1,625 research outputs found
Triplon mean-field analysis of an antiferromagnet with degenerate Shastry-Sutherland ground states
We look into the quantum phase diagram of a spin-
antiferromagnet on the square lattice with degenerate Shastry-Sutherland ground
states, for which only a schematic phase diagram is known so far. Many exotic
phases were proposed in the schematic phase diagram by the use of exact
diagonalization on very small system sizes. In our present work, an important
extension of this antiferromagnet is introduced and investigated in the
thermodynamic limit using triplon mean-field theory. Remarkably, this
antiferromagnet shows a stable plaquette spin-gapped phase like the original
Shastry-Sutherland antiferromagnet, although both of these antiferromagnets
differ in the Hamiltonian construction and ground state degeneracy. We propose
a sublattice columnar dimer phase which is stabilized by the second and third
neighbor antiferromagnetic Heisenberg exchange interactions. There are also
some commensurate and incommensurate magnetically ordered phases, and other
spin-gapped phases which find their places in the quantum phase diagram.
Mean-field results suggest that there is always a level-crossing phase
transition between two spin gapped phases, whereas in other situations, either
a level-crossing or a continuous phase transition happens
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
Carcinogenesis in tissue culture. 22. Malignant transformation of cloned rat liver cells treated in culture with 4-dimethylaminoazobenzene and properties of the transformed cells
Cultured rat liver cells which were cloned from a single
cell were transformed into malignant cells by a chemical carcinogen, 4-dimethylaminoazobenzene (DAB). The DAB-transformed cells produced tumors when back-transplanted into new-born rats but the carcinogen-untreated control cells did not. Characteristics of the transformed liver cells were compared to those of DAB-untreated control cells in regard to the morphology, the consumption of DAB
from the culture medium by the cells, the incorporation of 3H.DAB into the cells, and the aggregate.forming ability of the cells in rotation culture. The results showed that no significant parameter of malig. nant transformation in culture was detectable except the tumorigenicity of the transformed cells upon the inoculation into animals.</p
Unconventional magnetization plateaus in a Shastry-Sutherland spin tube
Using density matrix renormalization group (DMRG) and perturbative continuous
unitary transformations (PCUTs), we study the magnetization process in a
magnetic field for all coupling strengths of a quasi-1D version of the 2D
Shastry-Sutherland lattice, a frustrated spin tube made of two orthogonal dimer
chains. At small inter-dimer coupling, plateaus in the magnetization appear at
1/6, 1/4, 1/3, 3/8, and 1/2. As in 2D, they correspond to a Wigner crystal of
triplons. However, close to the boundary of the product singlet phase, plateaus
of a new type appear at 1/5 and 3/4. They are stabilized by the localization of
{\it bound states} of triplons. Their magnetization profile differs
significantly from that of single triplon plateaus and leads to specific NMR
signatures. We address the possibility to stabilize such plateaus in further
geometries by analyzing small finite clusters using exact diagonalizations and
the PCUTs.Comment: Final version as published in EP
Field Induced Staggered Magnetization and Magnetic Ordering in
We present a D NMR investigation of the gapped spin-1/2 compound . Our measurements reveal the presence of a magnetic
field induced transverse staggered magnetization (TSM) which persists well
below and above the field-induced 3D long-range magnetically ordered (FIMO)
phase. The symmetry of this TSM is different from that of the TSM induced by
the order parameter of the FIMO phase. Its origin, field dependence and
symmetry can be explained by an intra-dimer Dzyaloshinskii-Moriya interaction,
as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict
that the transition into the FIMO phase is not in the BEC universality class.Comment: 4 page
Orbital Ordering in ferromagnetic Lu2V2O7
We have observed the orbital ordering in the ferromagnetic Mott-insulator
Lu2V2O7 by the polarized neutron diffraction technique. The orbital ordering
