1,721,346 research outputs found
Electron-phonon coupling and electron self-energy in electron-doped graphene: Calculation of angular-resolved photoemission spectra
No full textWe obtain analytical expressions for the electron self-energy and the electron-phonon coupling in electron-doped graphene using electron-phonon matrix elements extracted from density functional theory simulations. From the electron self-energies we calculate angle-resolved photoemission spectra (ARPES). We demonstrate that the measured kink at approximate to-0.2 eV from the Fermi level is actually composed of two features, one at approximate to-0.195 eV due to the twofold-degenerate E-2g mode, and a second one at approximate to-0.16 eV due to the A(1)(') mode. The electron-phonon coupling extracted from the kink observed in ARPES experiments is roughly a factor of 5.5 larger than the calculated one. This disagreement can be only partially reconciled by the inclusion of resolution effects. Indeed, we show that a finite resolution increases the apparent electron-phonon coupling by underestimating the renormalization of the electron velocity at energies larger than the kink positions. The discrepancy between theory and experiments is thus reduced to a factor of approximate to 2.5. From the linewidth of the calculated ARPES we obtain the electron relaxation time. A comparison with available experimental data in graphene shows that the electron relaxation time detected in ARPES is almost two orders of magnitudes smaller than that measured by other experimental techniques
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No full textHuge anharmonic effects in superconducting hydrides and transition metal dichalcogenides
No full textEven if the harmonic approximation describing the vibrations of atoms in solids suffices to interpret experimental measurements in many occasions, it can completely break down when the displacements of the atoms exceed the range in which the harmonic potential is valid. The stochastic self-consistent harmonic approximation method is precisely devised to calculate theoretically vibrational properties in strongly anharmonic solids in which the harmonic theory fails. We apply this method to palladium hydrides and 2H-NbSe2, two strongly anharmonic systems that exemplify the importance of anharmonic effects in metallic hydrides and transition metal dichalcogenides. First of all, we explain that the inversion of the isotope effect in palladium hydrides is a consequence of huge anharmonic effects. The temperature dependence of the phonon spectra in PdH, PdD, and PdT is also presented, where qualitative differences are predicted depending on the isotope. Secondly, we demonstrate that the high-temperature 2H-NbSe2 structure is fully stabilized dynamically by anharmonicity. The softening with temperature of the acoustic longitudinal mode in 2H-NbSe2 at the CDW momentum is predicted as well by our calculation. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
High-field transport in graphene: the impact of Zener tunneling
No full textWe study, theoretically, the impact of Zener tunneling (the generation of electron-hole pairs by the electric field) on the charge-transport properties of graphene in the high-field regime. We model Zener tunneling in a rigorous way, using the quantum master equation for the density matrix. In the presence of Zener tunneling, a steady-state can be reached only by further including an efficient mechanism for the electron thermalization such as electron-electron scattering. We treat the effects of electron-electron relaxation within a simplified model, that assumes an instantaneous separate thermalization of the electrons in the conduction band and of the holes in the valence band. The inclusion of both Zener tunneling and electron-electron relaxation improves the agreement with measurements performed in graphene in the high-field regime at low doping
Universal Increase in the Superconducting Critical Temperature of Two-Dimensional Semiconductors at Low Doping by the Electron-Electron Interaction
No full textIn two-dimensional multivalley semiconductors, at low doping, even a moderate electron-electron interaction enhances the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the electron-electron interaction results in an enhancement of the superconducting critical temperature. By performing first-principles calculations beyond density functional theory, we prove that this effect accounts for the unconventional doping dependence of the superconducting transition temperature (T-c) and of the magnetic susceptibility measured in LixZrNCI. Finally, we discuss what are the conditions for a maximal T-c enhancement in weakly doped two-dimensional semiconductors
First-Principles Nuclear Magnetic Resonance Structural Analysis of Vitreous Silica
