ." Καλοφωνικός ειρμός εις ήχος βαρύν. Εν χειρογράφω καλοφωνικώ ειρμολογίω υπάρχει η εξής επιγραφή: "Τον παρόντα φασίν είναι του Μπερεκέτου, αλλ' ουκ έστιν" (Ίσως του Γούτα εστίν)"., Αστάχωτο. Φυλάσσεται σε υποφάκελο λυτό, σε πράσινο χρώμα με στάμπα μορφής δέρματος κροκοδείλου. Τετράδια 10. Τα περισσότερα πρωτογράμματα ελλείπουν., Without binding. It is kept in an unbound green sub-file which is stamped a croco style design. The initial letters are missing., Sans reliure. Le manuscrit est conservé dans une chemise de papier vert, décoré avec des estampes de peau crocodile. Sans lettres initiales. more...

Πρωτότυπη, Middle, Ronde, Αστάχωτο, φυλάσσεται σε λυτό υποφάκελο χρώματος πράσινου με στάμπα μορφής δέρματος κροκοδείλου., Without binding. It is kept in an unbound green sub-file which is stamped a croco style design., Sans reliure. Le manuscrit est conservé dans une chemise de papier vert, décoré avec des estampes de peau de crocodile. more...

Πρωτότυπη, Middle, Ronde, Αστάχωτος. Φυλάσσεται σε λυτό υποφάκελο, σε πράσινο χρώμα, με στάμπα μορφής δέρματος κροκοδείλου., Without binding. It is kept in an unbound green sub-file which is stamped a croco style design., Sans reliure. Le manuscrit est conservé dans une chemise de papier vert, décoré avec des estampes de peau de crocodile. more...

Πρωτότυπη, Middle, Ronde, Αστάχωτο. Φυλάσσεται σε υποφάκελο λυτό σε πράσινο χρώμα με στάμπα μορφής δέρματος κροκοδείλου. Τετράδια 3., Without binding. It is kept in an unbound green sub-file which is stamped a croco style design., Sans reliure. Le manuscrit est conservé dans une chemise de papier vert, décoré avec des estampes de peau crocodile. more...