188 research outputs found
Spin-Wave Theory of the Multiple-Spin Exchange Model on a Triangular Lattice in a Magnetic Field : 3-Sublattice Structures
We study the spin wave in the S=1/2 multiple-spin exchange model on a
triangular lattice in a magnetic field within the linear spin-wave theory. We
take only two-, three- and four-spin exchange interactions into account and
restrict ourselves to the region where a coplanar three-sublattice state is the
mean-field ground state. We found that the Y-shape ground state survives
quantum fluctuations and the phase transition to a phase with a 6-sublattice
structure occurs with softening of the spin wave. We estimated the quantum
corrections to the ground state sublattice magnetizations due to zero-point
spin-wave fluctuations.Comment: 8 pages, 20 figure
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
Noble internal transport barriers and radial subdiffusion of toroidal magnetic lines
Single trajectories of magnetic line motion indicate the persistence of a
central protected plasma core, surrounded by a chaotic shell enclosed in a
double-sided transport barrier : the latter is identified as being composed of
two Cantori located on two successive "most-noble" numbers values of the
perturbed safety factor, and forming an internal transport barrier (ITB).
Magnetic lines which succeed to escape across this barrier begin to wander in a
wide chaotic sea extending up to a very robust barrier (as long as L<1) which
is identified mathematically as a robust KAM surface at the plasma edge. In
this case the motion is shown to be intermittent, with long stages of
pseudo-trapping in the chaotic shell, or of sticking around island remnants, as
expected for a continuous time random walk.Comment: TEX file, 84 pages including 32 color figures. Higher quality figures
can be seen on the PDF file at
http://membres.lycos.fr/fusionbfr/JHM/Tokamap/JSP.pd
Atomic Fermi gas in the trimerized Kagom\'e lattice at the filling 2/3
We study low temperature properties of an atomic spinless interacting Fermi
gas in the trimerized Kagom\'e lattice for the case of two fermions per trimer.
The system is described by a quantum spin 1/2 model on the triangular lattice
with couplings depending on bonds directions. Using exact diagonalizations we
show that the system exhibits non-standard properties of a {\it quantum
spin-liquid crystal}, combining a planar antiferromagnetic order with an
exceptionally large number of low energy excitations.Comment: 4 pages & 4 figures + 2 tables, better version of Fig.
Doping quantum dimer models on the square lattice
A family of models is proposed to describe the motion of holes in a
fluctuating quantum dimer background on the square lattice. Following
Castelnovo et al. [Ann. Phys. (NY) 318, 316 (2005)], a generalized
Rokhsar-Kivelson Hamiltonian at **finite doping** which can be mapped on a
**doped** interacting classical dimer model is constructed. A simple physical
extension of this model is also considered. Using numerical computations and
simple considerations based on the above exact mapping, we determine the phase
diagram of the model showing a number of quantum phases typical of a doped Mott
insulator. The two-hole correlation function generically exhibits short-range
or long-range algebraic correlations in the solid (columnar) and liquid
(critical) phases of the model, respectively. Evidence for an extended region
of a doped VBS phase exhibiting holon pairing but **no** phase separation is
given. In contrast, we show that hole deconfinement occurs in the staggered
dimer phase.Comment: 5 page
A Generalized Quantum Dimer Model Applied to the Frustrated Heisenberg Model on the Square Lattice: Emergence of a Mixed Columnar-Plaquette Phase
Aiming to describe frustrated quantum magnets with non-magnetic singlet
ground states, we have extended the Rokhsar-Kivelson (RK) loop-expansion to
derive a generalized Quantum Dimer Model containing only connected terms up to
arbitrary order. For the square lattice frustrated Heisenberg antiferromagnet
(J1-J2-J3 model), an expansion up to 8th order shows that the leading
correction to the original RK model comes from dimer moves on length-6 loops.
This model free of the original sign problem is treated by advanced numerical
techniques. The results suggest that a rotationally anisotropic plaquette phase
is the ground state of the Heisenberg model in the parameter region of maximum
frustration.Comment: 9 pages, 8 figure
Soliton binding and low-lying singlets in frustrated odd-legged S=1/2 spin tubes
Motivated by the intriguing properties of the vanadium spin tube Na2V3O7, we
show that an effective spin-chirality model similar to that of standard
Heisenberg odd-legged S=1/2 spin tubes can be derived for frustrated inter-ring
couplings, but with a spin-chirality coupling constant alpha that can be
arbitrarily small. Using density matrix renormalization group and analytical
arguments, we show that, while spontaneous dimerization is always present,
solitons become bound into low-lying singlets as alpha is reduced. Experimental
implications for strongly frustrated tubes are discussed.Comment: 4 pages, 4 figure
Emergent Ising degrees of freedom in frustrated two-leg ladder and bilayer Heisenberg antiferromagnets
Based on exact diagonalization data for finite quantum Heisenberg
antiferromagnets on two frustrated lattices (two-leg ladder and bilayer) and
analytical arguments we map low-energy degrees of freedom of the spin models in
a magnetic field on classical lattice-gas models. Further we use
transfer-matrix calculations and classical Monte Carlo simulations to give a
quantitative description of low-temperature thermodynamics of the quantum spin
models. The classical lattice-gas model yields an excellent description of the
quantum spin models up to quite large temperatures. The main peculiarity of the
considered frustrated bilayer is a phase transition which occurs at low
temperatures for a wide range of magnetic fields below the saturation magnetic
field and belongs to the two-dimensional Ising model universality class.Comment: 17 pages, 8 figure
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