481 research outputs found
Magnetic order in a spin-1/2 interpolating kagome-square Heisenberg antiferromagnet
The coupled cluster method is applied to a spin-half model at zero
temperature (), which interpolates between Heisenberg antiferromagnets
(HAF's) on a kagome and a square lattice. With respect to an underlying
triangular lattice the strengths of the Heisenberg bonds joining the
nearest-neighbor (NN) kagome sites are along two of the
equivalent directions and along the third. Sites connected by
bonds are themselves connected to the missing NN non-kagome sites of
the triangular lattice by bonds of strength . When
and the model reduces to the square-lattice HAF. The
magnetic ordering of the system is investigated and its phase diagram
discussed. Results for the kagome HAF limit are among the best available.Comment: 21 pages, 8 figure
Coupled Cluster Treatment of the XY model
We study quantum spin systems in the 1D, 2D square and 3D cubic lattices with
nearest-neighbour XY exchange. We use the coupled-cluster method (CCM) to
calculate the ground-state energy, the T=0 sublattice magnetisation and the
excited state energies, all as functions of the anisotropy parameter .
We consider in detail and give some results for higher . In 1D these
results are compared with the exact results and in 2D with Monte-Carlo
and series expansions. We obtain critical points close to the expected value
and our extrapolated LSUBn results for the ground-state energy are
well converged for all except very close to the critical point.Comment: 11 pages, Latex, 4 postscript figure, accepted by J.Phys.: Condens.
Matte
Quantum Phase Transitions in Spin Systems
We discuss the influence of strong quantum fluctuations on zero-temperature
phase transitions in a two-dimensional spin-half Heisenberg system. Using a
high-order coupled cluster treatment, we study competition of magnetic bonds
with and without frustration. We find that the coupled cluster treatment is
able to describe the zero-temperature transitions in a qualitatively correct
way, even if frustration is present and other methods such as quantum Monte
Carlo fail.Comment: 8 pages, 12 Postscipt figures; Accepted for publication in World
Scientifi
Coupled Cluster Treatment of the Shastry-Sutherland Antiferromagnet
We consider the zero-temperature properties of the spin-half two-dimensional
Shastry-Sutherland antiferromagnet by using a high-order coupled cluster method
(CCM) treatment. We find that this model demonstrates various groundstate
phases (N\'{e}el, magnetically disordered, orthogonal dimer), and we make
predictions for the positions of the phase transition points. In particular, we
find that orthogonal-dimer state becomes the groundstate at . For the critical point where the semi-classical N\'eel
order disappears we obtain a significantly lower value than ,
namely, in the range . We therefore conclude that
an intermediate phase exists between the \Neel and the dimer phases. An
analysis of the energy of a competing spiral phase yields clear evidence that
the spiral phase does not become the groundstate for any value of . The
intermediate phase is therefore magnetically disordered but may exhibit
plaquette or columnar dimer ordering.Comment: 6 pages, 5 figure
The frustrated Heisenberg antiferromagnet on the honeycomb lattice: -- model
We study the ground-state (gs) phase diagram of the frustrated spin-1/2
-- antiferromagnet with () on the
honeycomb lattice, using the coupled-cluster method. We present results for the
ground-state energy, magnetic order parameter and plaquette valence-bond
crystal (PVBC) susceptibility. We find a paramagnetic PVBC phase for
, where and . The transition at
to the N\'{e}el phase seems to be a continuous deconfined
transition (although we cannot exclude a very narrow intermediate phase in the
range ), while that at is of
first-order type to another quasiclassical antiferromagnetic phase that occurs
in the classical version of the model only at the isolated and highly
degenerate critical point . The spiral phases that are present
classically for all values are absent for all .Comment: 6 pages, 5 figure
The relationship between body mass index (BMI) and sedentary behavior is mediated by negative peer interaction in boys
To determine if self-reported negative social interaction mediates the relationship between sedentary behavior and body mass index (BMI) percentile in boys. Twelve overweight/obese (≥85th BMI percentile) and 14 non-overweight (\u3c85th BMI percentile) boys (10.5 ± 1.5 years old) completed surveys assessing overt peer victimization and relational victimization. Children were individually given access to a gymnasium with physical activity equipment and sedentary alternatives for 30 minutes. Children could play with the equipment in any pattern they wished and the amount of time allocated to sedentary activities (sitting time) was recorded. Overt and relational victimization were moderately and positively associated with BMI percentile (r ≥ 0.40, p ≤ 0.04) and sitting time (r ≥ 0.40, p ≤ 0.05) and sitting time was positively associated with BMI percentile (r = 0.4, p = 0.05). After controlling for overt and relational victimization the correlation between sitting time and BMI percentile was non-significant (r ≤ 0.28, p ≥ 0.18). The positive relationship between BMI percentile and sedentary behavior was mediated by measures of negative social interaction
