1,521 research outputs found
Degeneracy and ordering of the non-coplanar phase of the classical bilinear-biquadratic Heisenberg model on the triangular lattice
We investigate the zero-temperature behavior of the classical Heisenberg
model on the triangular lattice in which the competition between exchange
interactions of different orders favors a relative angle between neighboring
spins in the interval (0,2pi/3). In this situation, the ground states are
noncoplanar and have an infinite discrete degeneracy. In the generic case, the
set of the ground states is in one to one correspondence (up to a global
rotation) with the non-crossing loop coverings of the three equivalent
honeycomb sublattices into which the bonds of the triangular lattice can be
partitioned. This allows one to identify the order parameter space as an
infinite Cayley tree with coordination number 3. Building on the duality
between a similar loop model and the ferromagnetic O(3) model on the honeycomb
lattice, we argue that a typical ground state should have long-range order in
terms of spin orientation. This conclusion is further supported by the
comparison with the four-state antiferromagnetic Potts model [describing the
case when the angle between neighboring spins is equal to arccos(-1/3)], which
at zero temperature is critical and in terms of the solid-on-solid
representation is located exactly at the point of roughening transition. At
other values of the angle between neighboring spins an additional constraint
appears, whose presence drives the system into an ordered phase (unless this
angle is equal to pi/2, when another constraint is removed and the model
becomes trivially exactly solvable).Comment: 10 pages, 5 figure
Static impurities in the kagome lattice: dimer freezing and mutual repulsion
We consider the effects of doping the S = 1/2 kagome lattice with static
impurities. We demonstrate that impurities lower the number of low-lying
singlet states, induce dimer-dimer correlations of considerable spatial extent,
and do not generate free spin degrees of freedom. Most importantly, they
experience a highly unconventional mutual repulsion as a direct consequence of
the strong spin frustration. These properties are illustrated by exact
diagonalization, and reproduced to semi-quantitative accuracy within a dimer
resonating-valence-bond description which affords access to longer length
scales. We calculate the local magnetization induced by doped impurities, and
consider its implications for nuclear magnetic resonance measurements on known
kagome systems.Comment: 9 pages, 12 figure
Absence of single particle Bose-Einstein condensation at low densities for bosons with correlated-hopping
Motivated by the physics of mobile triplets in frustrated quantum magnets,
the properties of a two dimensional model of bosons with correlated-hopping are
investigated. A mean-field analysis reveals the presence of a pairing phase
without single particle Bose-Einstein condensation (BEC) at low densities for
sufficiently strong correlated-hopping, and of an Ising quantum phase
transition towards a BEC phase at larger density. The physical arguments
supporting the mean-field results and their implications for bosonic and
quantum spin systems are discussed.Comment: revtex, 4 pages, 5 figure
Phase diagram of the fully frustrated transverse-field Ising model on the honeycomb lattice
Motivated by the current interest in the quantum dimer model on the
triangular lattice, we investigate the phase diagram of the closely related
fully-frustrated transverse field Ising model on the honeycomb lattice using
classical and semi-classical approximations. We show that, in addition to the
fully polarized phase at large field, the classical model possesses a multitude
of phases that break the translational symmetry which in the dimer language,
correspond to a plaquette phase and a columnar phase separated by an infinite
cascade of mixed phases. The modification of the phase diagram by quantum
fluctuations has been investigated in the context of linear spin-wave theory.
The extrapolation of the semiclassical energies suggests that the plaquette
phase extends down to zero field for spin 1/2, in agreement with the
phase of the quantum dimer model on the triangular
lattice with only kinetic energy.Comment: 15 Pages, 11 Figures, Accepted for PR
Magnetization plateaux in an extended Shastry-Sutherland model
We study an extended two-dimensional Shastry-Sutherland model in a magnetic
field where besides the usual Heisenberg exchanges of the Shastry-Sutherland
model two additional SU(2) invariant couplings are included. Perturbative
continous unitary transformations are used to determine the leading order
effects of the additional couplings on the pure hopping and on the long-range
interactions between the triplons which are the most relevant terms for small
magnetization. We then compare the energy of various magnetization plateaux in
the classical limit and we discuss the implications for the two-dimensional
quantum magnet SrCu(BO).Comment: 8 pages, Proceedings of the HFM2008 Conferenc
Adaptive Guidance: Effects On Self-Regulated Learning In Technology-Based Training
Guidance provides trainees with the information necessary to make effective use of the learner control inherent in technology-based training, but also allows them to retain a sense of control over their learning (Bell & Kozlowski, 2002). One challenge, however, is determining how much learner control, or autonomy, to build into the guidance strategy. We examined the effects of alternative forms of guidance (autonomy supportive vs. controlling) on trainees’ learning and performance, and examined trainees’ cognitive ability and motivation to learn as potential moderators of these effects. Consistent with our hypotheses, trainees receiving adaptive guidance had higher levels of knowledge and performance than trainees in a learner control guidance. Controlling guidance had the most consistent positive impact on the learning outcomes, while autonomy supportive guidance demonstrated utility for more strategic outcomes. In addition, guidance was generally more effective for trainees with higher levels of cognitive ability and autonomy guidance served to enhance the positive effects of motivation to learn on the training outcomes
Crystallization of the resonating valence bond liquid as vortex condensation
We show that the liquid-to-crystal quantum phase transition in the
Rokhsar--Kivelson dimer model on the two-dimensional triangular lattice occurs
as a condensation of vortex-like excitations called ``visons''. This conclusion
is drawn from the numerical studies of the vison spectrum in the liquid phase
by using the Green's function Monte Carlo method. We find that visons remain
the lowest excitation throughout the liquid phase and that their gap decreases
continuously to zero at the phase transition. The nature of the crystal phase
and the second order of the phase transition are in agreement with the earlier
prediction of Moessner and Sondhi [Phys. Rev. B 63, 224401 (2001)].Comment: 4 pages, 4 figure
Frustrated three-leg spin tubes: from spin 1/2 with chirality to spin 3/2
Motivated by the recent discovery of the spin tube
[(CuCltachH)Cl]Cl, we investigate the properties of a frustrated
three-leg spin tube with antiferromagnetic intra-ring and inter-ring couplings.
We pay special attention to the evolution of the properties from weak to strong
inter-ring coupling and show on the basis of extensive density matrix
renormalization group and exact diagonalization calculations that the system
undergoes a first-order phase transition between a dimerized gapped phase at
weak coupling that can be described by the usual spin-chirality model and a
gapless critical phase at strong coupling that can be described by an effective
spin-3/2 model. We also show that there is a magnetization plateau at 1/3 in
the gapped phase and slightly beyond. The implications for
[(CuCltachH)Cl]Cl are discussed, with the conclusion that this
system behaves essentially as a spin-3/2 chain.Comment: 8 pages, 9 figures, revised versio
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