622 research outputs found
Lattice dynamics and electron-phonon interaction in (3,3) carbon nanotubes
We present a detailed study of the lattice dynamics and electron-phonon
coupling for a (3,3) carbon nanotube which belongs to the class of small
diameter based nanotubes which have recently been claimed to be
superconducting. We treat the electronic and phononic degrees of freedom
completely by modern ab-initio methods without involving approximations beyond
the local density approximation. Using density functional perturbation theory
we find a mean-field Peierls transition temperature of approx 40K which is an
order of magnitude larger than the calculated superconducting transition
temperature. Thus in (3,3) tubes the Peierls transition might compete with
superconductivity. The Peierls instability is related to the special 2k_F
nesting feature of the Fermi surface. Due to the special topology of the (n,n)
tubes also a q=0 coupling between the two bands crossing the Fermi energy at
k_F is possible which leads to a phonon softening at the Gamma point.Comment: 4 pages, 3 figures; to be published in Phys. Rev. Let
Microscopic calculation of the phonon dynamics of SrRuO compared with LaCuO
The phonon dynamics of the low-temperature superconductor SrRuO
is calculated quantitatively in linear response theory and compared with the
structurally isomorphic high-temperature superconductor LaCuO. Our
calculation corrects for a typical deficit of LDA-based calculations which
always predict a too large electronic -dispersion insufficient to
describe the c-axis response in the real materials. With a more realistic
computation of the electronic band structure the frequency and wavevector
dependent irreducible polarization part of the density response function is
determined and used for adiabatic and nonadiabatic phonon calculations. Our
analysis for SrRuO reveals important differences from the lattice
dynamics of - and -doped cuprates. Consistent with experimental evidence
from inelastic neutron scattering the anomalous doping related softening of the
strongly coupling high-frequency oxygen bond-stretching modes (OBSM) which is
generic for the cuprate superconductors is largely suppressed or completely
absent, respectively, depending on the actual value of the on-site Coulomb
repulsion of the Ru4d orbitals. Also the presence of a characteristic
-mode with a very steep dispersion coupling strongly with the
electrons is missing in SrRuO. Moreover, we evaluate the
possibility of a phonon-plasmon scenario for SrRuO which has been
shown recently to be realistic for LaCuO. In contrast to
LaCuO in SrRuO the very low lying plasmons are
overdamped along the c-axis.Comment: 30 pages, 16 figures, 4 tables, 33 reference
Lattice dynamics and electron-phonon coupling in transition metal diborides
The phonon density-of-states of transition metal diborides TMB2 with TM = Ti,
V, Ta, Nb and Y has been measured using the technique of inelastic neutron
scattering. The experimental data are compared with ab initio density
functional calculations whereby an excellent agreement is registered. The
calculations thus can be used to obtain electron-phonon spectral functions
within the isotropic limit. A comparison to similar data for MgB2 and AlB2
which were subject of prior publications as well as parameters important for
the superconducting properties are part of the discussion.Comment: 4 pages, 3 figure
Relação entre alumínio e silício em genótipos de milho resistente e sensível a toxidez de alumínio.
