7,960 research outputs found
Magnetic behavior of a spin-1 Blume-Emery-Griffiths model
I study the one-dimensional spin-1 Blume-Emery-Griffiths model with bilinear
and biquadratic exchange interactions and single-ion crystal field under an
applied magnetic field. This model can be exactly mapped into a tight-binding
Hubbard model - extended to include intersite interactions - provided one
renormalizes the chemical and the on-site potentials, which become temperature
dependent. After this transformation, I provide the exact solution of the
Blume-Emery-Griffiths model in one dimension by means of the Green's functions
and equations of motion formalism. I investigate the magnetic variations of
physical quantities - such as magnetization, quadrupolar moment, susceptibility
- for different values of the interaction parameters and of the applied field,
focusing on the role played by the biquadratic interaction in the breakdown of
the magnetization plateaus.Comment: 4 pages, 5 figures. ICM 2009 (Karlsruhe) Conference proceeding
Advanced vehicle separation apparatus
A method of obtaining test data from two independent models or bodies in a conventional wind tunnel is described. The system makes efficient use of wind tunnel test time with computer control performing complex coordinate transformations necessary for model positioning. The apparatus is designed to be used in any of the three Unitary Wind Tunnels at NASA-Ames Research Center. Mechanical design details and a brief description of the control system for the separation apparatus are presented
Equations of motion approach to the spin-1/2 Ising model on the Bethe lattice
We exactly solve the ferromagnetic spin-1/2 Ising model on the Bethe lattice
in the presence of an external magnetic field by means of the equations of
motion method within the Green's function formalism. In particular, such an
approach is applied to an isomorphic model of localized Fermi particles
interacting via an intersite Coulomb interaction. A complete set of
eigenoperators is found together with the corresponding eigenvalues. The
Green's functions and the correlation functions are written in terms of a
finite set of parameters to be self-consistently determined. A procedure is
developed, that allows us to exactly fix the unknown parameters in the case of
a Bethe lattice with any coordination number z. Non-local correlation functions
up to four points are also provided together with a study of the relevant
thermodynamic quantities.Comment: RevTex, 29 pages, 13 figure
Blind encoding into qudits
We consider the problem of encoding classical information into unknown qudit
states belonging to any basis, of a maximal set of mutually unbiased bases, by
one party and then decoding by another party who has perfect knowledge of the
basis. Working with qudits of prime dimensions, we point out a no-go theorem
that forbids shift operations on arbitrary unknown states. We then provide the
necessary conditions for reliable encoding/decoding.Comment: To appear in Physics Letters
Bosonic sector of the two-dimensional Hubbard model studied within a two-pole approximation
The charge and spin dynamics of the two-dimensional Hubbard model in the
paramagnetic phase is first studied by means of the two-pole approximation
within the framework of the Composite Operator Method. The fully
self-consistent scheme requires: no decoupling, the fulfillment of both Pauli
principle and hydrodynamics constraints, the simultaneous solution of fermionic
and bosonic sectors and a very rich momentum dependence of the response
functions. The temperature and momentum dependencies, as well as the dependency
on the Coulomb repulsion strength and the filling, of the calculated charge and
spin susceptibilities and correlation functions are in very good agreement with
the numerical calculations present in the literature
Refined physical properties and g',r',i',z',J,H,K transmission spectrum of WASP-23b from the ground
Multi-band observations of planetary transits using the telescope defocus
technique may yield high-quality light curves suitable for refining the
physical properties of exoplanets even with small or medium size telescopes.
Such observations can be used to construct a broad-band transmission spectrum
of transiting planets and search for the presence of strong absorbers. We have
thoroughly characterised the orbital ephemeris and physical properties of the
transiting planet and host star in the WASP-23b system, constructed a
broad-band transmission spectrum of WASP-23b and performed a comparative
analysis with theoretical models of hot Jupiters. We observed a complete
transit of WASP-23b in seven bands simultaneously, using the GROND instrument
on the MPG/ESO 2.2m telescope at La Silla Observatory and telescope
defocussing. The optical data were taken in the Sloan g',r',i' and z' bands.
The resulting light curves are of high quality, with a root-mean-square scatter
of the residual as low as 330ppm in the z'-band, with a cadence of 90s.
Near-infrared data were obtained in the JHK bands. We performed MCMC analysis
of our photometry plus existing radial velocity data to refine measurements of
the ephemeris and physical properties of the WASP-23. We constructed a
broad-band transmission spectrum of WASP-23b and compared it with a theoretical
transmission spectrum of a Hot Jupiter. We measured the central transit time
with a precision about 8s. From this and earlier observations we obtain an
orbital period of P=2.9444300+/-0.0000011d. Our analysis also yielded a larger
radius and mass for the planet (Rp=1.067+0.045-0.038 RJup and,
Mp=0.917+0.040-0.039MJup). The transmission spectrum is marginally flat, given
the limited precision of the measurements for the planet radius and poor
spectral resolution of the data.Comment: 8 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
High--order connected moments expansion for the Rabi Hamiltonian
We analyze the convergence properties of the connected moments expansion
(CMX) for the Rabi Hamiltonian. To this end we calculate the moments and
connected moments of the Hamiltonian operator to a sufficiently large order.
Our large--order results suggest that the CMX is not reliable for most
practical purposes because the expansion exhibits considerable oscillations.Comment: 12 pages, 5 figures, 1 tabl
A Study of the Antiferromagnetic Phase in the Hubbard Model by means of the Composite Operator Method
We have investigated the antiferromagnetic phase of the 2D, the 3D and the
extended Hubbard models on a bipartite cubic lattice by means of the Composite
Operator Method within a two-pole approximation. This approach yields a fully
self-consistent treatment of the antiferromagnetic state that respects the
symmetry properties of both the model and the algebra. The complete phase
diagram, as regards the antiferromagnetic and the paramagnetic phases, has been
drawn. We firstly reported, within a pole approximation, three kinds of
transitions at half-filling: Mott-Hubbard, Mott-Heisenberg and Heisenberg. We
have also found a metal-insulator transition, driven by doping, within the
antiferromagnetic phase. This latter is restricted to a very small region near
half filling and has, in contrast to what has been found by similar approaches,
a finite critical Coulomb interaction as lower bound at half filling. Finally,
it is worth noting that our antiferromagnetic gap has two independent
components: one due to the antiferromagnetic correlations and another coming
from the Mott-Hubbard mechanism.Comment: 20 pages, 37 figures, RevTeX, submitted to Phys. Rev.
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