9,025 research outputs found
On the presence of mid-gap states in CaV4O9
Using exact diagonalizations of finite clusters with up to 32 sites, we study
the model on the 1/5 depleted square lattice. Spin-spin correlation
functions are consistent with plaquette order in the spin gap phase which
exists for intermediate values of . Besides, we show that singlet
states will be present in the singlet-triplet gap if is not too small
(). We argue that this property should play a central
role in determining the exchange integrals in Comment: 4 pages, 5 postscript figure
Magnetic Susceptibility for
We examine experimental magnetic susceptibility for
CaVO by fitting with fitting function .
The function is a power series of 1/T and the lowest order
term is fixed as , where is the Curie constant as determined by the
experimental -value (g=1.96). Fitting parameters are , and
expansion coefficients except for the first one in .
We determine and as 0.73 and 0 for an
experimental sample. We interpret as the volume fraction of
CaVO in the sample and as the susceptibility for the
pure CaVO. The result of means that the sample includes
nonmagnetic components. This interpretation consists with the result of a
perturbation theory and a neutron scattering experiment.Comment: 4pages, 4figure
Orbital Order Effect of Two-Dimensional Spin Gap System for CaV4O9
Effects of possible orbital order in magnetic properties of two-dimensional
spin gap system for CaVO are investigated theoretically. After
analyzing experimental data, we show that single orbital models assumed in the
literature are insufficient to reproduce the data. To understand the origin of
the discrepancy, we assume that in state of V, and
orbitals have substantial contributions in the lowest-energy atomic level which
leads to a double-degeneracy. We study possible configurations of the orbital
order. By exact diagonalization and perturbation expansion, we calculate the
susceptibility, wavenumber dependence of low-lying excitations and equal-time
spin-spin correlations which is related to integrated intensity of the neutron
inelastic scattering. These quantities sensitively depend on the configuration
of the orbital order. The calculated results for some configurations of the
orbital order reproduce many experimental results much better than the previous
single-orbital models. However some discrepancy still remains to completely
reproduce all of the reported experimental results. To understand the origin of
these discrepancies, we point out the possible importance of the partially
occupied orbital in addition to orbital order of partially filled
and orbitals.Comment: 19 pages LATEX, 15 postscript figures, using jpsj.sty,to be published
in J.Phys.Soc.Jpn. Vol.67 No.2 (1998
The Heisenberg model on the 1/5-depleted square lattice and the CaV4O9 compound
We investigate the ground state structure of the Heisenberg model on the
1/5-depleted square lattice for arbitrary values of the first- and
second-neighbor exchange couplings. By using a mean-field Schwinger-boson
approach we present a unified description of the rich ground-state diagram,
which include the plaquette and dimer resonant-valence-bond phases, an
incommensurate phase and other magnetic orders with complex magnetic unit
cells. We also discuss some implications of ours results for the experimental
realization of this model in the CaV4O9 compound.Comment: 4 pages, Latex, 7 figures included as eps file
Effects of Possible Transitions in Neutral Meson Decays}
We explore the possibility that the existing data on like-sign dileptons at
the resonance consist of events arising from mixing and also from transitions. The
consequences of these nonstandard transitions for certain time-asymmetries
which are likely to be measured at the factories are studied.Comment: {\LARGE \bf 10 pages, no figures, process using latex, TIFR/TH/93-5
Isgur - Wise Functions for Confined Light Quarks in a Colour Electric Potential
We explore the influence on the Isgur-Wise function of the colour electric
potential between heavy and light quarks in mesons. It is shown that in bag
models, its inclusion tends to restore light quark flavour symmetry relative to
the MIT bag predictions, and that relative to this model it flattens the
Isgur-Wise function. Results compare very well with observations.Comment: 9 pages, 1 figure (available upon request), Latex, TPJU - 4/9
Phenomenology of Hybrid Scenarios of Neutrino Dark Energy
We study the phenomenology of hybrid scenarios of neutrino dark energy, where
in addition to a so-called Mass Varying Neutrino (MaVaN) sector a cosmological
constant (from a false vacuum) is driving the accelerated expansion of the
universe today. For general power law potentials we calculate the effective
equation of state parameter w_{eff}(z) in terms of the neutrino mass scale. Due
to the interaction of the dark energy field (acceleron) with the neutrino
sector, w_{eff}(z) is predicted to become smaller than -1 for z>0, which could
be tested in future cosmological observations. For the considered scenarios,
the neutrino mass scale additionally determines which fraction of the dark
energy is dynamical, and which originates from the cosmological constant like
vacuum energy of the false vacuum. On the other hand, the field value of the
acceleron field today as well as the masses of the right-handed neutrinos,
which appear in the seesaw-type mechanism for small neutrino masses, are not
fixed. This, in principle, allows to realise hybrid scenarios of neutrino dark
energy with a high-scale seesaw where the right-handed neutrino masses are
close to the GUT scale. We also comment on how MaVaN Hybrid Scenarios with
high-scale seesaw might help to resolve stability problems of dark energy
models with non-relativistic neutrinos.Comment: 22 pages, 5 figures; references and comments added; version to be
published in JCA
Ab Initio Calculation of Spin Gap Behavior in CaV4O9
Second neighbor dominated exchange coupling in CaV4O9 has been obtained from
ab initio density functional (DF) calculations. A DF-based self-consistent
atomic deformation model reveals that the nearest neighbor coupling is small
due to strong cancellation among the various superexchange processes. Exact
diagonalization of the predicted Heisenberg model yields spin-gap behavior in
good agreement with experiment. The model is refined by fitting to the
experimental susceptibility. The resulting model agrees very well with the
experimental susceptibility and triplet dispersion.Comment: 4 pages; 3 ps figures included in text; Revte
Exchange interactions and magnetic properties of the layered vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9
We have performed ab-initio calculations of exchange couplings in the layered
vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9. The uniform susceptibility of the
Heisenberg model with these exchange couplings is then calculated by quantum
Monte Carlo method; it agrees well with the experimental measurements. Based on
our results we naturally explain the unusual magnetic properties of these
materials, especially the huge difference in spin gap between CaV2O5 and
MgV2O5, the unusual long range order in CaV3O7 and the "plaquette resonating
valence bond (RVB)" spin gap in CaV4O9
Evidence for electronically-driven ferroelectricity in the family of strongly correlated dimerized BEDT-TTF molecular conductors
By applying measurements of the dielectric constants and relative length
changes to the dimerized molecular conductor
-(BEDT-TTF)Hg(SCN)Cl, we provide evidence for order-disorder
type electronic ferroelectricity which is driven by charge order within the
(BEDT-TTF) dimers and stabilized by a coupling to the anions. According to
our density functional theory calculations, this material is characterized by a
moderate strength of dimerization. This system thus bridges the gap between
strongly dimerized materials, often approximated as dimer-Mott systems at 1/2
filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge
order. Our results indicate that intra-dimer charge degrees of freedom are of
particular importance in correlated -(BEDT-TTF)X salts and can
create novel states, such as electronically-driven multiferroicity or
charge-order-induced quasi-1D spin liquids.Comment: 6 pages, 4 figures + Supplementary Information (8 pages, 8 figures
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