105 research outputs found
Ground State Properties of One Dimensional S=1/2 Heisenberg Model with Dimerization and Quadrumerization
The one dimensional S=1/2 Heisenberg model with dimerization and
quadrumerization is studied by means of the numerical exact diagonalization of
finite size systems. Using the phenomenological renormalization group and
finite size scaling law, the ground state phase diagram is obtained in the
isotropic case. It exhibits a variety of the ground states which contains the
S=1 Haldane state, S=1 dimer state and S=1/2 dimer state as limiting cases. The
gap exponent is also calculated which coincides with the value for the
dimerization transition of the isotropic Heisenberg chain. In the XY limit, the
phase diagram is obtained analytically and the comparison is made with the
isotropic case.Comment: 4 pages, 7 figure
Excitation Spectra of Structurally Dimerized and Spin-Peierls Chains in a Magnetic Field
The dynamical spin structure factor and the Raman response are calculated for
structurally dimerized and spin-Peierls chains in a magnetic field, using exact
diagonalization techniques. In both cases there is a spin liquid phase composed
of interacting singlet dimers at small fields h < h_c1, an incommensurate
regime (h_c1 < h < h_c2) in which the modulation of the triplet excitation
spectra adapts to the applied field, and a fully spin polarized phase above an
upper critical field h_c2. For structurally dimerized chains, the spin gap
closes in the incommensurate phase, whereas spin-Peierls chains remain gapped.
In the spin liquid regimes, the dominant feature of the triplet spectra is a
one-magnon bound state, separated from a continuum of states at higher
energies. There are also indications of a singlet bound state above the
one-magnon triplet.Comment: RevTex, 10 pages with 8 eps figure
Magnetic excitation spectrum of dimerized antiferromagnetic chains
Motivated by recent measurements on CuGeO the spectrum of magnetic
excitations of an antiferromagnetic chain with alternating
coupling strength is investigated. Wave vector dependent magnons and a
continuum with square root behavior at the band edges are found. The spectral
density of the continua is calculated. Spin rotation symmetry fixes the gap of
the continuum to be twice the elementary magnon gap. This is in excellent
agreement with experimental results. In addition, the existence of bound states
of two magnons is predicted: below the continuum a singlet and a triplet, above
the continuum an ``anti-bound'' quintuplet. The results are based on field
theoretic arguments, RPA calculations, and consideration of the limit of strong
alternation.Comment: 4 pages, 4 figures included, Revte
Spin-Peierls Dimerization of a s=1/2 Heisenberg Antiferromagnet on a Square Lattice
Dimerization of a spin-half Heisenberg antiferromagnet on a square lattice is
investigated for several possible dimerized configurations, some of which are
shown to have lower ground state energies than the others. In particular, the
lattice deformations resulting in alternate stronger and weaker couplings along
both the principal axes of a square lattice are shown to result in a larger
gain in magnetic energy. In addition, a `columnar' configuration is shown to
have a lower ground state energy and a faster increase in the energy gap
parameter than a `staggered' configuration. The inclusion of unexpanded
exchange coupling leads to a power law behaviour for the magnetic energy gain
and energy gap, which is qualitatively different from that reported earlier.
Instead of increasing as , the two quantities depend on
as This is true both in the near critical
regime as well as in the far regime . It is suggested that the unexpanded exchange coupling is as much a source
of the logarithmic dependence as a correction due to the contribution of
umklapp processes. Staggered magnetization is shown to follow the same -dependence in all the configurations in the small -regime, while for
, it follows the power law .Comment: 12 pages, 7 Postscript figures, RevTex forma
Density Matrix Renormalization Group Study of the Spin 1/2 Heisenberg Ladder with Antiferromagnetic Legs and Ferromagnetic Rungs
The ground state and low lying excitation of the spin 1/2 Heisenberg ladder
with antiferromagnetic leg () and ferromagnetic rung () interaction is studied by means of the density matrix renormalization
group method. It is found that the state remains in the Haldane phase even for
small suggesting the continuous transition to the gapless
phase at . The critical behavior for small is studied by
the finite size scaling analysis. The result is consistent with the recent
field theoretical prediction.Comment: 11 pages, revtex, figures upon reques
Return to work trajectories among employees with mental health problems:Insights from longitudinal sickness absence data and a multi-stakeholder expert meeting
IOSH, the Chartered body for health and safety professionals, is committed to evidence-based practice in workplace safety and health. We maintain a Research Fund to support research and inspire innovation as part of our work as a thought leader in health and safet
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