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Emmanuele Cappelluti - One of the best experts on this subject based on the ideXlab platform.

  • Nonadiabatic phenomenology in small Fermi Energy superconductors
    Journal of Physics and Chemistry of Solids, 2020
    Co-Authors: L. Pietronero, Emmanuele Cappelluti
    Abstract:

    Abstract High- T c and unconventional superconductivity appears in a number of different materials with different physical properties. A common characteristics shared by almost all these compounds is that they are ‘bad metal’ in the sense that their effective charge carrier density is quite small, leading to a small Fermi Energy E F . In such a situation the dynamics of the electronic and lattice degrees of freedom becomes comparable ( ω ph ∼ E F ) driving these systems towards a breakdown of the Born-Oppenheimer adiabatic principle ( ω ph ≪ E F ) ruled by the comparison between the phonon frequency scale ω ph and E F . The standard concept of Fermi liquid picture applied to the electron–phonon interaction needs thus to be deeply revised. We discuss the interesting implications of the onset of nonadiabatic effects on many properties of systems with small Fermi Energy.

  • Small Fermi Energy, zero-point fluctuations, and nonadiabaticity in Mg B 2
    Physical Review B, 2005
    Co-Authors: Lilia Boeri, Emmanuele Cappelluti, Luciano Pietronero
    Abstract:

    Small Fermi Energy effects are induced in MgB2 by the low hole doping in the s bands which are characterized by a Fermi Energy E F , 0.5 eV. We show that due to the particularly strong deformation potential relative to the E2g phonon mode, lattice fluctuations are reflected in strong fluctuations in the electronic band structure. Quantum fluctuations associated to the zero-point lattice motion are responsible for an uncertainty of the Fermi Energy of the order of the Fermi Energy itself, leading to the breakdown of the adiabatic principle underlying the Born-Oppenheimer approximation in MgB2 even if vph/ EF, 0.1˛ 0.2, where vph are the characteristic phonon frequencies. This amounts to a new nonadiabatic regime, which could be relevant to other unconventional superconductors.

  • Small Fermi Energy effects in MgB2 and related compounds
    Physica C-superconductivity and Its Applications, 2004
    Co-Authors: Emmanuele Cappelluti, Luciano Pietronero, Lilia Boeri, G.b. Bachelet, S. Ciuchi, C. Grimaldi, S. Strässler
    Abstract:

    Abstract Superconductivity at T c ≃40 K in MgB 2 is thought to origin from the strong electron–phonon (el–ph) coupling of the σ -bands, whereas the residual interband scattering gives rise to the multigap phenomenology. The extremely low charge density of the σ -bands is reflected in the small Fermi Energy E F σ , a fraction of eV, a common feature which is shared also by cuprates and fullerides. In our contribution we discuss the anomalous effects arising from the small Fermi Energy phenomenology, when E F σ becomes comparable with the other Energy scales of the systems. In particular we analyze the nonadiabatic effects arising from the finite adiabatic ratio ω ph / E F σ ; the anharmonic character of the E 2 g phonon mode, which is shown to be related to the smallness of the Fermi Energy with respect to the electron–phonon coupling g E 2 g : E F σ ∼ g E 2 g ; the anomalous effects of disorder when impurity scattering rate γ imp is compared with E F σ . We discuss also the possibility of an enhancement of the nonadiabatic character due to zero point quantum fluctuations.

  • Electron-phonon renormalization in small Fermi Energy systems
    Physical Review B, 2003
    Co-Authors: Emmanuele Cappelluti, Luciano Pietronero
    Abstract:

    The puzzling features of recent photoemission data in cuprates have been the object of several analyses in order to identity the nature of the underlying electron-boson interaction. In this paper we point out that many basic assumptions of the conventional analysis are expected to fail in small Fermi Energy systems, when, as in the cuprates, the Fermi Energy ${E}_{F}$ is comparable with the boson Energy scale. We discuss in detail the features appearing in the self-Energy of small Fermi Energy systems and the possible implications for the angle-resolved photoemission spectroscopy data in cuprates.

