530 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
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 spin-half Heisenberg antiferromagnet on two Archimedian lattices: From the bounce lattice to the maple-leaf lattice and beyond
We investigate the ground state of the two-dimensional Heisenberg
antiferromagnet on two Archimedean lattices, namely, the maple-leaf and bounce
lattices as well as a generalized - model interpolating between both
systems by varying from (bounce limit) to (maple-leaf
limit) and beyond. We use the coupled cluster method to high orders of
approximation and also exact diagonalization of finite-sized lattices to
discuss the ground-state magnetic long-range order based on data for the
ground-state energy, the magnetic order parameter, the spin-spin correlation
functions as well as the pitch angle between neighboring spins. Our results
indicate that the "pure" bounce () and maple-leaf () Heisenberg
antiferromagnets are magnetically ordered, however, with a sublattice
magnetization drastically reduced by frustration and quantum fluctuations. We
found that magnetic long-range order is present in a wide parameter range and that the magnetic order parameter varies only
weakly with . At a direct first-order transition to
a quantum orthogonal-dimer singlet ground state without magnetic long-range
order takes place. The orthogonal-dimer state is the exact ground state in this
large- regime, and so our model has similarities to the Shastry-Sutherland
model. Finally, we use the exact diagonalization to investigate the
magnetization curve. We a find a 1/3 magnetization plateau for and another one at 2/3 of saturation emerging only at large .Comment: 9 pages, 10 figure
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
Non-Invasive Hall Current Distribution Measurement in a Hall Effect Thruster
A means is presented to determine the Hall current density distribution in a closed drift thruster by remotely measuring the magnetic field and solving the inverse problem for the current density. The magnetic field was measured by employing an array of eight tunneling magnetoresistive (TMR) sensors capable of milligauss sensitivity when placed in a high background field. The array was positioned just outside the thruster channel on a 1.5 kW Hall thruster equipped with a center-mounted hollow cathode. In the sensor array location, the static magnetic field is approximately 30 G, which is within the linear operating range of the TMR sensors. Furthermore, the induced field at this distance is approximately tens of milligauss, which is within the sensitivity range of the TMR sensors. Because of the nature of the inverse problem, the induced-field measurements do not provide the Hall current density by a simple inversion; however, a Tikhonov regularization of the induced field does provide the current density distributions. These distributions are shown as a function of time in contour plots. The measured ratios between the average Hall current and the average discharge current ranged from 6.1 to 7.3 over a range of operating conditions from 1.3 kW to 2.2 kW. The temporal inverse solution at 1.5 kW exhibited a breathing mode frequency of 24 kHz, which was in agreement with temporal measurements of the discharge current
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
Numerical and approximate analytical results for the frustrated spin-1/2 quantum spin chain
We study the frustrated phase of the quantum spin-
system with nearest-neighbour and next-nearest-neighbour isotropic exchange
known as the Majumdar-Ghosh Hamiltonian. We first apply the coupled-cluster
method of quantum many-body theory based on a spiral model state to obtain the
ground state energy and the pitch angle. These results are compared with
accurate numerical results using the density matrix renormalisation group
method, which also gives the correlation functions. We also investigate the
periodicity of the phase using the Marshall sign criterion. We discuss
particularly the behaviour close to the phase transitions at each end of the
frustrated phase.Comment: 17 pages, Standard Latex File + 7 PostScript figures in separate
file. Figures also can also be requested from [email protected]
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
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
Ab Initio Treatments of the Ising Model in a Transverse Field
In this article, new results are presented for the zero-temperature
ground-state properties of the spin-half transverse Ising model on various
lattices using three different approximate techniques. These are, respectively,
the coupled cluster method, the correlated basis function method, and the
variational quantum Monte Carlo method. The methods, at different levels of
approximation, are used to study the ground-state properties of these systems,
and the results are found to be in excellent agreement both with each other and
with results of exact calculations for the linear chain and results of exact
cumulant series expansions for lattices of higher spatial dimension. The
different techniques used are compared and contrasted in the light of these
results, and the constructions of the approximate ground-state wave functions
are especially discussed.Comment: 28 Pages, 4 Figures, 1 Tabl
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