5,792 research outputs found
RVB gauge theory and the Topological degeneracy in the Honeycomb Kitaev model
We relate the Z gauge theory formalism of the Kitaev model to the SU(2)
gauge theory of the resonating valence bond (RVB) physics. Further, we
reformulate a known Jordan-Wigner transformation of Kitaev model on a torus in
a general way that shows that it can be thought of as a Z gauge fixing
procedure. The conserved quantities simplify in terms of the gauge invariant
Jordan-Wigner fermions, enabling us to construct exact eigen states and
calculate physical quantities. We calculate the fermionic spectrum for flux
free sector for different gauge field configurations and show that the ground
state is four-fold degenerate on a torus in thermodynamic limit. Further on a
torus we construct four mutually anti-commuting operators which enable us to
prove that all eigenstates of this model are four fold degenerate in
thermodynamic limit.Comment: 12 pages, 3 figures. Added affiliation and a new section,
'Acknowledgements'.Typos correcte
Galaxy size trends as a consequence of cosmology
We show that recently documented trends in galaxy sizes with mass and
redshift can be understood in terms of the influence of underlying cosmic
evolution; a holistic view which is complimentary to interpretations involving
the accumulation of discreet evolutionary processes acting on individual
objects. Using standard cosmology theory, supported with results from the
Millennium simulations, we derive expected size trends for collapsed cosmic
structures, emphasising the important distinction between these trends and the
assembly paths of individual regions. We then argue that the observed variation
in the stellar mass content of these structures can be understood to first
order in terms of natural limitations of cooling and feedback. But whilst these
relative masses vary by orders of magnitude, galaxy and host radii have been
found to correlate linearly. We explain how these two aspects will lead to
galaxy sizes that closely follow observed trends and their evolution, comparing
directly with the COSMOS and SDSS surveys. Thus we conclude that the observed
minimum radius for galaxies, the evolving trend in size as a function of mass
for intermediate systems, and the observed increase in the sizes of massive
galaxies, may all be considered an emergent consequence of the cosmic
expansion.Comment: 14 pages, 13 figures. Accepted by MNRA
A Physical Model for Co-evolution of QSOs and of their Spheroidal Hosts
At variance with most semi-analytic models, in the Anti-hierarchical Baryon
Collapse scenario (Granato et al. 2001, 2004) the main driver of the galaxy
formation and evolution is not the merging sequence but are baryon processes.
This approach emphasizes, still in the framework of the hierarchical clustering
paradigm for dark matter halos, feedback processes from supernova explosions
and from active nuclei, that tie together star formation in spheroidal galaxies
and the growth of black holes at their centers. We review some recent results
showing the remarkably successful predictive power of this scenario, which
allows us to account for the evolution with cosmic time of a broad variety of
properties of galaxies and active nuclei, which proved to be very challenging
for competing models.Comment: Invited talk at the Specola Vaticana Workshop on "AGN and Galaxy
Evolution", Castel Gandolfo, 3-6 October 2005, 10 pages, 2 figure
From First Galaxies to QSOs: feeding the baby monsters
We present a physical model for the coevolution of massive spheroidal
galaxies and active nuclei at their centers. Supernova heating is increasingly
effective in slowing down the star formation and in driving gas outflows in
smaller and smaller dark matter halos. Thus the more massive protogalaxies
virializing at early times are the sites of faster star formation. The
correspondingly higher radiation drag causes a faster angular momentum loss by
the gas and induces a larger accretion rate onto the central black hole. In
turn, the kinetic energy of the outflows powered by the active nuclei can
unbind the residual gas in a time shorter for larger halos. The model accounts
for a broad variety of dynamical, photometric and metallicity properties of
early-type galaxies, for the M_BH -- \sigma relation and for the local
supermassive black-hole mass function.Comment: 6 pages, contributed paper to Proceedings of the Conference on
"Growing Black Holes" held in Garching, Germany, on June 21-25, 2004, edited
by A. Merloni, S. Nayakshin and R. Sunyaev, Springer-Verlag series of "ESO
Astrophysics Symposia
Exact solution of a 2d random Ising model
The model considered is a d=2 layered random Ising system on a square lattice
with nearest neighbours interaction. It is assumed that all the vertical
couplings are equal and take the positive value J while the horizontal
couplings are quenched random variables which are equal in the same row but can
take the two possible values J and J-K in different rows. The exact solution is
obtained in the limit case of infinite K for any distribution of the horizontal
couplings. The model which corresponds to this limit can be seen as an ordinary
Ising system where the spins of some rows, chosen at random, are frozen in an
antiferromagnetic order. No phase transition is found if the horizontal
couplings are independent random variables while for correlated disorder one
finds a low temperature phase with some glassy properties.Comment: 10 pages, Plain TeX, 3 ps figures, submitted to Europhys. Let
The Allometry of Daily Energy Expenditure in Hummingbirds: An Energy Budget Approach
1. Within-clade allometric relationships represent standard laws of scaling between energy and size, and their outliers provide new avenues for physiological and ecological research. According to the metabolic-level boundaries hypothesis, metabolic rates as a function of mass are expected to scale closer to 0.67 when driven by surface-related processes (e.g. heat or water flux), while volume-related processes (e.g. activity) generate slopes closer to one.
