509 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 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 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 status of Rangifer tarandus caribou in Yukon, Canada
This paper summarizes the population trends as well as research and management programs for woodland caribou {Rangifer tarandus caribou) in Yukon. Most herds are stable although not all are counted regularly and systematic monitoring of herds remains an essential need. Over the past decade the Southern Lakes, Aishihik, and Finlayson herds have been well studied and provide valuable models for guiding Yukon management programs. Over harvest and the spread of agriculture, forestry and mining are ongoing human activities are of concern to caribou managers
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]
Purinergic junctional transmission and propagation of calcium waves in cultured spinal cord microglial networks
In order to elucidate the mechanisms of purinergic transmission of calcium (Ca(2 + )) waves between microglial cells, we have employed micro-photolithographic methods to form discrete patterns of microglia that allow quantitative measurements of Ca(2 + ) wave propagation. Microglia were confined to lanes 20–100 [Formula: see text] wide and Ca(2 + ) waves propagated from a point of mechanical stimulation, with a diminution in amplitude, for about 120 [Formula: see text]. The number of cells participating in propagation also decreased over this distance. Ca(2 + ) waves could propagate across a cell-free lane from one microglia lane to another if this distance of separation was less than about 60 [Formula: see text] , indicating that propagation involved diffusion of a chemical transmitter. This transmitter was identified as ATP since all Ca(2 + ) wave propagation was blocked by the purinoceptor antagonist suramin, which blocks P2Y(2) and P2Y(12) at relatively low concentrations. Antibodies to P2Y(12) showed these at very high density compared with P2Y(2), indicating a role for P2Y(12) receptors. These observations were quantitatively accounted for by a model in which the main determinants are the diffusion of ATP released from a stimulated microglial cell and differences in the dissociation constant of the purinoceptors on the microglial cells
Ground-state phases of the frustrated spin-1/2 ---- Heisenberg ferromagnet () on the honeycomb lattice with
We study the ground-state (gs) properties of the frustrated spin-1/2
---- Heisenberg model on a honeycomb lattice with
ferromagnetic (FM) nearest-neighbor () exchange and frustrating
antiferromagnetic (AFM) next-nearest-neighbor () and
next-next-nearest-neighbor () exchanges, for the case .
We use the coupled cluster method in high orders of approximation, complemented
by the exact diagonalization of a lattice with 32 sites, and calculate the gs
energy, magnetic order parameter, and spin-spin correlation functions. We find
a quantum phase transition between regions characterized by FM order and a form
of AFM ("striped") collinear order at . We
compare results for the FM case (with ) to previous results for the
corresponding AFM case (with ). While the magnetic order parameters
behave similarly for the FM and the AFM models for large values of the
frustration parameter , there are considerable differences between them
for . For example, the quasiclassical collinear
magnetic long-range order for the AFM model (with ) breaks down at
, whereas the "equivalent" point for the FM model
(with ) occurs at . Unlike in the AFM model
(with ), where a plaquette valence-bond crystal phase intrudes
between the two corresponding quasiclassical AFM phases (with N\'eel and
striped order) for , with , we find no clear indications in the FM model for an intermediate
magnetically disordered phase between the phases exhibiting FM and striped
order. Instead, the evidence points strongly to a direct first-order transition
between the two ordered phases of the FM model.Comment: 21 pages, 6 figures (a & b
High-Order Coupled Cluster Method (CCM) Calculations for Quantum Magnets with Valence-Bond Ground States
In this article, we prove that exact representations of dimer and plaquette
valence-bond ket ground states for quantum Heisenberg antiferromagnets may be
formed via the usual coupled cluster method (CCM) from independent-spin product
(e.g. N\'eel) model states. We show that we are able to provide good results
for both the ground-state energy and the sublattice magnetization for dimer and
plaquette valence-bond phases within the CCM. As a first example, we
investigate the spin-half -- model for the linear chain, and we show
that we are able to reproduce exactly the dimerized ground (ket) state at
. The dimerized phase is stable over a range of values for
around 0.5. We present evidence of symmetry breaking by considering
the ket- and bra-state correlation coefficients as a function of . We
then consider the Shastry-Sutherland model and demonstrate that the CCM can
span the correct ground states in both the N\'eel and the dimerized phases.
Finally, we consider a spin-half system with nearest-neighbor bonds for an
underlying lattice corresponding to the magnetic material CaVO (CAVO).
We show that we are able to provide excellent results for the ground-state
energy in each of the plaquette-ordered, N\'eel-ordered, and dimerized regimes
of this model. The exact plaquette and dimer ground states are reproduced by
the CCM ket state in their relevant limits.Comment: 34 pages, 13 figures, 2 table
Strong and weak coupling limits in optics of quantum well excitons
A transition between the strong (coherent) and weak (incoherent) coupling
limits of resonant interaction between quantum well (QW) excitons and bulk
photons is analyzed and quantified as a function of the incoherent damping rate
caused by exciton-phonon and exciton-exciton scattering. For confined QW
polaritons, a second, anomalous, damping-induced dispersion branch arises and
develops with increasing damping. In this case, the strong-weak coupling
transition is attributed to a critical damping rate, when the intersection of
the normal and damping-induced dispersion branches occurs. For the radiative
states of QW excitons, i.e., for radiative QW polaritons, the transition is
described as a qualitative change of the photoluminescence spectrum at grazing
angles along the QW structure. Furthermore, we show that the radiative
corrections to the QW exciton states with in-plane wavevector approaching the
photon cone are universally scaled by an energy parameter rather than diverge.
The strong-weak coupling transition rates are also proportional to the same
energy parameter. The numerical evaluations are given for a GaAs single quantum
well with realistic parameters.Comment: Published in Physical Review B. 29 pages, 12 figure
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