633 research outputs found
Ground state properties of two spin models with exactly known ground states on the square lattice
We introduce a new two-dimensional model with diagonal four spin exchange and
an exactly knownground-state. Using variational ansaetze and exact
diagonalisation we calculate upper and lower bounds for the critical coupling
of the model. Both for this model and for the Shastry-Sutherland model we study
periodic systems up to system size 6x6.Comment: to appear in IJMPC 17, 12 pages, 7 figure
Probing the light induced dipole-dipole interaction in momentum space
We theoretically investigate the mechanical effect of the light-induced
dipole-dipole interaction potential on the atoms in a Bose-Einstein condensate.
We present numerical calculations on the magnitude and shape of the induced
potentials for different experimentally accessible geometries. It is shown that
the mechanical effect can be distinguished from the effect of incoherent
scattering for an experimentally feasible setting
Gapped Heisenberg spin chains in a field
We consider the fully anisotropic Heisenberg spin-1/2 antiferromagnet in a
uniform magnetic field, whose ground-state is characterized by broken spin
rotation symmetry and gapped spinon excitations. We expand on a recent
mean-field approach to the problem by incorporating fluctuations in a loop
expansion. Quantitative results for the magnetization, excitation gap and
specific heat are obtained. We compare our predictions with new DMRG and exact
diagonalization data and, for zero field, with the exact solution of the
spin chain from the Bethe Ansatz.Comment: 11 pages, 14 figure
beta-Cu2V2O7: a spin-1/2 honeycomb lattice system
We report on band structure calculations and a microscopic model of the
low-dimensional magnet beta-Cu2V2O7. Magnetic properties of this compound can
be described by a spin-1/2 anisotropic honeycomb lattice model with the
averaged coupling \bar J1=60-66 K. The low symmetry of the crystal structure
leads to two inequivalent couplings J1 and J1', but this weak spatial
anisotropy does not affect the essential physics of the honeycomb spin lattice.
The structural realization of the honeycomb lattice is highly non-trivial: the
leading interactions J1 and J1' run via double bridges of VO4 tetrahedra
between spatially separated Cu atoms, while the interactions between structural
nearest neighbors are negligible. The non-negligible inter-plane coupling
Jperp~15 K gives rise to the long-range magnetic ordering at TN~26 K. Our model
simulations improve the fit of the magnetic susceptibility data, compared to
the previously assumed spin-chain models. Additionally, the simulated ordering
temperature of 27 K is in remarkable agreement with the experiment. Our study
evaluates beta-Cu2V2O7 as the best available experimental realization of the
spin-1/2 Heisenberg model on the honeycomb lattice. We also provide an
instructive comparison of different band structure codes and computational
approaches to the evaluation of exchange couplings in magnetic insulators.Comment: 11 pages, 10 figures, 2 tables: revised version, extended description
of simulation result
Acoustic Faraday effect in TbGaO
The transverse acoustic wave propagating along the [100] axis of the cubic
TbGaO (acoustic mode) is doubly degenerate. A magnetic
field applied in the direction of propagation lifts this degeneracy and leads
to the rotation of the polarization vector - the magneto-acoustic Faraday
rotation. Here, we report on the observation and analysis of the
magneto-acoustic Faraday-effect in TbGaO in static and pulsed
magnetic fields. We present also a theoretical model based on magnetoelastic
coupling of 4 electrons to both, acoustic and optical phonons and an
effective coupling between them. This model explains the observed linear
frequency dependence of the Faraday rotation angle
Rydberg trimers and excited dimers bound by internal quantum reflection
Quantum reflection is a pure wave phenomena that predicts reflection of a
particle at a changing potential for cases where complete transmission occurs
classically. For a chemical bond, we find that this effect can lead to
non-classical vibrational turning points and bound states at extremely large
interatomic distances. Only recently has the existence of such ultralong-range
Rydberg molecules been demonstrated experimentally. Here, we identify a broad
range of molecular lines, most of which are shown to originate from two
different novel sources: a single-photon associated triatomic molecule formed
by a Rydberg atom and two ground state atoms and a series of excited dimer
states that are bound by a so far unexplored mechanism based on internal
quantum reflection at a steep potential drop. The properties of the Rydberg
molecules identified in this work qualify them as prototypes for a new type of
chemistry at ultracold temperatures.Comment: 6 pages, 3 figures, 1 tabl
Renormalization of tensor-network states
We have discussed the tensor-network representation of classical statistical
or interacting quantum lattice models, and given a comprehensive introduction
to the numerical methods we recently proposed for studying the tensor-network
states/models in two dimensions. A second renormalization scheme is introduced
to take into account the environment contribution in the calculation of the
partition function of classical tensor network models or the expectation values
of quantum tensor network states. It improves significantly the accuracy of the
coarse grained tensor renormalization group method. In the study of the quantum
tensor-network states, we point out that the renormalization effect of the
environment can be efficiently and accurately described by the bond vector.
This, combined with the imaginary time evolution of the wavefunction, provides
an accurate projection method to determine the tensor-network wavfunction. It
reduces significantly the truncation error and enable a tensor-network state
with a large bond dimension, which is difficult to be accessed by other
methods, to be accurately determined.Comment: 18 pages 23 figures, minor changes, references adde
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