2. In birds, daily energy expenditure (DEE) scales with body mass (M) in the relationship log (DEE)=2.35+0.68×log (M), consistent with surface-level processes driving the relationship. However, taxon-specific patterns differ from the scaling slope of all birds.
3. Hummingbirds have the highest mass-specific metabolic rates among all vertebrates. Previous studies on a few hummingbird species, without accounting for the phylogeny, estimated that the DEE–body mass relationship for hummingbirds was log (DEE)=1.72+1.21×log (M). In Contrast to the theoretical expectations, this slope \u3e1 indicates that larger hummingbirds are less metabolically efficient than smaller hummingbirds.
4. We collected DEE and mass data for 12 hummingbird species, which, combined with published data, represented 17 hummingbird species in eight of nine hummingbird clades over a sixfold size range of body size (2.7–17.5 g).
5. After accounting for phylogenetic relatedness, we found DEE scales with body mass as log(DEE)=2.04+0.95×log (M). This slope of 0.95 is lower than previously estimated for hummingbirds, but much higher than the slope for all birds (0.68). The high slopes of torpor, hovering and flight potentially explain the high interspecific DEE slope for hummingbirds compared to other endotherms
Knots in a Spinor Bose-Einstein Condensate
We show that knots of spin textures can be created in the polar phase of a
spin-1 Bose-Einstein condensate, and discuss experimental schemes for their
generation and probe, together with their lifetime.Comment: 4 pages, 3 figure
Exact results for spin dynamics and fractionization in the Kitaev Model
We present certain exact analytical results for dynamical spin correlation
functions in the Kitaev Model. It is the first result of its kind in
non-trivial quantum spin models. The result is also novel: in spite of presence
of gapless propagating Majorana fermion excitations, dynamical two spin
correlation functions are identically zero beyond nearest neighbor separation,
showing existence of a gapless but short range spin liquid. An unusual,
\emph{all energy scale fractionization}of a spin -flip quanta, into two
infinitely massive -fluxes and a dynamical Majorana fermion, is shown to
occur. As the Kitaev Model exemplifies topological quantum computation, our
result presents new insights into qubit dynamics and generation of topological
excitations.Comment: 4 pages, 2 figures. Typose corrected, figure made better, clarifying
statements and references adde
Occupation of X-ray selected galaxy groups by X-ray AGN
We present the first direct measurement of the mean Halo Occupation
Distribution (HOD) of X-ray selected AGN in the COSMOS field at z < 1, based on
the association of 41 XMM and 17 C-COSMOS AGN with member galaxies of 189 X-ray
detected galaxy groups from XMM and Chandra data. We model the mean AGN
occupation in the halo mass range logM_200[Msun] = 13-14.5 with a rolling-off
power-law with the best fit index alpha = 0.06(-0.22;0.36) and normalization
parameter f_a = 0.05(0.04;0.06). We find the mean HOD of AGN among central
galaxies to be modelled by a softened step function at logMh > logMmin = 12.75
(12.10,12.95) Msun while for the satellite AGN HOD we find a preference for an
increasing AGN fraction with Mh suggesting that the average number of AGN in
satellite galaxies grows slower (alpha_s < 0.6) than the linear proportion
(alpha_s = 1) observed for the satellite HOD of samples of galaxies. We present
an estimate of the projected auto correlation function (ACF) of galaxy groups
over the range of r_p = 0.1-40 Mpc/h at = 0.5. We use the large-scale
clustering signal to verify the agreement between the group bias estimated by
using the observed galaxy groups ACF and the value derived from the group mass
estimates. We perform a measurement of the projected AGN-galaxy group
cross-correlation function, excluding from the analysis AGN that are within
galaxy groups and we model the 2-halo term of the clustering signal with the
mean AGN HOD based on our results.Comment: Accepted for publication in The Astrophysical Journa
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