  • NONADIABATIC EFFECTS AND THE ROLE OF SMALL Fermi Energy IN MgB 2
    International Journal of Modern Physics B, 2003
    Co-Authors: Emmanuele Cappelluti, Luciano Pietronero, Lilia Boeri, C. Grimaldi, S. Strässler, G.b. Bachelet
    Abstract:

    There is nowadays a general agreement on a key role of the σ bands in the superconducting properties of MgB2. We show that peculiar characteristics of the σ bands give rise to nonadiabatic and anharmonic effects which break the conventional Migdal-Eliashberg framework. Both these features are governed by the small value of the Fermi Energy due to the vicinity of the hole doping level to the top of the σ bands. In this context we discuss how the nonadiabatic theory leads to a coherent interpretation of the superconducting properties of MgB2 without invoking very large couplings and it naturally explains the role of the disorder on Tc. It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these type of compounds. Anharmonicity is also investigated by means of LDA calculations. We find that the anharmonic character of the E2g phonon is essentially driven by the small Fermi Energy of the σ holes. We present a simple analytic model which allows us to unde...

Luciano Pietronero - One of the best experts on this subject based on the ideXlab platform.

  • Small Fermi Energy, zero-point fluctuations, and nonadiabaticity in Mg B 2
    Physical Review B, 2005
    Co-Authors: Lilia Boeri, Emmanuele Cappelluti, Luciano Pietronero
    Abstract:

    Small Fermi Energy effects are induced in MgB2 by the low hole doping in the s bands which are characterized by a Fermi Energy E F , 0.5 eV. We show that due to the particularly strong deformation potential relative to the E2g phonon mode, lattice fluctuations are reflected in strong fluctuations in the electronic band structure. Quantum fluctuations associated to the zero-point lattice motion are responsible for an uncertainty of the Fermi Energy of the order of the Fermi Energy itself, leading to the breakdown of the adiabatic principle underlying the Born-Oppenheimer approximation in MgB2 even if vph/ EF, 0.1˛ 0.2, where vph are the characteristic phonon frequencies. This amounts to a new nonadiabatic regime, which could be relevant to other unconventional superconductors.

  • Small Fermi Energy effects in MgB2 and related compounds
    Physica C-superconductivity and Its Applications, 2004
    Co-Authors: Emmanuele Cappelluti, Luciano Pietronero, Lilia Boeri, G.b. Bachelet, S. Ciuchi, C. Grimaldi, S. Strässler
    Abstract:

    Abstract Superconductivity at T c ≃40 K in MgB 2 is thought to origin from the strong electron–phonon (el–ph) coupling of the σ -bands, whereas the residual interband scattering gives rise to the multigap phenomenology. The extremely low charge density of the σ -bands is reflected in the small Fermi Energy E F σ , a fraction of eV, a common feature which is shared also by cuprates and fullerides. In our contribution we discuss the anomalous effects arising from the small Fermi Energy phenomenology, when E F σ becomes comparable with the other Energy scales of the systems. In particular we analyze the nonadiabatic effects arising from the finite adiabatic ratio ω ph / E F σ ; the anharmonic character of the E 2 g phonon mode, which is shown to be related to the smallness of the Fermi Energy with respect to the electron–phonon coupling g E 2 g : E F σ ∼ g E 2 g ; the anomalous effects of disorder when impurity scattering rate γ imp is compared with E F σ . We discuss also the possibility of an enhancement of the nonadiabatic character due to zero point quantum fluctuations.

  • Electron-phonon renormalization in small Fermi Energy systems
    Physical Review B, 2003
    Co-Authors: Emmanuele Cappelluti, Luciano Pietronero
    Abstract:

    The puzzling features of recent photoemission data in cuprates have been the object of several analyses in order to identity the nature of the underlying electron-boson interaction. In this paper we point out that many basic assumptions of the conventional analysis are expected to fail in small Fermi Energy systems, when, as in the cuprates, the Fermi Energy ${E}_{F}$ is comparable with the boson Energy scale. We discuss in detail the features appearing in the self-Energy of small Fermi Energy systems and the possible implications for the angle-resolved photoemission spectroscopy data in cuprates.

