9,859 research outputs found

    Zero-temperature transition and correlation-length exponent of the frustrated XY model on a honeycomb lattice

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    Phase coherence and vortex order in the fully frustrated XY model on a two-dimensional honeycomb lattice are studied by extensive Monte Carlo simulations using the parallel tempering method and finite-size scaling. No evidence is found for an equilibrium order-disorder or a spin/vortex-glass transition, suggested in previous simulation works. Instead, the scaling analysis of correlations of phase and vortex variables in the full equilibrated system is consistent with a phase transition where the critical temperature vanishes and the correlation lengths diverge as a power-law with decreasing temperatures and corresponding critical exponents νph\nu_{ph} and νv\nu_{v}. This behavior and the near agreement of the critical exponents suggest a zero-temperature transition scenario where phase and vortex variables remain coupled on large length scales.Comment: 8 pages, 10 figure

    A New Technique for Heterodyne Spectroscopy: Least-Squares Frequency Switching (LSFS)

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    We describe a new technique for heterodyne spectroscopy, which we call Least-Squares Frequency Switching, or LSFS. This technique avoids the need for a traditional reference spectrum, which--when combined with the on-source spectrum--introduces both noise and systematic artifacts such as ``baseline wiggles''. In contrast, LSFS derives the spectrum directly, and in addition the instrumental gain profile. The resulting spectrum retains nearly the full theoretical sensitivity and introduces no systematic artifacts. Here we discuss mathematical details of the technique and use numerical experiments to explore optimum observing schemas. We outline a modification suitable for computationally difficult cases as the number of spectral channels grows beyond several thousand. We illustrate the method with three real-life examples. In one of practical interest, we created a large contiguous bandwidth aligning three smaller bandwidths end-to-end; radio astronomers are often faced with the need for a larger contiguous bandwidth than is provided with the available correlator.Comment: 37 pages, 8 figure

    Geometrical properties of the potential energy of the soft-sphere binary mixture

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    We report a detailed study of the stationary points (zero-force points) of the potential energy surface (PES) of a model structural glassformer. We compare stationary points found with two different algorithms (eigenvector following and square gradient minimization), and show that the mapping between instantaneous configuration and stationary points defined by those algorithms is as different as to strongly influence the instability index K vs. temperature plot, which relevance in analyzing the liquid dynamics is thus questioned. On the other hand, the plot of K vs. energy is much less sensitive to the algorithm employed, showing that the energy is the good variable to discuss geometric properties of the PES. We find new evidence of a geometric transition between a minima-dominated phase and a saddle-point-dominated one. We analyze the distances between instantaneous configurations and stationary points, and find that above the glass transition, the system is closer to saddle points than to minima

    Fermion Masses from SO(10) Hermitian Matrices

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    Masses of fermions in the SO(10) 16-plet are constructed using only the 10, 120 and 126 scalar multiplets. The mass matrices are restricted to be hermitian and the theory is constructed to have certain assumed quark masses, charged lepton masses and CKM matrix in accord with data. The remaining free parameters are found by fitting to light neutrino masses and MSN matrices result as predictions.Comment: 23 pages. Small textual additions for clarification; formalism and results unchanged. Version to appear in Phys. Rev.

    Simulation of complete many-body quantum dynamics using controlled quantum-semiclassical hybrids

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    A controlled hybridization between full quantum dynamics and semiclassical approaches (mean-field and truncated Wigner) is implemented for interacting many-boson systems. It is then demonstrated how simulating the resulting hybrid evolution equations allows one to obtain the full quantum dynamics for much longer times than is possible using an exact treatment directly. A collision of sodium BECs with 1.x10^5 atoms is simulated, in a regime that is difficult to describe semiclassically. The uncertainty of physical quantities depends on the statistics of the full quantum prediction. Cutoffs are minimised to a discretization of the Hamiltonian. The technique presented is quite general and extension to other systems is considered.Comment: Published version. Broader background and discussion, slightly shortened, less figures in epaps. Research part unchanged. Article + epaps (4+4 pages), 8 figure

    Quantum heat transfer in harmonic chains with self consistent reservoirs: Exact numerical simulations

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    We describe a numerical scheme for exactly simulating the heat current behavior in a quantum harmonic chain with self-consistent reservoirs. Numerically-exact results are compared to classical simulations and to the quantum behavior under the linear response approximation. In the classical limit or for small temperature biases our results coincide with previous calculations. At large bias and for low temperatures the quantum dynamics of the system fundamentally differs from the close-to-equilibrium behavior, revealing in particular the effect of thermal rectification for asymmetric chains. Since this effect is absent in the classical analog of our model, we conclude that in the quantum model studied here thermal rectification is a purely quantum phenomenon, rooted in the quantum statistics

    Determination of the zeta potential for highly charged colloidal suspensions

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    We compute the electrostatic potential at the surface, or zeta potential ζ\zeta, of a charged particle embedded in a colloidal suspension using a hybrid mesoscopic model. We show that for weakly perturbing electric fields, the value of ζ\zeta obtained at steady state during electrophoresis is statistically indistinguishable from ζ\zeta in thermodynamic equilibrium. We quantify the effect of counterions concentration on ζ\zeta. We also evaluate the relevance of the lattice resolution for the calculation of ζ\zeta and discuss how to identify the effective electrostatic radius.Comment: 8 pages, 3 figures with 2 panel

    Regularization of fields for self-force problems in curved spacetime: foundations and a time-domain application

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    We propose an approach for the calculation of self-forces, energy fluxes and waveforms arising from moving point charges in curved spacetimes. As opposed to mode-sum schemes that regularize the self-force derived from the singular retarded field, this approach regularizes the retarded field itself. The singular part of the retarded field is first analytically identified and removed, yielding a finite, differentiable remainder from which the self-force is easily calculated. This regular remainder solves a wave equation which enjoys the benefit of having a non-singular source. Solving this wave equation for the remainder completely avoids the calculation of the singular retarded field along with the attendant difficulties associated with numerically modeling a delta function source. From this differentiable remainder one may compute the self-force, the energy flux, and also a waveform which reflects the effects of the self-force. As a test of principle, we implement this method using a 4th-order (1+1) code, and calculate the self-force for the simple case of a scalar charge moving in a circular orbit around a Schwarzschild black hole. We achieve agreement with frequency-domain results to ~ 0.1% or better.Comment: 15 pages, 12 figures, 1 table. More figures, extended summar

    Statistical Tests for Scaling in the Inter-Event Times of Earthquakes in California

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    We explore in depth the validity of a recently proposed scaling law for earthquake interevent time distributions in the case of the Southern California, using the waveform cross-correlation catalog of Shearer et al. Two statistical tests are used: on the one hand, the standard two-sample Kolmogorov-Smirnov test is in agreement with the scaling of the distributions. On the other hand, the one-sample Kolmogorov-Smirnov statistic complemented with Monte Carlo simulation of the inter-event times, as done by Clauset et al., supports the validity of the gamma distribution as a simple model of the scaling function appearing on the scaling law, for rescaled inter-event times above 0.01, except for the largest data set (magnitude greater than 2). A discussion of these results is provided.Comment: proceedings of Erice conference, 200

    The motion of the freely falling chain tip

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    The dynamics of the tip of the falling chain is analyzed. Results of laboratory experiments are presented and compared with results of numerical simulations. Time dependences of the velocity and the acceleration of the chain tip for a number of different initial conformations of the chain are determined. A simple analytical model of the system is also considered.Comment: 29 pages, 13 figure