8,876 research outputs found
Finite temperature dynamics of vortices in the two dimensional anisotropic Heisenberg model
We study the effects of finite temperature on the dynamics of non-planar
vortices in the classical, two-dimensional anisotropic Heisenberg model with
XY- or easy-plane symmetry. To this end, we analyze a generalized
Landau-Lifshitz equation including additive white noise and Gilbert damping.
Using a collective variable theory with no adjustable parameters we derive an
equation of motion for the vortices with stochastic forces which are shown to
represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a
Green's function formalism and obtain the mean vortex trajectory and its
variance. We find a non-standard time dependence for the variance of the
components perpendicular to the driving force. We compare the analytical
results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition
temperature. Finally, we discuss the reasons why our approach is not
appropriate for higher temperatures as well as the discreteness effects
observed in the numerical simulations.Comment: 12 pages, 8 figures, accepted for publication in European Physical
Journal B (uses EPJ LaTeX
Stellar population analysis of MaNGA early-type galaxies: IMF dependence and systematic effects
We study systematics associated with estimating simple stellar population
(SSP) parameters -- age, metallicity [M/H], -enhancement [/Fe]
and IMF shape -- and associated gradients, of elliptical slow rotators
(E-SRs), fast rotators (E-FRs) and S0s from stacked spectra of galaxies in the
MaNGA survey. These systematics arise from (i) how one normalizes the spectra
when stacking; (ii) having to subtract emission before estimating absorption
line strengths; (iii) the decision to fit the whole spectrum or just a few
absorption lines; (iv) SSP model differences (e.g. isochrones, enrichment,
IMF). The MILES+Padova SSP models, fit to the H, Fe,
TiO and [MgFe] Lick indices in the stacks, indicate that out to
the half-light radius : (a) ages are younger and [/Fe] values are
lower in the central regions but the opposite is true of [M/H]; (b) the IMF is
more bottom-heavy in the center, but is close to Kroupa beyond about ;
(c) this makes about larger in the central regions than
beyond . While the models of Conroy et al. (2018) return similar [M/H]
and [/Fe] profiles, the age and (hence) profiles can differ
significantly even for solar abundances and a Kroupa IMF; different responses
to non-solar abundances and IMF parametrization further compound these
differences. There are clear (model independent) differences between E-SRs,
E-FRs and S0s: younger ages and less enhanced [/Fe] values suggest that
E-FRs and S0s are not SSPs, but relaxing this assumption is unlikely to change
their inferred gradients significantly.Comment: 22 pages, 23 figures, accepted for publication in MNRA
The half mass radius of MaNGA galaxies: Effect of IMF gradients
Gradients in the stellar populations (SP) of galaxies -- e.g., in age,
metallicity, stellar Initial Mass Function (IMF) -- can result in gradients in
the stellar mass to light ratio, . Such gradients imply that the
distribution of the stellar mass and light are different. For old SPs, e.g., in
early-type galaxies at , the gradients are weak if driven by
variations in age and metallicity, but significantly larger if driven by the
IMF. A gradient which has larger in the center increases the estimated
total stellar mass () and reduces the scale which contains half this mass
(), compared to when the gradient is ignored. For the IMF gradients
inferred from fitting MILES simple SP models to the H,
Fe, [MgFe] and TiO absorption lines measured in
spatially resolved spectra of early-type galaxies in the MaNGA survey, the
fractional change in can be significantly larger than that in ,
especially when the light is more centrally concentrated. The
correlation which results is offset by 0.3 dex to smaller sizes compared to
when these gradients are ignored. Comparisons with `quiescent' galaxies at
higher- must account for evolution in SP gradients (especially age and IMF)
and the light profile before drawing conclusions about how and
evolve. The implied merging between higher- and the present is less
contrived if at is closer to our IMF-driven gradient
calibration than to unity.Comment: 16 pages, 15 figures, accepted for publication in MNRA
Localized magnetic plasmons in all-dielectric mu<0 metastructures
Metamaterials are known to exhibit a variety of electromagnetic properties
non-existing in nature. We show that an all-dielectric (non-magnetic) system
consisting of deep subwavelength, high permittivity resonant spheres possess
effective negative magnetic permeability (dielectric permittivity being
positive and small). Due to the symmetry of the electromagnetic wave equations
in classical electrodynamics, localized "magnetic" plasmon resonances can be
excited in a metasphere made of such metamaterial. This is theoretically
demonstrated by the coupled-dipole approximation and numerically for real
spheres, in full agreement with the exact analytical solution for the
scattering process by the same metasphere with effective material properties
predicted by effective medium theory. The emergence of this phenomenon as a
function of structural order within the metastructures is also studied.
Universal conditions enabling effective negative magnetic permeability relate
subwavelength sphere permittivity and size with critical filling fraction. Our
proposal paves the way towards (all-dielectric) magnetic plasmonics, with a
wealth of fascinating applications.Comment: 7 pages, 4 figures; figure 3 modified and new figure (4) added, with
corresponding discussio
Enhanced suppresion of localization in a continuous Random-Dimer Model
We consider a one-dimensional continuous (Kronig-Penney) extension of the
(tight-binding) Random Dimer model of Dunlap et al. [Phys. Rev. Lett. 65, 88
(1990)]. We predict that the continuous model has infinitely many resonances
(zeroes of the reflection coefficient) giving rise to extended states instead
of the one resonance arising in the discrete version. We present exact,
transfer-matrix numerical calculations supporting, both realizationwise and on
the average, the conclusion that the model has a very large number of extended
states.Comment: 10 pages, 3 Figures available on request, REVTeX 3.0, MA/UC3M/1/9
Quiet Sun magnetic fields from simultaneous inversions of visible and infrared spectropolarimetric observations
We study the quiet Sun magnetic fields using spectropolarimetric observations
of the infrared and visible Fe I lines at 6301.5, 6302.5, 15648 and 15653 A.
Magnetic field strengths and filling factors are inferred by the simultaneous
fit of the observed Stokes profiles under the MISMA hypothesis. The
observations cover an intra-network region at the solar disk center. We analyze
2280 Stokes profiles whose polarization signals are above noise in the two
spectral ranges, which correspond to 40% of the field of view. Most of these
profiles can be reproduced only with a model atmosphere including 3 magnetic
components with very different field strengths, which indicates the
co-existence of kG and sub-kG fields in our 1.5" resolution elements. We
measure an unsigned magnetic flux density of 9.6 G considering the full field
of view. Half of the pixels present magnetic fields with mixed polarities in
the resolution element. The fraction of mixed polarities increases as the
polarization weakens. We compute the probability density function of finding
each magnetic field strength. It has a significant contribution of kG field
strengths, which concentrates most of the observed magnetic flux and energy.
This kG contribution has a preferred magnetic polarity, while the polarity of
the weak fields is balanced.Comment: 16 pages and 14 figure
- …