1,721,094 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
First-Principles Theory of Anharmonicity and the Inverse Isotope Effect in Superconducting Palladium-Hydride Compounds
No full textPalladium hydrides display the largest isotope effect anomaly known in
the literature. Replacement of hydrogen with the heavier isotopes leads
to higher superconducting temperatures, a behavior inconsistent with
harmonic theory. Solving the self-consistent harmonic approximation by a
stochastic approach, we obtain the anharmonic free energy, the thermal
expansion, and the superconducting properties fully ab initio. We find
that the phonon spectra are strongly renormalized by anharmonicity far
beyond the perturbative regime. Superconductivity is phonon mediated,
but the harmonic approximation largely overestimates the superconducting
critical temperatures. We explain the inverse isotope effect, obtaining
a -0.38 value for the isotope coefficient in good agreement with
experiments, hydrogen anharmonicity being mainly responsible for the
isotope anomaly
Huge 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
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
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
Electrical tuning of the magnetic properties of two-dimensional magnets: Cr2Ge2Te6
Full text linkMotivated by growing interest in atomically thin van der Waals magnetic materials, we present an ab initio theoretical study of the dependence of their magnetic properties on the electron / hole density rho induced via the electrical field effect. By focusing on the case of monolayer Cr2Ge2Te6 (a prototypical 2D Ising ferromagnet) and employing a hybrid functional, we first study the dependence of the gap and effective mass on the carrier concentration rho. We then investigate the robustness of magnetism by studying the dependencies of the exchange couplings and magneto crystalline anisotropy energy (MAE) on rho. In agreement with experimental results, we find that magnetism displays a bipolar electrically -tunable character, which is, however, much more robust for hole (rho > 0) rather than electron (rho < 0) doping. Indeed, the MAE vanishes for an electron density rho approximate to - 7.5 x 10(13) e x cm(-2) , signaling the failure of a localized description based on a Heisenberg -type anisotropic spin Hamiltonian. This is in agreement with the rapid increase of the coupling between fourth -neighbor atoms with increasing electron density
Long-Range Rhombohedral-Stacked Graphene through Shear
No full textThe discovery of superconductivity and correlated electronic states in the flat bands of twisted bilayer graphene has raised a lot of excitement. Flat bands also occur in multilayer graphene flakes that
present rhombohedral (ABC) stacking order on many consecutive layers. Although Bernal-stacked (AB) graphene is more stable, long-range ABC-ordered flakes involving up to 50 layers have been surprisingly observed in natural samples. Here, we present a microscopic atomistic model, based on first-principles density functional theory calculations, that demonstrates how shear stress can produce long-range ABC order. A stress-angle phase diagram shows under which conditions ABC-stacked graphene can be obtained, providing an experimental guide for its synthesis
Ab initio study of the LiH phase diagram at extreme pressures and temperatures
No full textThe effect of anharmonic vibrational contributions to the finite-temperature pressure-driven B1-B2 structural phase transition of LiH is studied by using the stochastic self-consistent harmonic approximation method in combination with ab initio density functional theory and the quasiharmonic approximation. Contrary to previous experimental results based on multiple-shock compression, we find that the B1-B2 transition pressure is not significantly reduced at high temperatures. Moreover, we find that the B2 phase is dynamically unstable at low temperatures within harmonic theory in a wide range of pressures where its enthalpy is lower than that of the B1 phase, and the inclusion of anharmonic effects stabilizes the B2 phase in this pressure range. Our results imply that a third, yet unknown phase must exist in the phase diagram of LiH, in addition to the B1 and B2 phases, in order to explain the shock compression result
Spin susceptibility and electron-phonon coupling of two-dimensional materials by range-separated hybrid density functionals: Case study of LixZrNCl
No full textWe investigate the capability of density functional theory (DFT) to appropriately describe the spin susceptibility, chi(s), and the intervalley electron-phonon coupling in LixZrNCl. At low doping, LixZrNCl behaves as a two-dimensional two-valley electron gas, with parabolic bands. In such a system, chi(s) increases with decreasing doping because of the electron-electron interaction. We show that DFT with local functionals (LDA/GGA) is not capable of reproducing this behavior. The use of exact exchange in Hartree-Fock (HF) or in DFT hybrid functionals enhances chi(s). HF, B3LYP, and PBE0 approaches overestimate chi(s), whereas the range-separated HSE06 functional leads to results similar to those obtained in the random phase approximation (RPA) applied to a two-valley two-spin electron gas. Within HF, LixZrNCl is even unstable towards a ferromagnetic state for x < 0.16. The intervalley phonons induce an imbalance in the valley occupation that can be viewed as the effect of a pseudomagnetic field. Thus, similarly to what happens for chi(s), the electron-phonon coupling of intervalley phonons is enhanced by the electron-electron interaction. Only hybrid DFT functionals capture such an enhancement and the HSE06 functional reproduces the RPA results presented in M. Calandra et al. [Phys. Rev. Lett. 114, 077001 (2015)]. These results imply that the description of the susceptibility and electron-phonon coupling with a range-separated hybrid functional would be important also in other two-dimensional weakly doped semiconductors, such as transition-metal dichalcogenides and graphene
Superconductivity in metal-coated graphene
No full textIn this work we explore, by first-principles density functional theory (DFT) calculations, the possibility of inducing electronphonon mediated superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms. We demonstrate that, contrary to what could be naively believed, simple exfoliation to one layer of superconducting graphite intercalated compounds (GICs) does not necessarily lead to superconducting graphene, as it is the case in CaC6. On the contrary, it is meaningful to look for superconductivity in monolayers obtained by exfoliating non-superconducting GICs. In particular, we demonstrate that Li coating and double-coating of graphene leads to superconductivity in graphene with T-c that could be as large as 18 K. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
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