A fim de obter informações sobre a relação entre alumínio e silício no crescimento de dois genótipos de milho, sendo um tolerante e outro sensível a toxidez de alumínio, foi realizado um estudo em casa de vegetação, na presença e ausência de alumínio e silício em dois genótipos de milho. Os tratamentos utilizados foram uma solução de 2,0 mmol L-1 de cálcio (CaCl2), na presença e na ausência de alumínio (0,025 mmol L-1- AlCl3) e de silício (0,14 mmol L-1 a partir de uma solução de SiO2). O experimento foi conduzido por um período de cinco dias e no final foram avaliados o comprimento de raiz, as concentrações de silício, alumínio e cálcio na parte aérea e nas raízes e o índice mitótico de células meristemáticas das raízes. O silício reduziu o efeito tóxico do alumínio no crescimento radicular dos genótipos de milho independente da sensibilidade ao alumínio
Properties of the phonon-induced pairing interaction in YBaCuO within the local density approximation
The properties of the phonon-induced interaction between electrons are
studied using the local density approximation (LDA). Restricting the electron
momenta to the Fermi surface we find generally that this interaction has a
pronounced peak for large momentum transfers and that the interband
contributions between bonding and antibonding band are of the same magnitude as
the intraband ones. Results are given for various symmetry averages of this
interaction over the Fermi surface. In particular, we find that the
dimensionless coupling constant in the d-wave channel , relevant for
superconductivity, is only 0.022, i.e., even about ten times smaller than the
small value of the s-wave channel. Similarly, the LDA contribution to the
resistivity is about a factor 10 times smaller than the observed resistivity
suggesting that phonons are not the important low-energy excitations in
high-T oxides.Comment: 6 pages, 7 figure
First-principles calculations of the dispersion of surface phonons of the unreconstructed and reconstructed Pt(110)
We present result of calculations of the surface phonon dispersion curves for
Pt(110) using density functional theory in the local density approximation and
norm conserving pseudopotentials in a mixed-basis approach. Linear response
theory is invoked and both the unreconstructed, and the missing row (1x2)
reconstructed surfaces are considered. We find that the reconstruction is not
driven by a phonon instability. Most of the observed phonon modes for the (1x2)
structure can be understood in terms of simple folding of the (1x1) Brillouin
zone onto that for the (1x2) surface. Largest changes in the phonon frequencies
on surface reconstruction occur close to the zone boundary in the (001)
direction. Detailed comparison of atomic force constants for the (1x1) and the
(1x2) surfaces and their bulk counterparts show that the bulk value is attained
after three layers. Our calculations reproduce nicely the Kohn anomaly observed
along the (110) direction in the bulk. We do not find a corresponding effect on
the surface
First-principles calculations of the structural, electronic, vibrational and magnetic properties of C_{60} and C_{48}N_{12}: a comparative study
In this work, we perform first-principles calculations of the structural,
electronic, vibrational and magnetic properties of a novel azafullerene. Full geometrical optimization shows that is characterized by several distinguishing features: only
one nitrogen atom per pentagon, two nitrogen atoms preferentially sitting in
one hexagon, symmetry, 6 unique nitrogen-carbon and 9 unique
carbon-carbon bond lengths. The highest occupied molecular orbital of is a doubly degenerate level of symmetry and its
lowest unoccupied molecular orbital is a nondegenerate level of
symmetry. Vibrational frequency analysis predicts that has in total 116 vibrational modes: 58 infrared-active and 58
Raman-active modes. is also characterized by 8
and 2 NMR spectral signals. Compared to , shows an enhanced third-order optical
nonlinearities which implies potential applications in optical limiting and
photonics.Comment: a long version of our manuscript submitted to J.Chem.Phy
Photoemission kinks and phonons in cuprates
One of the possible mechanisms of high Tc superconductivity is Cooper pairing
with the help of bosons, which change the slope of the electronic dispersion as
observed by photoemission. Giustino et al. calculated that in the high
temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations
(phonons) should have a negligible effect on photoemission spectra and
concluded that phonons do not play an important role. We show that the
calculations employed by Giustino et al. fail to reproduce huge influence of
electron-phonon coupling on important phonons observed in experiments. Thus one
would expect these calculations to similarly fail in explaining the role of
electron-phonon coupling for the electronic dispersion.Comment: To appear in Nature as a Brief Communiction Arisin
Heat Capacity and Magnetic Phase Diagram of the Low-Dimensional Antiferromagnet YBaCuO
A study by specific heat of a polycrystalline sample of the low-dimensional
magnetic system YBaCuO is presented. Magnetic fields up to 14 T are
applied and permit to extract the (,) phase diagram. Below
T, the N\'eel temperature, associated with a
three-dimensional antiferromagnetic long-range ordering, is constant and equals
K. Above , increases linearly with and a
field-induced increase of the entropy at is related to the presence of an
isosbestic point at K, where all the specific heat curves cross.
A comparison is made between YBaCuO and the quasi-two-dimensional
magnetic systems BaNiVO, SrCuOCl, and
PrCuO, for which very similar phase diagrams have been reported. An
effective field-induced magnetic anisotropy is proposed to explain these phase
diagrams.Comment: 14 pages, 7 figure
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