124 research outputs found
Can disorder alone destroy the eg' hole pockets of NaxCoO2? A Wannier function based first-principles method for disordered systems
We investigate from first principles the proposed destruction of the
controversial eg' pockets in the Fermi surface of NaxCoO2 due to Na disorder,
by calculating its k-dependent configurational averaged spectral function
. To this end, a Wannier function based method is developed that treats
the effects of disorder beyond the mean field. Remarkable spectral broadenings
of order ~eV are found for the oxygen orbitals, possibly explaining their
absence in the experiments. Contrary to the current lore, however, the eg'
pockets remain almost perfectly coherent. The developed method is expected to
generate exciting opportunities in the study of the countless functional
materials that owe their important electronic properties to disordered dopants
Enhanced superconductivity due to forward scattering in FeSe thin films on SrTiO3 substrates
We study the consequences of an electron-phonon (-) interaction that
is strongly peaked in the forward scattering () direction in a
two-dimensional superconductor using Migdal-Eliashberg theory. We find that
strong forward scattering results in an enhanced that is linearly
proportional to the strength of the dimensionless - coupling constant
in the weak coupling limit. This interaction also produces distinct
replica bands in the single-particle spectral function, similar to those
observed in recent angle-resolved photoemission experiments on FeSe monolayers
on SrTiO and BaTiO substrates. By comparing our model to photoemission
experiments, we infer an - coupling strength that can provide a
significant portion of the observed high in these systems.Comment: Main text 5 pages, 4 figures; and Supplementary Informatio
Unfolding first-principles band structures
A general method is presented to unfold band structures of first-principles
super-cell calculations with proper spectral weight, allowing easier
visualization of the electronic structure and the degree of broken
translational symmetry. The resulting unfolded band structures contain
additional rich information from the Kohn-Sham orbitals, and absorb the
structure factor that makes them ideal for a direct comparison with angular
resolved photoemission spectroscopy experiments. With negligible computational
expense via the use of Wannier functions, this simple method has great
practical value in the studies of a wide range of materials containing
impurities, vacancies, lattice distortions, or spontaneous long-range orders.Comment: 4 pages, 3 figure
What is the valence of Mn in GaMnN?
We investigate the current debate on the Mn valence in GaMnN, a
diluted magnetic semiconductor (DMSs) with a potentially high Curie
temperature. From a first-principles Wannier-function analysis, we
unambiguously find the Mn valence to be close to (), but in a mixed
spin configuration with average magnetic moments of 4. By integrating
out high-energy degrees of freedom differently, we further derive for the first
time from first-principles two low-energy pictures that reflect the intrinsic
dual nature of the doped holes in the DMS: 1) an effective picture ideal
for local physics, and 2) an effective picture suitable for extended
properties. In the latter, our results further reveal a few novel physical
effects, and pave the way for future realistic studies of magnetism. Our study
not only resolves one of the outstanding key controversies of the field, but
also exemplifies the general need for multiple effective descriptions to
account for the rich low-energy physics in many-body systems in general.Comment: 4 figure
Aspects of electron-phonon interactions with strong forward scattering in FeSe Thin Films on SrTiO substrates
Mono- and multilayer FeSe thin films grown on SrTiO and
BiTiO substrates exhibit a greatly enhanced superconductivity over
that found in bulk FeSe. A number of proposals have been advanced for the
mechanism of this enhancement. One possibility is the introduction of a
cross-interface electron-phonon (-) interaction between the FeSe
electrons and oxygen phonons in the substrates that is peaked in the forward
scattering (small ) direction due to the two-dimensional nature of the
interface system. Motivated by this, we explore the consequences of such an
interaction on the superconducting state and electronic structure of a
two-dimensional system using Migdal-Eliashberg theory. This interaction
produces not only deviations from the expectations of conventional
phonon-mediated pairing but also replica structures in the spectral function
and density of states, as probed by angle-resolved photoemission spectroscopy,
scanning tunneling microscopy/spectroscopy, and quasi-particle interference
imaging. We also discuss the applicability of Migdal-Eliashberg theory for a
situation where the \ep interaction is peaked at small momentum transfer and in
the FeSe/STO system
Recovering hidden Bloch character: Unfolding Electrons, Phonons, and Slabs
For a quantum state, or classical harmonic normal mode, of a system of
spatial periodicity "R", Bloch character is encoded in a wavevector "K". One
can ask whether this state has partial Bloch character "k" corresponding to a
finer scale of periodicity "r". Answering this is called "unfolding." A theorem
is proven that yields a mathematically clear prescription for unfolding, by
examining translational properties of the state, requiring no "reference
states" or basis functions with the finer periodicity (r,k). A question then
arises, how should one assign partial Bloch character to a state of a finite
system? A slab, finite in one direction, is used as the example. Perpendicular
components k_z of the wavevector are not explicitly defined, but may be hidden
in the state (and eigenvector |i>.) A prescription for extracting k_z is
offered and tested. An idealized silicon (111) surface is used as the example.
Slab-unfolding reveals surface-localized states and resonances which were not
evident from dispersion curves alone.Comment: 11 pages, 7 figure
Universality of scanning tunneling microscopy in cuprate superconductors
We consider the problem of local tunneling into cuprate superconductors,
combining model based calculations for the superconducting order parameter with
wavefunction information obtained from first principles electronic structure.
For some time it has been proposed that scanning tunneling microscopy (STM)
spectra do not reflect the properties of the superconducting layer in the
CuO plane directly beneath the STM tip, but rather a weighted sum of
spatially proximate states determined by the details of the tunneling process.
These "filter" ideas have been countered with the argument that similar
conductance patterns have been seen around impurities and charge ordered states
in systems with atomically quite different barrier layers. Here we use a
recently developed Wannier function based method to calculate topographies,
spectra, conductance maps and normalized conductance maps close to impurities.
We find that it is the local planar Cu Wannier function,
qualitatively similar for many systems, that controls the form of the tunneling
spectrum and the spatial patterns near perturbations. We explain how, despite
the fact that STM observables depend on the materials-specific details of the
tunneling process and setup parameters, there is an overall universality in the
qualitative features of conductance spectra. In particular, we discuss why STM
results on BiSrCaCuO and CaNaCuOCl are
essentially identical
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