348 research outputs found
Low temperature properties of the triangular-lattice antiferromagnet: a bosonic spinon theory
We study the low temperature properties of the triangular-lattice Heisenberg
antiferromagnet with a mean field Schwinger spin-1/2 boson scheme that
reproduces quantitatively the zero temperature energy spectrum derived
previously using series expansions. By analyzing the spin-spin and the boson
density-density dynamical structure factors, we identify the unphysical spin
excitations that come from the relaxation of the local constraint on bosons.
This allows us to reconstruct a free energy based on the physical excitations
only, whose predictions for entropy and uniform susceptibility seem to be
reliable within the temperature range $0< T <0.3J, which is difficult to access
by other methods. The high values of entropy, also found in high temperature
expansions studies, can be attributed to the roton-like narrowed dispersion at
finite temperatures.Comment: 16 pages, 5 figure
Magnons and Excitation Continuum in XXZ triangular antiferromagnetic model: Application to
We investigate the excitation spectrum of the triangular-lattice
antiferromagnetic model using series expansions and mean field Schwinger
bosons approaches. The single-magnon spectrum computed with series expansions
exhibits rotonic minima at the middle points of the edges of the Brillouin
zone, for all values of the anisotropy parameter in the range . Based on the good agreement with series expansions for the
single-magnon spectrum, we compute the full dynamical magnetic structure factor
within the mean field Schwinger boson approach to investigate the relevance of
the model for the description of the unusual spectrum found recently in
. In particular, we obtain an extended continuum above the spin
wave excitations, which is further enhanced and brought closer to those
observed in with the addition of a second neighbor exchange
interaction approximately 15% of the nearest-neighbor value. Our results
support the idea that excitation continuum with substantial spectral-weight are
generically present in two-dimensional frustrated spin systems and
fractionalization in terms of {\it bosonic} spinons presents an efficient way
to describe them.Comment: 8 pages, 4 figure
Small-Angle Scattering of X-Rays from Extragalactic Sources by Dust in Intervening Galaxies
Gamma-ray bursts are now known to be a cosmological population of objects,
which are often accompanied by X-ray and optical afterglows. The total energy
emitted in the afterglow can be similar to the energy radiated in the gamma-ray
burst itself. If a galaxy containing a large column density of dust is near the
line of sight to a gamma-ray burst, small-angle scattering of the X-rays due to
diffraction by the dust grains will give rise to an X-ray echo of the
afterglow. A measurement of the angular size of the echo at a certain time
after the afterglow is observed yields a combination of the angular diameter
distances to the scattering galaxy and the gamma-ray burst that can be used to
constrain cosmological models in the same way as a time delay in a
gravitational lens. The scattering galaxy will generally cause gravitational
lensing as well, and this should modify the shape of the X-ray echo from a
circular ring.
The main difficulty in detecting this phenomenon is the very low flux
expected for the echo. The flux can be increased when the gamma-ray burst is
highly magnified by gravitational lensing, or when the deflecting galaxy is at
low redshift. X-ray echos of continuous (but variable) sources, such as
quasars, may also be detectable with high-resolution instruments and would
allow similar measurements.Comment: To be published in Ap
An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies
We present an XMM-Newton observation of the bright, narrow-line, ultrasoft
Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved,
showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless
excess of emission at lower energies. The spectrum can be adequately
parameterised using a simple double power-law model. The source is strongly
variable over the course of the observation but shows only weak spectral
variability, with the fractional variability amplitude remaining approximately
constant over more than a decade in energy. The curved continuum shape and weak
spectral variability are discussed in terms of various physical models for the
soft X-ray excess emission, including reflection off the surface of an ionised
accretion disc, inverse-Compton scattering of soft disc photons by thermal
electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal
distribution. We emphasise the possibility that the strong soft excess may be
produced by dissipation of accretion energy in the hot, upper atmosphere of the
putative accretion disc.Comment: 9 pages, accepted for publication in MNRA
Evidence for the Galactic X-ray Bulge II
A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane
() reveals X-ray shadows in the keV band cast by
distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes
indicate that % and % of the diffuse X-ray background in this
direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the
molecular gas which is located at 3 kpc from the Sun. The implication of
the derived background X-ray flux beyond the absorbing molecular cloud is
consistent with, and lends further support to recent observations of a Galactic
X-ray bulge.Comment: 19 pages, 5 figures, 2 table
Spin wave analysis to the spatially-anisotropic Heisenberg antiferromagnet on triangular lattice
We study the phase diagram at T=0 of the antiferromagnetic Heisenberg model
on the triangular lattice with spatially-anisotropic interactions. For values
of the anisotropy very close to J_alpha/J_beta=0.50, conventional spin wave
theory predicts that quantum fluctuations melt the classical structures, for
S=1/2. For the regime J_beta<J_alpha, it is shown that the incommensurate
spiral phases survive until J_beta/J_alpha=0.27, leaving a wide region where
the ground state is disordered. The existence of such nonmagnetic states
suggests the possibility of spin liquid behavior for intermediate values of the
anisotropy.Comment: Revised version, 4 pages, Latex (twocolumn), 4 figures as eps files.
