214 research outputs found
Small Fermi energy, zero point fluctuations and nonadiabaticity in MgB
Small Fermi energy effects are induced in MgB by the low hole doping in
the bands which are characterized by a Fermi energy eV. We show that, due to the particularly strong deformation
potential relative to the 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 MgB even if , where are the characteristic phonon
frequencies. This amounts to a new nonadiabatic regime, which could be relevant
to other unconventional superconductors.Comment: to appear on Physical Review
Electron-phonon coupling and phonon self-energy in MgB: do we really understand MgB Raman spectra ?
We consider a model Hamiltonian fitted on the ab-initio band structure to
describe the electron-phonon coupling between the electronic bands and
the phonon E mode in MgB. The model allows for analytical
calculations and numerical treatments using very large k-point grids. We
calculate the phonon self-energy of the E mode along two high symmetry
directions in the Brillouin zone. We demonstrate that the contribution of the
bands to the Raman linewidth of the E mode via the
electron-phonon coupling is zero. As a consequence the large resonance seen in
Raman experiments cannot be interpreted as originated from the mode at
. We examine in details the effects of Fermi surface singularities in
the phonon spectrum and linewidth and we determine the magnitude of finite
temperature effects in the the phonon self-energy. From our findings we suggest
several possible effects which might be responsible for the MgB Raman
spectra.Comment: 10 pages, 9 figure
Pauli susceptibility of A3C60 (A=K, Rb)
The Pauli paramagnetic susceptibility of A3C60 (A= K, Rb) compounds is
calculated. A lattice quantum Monte Carlo method is applied to a multi-band
Hubbard model, including the on-site Coulomb interaction U. It is found that
the many-body enhancement of the susceptibility is of the order of a factor of
three. This reconciles estimates of the density of states from the
susceptibility with other estimates. The enhancement is an example of a
substantial many-body effect in the doped fullerenes.Comment: 4 pages, revtex, 2 figures, submitted to Phys. Rev. B more
information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
Polaron in t-J model
We present numeric results for ground state and angle resolved photoemission
spectra (ARPES) for single hole in t-J model coupled to optical phonons. The
systematic-error free diagrammatic Monte Carlo is employed where the Feynman
graphs for the Matsubara Green function in imaginary time are summed up
completely with respect to phonons variables, while magnetic variables are
subjected to non-crossing approximation. We obtain that at electron-phonon
coupling constants relevant for high Tc cuprates the polaron undergoes
self-trapping crossover to strong coupling limit and theoretical ARPES
demonstrate features observed in experiment: a broad peak in the bottom of the
spectra has momentum dependence which coincides with that of hole in pure t-J
model.Comment: 4 pages, 4 figure
The Electron-Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering
Atom scattering is becoming recognized as a sensitive probe of the
electron-phonon interaction parameter at metal and metal-overlayer
surfaces. Here, the theory is developed linking to the thermal
attenuation of atom scattering spectra (in particular, the Debye-Waller
factor), to conducting materials of different dimensions, from quasi-one
dimensional systems such as W(110):H(11) and Bi(114), to quasi-two
dimensional layered chalcogenides and high-dimensional surfaces such as
quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of
obtained using He atoms compare favorably with known values for the bulk
materials. The corresponding analysis indicates in addition the number of
layers contributing to the electron-phonon interaction that is measured in an
atom surface collision.Comment: 23 pages, 5 figures, 1 tabl
Effect of electron-phonon interaction on spectroscopies in graphene
We calculate the effect of the electron-phonon interaction on the electronic
density of states (DOS), the quasiparticle properties and on the optical
conductivity of graphene. In metals with DOS constant on the scale of phonon
energies, the electron-phonon renormalizations drop out of the dressed DOS,
however, due to the Dirac nature of the electron dynamics in graphene, the band
DOS is linear in energy and phonon structures remain, which can be emphasized
by taking an energy derivative. There is a shift in the chemical potential and
in the position in energy of the Dirac point. Also, the DOS can be changed from
a linear dependence out of value zero at the Dirac point to quadratic out of a
