13,143 research outputs found
Essential Role of the Cooperative Lattice Distortion in the Charge, Orbital and Spin Ordering in doped Manganites
The role of lattice distortion in the charge, orbital and spin ordering in
half doped manganites has been investigated. For fixed magnetic ordering, we
show that the cooperative lattice distortion stabilize the experimentally
observed ordering even when the strong on-site electronic correlation is taken
into account. Furthermore, without invoking the magnetic interactions, the
cooperative lattice distortion alone may lead to the correct charge and orbital
ordering including the charge stacking effect, and the magnetic ordering can be
the consequence of such a charge and orbital ordering. We propose that the
cooperative nature of the lattice distortion is essential to understand the
complicated charge, orbital and spin ordering observed in doped manganites.Comment: 5 pages,4 figure
Single cell mechanics: stress stiffening and kinematic hardening
Cell mechanical properties are fundamental to the organism but remain poorly
understood. We report a comprehensive phenomenological framework for the
nonlinear rheology of single fibroblast cells: a superposition of elastic
stiffening and viscoplastic kinematic hardening. Our results show, that in
spite of cell complexity its mechanical properties can be cast into simple,
well-defined rules, which provide mechanical cell strength and robustness via
control of crosslink slippage.Comment: 4 pages, 6 figure
Coherent pion production in neutrino nucleus collision in the 1 GeV region
We calculate cross sections for coherent pion production in nuclei induced by
neutrinos and antineutrinos of the electron and muon type. The analogies and
differences between this process and the related ones of coherent pion
production induced by photons, or the (p,n) and reactions are
discussed. The process is one of the several ones occurring for intermediate
energy neutrinos, to be considered when detecting atmospheric neutrinos. For
this purpose the results shown here can be easily extrapolated to other
energies and other nuclei.Comment: 13 pages, LaTex, 8 post-script figures available at
[email protected]
Microscopic Calculation of Spin Torques in Disordered Ferromagnets
Effects of conduction electrons on magnetization dynamics, represented by
spin torques, are calculated microscopically in the first order in spatial
gradient and time derivative of magnetization. Special attention is paid to the
so-called -term and the Gilbert damping, , in the presence of
electrons' spin-relaxation processes, which are modeled by quenched magnetic
(and spin-orbit) impurities. The obtained results such as
hold for localized as well as itinerant ferromagnetism.Comment: 4 page
Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances
We analyze a sample of 3,944 low-resolution (R ~ 2000) optical spectra from
the Sloan Digital Sky Survey (SDSS), focusing on stars with effective
temperatures 5800 < Teff < 6300 K, and distances from the Milky Way plane in
excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. We followed
the same methodology as in the previous paper in this series, deriving
atmospheric parameters by chi2 minimization, but this time we obtained the
abundances of individual elements by fitting their associated spectral lines.
Distances were calculated from absolute magnitudes obtained by a statistical
comparison of our stellar parameters with stellar-evolution models. The
observations reveal a decrease in the abundances of iron, calcium, and
magnesium at large distances from the Galactic center. The median abundances
for the halo stars analyzed are fairly constant up to a Galactocentric distance
r ~ 20 kpc, rapidly decrease between r ~ 20 and r ~ 40 kpc, and flatten out to
significantly lower values at larger distances, consistent with previous
studies. In addition, we examine the [Ca/Fe] and [Mg/Fe] as a function of Fe/H
and Galactocentric distance. Our results show that the most distant parts of
the halo show a steeper variation of the [Ca/Fe] and [Mg/Fe] with iron. We
found that at the range -1.6 < [Fe/H] < -0.4 [Ca/Fe] decreases with distance,
in agreement with earlier results based on local stars. However, the opposite
trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the
halo are more metal-poor than the inner regions, based on in situ observations
of distant stars, agrees with recent results based on inferences from the
kinematics of more local stars, and with predictions of recent galaxy formation
simulations for galaxies similar to the Milky Way
PT-Symmetric Quantum Theory Defined in a Krein Space
We provide a mathematical framework for PT-symmetric quantum theory, which is
applicable irrespective of whether a system is defined on R or a complex
contour, whether PT symmetry is unbroken, and so on. The linear space in which
PT-symmetric quantum theory is naturally defined is a Krein space constructed
by introducing an indefinite metric into a Hilbert space composed of square
integrable complex functions in a complex contour. We show that in this Krein
space every PT-symmetric operator is P-Hermitian if and only if it has
transposition symmetry as well, from which the characteristic properties of the
PT-symmetric Hamiltonians found in the literature follow. Some possible ways to
construct physical theories are discussed within the restriction to the class
K(H).Comment: 8 pages, no figures; Refs. added, minor revisio
Origin of the Charge-Orbital Stripe Structure in La_(1-x)Ca_(x)MnO_(3) (x=1/2, 2/3)
We propose the origin of the charge-ordered stripe structure with the orbital
ordering observed experimentally in La_(1-x)Ca_(x)MnO_(3) (x=1/2, 2/3), in
which the long-range Coulomb interaction plays an essential role. We study a
Hubbard model with doubly-degenerate e_g orbitals, and treat the on-site
Coulomb interaction (U) and the nearest-neighbor one (V) with the Hartree-Fock
approximation. Both the charge and orbital ordering structures observed in
experiments are reproduced in a wide region of the U-V phase diagram determined
by the present study. The stability of the orbital ordering is also confirmed
by the perturbation theory.Comment: 4 pages, 5 Postscript figures, REVTeX, submitted to Phys. Rev. Let
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