77,932 research outputs found
Metallic Continuum Quantum Ferromagnets at Finite Temperature
We study via renormalization group (RG) and large N methods the problem of
continuum SU(N) quantum Heisenberg ferromagnets (QHF) coupled to gapless
electrons. We establish the phase diagram of the dissipative problem and
investigate the changes in the Curie temperature, magnetization, and magnetic
correlation length due to dissipation and both thermal and quantum
fluctuations. We show that the interplay between the topological term (Berry's
phase) and dissipation leads to non-trivial effects for the finite temperature
critical behavior.Comment: Corrected typos, new discussion of T=0 results, to appear in
Europhys. Let
Combining Physical galaxy models with radio observations to constrain the SFRs of high-z dusty star forming galaxies
We complement our previous analysis of a sample of z~1-2 luminous and
ultra-luminous infrared galaxies ((U)LIRGs), by adding deep VLA radio
observations at 1.4 GHz to a large data-set from the far-UV to the sub-mm,
including Spitzer and Herschel data. Given the relatively small number of
(U)LIRGs in our sample with high S/N radio data, and to extend our study to a
different family of galaxies, we also include 6 well sampled near IR-selected
BzK galaxies at z~1.5. From our analysis based on the radiative transfer
spectral synthesis code GRASIL, we find that, while the IR luminosity may be a
biased tracer of the star formation rate (SFR) depending on the age of stars
dominating the dust heating, the inclusion of the radio flux offers
significantly tighter constraints on SFR. Our predicted SFRs are in good
agreement with the estimates based on rest-frame radio luminosity and the Bell
(2003) calibration. The extensive spectro-photometric coverage of our sample
allows us to set important constraints on the SF history of individual objects.
For essentially all galaxies we find evidence for a rather continuous SFR and a
peak epoch of SF preceding that of the observation by a few Gyrs. This seems to
correspond to a formation redshift of z~5-6. We finally show that our physical
analysis may affect the interpretation of the SFR-M* diagram, by possibly
shifting, with respect to previous works, the position of the most dust
obscured objects to higher M* and lower SFRs.Comment: 26 pages, 15 figures, 3 tables, accepted for publication in MNRAS on
Dec. 4th, 201
Magnetic monopole and string excitations in a two-dimensional spin ice
We study the magnetic excitations of a square lattice spin-ice recently
produced in an artificial form, as an array of nanoscale magnets. Our analysis,
based upon the dipolar interaction between the nanomagnetic islands, correctly
reproduces the ground-state observed experimentally. In addition, we find
magnetic monopole-like excitations effectively interacting by means of the
usual Coulombic plus a linear confining potential, the latter being related to
a string-like excitation binding the monopoles pairs, what indicates that the
fractionalization of magnetic dipoles may not be so easy in two dimensions.
These findings contrast this material with the three-dimensional analogue,
where such monopoles experience only the Coulombic interaction. We discuss,
however, two entropic effects that affect the monopole interactions: firstly,
the string configurational entropy may loose the string tension and then, free
magnetic monopoles should also be found in lower dimensional spin ices;
secondly, in contrast to the string configurational entropy, an entropically
driven Coulomb force, which increases with temperature, has the opposite effect
of confining the magnetic defects.Comment: 8 pages. Accepted by Journal of Applied Physics (2009
Spatial-temporal evolution of the current filamentation instability
The spatial-temporal evolution of the purely transverse current filamentation
instability is analyzed by deriving a single partial differential equation for
the instability and obtaining the analytical solutions for the spatially and
temporally growing current filament mode. When the beam front always encounters
fresh plasma, our analysis shows that the instability grows spatially from the
beam front to the back up to a certain critical beam length; then the
instability acquires a purely temporal growth. This critical beam length
increases linearly with time and in the non-relativistic regime it is
proportional to the beam velocity. In the relativistic regime the critical
length is inversely proportional to the cube of the beam Lorentz factor
. Thus, in the ultra-relativistic regime the instability
immediately acquires a purely temporal growth all over the beam. The analytical
results are in good agreement with multidimensional particle-in-cell
simulations performed with OSIRIS. Relevance of current study to recent and
future experiments on fireball beams is also addressed
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