363 research outputs found
Phase diagram of the lattice Wess-Zumino model from rigorous lower bounds on the energy
We study the lattice N=1 Wess-Zumino model in two dimensions and we construct
a sequence of exact lower bounds on its ground state energy
density , converging to in the limit . The bounds
can be computed numerically on a finite lattice with sites and
can be exploited to discuss dynamical symmetry breaking. The transition point
is determined and compared with recent results based on large-scale Green
Function Monte Carlo simulations with good agreement.Comment: 32 pages, 12 figure
Global dynamics above the ground state for the nonlinear Klein-Gordon equation without a radial assumption
We extend our previous result on the focusing cubic Klein-Gordon equation in
three dimensions to the non-radial case, giving a complete classification of
global dynamics of all solutions with energy at most slightly above that of the
ground state.Comment: 40 page
Ground and excited states Gamow-Teller strength distributions of iron isotopes and associated capture rates for core-collapse simulations
This paper reports on the microscopic calculation of ground and excited
states Gamow-Teller (GT) strength distributions, both in the electron capture
and electron decay direction, for Fe. The associated electron and
positron capture rates for these isotopes of iron are also calculated in
stellar matter. These calculations were recently introduced and this paper is a
follow-up which discusses in detail the GT strength distributions and stellar
capture rates of key iron isotopes. The calculations are performed within the
framework of the proton-neutron quasiparticle random phase approximation
(pn-QRPA) theory. The pn-QRPA theory allows a microscopic
\textit{state-by-state} calculation of GT strength functions and stellar
capture rates which greatly increases the reliability of the results. For the
first time experimental deformation of nuclei are taken into account. In the
core of massive stars isotopes of iron, Fe, are considered to be
key players in decreasing the electron-to-baryon ratio () mainly via
electron capture on these nuclide. The structure of the presupernova star is
altered both by the changes in and the entropy of the core material.
Results are encouraging and are compared against measurements (where possible)
and other calculations. The calculated electron capture rates are in overall
good agreement with the shell model results. During the presupernova evolution
of massive stars, from oxygen shell burning stages till around end of
convective core silicon burning, the calculated electron capture rates on
Fe are around three times bigger than the corresponding shell model
rates. The calculated positron capture rates, however, are suppressed by two to
five orders of magnitude.Comment: 18 pages, 12 figures, 10 table
First- principle calculations of magnetic interactions in correlated systems
We present a novel approach to calculate the effective exchange interaction
parameters based on the realistic electronic structure of correlated magnetic
crystals in local approach with the frequency dependent self energy. The analog
of ``local force theorem'' in the density functional theory is proven for
highly correlated systems. The expressions for effective exchange parameters,
Dzialoshinskii- Moriya interaction, and magnetic anisotropy are derived. The
first-principle calculations of magnetic excitation spectrum for ferromagnetic
iron, with the local correlation effects from the numerically exact QMC-scheme
is presented.Comment: 17 pages, 3 Postscript figure
Bosonic representation of one-dimensional Heisenberg ferrimagnets
The energy structure and the thermodynamics of ferrimagnetic Heisenberg
chains of alternating spins S and s are described in terms of the Schwinger
bosons and modified spin waves. In the Schwinger representation, we average the
local constraints on the bosons and diagonalize the Hamiltonian at the
Hartree-Fock level. In the Holstein-Primakoff representation, we optimize the
free energy in two different ways introducing an additional constraint on the
staggered magnetization. A new modified spin-wave scheme, which employs a
Lagrange multiplier keeping the native energy structure free from temperature
and thus differs from the original Takahashi Scheme, is particularly stressed
as an excellent language to interpret one-dimensional quantum ferrimagnetism.
