340 research outputs found
Pressure-induced hole doping of the Hg-based cuprate superconductors
We investigate the electronic structure and the hole content in the
copper-oxygen planes of Hg based high Tc cuprates for one to four CuO2 layers
and hydrostatic pressures up to 15 GPa. We find that with the pressure-induced
additional number of holes of the order of 0.05e the density of states at the
Fermi level changes approximately by a factor of 2. At the same time the saddle
point is moved to the Fermi level accompanied by an enhanced k_z dispersion.
This finding explains the pressure behavior of Tc and leads to the conclusion
that the applicability of the van Hove scenario is restricted. By comparison
with experiment, we estimate the coupling constant to be of the order of 1,
ruling out the weak coupling limit.Comment: 4 pages, 4 figure
Investigation of A1g phonons in YBa2Cu3O7 by means of LAPW atomic-force calculations
We report first-principles frozen-phonon calculations for the determination
of the force-free geometry and the dynamical matrix of the five Raman-active
A1g modes in YBa2Cu3O7. To establish the shape of the phonon potentials atomic
forces are calculated within the LAPW method. Two different schemes - the local
density approximation (LDA) and a generalized gradient approximation (GGA) -
are employed for the treatment of electronic exchange and correlation effects.
We find that in the case of LDA the resulting phonon frequencies show a
deviation from experimental values of approximately -10%. Invoking GGA the
frequency values are significantly improved and also the eigenvectors are in
very good agreement with experimental findings.Comment: 15 page
Band-structure topologies of graphene: spin-orbit coupling effects from first principles
The electronic band structure of graphene in the presence of spin-orbit
coupling and transverse electric field is investigated from first principles
using the linearized augmented plane-wave method. The spin-orbit coupling opens
a gap at the -point of the magnitude of 24 eV (0.28 K). This
intrinsic splitting comes 96% from the usually neglected and higher
orbitals. The electric field induces an additional (extrinsic)
Bychkov-Rashba-type splitting of 10 eV (0.11 K) per V/nm, coming from the
- mixing. A 'mini-ripple' configuration with every other atom is
shifted out of the sheet by less than 1% differs little from the intrinsic
case.Comment: 4 pages, 4 figure
Lithiation of InSb and CuSb : A Theoretical Investigation
In this work the mechanism of Li insertion/intercalation in the anode
materials InSb and CuSb is investigated by means of the first principles
total energy calculations. The total charge densities for the lithiated
products of the two compounds are presented. Based on these results the change
in the bonding character on lithiation is discussed. Further, the isomer shift
for InSb and CuSb and there various lithiated products is reported. The
average insertion/intercalation voltage and volume expansion for transitions
from InSb to LiInSb and CuSb to LiCuSb are calculated and found to
be in good agreement with the experimental values. These findings help to
resolve the controversy regarding the lithiation mechanism in InSb.Comment: 5 pages 3 figure
Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach
The crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Δ, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Δ, from below to above the thermal energy kBT at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices
Optical Properties and Correlation Effects in NaxCoO2
We have calculated the optical spectra of NaCoO for =0.3, 0.5,
and 0.7 within the LDA. We compare our results to available experimental data
and show that the important features and trends are reproduced well, but there
is a nearly uniform shift of peak positions and poor agreement in intensities.
We show, through application of a simple model, that these differences can be
attributed to overhybridization between Co and O orbitals and spin fluctuations
which renormalize the bandwidth. Applying the LDA+U procedure shifts the
optical peaks further from their experimental locations, indicating that this
method of incorporating correlation effects is ill-suited for the case NaxCoO2
Optical properties, electron-phonon coupling, and Raman scattering of vanadium ladder compounds
The electronic structure of two V-based ladder compounds, the quarter-filled
NaVO in the symmetric phase and the iso-structural half-filled
CaVO is investigated by ab initio calculations. Based on the
bandstructure we determine the dielectric tensor of these
systems in a wide energy range. The frequencies and eigenvectors of the fully
symmetric A phonon modes and the corresponding electron-phonon and
spin-phonon coupling parameters are also calculated from first-principles. We
determine the Raman scattering intensities of the A phonon modes as a
function of polarization and frequency of the exciting light.
All results, i.e. shape and magnitude of the dielectric function, phonon
frequencies and Raman intensities show very good agreement with available
experimental data.Comment: 11 pages, 10 figure
Fermi surface induced lattice distortion in NbTe
The origin of the monoclinic distortion and domain formation in the quasi
two-dimensional layer compound NbTe is investigated. Angle-resolved
photoemission shows that the Fermi surface is pseudogapped over large portions
of the Brillouin zone. Ab initio calculation of the electron and phonon
bandstructure as well as the static RPA susceptibility lead us to conclude that
Fermi surface nesting and electron-phonon coupling play a key role in the
lowering of the crystal symmetry and in the formation of the charge density
wave phase
Electronic States and Superconducting Transition Temperature based on the Tomonaga-Luttinger liquid in PrBaCuO
An NQR experiment revealed superconductivity of
PrBaCuO (Pr247) to be realized on CuO double chain
layers and suggests possibility of novel one-dimensional(1D) superconductivity.
To clarify the nature of the 1D superconductivity, we calculate the band
dispersions of Pr247 by using the generalized gradient approximation(GGA). It
indicates that Fermi surface of CuO double chains is well described to the
electronic structure of a quasi-1D system.
Assuming the zigzag Hubbard chain model to be an effective model of the
system, we derive tight binding parameters of the model from a fit to the
result of GGA. Based on the Tomonaga-Luttinger liquid theory, we estimate
transition temperature () of the quasi-1D zigzag Hubbard model from the
calculated value of the Luttinger liquid parameter . The result of
is consistent with that of experiments in Pr247 and it suggests that the
mechanism of the superconductivity is well understood within the concept of the
Tomonaga-Luttinger liquid.Comment: 4 pages, 5 figure
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