319 research outputs found
Magnetization Plateaus in the Shastry-Sutherland Model for SrCu_2(BO_3)_2: Results of Fourth-Order Perturbation Expansion with a Low-Density Approximation
Magnetization plateaus in the Shastry-Sutherland model for are studied by the perturbation expansion method. The
fourth-order effective Hamiltonian which describes the dynamics of triplet
dimers (TD's) with in the singlet sea is derived and then
partially diagonalized for the space that consists of the TD configurations
with the lowest second-order energy. The fourth-order terms are treated within
a low-density approximation. Our procedure makes clear how TD interactions are
responsible for the formation of magnetization plateaus. Particularly, the
1/4-plateau is obtained by the fourth-neighbor TD repulsion in the fourth-order
perturbation, and a diagonal stripe arrangement of TD's appears at this
plateau.Comment: 8 pages, 12 figure
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
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
Field-Induced Effects of Anisotropic Magnetic Interactions in SrCu2(BO3)2
We observed a field-induced staggered magnetization in the 2D frustrated
dimer-singlet spin system SrCu2(BO3)2 by 11B NMR, from which the magnitudes of
the intradimer Dzyaloshinsky-Moriya interaction and the staggered g-tensor were
determined. These anisotropic interactions cause singlet-triplet mixing and
eliminate a quantum phase transition at the expected critical field Hc for gap
closing. They also provide a quantitative account for some puzzling phenomena
such as the onset of a uniform magnetization below the and the persistence of
the excitation gap above Hc. The gap was accurately determined from the
activation energy of the nuclear relaxation rate.Comment: 8 pages, 5 figures, published versio
Far infrared study of the two dimensional dimer spin system SrCu_2(BO_3)_2
Using far-infrared spectroscopy in magnetic fields up to 12T we have studied
a two-dimensional dimer spin gap system SrCu_2(BO_3)_2. We found several
infrared active modes in the dimerized state (below 10K) in the frequency range
from 3 to 100cm^-1. The measured splitting from the ground state to the excited
triplet M_S=0 sublevel is Delta_1=24.2cm^-1 and the other two triplet state
sublevels in zero magnetic field are 1.4cm^-1 below and above the M_S=0
sublevel. Another multiplet is at Delta_2=37.6cm^-1 from the ground state. A
strong electric dipole active transition polarized in the (ab)-plane is
activated in the dimer spin system below 15K at 52cm^-1.Comment: 4 pages including 5 figures, submitted to PRB, instrumental arte
facts remove
Numerical Study of Excited States in the Shastry-Sutherland Model
We investigate excited states of the Shastry-Sutherland model using a kind of
variational method. Starting from various trial states which include one or two
triplet dimers, we numerically pursue the best evaluation of the energy for
each set of quantum numbers. We present the energy difference as a function of
either the coupling ratio or the momentum and compare them with the
perturbative calculations. Our data suggest that the helical order phase exists
between the singlet dimer phase and the magnetically ordered phase. In
comparison with the experimental data we can estimate the intra-dimer coupling
J and the inter-dimer coupling J' for
SrCu2(BO3)2 : J'/J =0.65 and J = 87K.Comment: 15pages, 5figures to be published in JPS
Magnetization plateaus of SrCu_2(BO_3)_2 from a Chern-Simons theory
The antiferromagnetic Heisenberg model on the frustrated Shastry-Sutherland
lattice is studied by a mapping onto spinless fermions carrying one quantum of
statistical flux. Using a mean-field approximation these fermions populate the
bands of a generalized Hofstadter problem. Their filling leads to the
magnetization curve. For SrCu_2(BO_3)_2 we reproduce plateaus at 1/3 and 1/4 of
the saturation moment and predict a new one at 1/2. Gaussian fluctuations are
shown to be massive at these plateau values.Comment: 4 pages, 5 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
Dzyaloshinski-Moriya Interaction in the 2D Spin Gap System SrCu2(BO3)2
The Dzyaloshinski-Moriya interaction partially lifts the magnetic frustration
of the spin-1/2 oxide SrCu2(BO3)2. It explains the fine structure of the
excited triplet state and its unusual magnetic field dependence, as observed in
previous ESR and new neutron inelastic scattering experiments. We claim that it
is mainly responsible for the dispersion. We propose also a new mechanism for
the observed ESR transitions forbidden by standard selection rules, that relies
on an instantaneous Dzyaloshinski-Moriya interaction induced by spin-phonon
couplings.Comment: 5 pages, 4 figures, symmetries clarified, added references, (v3)
correct addres
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