Other types of one-dimensional ferrimagnets and the antiferromagnetic limit S=s
are also mentioned.Comment: to be published in Phys. Rev. B 69, No. 6, 0644XX (2004
DDW Order and its Role in the Phase Diagram of Extended Hubbard Models
We show in a mean-field calculation that phase diagrams remarkably similar to
those recently proposed for the cuprates arise in simple microscopic models of
interacting electrons near half-filling. The models are extended Hubbard models
with nearest neighbor interaction and correlated hopping. The underdoped region
of the phase diagram features density-wave (DDW) order. In a
certain regime of temperature and doping, DDW order coexists with
antiferromagnetic (AF) order. For larger doping, it coexists with
superconductivity (DSC). While phase diagrams of this form
are robust, they are not inevitable. For other reasonable values of the
coupling constants, drastically different phase diagrams are obtained. We
comment on implications for the cuprates.Comment: 7 pages, 3 figure
Status of a Supersymmetric Flavour Violating Solution to the Solar Neutrino Puzzle with Three Generations
We present a general study of a three neutrino flavour transition model based
on the supersymmetric interactions which violate R-parity. These interactions
induce flavour violating scattering reactions between solar matter and
neutrinos. The model does not contain any vacuum mass or mixing angle for the
first generation neutrino. Instead, the effective mixing in the first
generation is induced via the new interactions. The model provides a natural
interpretation of the atmospheric neutrino anomaly, and is consistent with
reactor experiments. We determine all R-parity violating couplings which can
contribute to the effective neutrino oscillations, and summarize the present
laboratory bounds. Independent of the specific nature of the (supersymmetric)
flavour violating model, the experimental data on the solar neutrino rates and
the recoil electron energy spectrum are inconsistent with the theoretical
predictions. The confidence level of the -analysis ranges between and . The incompatibility, is due to the new SNO
results, and excludes the present model. We conclude that a non-vanishing
vacuum mixing angle for the first generation neutrino is necessary in our
model. We expect this also to apply to the solutions based on other flavour
violating interactions having constraints of the same order of magnitude.Comment: 17 pages, Latex fil
Weak noise approach to the logistic map
Using a nonperturbative weak noise approach we investigate the interference
of noise and chaos in simple 1D maps. We replace the noise-driven 1D map by an
area-preserving 2D map modelling the Poincare sections of a conserved dynamical
system with unbounded energy manifolds. We analyze the properties of the 2D map
and draw conclusions concerning the interference of noise on the nonlinear time
evolution. We apply this technique to the standard period-doubling sequence in
the logistic map. From the 2D area-preserving analogue we, in addition to the
usual period-doubling sequence, obtain a series of period doubled cycles which
are elliptic in nature. These cycles are spinning off the real axis at
parameters values corresponding to the standard period doubling events.Comment: 22 pages in revtex and 8 figures in ep
Ground state of the three-band Hubbard model
The ground state of the two-dimensional three-band Hubbard model in oxide
superconductors is investigated by using the variational Monte Carlo method.
The Gutzwiller-projected BCS and spin- density wave (SDW) functions are
employed in the search for a possible ground state with respect to dependences
on electron density. Antiferromagnetic correlations are considerably enhanced
near half-filling. It is shown that the d-wave state may exist away from
half-filling for both the hole and electron doping cases. The overall structure
of the phase diagram obtained by the calculations qualitatively agrees with
experimental indications. The superconducting condensation energy is in
reasonable agreement with the experimental value obtained from specific heat
and critical magnetic field measurements for optimally doped samples. The
inhomogeneous SDW state is also examined near 1/8-hole doping.Comment: 10 pages, 17 figure
Lattice Pseudospin Model for Quantum Hall Bilayers
We present a new theoretical approach to the study of quantum Hall
bilayer that is based on a systematic mapping of the microscopic Hamiltonian to
an anisotropic SU(4) spin model on a lattice. To study the properties of this
model we generalize the Heisenberg model Schwinger boson mean field theory
(SBMFT) of Arovas and Auerbach to spin models with anisotropy. We calculate the
temperature dependence of experimentally observable quantities, including the
spin magnetization, and the differential interlayer capacitance. Our theory
represents a substantial improvement over the conventional Hartree-Fock picture
which neglects quantum and thermal fluctuations, and has advantages over
long-wavelength effective models that fail to capture important microscopic
physics at all realistic layer separations. The formalism we develop can be
generalized to treat quantum Hall bilayers at filling factor .Comment: 26 pages, 10 figures. The final version, to appear in PR
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