pattern determined from the observed magnetic form factors can be explained in
terms of a linear combination of wave functions |yz>, |zx> and |xy>; |0> =
(1/3)^(1/2) |xy> + (1/3)^(1/2)|yz> + (1/3)^(1/2) |zx> which is proportional to
|(x + y + z)^2 - r^2>; where each orbital is extended toward the center-of-mass
of the V tetrahedron. We discuss the stability of the ferromagnetic Lu2V2O7,
using a Hubbard Hamiltonian with these three orbitals.Comment: 17pages. to be published in J. Phys. Soc. Jpn. 74 (2005
Hole Dynamics in the Orthogonal-Dimer Spin System
The dynamics of a doped hole in the orthogonal-dimer spin system is
investigated systematically in one, two and three dimensions. By combining the
bond-operator method with the self-consistent
Born approximation, we argue that a dispersive quasi-particle state in the
dimer phase is well defined even for quasi-two-dimensional systems. On the
other hand, a doped hole in the plaquette-singlet phase hardly itinerates,
forming an almost localized mode. We further clarify that although the
quasi-particle weight in the dimer phase is decreased in the presence of the
interchain coupling, it is not suppressed but even enhanced upon the
introduction of the interlayer coupling.Comment: 8 pages, 10 figure
Competing Spin-Gap Phases in a Frustrated Quantum Spin System in Two Dimensions
We investigate quantum phase transitions among the spin-gap phases and the
magnetically ordered phases in a two-dimensional frustrated antiferromagnetic
spin system, which interpolates several important models such as the
orthogonal-dimer model as well as the model on the 1/5-depleted square lattice.
By computing the ground state energy, the staggered susceptibility and the spin
gap by means of the series expansion method, we determine the ground-state
phase diagram and discuss the role of geometrical frustration. In particular,
it is found that a RVB-type spin-gap phase proposed recently for the
orthogonal-dimer system is adiabatically connected to the plaquette phase known
for the 1/5-depleted square-lattice model.Comment: 6 pages, to appear in JPSJ 70 (2001
Uniform and staggered magnetizations induced by Dzyaloshinskii-Moriya interactions in isolated and coupled spin 1/2 dimers in a magnetic field
We investigate the interplay of Dzyaloshinskii-Moriya interactions and an
external field in spin 1/2 dimers. For isolated dimers and at low field, we
derive simple expressions for the staggered and uniform magnetizations which
show that the orientation of the uniform magnetization can deviate
significantly from that of the external field. In fact, in the limit where the
vector of the Dzyaloshinskii-Moriya interaction is parallel to the
external field, the uniform magnetization actually becomes {\it perpendicular}
to the field. For larger fields, we show that the staggered magnetization of an
isolated dimer has a maximum close to one-half the polarization, with a large
maximal value of in the limit of very small Dzyaloshinskii-Moriya
interaction. We investigate the effect of inter-dimer coupling in the context
of ladders with Density Matrix Renormalization Group (DMRG) calculations and
show that, as long as the values of the Dzyaloshinskii-Moriya and of the
exchange interaction are compatible with respect to the development of a
staggered magnetization, the simple picture that emerges for isolated dimers is
also valid for weakly coupled dimers with minor modifications. The results are
compared with torque measurements on
Cu(CHN)Cl.Comment: 8 pages, 9 figure
High Resolution Study of Spin Excitations in the Shastry-Sutherland Singlet Ground State of SrCu2(BO3)2
High resolution, inelastic neutron scattering measurements on SrCu2(BO3)2
reveal the dispersion of the three single triplet excitations continuously
across the (H,0) direction within its tetragonal basal plane. These
measurements also show distinct Q dependencies for the single and multiple
triplet excitations, and that these excitations are largely dispersionless
perpendicular to this plane. The temperature dependence of the intensities of
these excitations is well described as the complement of the dc-susceptibility
of SrCu2(BO3)2.Comment: 4 pages, 4 figures. Submitted to PR
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