finite value. The optical scattering rate sets the energy scale for
the rise of the optical conductivity from its universal DC value
(expected in the simplest theory when chemical potential and temperature are
both ) to its universal AC background value . As in ordinary metals the DC conductivity remains unrenormalized
while its AC value is changed. The optical spectral weight under the intraband
Drude is reduced by a mass renormalization factor as is the effective
scattering rate. Optical weight is transferred to an Holstein phonon-assisted
side band. Due to Pauli blocking the interband transitions are sharply
suppressed, but also nearly constant, below twice the value of renormalized
chemical potential and also exhibit a phonon-assisted contribution. The
universal background conductivity is reduced below at large
energies.Comment: 22 pages, 19 figures, submitted to PR
Effects of thermal- and spin- fluctuations on the band structure of purple bronze LiMoO
The band structures of ordered and thermally disordered
LiMoO are calculated by use of ab-initio DFT-LMTO method. The
unusual, very 1-dimensional band dispersion obtained in previous band
calculations is confirmed for the ordered structure, and the overall band
structure agrees reasonably with existing photoemission data. Dispersion and
bandstructure perpendicular to the main dispersive direction is obtained. A
temperature dependent band broadening is calculated from configurations with
thermal disorder of the atomic positions within the unit cell. This leads a
band broadening of the two bands at the Fermi energy which can become
comparable to their energy separation. The bands are particularly sensitive to
in-plane movements of Mo sites far from the Li-sites, where the
density-of-states is highest. The latter fact makes the effect of Li vacancies
on the two bands relatively small. Spin-polarized band results for the ordered
structure show a surprisingly large exchange enhancement on the high DOS Mo
sites. Consequences for spin fluctuations associated with a cell doubling along
the conducting direction are discussed
Electron-phonon interaction at the Be(0001) surface
We present a first principle study of the electron-phonon (e-p) interaction
at the Be(0001) surface. The real and imaginary part of the e-p self energy are
calculated for the surface state in the binding energy range from the
point to the Fermi level. Our calculation shows an overall good
agreement with several photoemission data measured at high and low
temperatures. Additionally, we show that the energy derivative of real part of
the self-energy presents a strong temperature and energy variation close to
, making it difficult to measure its value just at .Comment: Accepted in Phys. Rev. Lett., 5 figure
Phonon spectroscopy through the electronic density of states in graphene
We study how phonon structure manifests itself in the electronic density of
states of graphene. A procedure for extracting the value of the electron-phonon
renormalization is developed. In addition, we identify direct phonon
structures. With increasing doping, these structures, along with ,
grow in amplitude and no longer display particle-hole symmetry.Comment: 5 page
Adiabatic and non-adiabatic phonon dispersion in a Wannier function approach
We develop a first-principles scheme to calculate adiabatic and non-adiabatic
phonon frequencies in the full Brillouin zone. The method relies on the
variational properties of a force-constants functional with respect to the
first-order perturbation of the electronic charge density and on the
localization of the deformation potential in the Wannier function basis. This
allows for calculation of phonon dispersion curves free from convergence issues
related to Brillouin zone sampling. In addition our approach justify the use of
the static screened potential in the calculation of the phonon linewidth due to
decay in electron-hole pairs. We apply the method to the calculation of the
phonon dispersion and electron-phonon coupling in MgB and CaC. In both
compounds we demonstrate the occurrence of several Kohn anomalies, absent in
previous calculations, that are manifest only after careful electron and phonon
momentum integration. In MgB, the presence of Kohn anomalies on the
E branches improves the agreement with measured phonon spectra and
affects the position of the main peak in the Eliashberg function. In CaC we
show that the non-adiabatic effects on in-plane carbon vibrations are not
localized at zone center but are sizable throughout the full Brillouin zone.
Our method opens new perspectives in large-scale first-principles calculations
of dynamical properties and electron-phonon interaction.Comment: 18 pages, 8 figure
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