To appear in PR
Rotational invariance and order-parameter stiffness in frustrated quantum spin systems
We compute, within the Schwinger-boson scheme, the Gaussian-fluctuation
corrections to the order-parameter stiffness of two frustrated quantum spin
systems: the triangular-lattice Heisenberg antiferromagnet and the J1-J2 model
on the square lattice. For the triangular-lattice Heisenberg antiferromagnet we
found that the corrections weaken the stiffness, but the ground state of the
system remains ordered in the classical 120 spiral pattern. In the case of the
J1-J2 model, with increasing frustration the stiffness is reduced until it
vanishes, leaving a small window 0.53 < J2/J1 < 0.64 where the system has no
long-range magnetic order. In addition, we discuss several methodological
questions related to the Schwinger-boson approach. In particular, we show that
the consideration of finite clusters which require twisted boundary conditions
to fit the infinite-lattice magnetic order avoids the use of ad hoc factors to
correct the Schwinger-boson predictions.Comment: 9 pages, Latex, 6 figures as ps files, fig.1 changed and minor text
corrections, to appear in Phys.Rev.
The North Ecliptic Pole Supercluster
We have used the ROSAT All-Sky Survey to detect a known supercluster at
z=0.087 in the North Ecliptic Pole region. The X-ray data greatly improve our
understanding of this supercluster's characteristics, approximately doubling
our knowledge of the structure's spatial extent and tripling the cluster/group
membership compared to the optical discovery data. The supercluster is a rich
structure consisting of at least 21 galaxy clusters and groups, 12 AGN, 61 IRAS
galaxies, and various other objects. A majority of these components were
discovered with the X-ray data, but the supercluster is also robustly detected
in optical, IR, and UV wavebands. Extending 129 x 102 x 67 (1/h50 Mpc)^3, the
North Ecliptic Pole Supercluster has a flattened shape oriented nearly edge-on
to our line-of-sight. Owing to the softness of the ROSAT X-ray passband and the
deep exposure over a large solid angle, we have detected for the first time a
significant population of X-ray emitting galaxy groups in a supercluster. These
results demonstrate the effectiveness of X-ray observations with contiguous
coverage for studying structure in the Universe.Comment: Accepted for publication in The Astrophysical Journal; 5 pages with 2
embedded figures; uses emulateapj.sty; For associated animations, see
http://www.ifa.hawaii.edu/~mullis/nep3d.html; A high-resolution color
postscript version of the full paper is available at
http://www.ifa.hawaii.edu/~mullis/papers/nepsc.ps.g
Quasiparticle vanishing driven by geometrical frustration
We investigate the single hole dynamics in the triangular t-J model. We study
the structure of the hole spectral function, assuming the existence of a 120
magnetic Neel order. Within the self-consistent Born approximation (SCBA) there
is a strong momentum and t sign dependence of the spectra, related to the
underlying magnetic structure and the particle-hole asymmetry of the model. For
positive t, and in the strong coupling regime, we find that the low energy
quasiparticle excitations vanish outside the neighbourhood of the magnetic
Goldstone modes; while for negative t the quasiparticle excitations are always
well defined. In the latter, we also find resonances of magnetic origin whose
energies scale as (J/t)^2/3 and can be identified with string excitations. We
argue that this complex structure of the spectra is due to the subtle interplay
between magnon-assisted and free hopping mechanisms. Our predictions are
supported by an excellent agreement between the SCBA and the exact results on
finite size clusters. We conclude that the conventional quasiparticle picture
can be broken by the effect of geometrical magnetic frustration.Comment: 6 pages, 7 figures. Published versio
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