The frustrated Heisenberg antiferromagnet on the honeycomb lattice: A candidate for deconfined quantum criticality
We study the ground-state (gs) phase diagram of the frustrated spin-1/2
-- antiferromagnet with on the
honeycomb lattice, using coupled-cluster theory and exact diagonalization
methods. We present results for the gs energy, magnetic order parameter,
spin-spin correlation function, and plaquette valence-bond crystal (PVBC)
susceptibility. We find a N\'eel antiferromagnetic (AFM) phase for , a collinear striped AFM phase for , and a paramagnetic PVBC phase for . The transition at
appears to be of first-order type, while that at is
continuous. Since the N\'eel and PVBC phases break different symmetries our
results favor the deconfinement scenario for the transition at
User needs, benefits and integration of robotic systems in a space station laboratory
The methodology, results and conclusions of the User Needs, Benefits, and Integration Study (UNBIS) of Robotic Systems in the Space Station Microgravity and Materials Processing Facility are summarized. Study goals include the determination of user requirements for robotics within the Space Station, United States Laboratory. Three experiments were selected to determine user needs and to allow detailed investigation of microgravity requirements. A NASTRAN analysis of Space Station response to robotic disturbances, and acceleration measurement of a standard industrial robot (Intelledex Model 660) resulted in selection of two ranges of low gravity manipulation: Level 1 (10-3 to 10-5 G at greater than 1 Hz.) and Level 2 (less than = 10-6 G at 0.1 Hz). This included an evaluation of microstepping methods for controlling stepper motors and concluded that an industrial robot actuator can perform milli-G motion without modification. Relative merits of end-effectors and manipulators were studied in order to determine their ability to perform a range of tasks related to the three low gravity experiments. An Effectivity Rating was established for evaluating these robotic system capabilities. Preliminary interface requirements were determined such that definition of requirements for an orbital flight demonstration experiment may be established
Spin-1/2 Heisenberg antiferromagnet on an anisotropic kagome lattice
We use the coupled cluster method to study the zero-temperature properties of
an extended two-dimensional Heisenberg antiferromagnet formed from spin-1/2
moments on an infinite spatially anisotropic kagome lattice of corner-sharing
isosceles triangles, with nearest-neighbor bonds only. The bonds have exchange
constants along two of the three lattice directions and along the third. In the classical limit the ground-state (GS)
phase for has collinear ferrimagnetic (N\'{e}el) order where
the -coupled chain spins are ferromagnetically ordered in one direction
with the remaining spins aligned in the opposite direction, while for there exists an infinite GS family of canted ferrimagnetic spin states,
which are energetically degenerate. For the spin-1/2 case we find that quantum
analogs of both these classical states continue to exist as stable GS phases in
some regions of the anisotropy parameter , namely for
for the N\'{e}el state and for (at least part of)
the region for the canted phase. However, they are now
separated by a paramagnetic phase without either sort of magnetic order in the
region , which includes the isotropic
kagome point where the stable GS phase is now believed to be a
topological () spin liquid. Our best numerical estimates are
and
High-Order Coupled Cluster Method Calculations for the Ground- and Excited-State Properties of the Spin-Half XXZ Model
In this article, we present new results of high-order coupled cluster method
(CCM) calculations, based on a N\'eel model state with spins aligned in the
-direction, for both the ground- and excited-state properties of the
spin-half {\it XXZ} model on the linear chain, the square lattice, and the
simple cubic lattice. In particular, the high-order CCM formalism is extended
to treat the excited states of lattice quantum spin systems for the first time.
Completely new results for the excitation energy gap of the spin-half {\it XXZ}
model for these lattices are thus determined. These high-order calculations are
based on a localised approximation scheme called the LSUB scheme in which we
retain all -body correlations defined on all possible locales of
adjacent lattice sites (). The ``raw'' CCM LSUB results are seen to
provide very good results for the ground-state energy, sublattice
magnetisation, and the value of the lowest-lying excitation energy for each of
these systems. However, in order to obtain even better results, two types of
extrapolation scheme of the LSUB results to the limit (i.e.,
the exact solution in the thermodynamic limit) are presented. The extrapolated
results provide extremely accurate results for the ground- and excited-state
properties of these systems across a wide range of values of the anisotropy
parameter.Comment: 31 Pages, 5 Figure
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