308 research outputs found
Instability of isolated triplet excitations on the Shastry-Sutherland lattice (SSL)
Configurations of singlets and triplets on the SSL have been proposed in the
literature as variational ground states of the Shastry-Sutherland model at
fixed magnetization M. We prove, that isolated triplet excitations on the SSL
are unstable if the coupling alpha falls below a critical value alpha_c=2.0
(approx.). The instability should be visible in the compound SrCu_2(BO_3)_2
where a coupling alpha^*=1.48 is realized.Comment: 4 pages, 4 figures, RevTe
Magnetization plateaus as insulator-superfluid transitions in quantum spin systems
We study the magnetization process in two-dimensional S=1/2 spin systems, to
discuss the appearance of a plateau structure. The following three cases are
considered: (1) the Heisenberg antiferromagnet and multiple-spin exchange model
on the triangular lattice, (2) Shastry-Sutherland type lattice, [which is a
possible model for SrCu2(BO3)2,] (3) 1/5-depleted lattice (for CaV4O9). We find
in these systems that magnetization plateaus can appear owing to a transition
from superfluid to a Mott insulator of magnetic excitations. The plateau states
have CDW order of the excitations. The magnetizations of the plateaus depend on
components of the magnetic excitations, range of the repulsive interaction, and
the geometry of the lattice.Comment: 5 pages, RevTeX, 7 figures, note and reference adde
Duality relations and exotic orders in electronic ladder systems
We discuss duality relations in correlated electronic ladder systems to
clarify mutual relations between various conventional and unconventional
phases. For the generalized two-leg Hubbard ladder, we find two exact duality
relations, and also one asymptotic relation which holds in the low-energy
regime. These duality relations show that unconventional (exotic) density-wave
orders such as staggered flux or circulating spin-current are directly mapped
to conventional density-wave orders, which establishes the appearance of
various exotic states with time-reversal and/or spin symmetry breaking. We also
study duality relations in the SO(5) symmetry that was proposed to unify
antiferromagnetism and d-wave superconductivity. We show that the same SO(5)
symmetry also unifies circulating spin current order and s-wave
superconductivity.Comment: 9 pages, 2 figures; Proceedings of SPQS2004 (Sendai
Low-Lying Magnetic Excitation of the Shastry-Sutherland Model
By using perturbation calculation and numerical diagonalization, low-energy
spin dynamics of the Shastry-Sutherland model is investigated paying particular
attention to the two-particle coherent motion. In addition to spin-singlet- and
triplet bound states, we find novel branches of coherent motion of a bound
quintet pair, which are usually unstable because of repulsion. Unusual
dispersion observed in neutron-scattering measurements are explained by the
present theory. The importance of the effects of phonon is also pointed out.Comment: 4 pages, 6 figures combined into 1 PSfile. Final version to appear in
Phys.Rev.Lett. (Jan. 2000
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
Multi-Triplet Magnons in SrCu(BO) Studied by Thermal Conductivity Measurements in Magnetic Fields
We have measured the thermal conductivity parallel to the a-axis of the
Zn-free and 1% Zn-substituted SrCuZn(BO) in magnetic fields
up to 14 T, in order to examine the thermal conductivity due to the
multi-triplet magnons. It has been found that the thermal conductivity peak
observed in the spin gap state is suppressed by the substitution of Zn for Cu
in high magnetic fields above 6 T, while it is not changed in low magnetic
fields below 6 T. The results suggest that the thermal conductivity peak in the
spin-gap state of SrCu(BO) is composed of not only thermal
conductivity due to phonons but also that due to the multi-triplet magnons in
high fields above 6 T.Comment: 7 pages, 2 figure
Quantum fluctuations in quantum lattice-systems with continuous symmetry
We discuss conditions for the absence of spontaneous breakdown of continuous
symmetries in quantum lattice systems at . Our analysis is based on
Pitaevskii and Stringari's idea that the uncertainty relation can be employed
to show quantum fluctuations. For the one-dimensional systems, it is shown that
the ground state is invariant under the continuous transformation if a certain
uniform susceptibility is finite. For the two- and three-dimensional systems,
it is shown that truncated correlation functions cannot decay any more rapidly
than whenever the continuous symmetry is spontaneously broken.
Both of these phenomena occur owing to quantum fluctuations. Our theorems cover
a wide class of quantum lattice-systems having not-too-long-range interactions.Comment: 14 pages. To appear in J.Stat.Phy
Quantum melting of incommensurate domain walls in two dimensions
Quantum fluctuations of periodic domain-wall arrays in two-dimensional
incommensurate states at zero temperature are investigated using the elastic
theory in the vicinity of the commensurate-incommensurate transition point.
Both stripe and honeycomb structures of domain walls with short-range
interactions are considered. It is revealed that the stripes melt and become a
stripe liquid in a large-wall-spacing (low-density) region due to dislocations
created by quantum fluctuations. This quantum melting transition is of second
order and characterized by the three-dimensional XY universality class.
Zero-point energies of the stripe and honeycomb structures are calculated. As a
consequence of these results, phase diagrams of the domain-wall solid and
liquid phases in adsorbed atoms on graphite are discussed for various
domain-wall masses. Quantum melting of stripes in the presence of long-range
interactions that fall off as power laws is also studied. These results are
applied to incommensurate domain walls in two-dimensional adsorbed atoms on
substrates and in doped antiferromagnets, e.g. cuprates and nickelates.Comment: 11 pages, 5 figure
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