3,341 research outputs found
Dispersive Gap Mode of Phonons in Anisotropic Superconductors
We estimate the effect of the superconducting gap anisotropy in the
dispersive gap mode of phonons, which is observed by the neutron scattering on
borocarbide superconductors. We numerically analyze the phonon spectrum
considering the electron-phonon coupling, and examine contributions coming from
the gap suppression and the sign change of the pairing function on the Fermi
surface. When the sign of the pairing function is changed by the nesting
translation, the gap mode does not appear. We also discuss the suppression of
the phonon softening of the Kohn anomaly due to the onset of superconductivity.
We demonstrate that observation of the gap dispersive mode is useful for
sorting out the underlying superconducting pairing function.Comment: 7 pages, 12 figures, to be published in J. Phys. Soc. Jp
Spinor Parallel Propagator and Green's Function in Maximally Symmetric Spaces
We introduce the spinor parallel propagator for maximally symmetric spaces in
any dimension. Then, the Dirac spinor Green's functions in the maximally
symmetric spaces R^n, S^n and H^n are calculated in terms of intrinsic
geometric objects. The results are covariant and coordinate-independent.Comment: 7 page
Magnetic neutron scattering study of YVO3: Evidence for an orbital Peierls state
Neutron spectroscopy has revealed a highly unusual magnetic structure and
dynamics in YVO, an insulating pseudocubic perovskite that undergoes a
series of temperature induced phase transitions between states with different
spin and orbital ordering patterns. A good description of the neutron data is
obtained by a theoretical analysis of the spin and orbital correlations of a
realistic one-dimensional model. This leads to the tentative identification of
one of the phases of YVO with the ``orbital Peierls state'', a
theoretically proposed many-body state comprised of orbital singlet bonds.Comment: final version, to appear in PR
Crystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_{4} : role of the orbital degree of freedom
The crystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_4 (0<x<0.7) has
been studied by diffraction techniques and high resolution capacitance
dilatometry. There is no evidence for a structural phase transition like those
found in isostructural cuprates or nickelates, but there are significant
structural changes induced by the variation of temperature and doping which we
attribute to a rearrangement of the orbital occupation.Comment: 8 pages, 6 figures, submitted to PR
Interplay of charge and orbital ordering in manganese perovskites
A model of localized classical electrons coupled to lattice degrees of
freedom and, via the Coulomb interaction, to each other, has been studied to
gain insight into the charge and orbital ordering observed in lightly doped
manganese perovskites. Expressions are obtained for the minimum energy and
ionic displacements caused by given hole and electron orbital configurations.
The expressions are analyzed for several hole configurations, including that
experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find
that, although the preferred charge and orbital ordering depend sensitively on
parameters, there are ranges of the parameters in which the experimentally
observed hole configuration has the lowest energy. For these parameter values
we also find that the energy differences between different hole configurations
are on the order of the observed charge ordering transition temperature. The
effects of additional strains are also studied. Some results for
La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately
describe this material because the high temperature phase is metallic.Comment: 12 pages in RevTex, 5 figures in PS files, to appear in Phys. Rev. B
(New paragraphs and references added, typos corrected
Entropy Driven Dimerization in a One-Dimensional Spin-Orbital Model
We study a new version of the one-dimensional spin-orbital model with spins
S=1 relevant to cubic vanadates. At small Hund's coupling J_H we discover
dimerization in a pure electronic system solely due to a dynamical spin-orbital
coupling. Above a critical value J_H, a uniform ferromagnetic state is
stabilized at zero temperature. More surprisingly, we observe a temperature
driven dimerization of the ferrochain, which occurs due to a large entropy
released by dimer states. This dynamical dimerization seems to be the mechanism
driving the peculiar intermediate phase of YVO_3.Comment: 5 pages, 4 figure
Charge and orbital ordering in underdoped La1-xSrxMnO3
We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0
< x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At
x=1/8, several charge and orbitally modulated states are found to be stable and
almost degenerate in energy with a homogeneous ferromagnetic state. The present
calculation indicates that a ferromagnetic state with a charge modulation along
the c-axis which is consistent with the experiment by Yamada et al. might be
responsible for the anomalous behavior around x = 1/8.Comment: 5 pages, 5 figure
Restrictions on the lifetime of sterile neutrinos from primordial nucleosynthesis
We analyze the influence of decaying sterile neutrinos with the masses in the
range 1-140 MeV on the primordial Helium-4 abundance, explicitly solving the
Boltzmann equations for all particle species, taking into account neutrino
flavour oscillations, and paying special attention to systematic uncertainties.
We show that the Helium abundance depends only on the sterile neutrino lifetime
and not on the way the active-sterile mixing is distributed between flavours,
and derive an upper bound on the lifetime. We also demonstrate that the recent
results of Izotov & Thuan [arXiv:1001.4440], who find 2sigma higher than
predicted by the standard primordial nucleosynthesis value of Helium-4
abundance, are consistent with the presence in the plasma of sterile neutrinos
with the lifetime 0.01-2 seconds. The decay of these particles perturbs the
spectra of (decoupled) neutrinos and heats photons, changing the ratio of
neutrino to photon energy density, that can be interpreted as extra neutrino
species at the recombination epoch.Comment: 17 pp. + Appendices. Analysis of deuterium bounds and more accurate
account of CMB bounds on Helium-4 is added. Final version to appear in JCA
Heating process in the pre-Breakdown regime of the Quantum Hall Efect : a size dependent effect
Our study presents experimental measurements of the contact and longitudinal
voltage drops in Hall bars, as a function of the current amplitude. We are
interested in the heating phenomenon which takes place before the breakdown of
the quantum Hall effect, i.e. the pre-breakdown regime. Two types of samples
has been investigated, at low temperature (4.2 and 1.5K) and high magnetic
field (up to 13 T). The Hall bars have several different widths, and our
observations clearly demonstrate that the size of the sample influences the
heating phenomenon. By measuring the critical currents of both contact and
longitudinal voltages, as a function of the filling factor (around ), we
highlight the presence of a high electric field domain near the source contact,
which is observable only in samples whose width is smaller than 400 microns.Comment: 4 pages, 5 igures, 7th International Symposium of Research in High
Magnetic Fields, to be published in physica
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