  • NONADIABATIC EFFECTS AND THE ROLE OF SMALL Fermi Energy IN MgB 2
    International Journal of Modern Physics B, 2003
    Co-Authors: Emmanuele Cappelluti, Luciano Pietronero, Lilia Boeri, C. Grimaldi, S. Strässler, G.b. Bachelet
    Abstract:

    There is nowadays a general agreement on a key role of the σ bands in the superconducting properties of MgB2. We show that peculiar characteristics of the σ bands give rise to nonadiabatic and anharmonic effects which break the conventional Migdal-Eliashberg framework. Both these features are governed by the small value of the Fermi Energy due to the vicinity of the hole doping level to the top of the σ bands. In this context we discuss how the nonadiabatic theory leads to a coherent interpretation of the superconducting properties of MgB2 without invoking very large couplings and it naturally explains the role of the disorder on Tc. It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these type of compounds. Anharmonicity is also investigated by means of LDA calculations. We find that the anharmonic character of the E2g phonon is essentially driven by the small Fermi Energy of the σ holes. We present a simple analytic model which allows us to unde...

  • Spin susceptibility in small Fermi Energy systems: effects of nonmagnetic impurities
    European Physical Journal B, 2002
    Co-Authors: Emmanuele Cappelluti, C. Grimaldi, Luciano Pietronero
    Abstract:

    In small Fermi Energy metals, disorder can deeply modify superconducting state properties leading to a strong suppression of the critical temperature Tc. In this paper, we show that also normal state properties can be seriously influenced by disorder when the Fermi Energy EF is sufficiently small. We calculate the normal state spin susceptibility χ for a narrow band electron-phonon coupled metal as a function of the non-magnetic impurity scattering rate \(\). We find that as soon as \(\) is comparable to EF, χ is strongly reduced with respect to its value in the clean limit. The effects of the electron-phonon interaction including the nonadiabatic corrections are discussed. Our results strongly suggest that the recent finding on irradiated MgB2 samples can be naturally explained in terms of small EF values associated with the σ-bands of the boron plane, sustaining therefore the hypothesis that MgB2 is a nonadiabatic metal.

Mildred S Dresselhaus - One of the best experts on this subject based on the ideXlab platform.

  • Fermi Energy dependence of first and second order raman spectra in graphene kohn anomaly and quantum interference effect
    Physical Review B, 2016
    Co-Authors: Eddwi H Hasdeo, Ahmad R T Nugraha, Mildred S Dresselhaus, Riichiro Saito
    Abstract:

    Intensities of the first- and the second-order Raman spectra are calculated as a function of the Fermi Energy. We show that the Kohn anomaly effect, i.e., phonon frequency renormalization, in the first-order Raman spectra originates from the phonon renormalization by the interband electron-hole excitation, whereas in the second-order Raman spectra, a competition between the interband and intraband electron-hole excitations takes place. By this calculation, we confirm the presence of different dispersive behaviors of the Raman peak frequency as a function of the Fermi Energy for the first- and the second-order Raman spectra, as observed in some previous experiments. Moreover, the calculated results of the Raman intensity sensitively depend on the Fermi Energy for both the first- and the second-order Raman spectra, indicating the presence of the quantum interference effect. The electron-phonon matrix element plays an important role in the intensity increase (decrease) of the combination (overtone) phonon modes as a function of the Fermi Energy.

