9,297 research outputs found
An extended Agassi model: algebraic structure, phase diagram, and large size limit
The Agassi model is a schematic two-level model that involves pairing and
monopole-monopole interactions. It is, therefore, an extension of the well
known Lipkin-Meshkov-Glick (LMG) model. In this paper we review the algebraic
formulation of an extension of the Agassi model as well as its bosonic
realization through the Schwinger representation. Moreover, a mean-field
approximation for the model is presented and its phase diagram discussed.
Finally, a analysis, with proportional to the degeneracy of each
level, is worked out to obtain the thermodynamic limit of the ground state
energy and some order parameters from the exact Hamiltonian diagonalization for
finite.Comment: Accepted in Physica Scripta. Focus on SSNET 201
Phase diagram of an extended Agassi model
Background: The Agassi model is an extension of the Lipkin-Meshkov-Glick
model that incorporates the pairing interaction. It is a schematic model that
describes the interplay between particle-hole and pair correlations. It was
proposed in the 1960's by D. Agassi as a model to simulate the properties of
the quadrupole plus pairing model.
Purpose: The aim of this work is to extend a previous study by Davis and
Heiss generalizing the Agassi model and analyze in detail the phase diagram of
the model as well as the different regions with coexistence of several phases.
Method: We solve the model Hamiltonian through the Hartree-Fock-Bogoliubov
(HFB) approximation, introducing two variational parameters that play the role
of order parameters. We also compare the HFB calculations with the exact ones.
Results: We obtain the phase diagram of the model and classify the order of
the different quantum phase transitions appearing in the diagram. The phase
diagram presents broad regions where several phases, up to three, coexist.
Moreover, there is also a line and a point where four and five phases are
degenerated, respectively.
Conclusions: The phase diagram of the extended Agassi model presents a rich
variety of phases. Phase coexistence is present in extended areas of the
parameter space. The model could be an important tool for benchmarking novel
many-body approximations.Comment: Accepted for publication in PR
Squeezing enhancement by competing nonlinearities: Almost perfect squeezing without instabilities
4 págs.; 3 figs.; PACS number~s!: 42.50.Dv, 42.50.Lc, 42.65.KyThe competition between the χ(2) nonlinearity of a resonant second-harmonic-generation (SHG) system and an added χ(3) nonlinearity shifts the Hopf bifurcation of the standard SHG towards higher photon numbers eventually completely stabilizing the system. Remarkably, perfect squeezing survives the stabilization. Two important consequences are discussed, namely, efficient bright squeezing generation and strong suppression of the excess noise which, for parameters corresponding to an experiment reported in the literature, can be reduced by two orders of magnitude without diminishing the squeezing. Possible experimental implementations are finally discussed. ©1997 American Physical SocietyThis work was supported in part by Project No. TIC95-
0563-C05-03 (CICYT, Spain).Peer Reviewe
Creation of entangled states of distant atoms by interference
9 págs.; 2 figs.; app.; PACS number~s!: 03.65.Bz, 42.50.VkWe propose a scheme to create distant entangled atomic states. It is based on driving two (or more) atoms with a weak laser pulse, so that the probability that two atoms are excited is negligible. If the subsequent spontaneous emission is detected, the entangled state is created. We have developed a model to analyze the fidelity of the resulting state as a function of the dimensions and location of the detector, and the motional properties of the atoms. ©1999 American Physical SocietyThis work was supported in part by the
Acciones Integradas No. HU/997-0030 (Spain-Austria),Grant Nos. TIC95-0563-C05-03 and PB96-00819 from
CICYT (Spain), the Comunidad de Madrid under Grant No. 06T/039/96 (Spain), the FWF (Austrian Science Foundation)
and TMR Network No. ERB–FMRX–CT96–0087.Peer Reviewe
Giant magnetic anisotropy at nanoscale: overcoming the superparamagnetic limit
It has been recently observed for palladium and gold nanoparticles, that the
magnetic moment at constant applied field does not change with temperature over
the range comprised between 5 and 300 K. These samples with size smaller than
2.5 nm exhibit remanence up to room temperature. The permanent magnetism for so
small samples up to so high temperatures has been explained as due to blocking
of local magnetic moment by giant magnetic anisotropies. In this report we
show, by analysing the anisotropy of thiol capped gold films, that the orbital
momentum induced at the surface conduction electrons is crucial to understand
the observed giant anisotropy. The orbital motion is driven by localised charge
and/or spin through spin orbit interaction, that reaches extremely high values
at the surfaces. The induced orbital moment gives rise to an effective field of
the order of 103 T that is responsible of the giant anisotropy.Comment: 15 pages, 2 figures, submitted to PR
Effects of nonlinear dispersion on squeezed states in two-photon devices
7 págs.; 4 figs.; 1 apémdiceThe deleterious effects of nonlinear dispersion on squeezed light in two-photon devices when absorption losses are included have been analyzed making use of the variables called two-mode quadrature-phase amplitudes. The uncertainties in the quadrature amplitudes have been computed from a generalized Fokker-Planck equation. The dependence of squeezing on the nonlinear coupling, the modulation (including the case of high modulations), the pumping phase (where nonlinear dispersion causes an intensity-dependent shift in the minimum squeezing), and the absorption losses have been studied. © 1989 The American Physical Society.Peer Reviewe
Low-noise properties of squeezed light in transmission chains formed from nonlinear alternating attenuators and amplifiers
4 págs.; 4 figs.An idealized model of an optical communication link in which identical sections of nonlinear attenuating fiber alternate with identical amplifiers is considered. Signal-to-noise ratios of chains of these elements for input squeezed and coherent light have been numerically evaluated. It is found that input squeezed light and nonlinear attenuators improve the signal-to-noise ratios with respect to coherent inputs and linear chains. In the case of three-photon attenuators, the noise and signal-to-noise ratio remain practically constant from certain elements of the chain. Moreover, the influence of the attenuation coefficient and the input mean photon number on the signal-to-noise ratios was considered, as well as chains with nonlinear amplifiers and chains in which the amplifier gain does not exactly compensate for the fiber attenuation. © 1988 The American Physical Society.Peer Reviewe
- …