1,721,040 research outputs found
Spectroscopic signatures of massless gap opening in graphene
No full textGap opening in graphene is usually discussed in terms of a semiconductinglike spectrum, where the appearance of a finite gap at the Dirac point is accompanied by a finite mass for the fermions. In this paper we propose a gap scenario from graphene which preserves the massless characters of the carriers. This approach explains recent spectroscopic measurements carried out in epitaxially grown graphene, ranging from photoemission to optical transmission
Hopping-resolved electron-phonon coupling in bilayer graphene
No full textIn this paper we investigate the electron-phonon coupling in bilayer graphene, as a paradigmatic case for multilayer graphenes where interlayer hoppings are relevant. Using a frozen-phonon approach within the context of density functional theory (DFT) and using different optical phonon displacements, we are able to evaluate quantitatively the electron-phonon coupling alpha(i) associated with each hopping term gamma(i). This analysis also reveals a simple scaling law between the hopping terms gamma(i) and the electron-phonon coupling alpha(i) which goes beyond the specific DFT technique employed
Vertex renormalization in dc conductivity of doped chiral graphene
No full textThe remarkable transport properties of graphene follow not only from the Dirac-type energy dispersion, but also from the chiral nature of its excitations, which makes unclear the limits of applicability of the standard semiclassical Boltzmann approach. In this paper we provide a quantum derivation of the transport scattering time in graphene in the case of electron-phonon interaction. By using the Kubo formalism, we compute explicitly the vertex corrections to the dc conductivity by retaining the full chiral matrix structure of graphene. We show that at least in the regime of large chemical potential the Boltzmann picture is justified. This result is also robust against a small sublattice inequivalence, which partly spoils the role of chirality and leads to a doping dependence of the resistivity that can be relevant to recent transport experiments in doped graphene samples
Band structure and electron-phonon coupling in H3S: A tight-binding model
Full text linkWe present a robust tight-binding description, based on the Slater-Koster formalism, of the band structure of H3S in the Im (3) over barm structure, stable in the range of pressure P = 180-220 GPa. We show that the interatomic hopping between the 3s and 3p orbitals (and partially between the 3p orbitals themselves) of sulfur is fundamental to capturing the relevant physics associated with the Van Hove singularities close to the Fermi level. Comparing the model so defined with density functional theory calculations we obtain a very good agreement not only of the overall band structure but also of the low-energy states and the Fermi surface properties. The description in terms of Slater-Koster parameters permits us also to evaluate at a microscopic level a hopping-resolved linear electron-lattice coupling which can be employed for further tight-binding analyses also at a local scale
Fermi-surface Shrinking, Interband Coupling and Multiple Gaps in Iron-based Pnictides
No full textIn this contribution we present a comprehensive explanation for the origin of the band shifts observed in dHvA and ARPES experiments. Using a four-band Eliashberg analysis, we show that they are a natural consequence of the multiband character of these systems and of the strong particle-hole asymmetry of the bands. We also show that the relative sign of such shifts provides a direct experimental evidence of a dominant interband scattering. A quantitative analysis in LaFePO yields a spin-mediated interband coupling of the order V approximate to 0.46 eV, which corresponds to a mass enhancement Z approximate to 1.4. We also employ such four-band model to investigate the magnitude of the superconducting gap on different Fermi sheets of Ba0.6K0.4Fe2As2, and we show that the same four-band model provides a simple explanation of the different gap values on different Fermi sheets and of the thermodynamics properties (specific heat, superfluid density,......)
Infrared phonon activity and Fano interference in multilayer graphenes
No full textRecent optical measurements in bilayer graphene have reported a strong dependence on phonon peak intensity, as well on the asymmetric Fano lineshape, on the charge doping and on the bandgap, tuned by gate voltage. In this paper, we show how these features can be analyzed and predicted on a microscopic quantitative level using the charge-phonon theory applied to the specific case of graphene systems. We present a phase diagram where the infrared activity of both the symmetric (E-g) and antisymmetric (E-u) phonon modes is evaluated as a function of doping and gap. We also show how a switching mechanism between these two modes can occur, governing the dominance of the optical response of one mode with respect to the other. The theory presented here can be also generalized to bulk graphite and to multilayer systems with different stacking orders, providing a useful roadmap for the characterization of graphenic systems by optical infrared means
Effects of the Fermi-surface shrinking on the optical sum rule in pnictides
No full textIn this paper, we investigate the effects of the band shifts induced by the interband spin-fluctuation coupling on the optical sum rule in pnictides. We show that, despite the shrinking of the Fermi surfaces with respect to first-principles calculations, the charge-carrier concentration in each band is almost unchanged, with a substantial conservation of the total optical sum rule. However, a significant transfer of spectral weight occurs from low-energy coherent processes to incoherent ones that is carried out integrating the data up to a finite cutoff, with practical consequences on the experimental estimate of the sum rule. This has profound consequences both on the absolute value of the sum rule and on its temperature dependence, which must be taken into account while discussing optical experiments in these systems
The meaning of strange momentum structures in the gap
No full textThe anisotropy of the superconducting gap is usually associated with the anisotropy of the pairing scattering, and in the exception of the gap symmetry structure, like the nodes, it is expected to be smooth and predictable. There is, however, the alternative approach of momentum decoupling (dominantly forward scattering), in which the anisotropy of the gap follows the anisotropy of the electronic density of states in all gap symmetries. In this last case, the gap can have strange anisotropies in addition to the symmetry-related nodes. We argue that the hump centered in the Γ–X direction of the Bi2212 gap seen in ARPES, reflects the corresponding hump in the electronic density of states when for overdoped and/or contaminated samples the gap symmetry is switched from d-wave to s-wave and the node in this direction disappears (a situation plausible in the Momentum Decoupling regime). This correlation of the gap anisotropy with the DOS anisotropy implies a quasi-Kronecker momentum structure for the scattering amplitude
Charged-phonon theory and Fano effect in the optical spectroscopy of bilayer graphene
No full textSince their discovery, graphene-based systems represent an exceptional playground to explore the emergence of peculiar quantum effects. The present paper focuses on the anomalous appearance of strong infrared phonon resonances in the optical spectroscopy of bilayer graphene and on their pronounced Fano-like asymmetry, both tunable in gated devices. By developing a full microscopic many-body approach for the optical-phonon response we explain how both effects can be quantitatively accounted for by the quantum interference of electronic and phononic excitations. We show that the phonon modes borrow a large dipole intensity from the electronic background, the so-called charged-phonon effect, and at the same time interfere with it, leading to a typical Fano response. Our approach allows one to disentangle the correct selection rules that control the relative importance of the two (symmetric and antisymmetric) relevant phonon modes for different values of the doping and/or of the gap in bilayer graphene. Finally, we discuss the extension of the same theoretical scheme to the Raman spectroscopy, to explain the lack of the same features on the Raman phononic spectra. Besides its remarkable success in explaining the existing experimental data in graphene-based systems, the present theoretical approach offers a general scheme for the microscopic understanding of Fano-like features in a wide variety of other systems
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