No full textGauge including projector augmented wave (GIPAW) NMR calculations combined with hybrid Monte Carlo/molecular dynamics simulations are carried out in order to investigate the relationships between the oxygen-17 and silicon-29 NMR spectra of vitreous silica and its local structure in terms of the Si-O-Si bond angle and Si-O distance distributions. Special attention is paid to the structure and NMR parameters of,three- and four-membered rings, and the effect of their concentration on glass density is studied. It is shown that our simulations provide a new insight into the features of the O-17 NMR parameters distribution. Accordingly, a new analytical model is presented and applied for the reconstruction of the Si-O-Si angle from the NMR spectrum. The reliability of the procedure is demonstrated conclusively through the excellent consistency of the analysis of the oxygen-17 and silicon-29 NMR experimental data of vitreous silica. Si-O-Si angle distribution mean values of 147.1 degrees and 148.4 degrees, respectively, and standard deviations of 11.2 degrees and 10.8 degrees, respectively, are obtained from the oxygen-17 and silicon-29 NMR experimental spectrum (Clark et al., ref 4) of the same sample
Zener tunneling in the electrical transport of quasimetallic carbon nanotubes
No full textWe study theoretically the impact of Zener tunneling on the charge-transport properties of quasimetallic (Qm) carbon nanotubes (characterized by forbidden band gaps of a few tens of meV). We also analyze the interplay between Zener tunneling and elastic scattering on defects. To this purpose we use a model based on the master equation for the density matrix, which takes into account the interband Zener transitions induced by the electric field (a quantum mechanical effect), the electron-defect scattering, and the electron-phonon scattering. In the presence of Zener tunneling the Qm tubes support an electrical current even when the Fermi energy lies in the forbidden band gap. In the absence of elastic scattering (in high-quality samples), the small size of the band gap of Qm tubes enables Zener tunneling for realistic values of the the electric field (above similar to 1 V/mu m). The presence of a strong elastic scattering (in low-quality samples) further decreases the values of the field required to observe Zener tunneling. Indeed, for elastic-scattering lengths of the order of 50 nm, Zener tunneling affects the current-voltage characteristic already in the linear regime. In other words, in quasimetallic tubes, Zener tunneling is made more visible by defects
Ab-initio energetics of graphite and multilayer graphene: stability of Bernal versus rhombohedral stacking
Full text linkThere has been a lot of excitement around the observation of superconductivity in twisted bilayer graphene, associated to flat bands close to the Fermi level. Such correlated electronic states also occur in multilayer rhombohedral stacked graphene (RG), which has been receiving increasing attention in the last years. In both natural and artificial samples however, multilayer stacked Bernal graphene (BG) occurs more frequently, making it desirable to determine what is their relative stability and under which conditions RG might be favored. Here, we study the energetics of BG and RG in bulk and also multilayer stacked graphene using first-principles calculations. It is shown that the electronic temperature, not accounted for in previous studies, plays a crucial role in determining which phase is preferred. We also show that the low energy states at room temperature consist of BG, RG and mixed BG-RG systems with a particular type of interface. Energies of all stacking sequences (SSs) are calculated for N = 12 layers, and an Ising model is used to fit them, which can be used for larger N as well. In this way, the ordering of low energy SSs can be determined and analyzed in terms of a few parameters. Our work clarifies inconsistent results in the literature, and sets the basis to studying the effect of external factors on the stability of multilayer graphene systems in first principles calculations
Oxygen K-edge XANES of germanates investigated using first-principles calculations
No full textO K-edge x-ray absorption near-edge structure (XANES) spectra of alpha-quartz-type and rutile-type GeO2 polymorphs and of K2Ge8O17 have been analyzed using first-principles plane-wave pseudopotential calculations. XANES spectra have been calculated using supercell including core-hole effects and good agreement with experiment has been obtained. In the the case of GeO2 polymorphs, local density of empty states has been performed and peaks in the experimental spectra can be assigned to transitions involving hybridization of the O p orbitals with the Ge s, Ge p, Ge sp, and Ge d orbitals. Furthermore, peak positions in the theoretical spectra appear to be correlated with changes in the Ge-O-Ge angle as well as indirectly with the Ge coordination geometry. Analysis of O K-edge XANES spectra for individual O sites in K2Ge8O17 shows that oxygens shared between two fivefold Ge atoms or one fourfold and one fivefold Ge atom exhibit subtle shifts to lower energy of the peaks, which have been previously observed in alkali germanate glasses at and above the germanate anomaly
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