  • Fermi-Energy-Dependent Structural Deformation of Chiral Single-Wall Carbon Nanotubes
    Physical review applied, 2014
    Co-Authors: Bruno G M Vieira, Eduardo B Barros, Daniel G Vercosa, Georgy Samsonidze, Antonio G. Souza Filho, Mildred S Dresselhaus
    Abstract:

    In this work, we use an extended tight-binding approach for calculating the Fermi-Energy dependence of the structural deformation of chiral single-wall carbon nanotubes (SWNTs). We show that, in general, nanotube strains occur in such a way as to avoid a net charge from being accumulated on the nanotube. We also investigate the effect of the Fermi-Energy-induced strains on the electronic structure of SWNTs, showing that the optical transition energies change by up to 0.5 eV due to the induced strains and that this change is nearly independent of how the nanotube is deformed. Finally, we also consider the contribution of the electron-electron Coulomb repulsion to the total Energy by using an effective regularized potential Energy model. We show that the inclusion of the Coulomb repulsion leads to larger strains and smaller net charges transferred to the nanotube.National Science Foundation (U.S.) (Grant DMR-1004147

  • Fermi Energy dependent structural deformation of chiral single wall carbon nanotubes
    Physical Review Letters, 2014
    Co-Authors: Bruno G M Vieira, Eduardo B Barros, Daniel G Vercosa, Georgy Samsonidze, Antonio Souza G Filho, Mildred S Dresselhaus
    Abstract:

    In this work, we use an extended tight-binding approach for calculating the Fermi-Energy dependence of the structural deformation of chiral single-wall carbon nanotubes (SWNTs). We show that, in general, nanotube strains occur in such a way as to avoid a net charge from being accumulated on the nanotube. We also investigate the effect of the Fermi-Energy-induced strains on the electronic structure of SWNTs, showing that the optical transition energies change by up to 0.5 eV due to the induced strains and that this change is nearly independent of how the nanotube is deformed. Finally, we also consider the contribution of the electron-electron Coulomb repulsion to the total Energy by using an effective regularized potential Energy model. We show that the inclusion of the Coulomb repulsion leads to larger strains and smaller net charges transferred to the nanotube.

  • Fermi Energy dependence of the g band resonance raman spectra of single wall carbon nanotubes
    Physical Review B, 2009
    Co-Authors: Jin Sung Park, Riichiro Saito, Kenichi Sasaki, Wataru Izumida, Martin Kalbac, Hootan Farhat, G Dresselhaus, Mildred S Dresselhaus
    Abstract:

    The Fermi Energy dependence of the $G$-band resonance Raman spectra of single-wall carbon nanotubes (SWNTs) is calculated, including the Kohn anomaly effect for metallic tubes. The gate voltage dependence of the $G$-band Raman spectra for SWNTs shows chirality-dependent ${G}^{+}/{G}^{\ensuremath{-}}$ spectra, reflecting their dependence on the eigenvector direction of the optical (LO and TO) phonon modes and the nanotube chirality.

Akira Toriumi - One of the best experts on this subject based on the ideXlab platform.

Francisco Guinea - One of the best experts on this subject based on the ideXlab platform.

  • electronic band structure and pinning of Fermi Energy to van hove singularities in twisted bilayer graphene a self consistent approach
    Physical Review B, 2019
    Co-Authors: Niels R Walet, Francisco Guinea
    Abstract:

    The emergence of flat bands in twisted bilayer graphene leads to an enhancement of interaction effects, and thus to insulating and superconducting phases at low temperatures, even though the exact mechanism is still widely debated. The position and splitting of the flat bands is also very sensitive to the residual interactions. Moreover, the low Energy bands of twisted graphene bilayers show a rich structure of singularities in the density of states, van Hove singularities, which can enhance further the role of interactions. We study the effect of the long-range interactions on the band structure and the van Hove singularities of the low Energy bands of twisted graphene bilayers. Reasonable values of the long-range electrostatic interaction lead to a band dispersion with a significant dependence on the filling. The change of the shape and position of the bands with electronic filling implies that the van Hove singularities remain close to the Fermi Energy for a broad range of fillings. This result can be described as an effective pinning of the Fermi Energy at the singularity. The sensitivity of the band structure to screening by the environment may open new ways of manipulating the system.