34,138 research outputs found
Modulated phases in a three-dimensional Maier-Saupe model with competing interactions
This work is dedicated to the study of the discrete version of the Maier-Saupe model in the presence of competing interactions. The competition between interactions favoring different orientational ordering produces a rich phase diagram including modulated phases. Using a mean-field approach and Monte Carlo simulations, we show that the proposed model exhibits isotropic and nematic phases and also a series of modulated phases that meet at a multicritical point, a Lifshitz point. Though the Monte Carlo and mean-field phase diagrams show some quantitative disagreements, the Monte Carlo simulations corroborate the general behavior found within the mean-field approximation.We thank P. Gomes, R. Kaul, G. Landi, M. Oliveira, R. Oliveira, and S. Salinas for useful discussions and suggestions. P.F.B. was supported by Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) and the Condensed Matter Theory Visitors Program at Boston University. N.X. and A.W.S. were funded in part by the NSF under Grant No. DMR-1410126. Some of the calculations were carried out on Boston University's Shared Computing Cluster. (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP); Condensed Matter Theory Visitors Program at Boston University; DMR-1410126 - NSF)Accepted manuscrip
Dual time scales in simulated annealing of a two-dimensional Ising spin glass
We apply a generalized Kibble-Zurek out-of-equilibrium scaling ansatz to
simulated annealing when approaching the spin-glass transition at temperature
of the two-dimensional Ising model with random couplings.
Analyzing the spin-glass order parameter and the excess energy as functions of
the system size and the annealing velocity in Monte Carlo simulations with
Metropolis dynamics, we find scaling where the energy relaxes slower than the
spin-glass order parameter, i.e., there are two different dynamic exponents.
The values of the exponents relating the relaxation time scales to the system
length, , are for the relaxation of the order
parameter and for the energy relaxation. We argue that the
behavior with dual time scales arises as a consequence of the entropy-driven
ordering mechanism within droplet theory. We point out that the dynamic
exponents found here for simulated annealing are different from the
temperature-dependent equilibrium dynamic exponent , for which
previous studies have found a divergent behavior; . Thus, our study shows that, within Metropolis dynamics, it is easier
to relax the system to one of its degenerate ground states than to migrate at
low temperatures between regions of the configuration space surrounding
different ground states. In a more general context of optimization, our study
provides an example of robust dense-region solutions for which the excess
energy (the conventional cost function) may not be the best measure of success.Comment: 13 pages, 16 figure
Meson and Baryon dispersion relations with Brillouin fermions
We study the dispersion relations of mesons and baryons built from Brillouin
quarks on one N_f=2 gauge ensemble provided by QCDSF. For quark masses up to
the physical strange quark mass, there is hardly any improvement over the
Wilson discretization, if either action is link-smeared and tree-level clover
improved. For quark masses in the range of the physical charm quark mass, the
Brillouin action still shows a perfect relativistic behavior, while the Wilson
action induces severe cut-off effects. As an application we determine the
masses of the \Omega_c^0, \Omega_{cc}^+ and \Omega_{ccc}^{++} baryons on that
ensemble.Comment: 16 pages, 9 figures, 4 tables; v2: one Reference added, matches
published versio
Effect of Imperceptible Vibratory Noise Applied to Wrist Skin On Fingertip Touch Evoked Potentials – An EEG Study
Random vibration applied to skin can change the sense of touch. Specifically, low amplitude white-noise vibration can improve fingertip touch perception. In fact, fingertip touch sensation can improve even when imperceptible random vibration is applied to other remote upper extremity areas such as wrist, dorsum of the hand, or forearm. As such, vibration can be used to manipulate sensory feedback and improve dexterity, particularly during neurological rehabilitation. Nonetheless, the neurological bases for remote vibration enhanced sensory feedback are yet poorly understood. This study examined how imperceptible random vibration applied to the wrist changes cortical activity for fingertip sensation. We measured somatosensory evoked potentials to assess peak-to-peak response to light touch of the index fingertip with applied wrist vibration versus without. We observed increased peak-to-peak somatosensory evoked potentials with wrist vibration, especially with increased amplitude of the later component for the somatosensory, motor, and premotor cortex with wrist vibration. These findings corroborate an enhanced cortical-level sensory response motivated by vibration. It is possible that the cortical modulation observed here is the result of the establishment of transient networks for improved perception
Evolution of transport properties of BaFe2-xRuxAs2 in a wide range of isovalent Ru substitution
The effects of isovalent Ru substitution at the Fe sites of BaFe2-xRuxAs2 are
investigated by measuring resistivity and Hall coefficient on high-quality
single crystals in a wide range of doping (0 < x < 1.4). Ru substitution
weakens the antiferromagnetic (AFM) order, inducing superconductivity for
relatively high doping level of 0.4 < x < 0.9. Near the AFM phase boundary, the
transport properties show non-Fermi-liquid-like behaviors with a
linear-temperature dependence of resistivity and a strong temperature
dependence of Hall coefficient with a sign change. Upon higher doping, however,
both of them recover conventional Fermi-liquid behaviors. Strong doping
dependence of Hall coefficient together with a small magnetoresistance suggest
that the anomalous transport properties can be explained in terms of
anisotropic charge carrier scattering due to interband AFM fluctuations rather
than a conventional multi-band scenario.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Quantum simulation of Fermi-Hubbard models in semiconductor quantum dot arrays
We propose a device for studying the Fermi-Hubbard model with long-range
Coulomb interactions using an array of quantum dots defined in a semiconductor
two-dimensional electron gas system. Bands with energies above the lowest
energy band are used to form the Hubbard model, which allows for an
experimentally simpler realization of the device. We find that depending on
average electron density, the system is well described by a one- or two-band
Hubbard model. Our device design enables the control of the ratio of the
Coulomb interaction to the kinetic energy of the electrons independently to the
filling of the quantum dots, such that a large portion of the Hubbard phase
diagram may be probed. Estimates of the Hubbard parameters suggest that a
metal-Mott insulator quantum phase transition and a d-wave superconducting
phase should be observable using current fabrication technologies.Comment: 12 pages, 3 figures, 1 table
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