39,545 research outputs found
Study of a microcanonical algorithm on the spin glass model in d=3
We consider a microcanonical local algorithm to be applied on the
spin glass model. We have compared the results coming from a microcanonical
Monte Carlo simulation with those from a canonical one: Thermalization times,
spin glass susceptibilities and Binder parameters. For a fixed lattice size we
found different results between the two thermodynamic ensembles, which tend to
vanish at bigger volumes. Moreover, microcanonical thermalization times are
longer than the canonical ones. Finally we have checked that one of the Guerra
relations is satisfied with good precision for the two largest lattices.Comment: Revised version. Latex 14 pages, 6 figures. To be published in
Comput. Phys. Commu
DSP-based ionospheric radiolink using DS-CDMA and on-line channel estimation
In this paper, a new blind multiuser detection algorithm is presented. It can both cancel multiuser interference and estimate the multipath channel response in a blind way. The method has been specially conceived for low coherence bandwidth channels such as the ionospheric channel and exhibits very low computational requirements. Real-time measurements from a fully digital HF radio-link are presented that confirm the reliability of the method for the ionospheric channel.Peer ReviewedPostprint (published version
Chiral Lagrangian at finite temperature from the Polyakov-Chiral Quark Model
We analyze the consequences of the inclusion of the gluonic Polyakov loop in
chiral quark models at finite temperature. Specifically, the low-energy
effective chiral Lagrangian from two such quark models is computed. The tree
level vacuum energy density, quark condensate, pion decay constant and
Gasser-Leutwyler coefficients are found to acquire a temperature dependence.
This dependence is, however, exponentially small for temperatures below the
mass gap in the full unquenched calculation. The introduction of the Polyakov
loop and its quantum fluctuations is essential to achieve this result and also
the correct large counting for the thermal corrections. We find that new
coefficients are introduced at to account for the Lorentz
breaking at finite temperature. As a byproduct, we obtain the effective
Lagrangian which describes the coupling of the Polyakov loop to the Goldstone
bosons.Comment: 16 pages, no figure
Polyakov loop in chiral quark models at finite temperature
We describe how the inclusion of the gluonic Polyakov loop incorporates large
gauge invariance and drastically modifies finite temperature calculations in
chiral quark models after color neutral states are singled out. This generates
an effective theory of quarks and Polyakov loops as basic degrees of freedom.
We find a strong suppression of finite temperature effects in hadronic
observables triggered by approximate triality conservation (Polyakov cooling),
so that while the center symmetry breaking is exponentially small with the
constituent quark mass, chiral symmetry restoration is exponentially small with
the pion mass. To illustrate the point we compute some low energy observables
at finite temperature and show that the finite temperature corrections to the
low energy coefficients are suppressed due to color average of the
Polyakov loop. Our analysis also shows how the phenomenology of chiral quark
models at finite temperature can be made compatible with the expectations of
chiral perturbation theory. The implications for the simultaneous center
symmetry breaking-chiral symmetry restoration phase transition are also
discussed.Comment: 24 pages, 8 ps figures. Figure and appendix added. To appear in
Physical Review
Quantum Phase Transitions detected by a local probe using Time Correlations and Violations of Leggett-Garg Inequalities
In the present paper we introduce a way of identifying quantum phase
transitions of many-body systems by means of local time correlations and
Leggett-Garg inequalities. This procedure allows to experimentally determine
the quantum critical points not only of finite-order transitions but also those
of infinite order, as the Kosterlitz-Thouless transition that is not always
easy to detect with current methods. By means of simple analytical arguments
for a general spin- Hamiltonian, and matrix product simulations of
one-dimensional and anisotropic models, we argue that
finite-order quantum phase transitions can be determined by singularities of
the time correlations or their derivatives at criticality. The same features
are exhibited by corresponding Leggett-Garg functions, which noticeably
indicate violation of the Leggett-Garg inequalities for early times and all the
Hamiltonian parameters considered. In addition, we find that the infinite-order
transition of the model at the isotropic point can be revealed by the
maximal violation of the Leggett-Garg inequalities. We thus show that quantum
phase transitions can be identified by purely local measurements, and that
many-body systems constitute important candidates to observe experimentally the
violation of Leggett-Garg inequalities.Comment: Minor changes, 11 pages, 11 figures. Final version published in Phys.
Rev.
Non-linear response of single-molecule magnets: field-tuned quantum-to-classical crossovers
Quantum nanomagnets can show a field dependence of the relaxation time very
different from their classical counterparts, due to resonant tunneling via
excited states (near the anisotropy barrier top). The relaxation time then
shows minima at the resonant fields H_{n}=n D at which the levels at both sides
of the barrier become degenerate (D is the anisotropy constant). We showed that
in Mn12, near zero field, this yields a contribution to the nonlinear
susceptibility that makes it qualitatively different from the classical curves
[Phys. Rev. B 72, 224433 (2005)]. Here we extend the experimental study to
finite dc fields showing how the bias can trigger the system to display those
quantum nonlinear responses, near the resonant fields, while recovering an
classical-like behaviour for fields between them. The analysis of the
experiments is done with heuristic expressions derived from simple balance
equations and calculations with a Pauli-type quantum master equation.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. B, brief report
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