140 research outputs found
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.
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
The Heisenberg model on the 1/5-depleted square lattice and the CaV4O9 compound
We investigate the ground state structure of the Heisenberg model on the
1/5-depleted square lattice for arbitrary values of the first- and
second-neighbor exchange couplings. By using a mean-field Schwinger-boson
approach we present a unified description of the rich ground-state diagram,
which include the plaquette and dimer resonant-valence-bond phases, an
incommensurate phase and other magnetic orders with complex magnetic unit
cells. We also discuss some implications of ours results for the experimental
realization of this model in the CaV4O9 compound.Comment: 4 pages, Latex, 7 figures included as eps file
Does Sub-millisecond Pulsar XTE J1739-285 Contain a Low Magnetic Neutron Star or Quark Star ?
With the possible detection of the fastest spinning nuclear-powered pulsar
XTE J1739-285 of frequency 1122 Hz (0.8913 ms), it arouses us to constrain the
mass and radius of its central compact object and to imply the stellar matter
compositions: neutrons or quarks. Spun-up by the accreting materials to such a
high rotating speed, the compact star should have either a small radius or
short innermost stable circular orbit. By the empirical relation between the
upper kHz quasi-periodic oscillation frequency and star spin frequency, a
strong constraint on mass and radius is obtained as 1.51 solar masses and 10.9
km, which excludes most equations of states (EOSs) of normal neutrons and
strongly hints the star promisingly to be a strange quark star. Furthermore,
the star magnetic field is estimated to be about , which reconciles with those of millisecond radio pulsars,
revealing the clues of the evolution linkage of two types of astrophysical
objects.Comment: 10 pages, 2 figures, accepted by PASP 200
Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions in the "natural" gauge
We compute the Gaussian-fluctuation corrections to the saddle-point
Schwinger-boson results using collective coordinate methods. Concrete
application to investigate the frustrated J1-J2 antiferromagnet on the square
lattice shows that, unlike the saddle-point predictions, there is a quantum
nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by
considering the corrections to the spin stiffness on large lattices and
extrapolating to the thermodynamic limit, which avoids the infinite-lattice
infrared divergencies associated to Bose condensation. The very good agreement
of our results with exact numerical values on finite clusters lends support to
the calculational scheme employed.Comment: 4 pages, Latex, 3 figures included as eps files,minor correction
1/T_1 nuclear relaxation time of \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl : effects of magnetic frustration
We study the role played by the magnetic frustration in the antiferromagnetic
phase of the organic salt \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl. Using the
spatially anisotropic triangular Heisenberg model we analyze previous and new
performed NMR experiments. We compute the 1/T_1 relaxation time by means of the
modified spin wave theory. The strong suppression of the nuclear relaxation
time observed experimentally under varying pressure and magnetic field is
qualitatively well reproduced by the model. Our results suggest the existence
of a close relation between the effects of pressure and magnetic frustration.Comment: 10 pages, 9 figures, to appear in Journal of Phys.: Condens Matte
Long range Neel order in the triangular Heisenberg model
We have studied the Heisenberg model on the triangular lattice using several
Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact
diagonalization (ED) (up to 36 sites). By studying the spin gap as a function
of the system size we have obtained a robust evidence for a gapless spectrum,
confirming the existence of long range Neel order. Our best estimate is that in
the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by
about 59% from its classical value and the ground state energy per site is
e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the
important ground state correlations at short distance.Comment: 4 pages, RevTeX + 4 encapsulated postscript figure
The ROSAT All-Sky Survey: a Catalog of Clusters of Galaxies in a Region of 1 Ster around the South Galactic Pole
A field of 1.013 ster in the ROSAT all-sky survey (RASS), centered on the
south galactic pole (SGP), has been searched in a systematic, objective manner
for clusters of galaxies. The procedure relied on a correlation of the X-ray
positions and properties of ROSAT sources with the distribution of galaxies in
the COSMOS digitised data base, which was obtained by scanning the plates of
the UK Schmidt IIIa-J optical southern sky survey. The study used the second
ROSAT survey data base (RASS-2) and included several optical observing
campaigns to measure redshifts. The search, a precursor to the larger REFLEX
survey of the whole southern sky, reached the detection limits of the RASS and
the COSMOS data, and yielded a catalog of 186 clusters in which the lowest flux
is 1.5e-12 erg/cm2/s in the 0.1-2.4 keV band. Of these 157 have measured
redshifts. Using a limit of 3.0e-12 erg/cm2/s a complete subset of 112 clusters
was obtained, of which 110 have measured redshifts. The spatial distribution of
the X-ray clusters out to z = 0.15 shows an extension of the Local Supercluster
to the Pisces-Cetus supercluster (z<~0.07), and a more distant orthogonal
structure at 0.07<z<0.15.Comment: To be published in ApJ Supplements in February 2002: 53 pages: 18
figure
The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray Properties of Clusters Detected as Extended X-ray Sources
In the construction of an X-ray selected sample of galaxy clusters for
cosmological studies, we have assembled a sample of 495 X-ray sources found to
show extended X-ray emission in the first processing of the ROSAT All-Sky
Survey. The sample covers the celestial region with declination and galactic latitude and comprises sources with
a count rate counts s and a source extent likelihood of 7. In
an optical follow-up identification program we find 378 (76%) of these sources
to be clusters of galaxies. ...Comment: 61 pages; ApJS in press; fixed bug in table file; also available at
(better image quality) http://www.xray.mpe.mpg.de/theorie/NORAS
Purely radiative perfect fluids
We study `purely radiative' (div E = div H = 0) and geodesic perfect fluids
with non-constant pressure and show that the Bianchi class A perfect fluids can
be uniquely characterized --modulo the class of purely electric and
(pseudo-)spherically symmetric universes-- as those models for which the
magnetic and electric part of the Weyl tensor and the shear are simultaneously
diagonalizable. For the case of constant pressure the same conclusion holds
provided one also assumes that the fluid is irrotational.Comment: 12 pages, minor grammatical change
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