899 research outputs found
Ab initio calculations of the physical properties of transition metal carbides and nitrides and possible routes to high-Tc
Ab initio linear-response calculations are reported of the phonon spectra and
the electron-phonon interaction for several transition metal carbides and
nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and
superconducting properties are calculated in detail at various pressures and
the normal-pressure results are found to well agree with the experiment.
Factors accounting for the relatively low critical temperatures Tc in
transition metal compounds with light elements are considered and the possible
ways of increasing Tc are discussed.Comment: 19 pages, 7 figure
Phonon assisted tunneling in Josephson junctions
The expression for additional subgap current in the presence of
electron-phonon interaction is derived. We show that the phonon assisted
tunneling leads to appearance of peaks on current-voltage characteristics at
the Josephson frequencies corresponding to the Raman-active phonons. The
relation of the obtained results to experimental observations are discussed.Comment: 8 pages, submitted to PR
Bosonic Spectral Function and The Electron-Phonon Interaction in HTSC Cuprates
In Part I we discuss accumulating experimental evidence related to the
structure and origin of the bosonic spectral function in high-temperature
superconducting (HTSC) cuprates at and near optimal doping. Some global
properties of the spectral function, such as number and positions of peaks, are
extracted by combining optics, neutron scattering, ARPES and tunnelling
measurements. These methods give convincing evidence for strong electron-phonon
interaction (EPI) with the coupling constant between 1-3 in cuprates near
optimal doping. Here we clarify how these results are in favor of the
Eliashberg-like theory for HTSC cuprates near optimal doping. In Part II we
discuss some theoretical ingredients - such as strong EPI, strong correlations
- which are necessary to explain the experimental results related to the
mechanism of d-wave pairing in optimally doped cuprates. These comprise the
Migdal-Eliashberg theory for EPI in strongly correlated systems which give rise
to the forward scattering peak. The latter is further supported by the weakly
screened Madelung interaction in the ionic-metallic structure of layered
cuprates. In this approach EPI is responsible for the strength of pairing while
the residual Coulomb interaction (by including spin fluctuations) triggers the
d-wave pairing.Comment: 59 pages, 38 figures, review articl
Critical temperature and giant isotope effect in presence of paramagnons
We reconsider the long-standing problem of the effect of spin fluctuations on
the critical temperature and isotope effect in a phonon-mediated
superconductor. Although the general physics of the interplay between phonons
and paramagnons had been rather well understood, the existing approximate
formulas fail to describe the correct behavior of for general phonon
and paramagnon spectra. Using a controllable approximation, we derive an
analytical formula for which agrees well with exact numerical solutions
of the Eliashberg equations for a broad range of parameters. Based on both
numerical and analytical results, we predict a strong enhancement of the
isotope effect when the frequencies of spin fluctuation and phonons are of the
same order. This effect may have important consequences for near-magnetic
superconductors such as MgCNiComment: 5 pages, 2 figure
Doping dependent optical properties of Bi2201
An experimental study of the in-plane optical conductivity of
(Pb,Bi)(LaSr)CuO (Bi2201) is presented
for a broad doping and temperature range. The in-plane conductivity is analyzed
within a strong coupling formalism. We address the interrelationship between
the optical conductivity (), the single particle self energy,
and the electron-boson spectral function. We find that the frequency and
temperature dependence can be well described within this formalism. We present
a universal description of optical, ARPES and tunneling spectra. The full
frequency and temperature dependence of the optical spectra and single particle
self-energy is shown to result from an electron-boson spectral function, which
shows a strong doping dependence and weak temperature dependence.Comment: 20 pages, 9 figures. To appear in special focus issue
"Superconductors with Exotic Symmetries", New Journal of Physic
A spectral function tour of electron-phonon coupling outside the Migdal limit
We simulate spectral functions for electron-phonon coupling in a filled band
system - far from the asymptotic limit often assumed where the phonon energy is
very small compared to the Fermi energy in a parabolic band and the Migdal
theorem predicting 1+lambda quasiparticle renormalizations is valid. These
spectral functions are examined over a wide range of parameter space through
techniques often used in angle-resolved photoemission spectroscopy (ARPES).
Analyzing over 1200 simulations we consider variations of the microscopic
coupling strength, phonon energy and dimensionality for two models: a
momentum-independent Holstein model, and momentum-dependent coupling to a
breathing mode phonon. In this limit we find that any `effective coupling',
lambda_eff, inferred from the quasiparticle renormalizations differs from the
microscopic dimensionless coupling characterizing these Hamiltonians, lambda,
and could drastically either over- or under-estimate it depending on the
particular parameters and model. In contrast, we show that perturbation theory
retains good predictive power for low coupling and small momenta, and that the
momentum-dependence of the self-energy can be revealed via the relationship
between velocity renormalization and quasiparticle strength. Additionally we
find that (although not strictly valid) it is often possible to infer the
self-energy and bare electronic structure through a self-consistent
Kramers-Kronig bare-band fitting; and also that through lineshape alone, when
Lorentzian, it is possible to reliably extract the shape of the imaginary part
of a momentum-dependent self-energy without reference to the bare-band.Comment: 15 pages, 11 figures. High resolution available here:
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/sf_tour.pd
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