7,319 research outputs found
Dimerization-induced enhancement of the spin gap in the quarter-filled two-leg rectangular ladder
We report density-matrix renormalization group calculations of spin gaps in
the quarter-filled correlated two-leg rectangular ladder with bond-dimerization
along the legs of the ladder. In the small rung-coupling region, dimerization
along the leg bonds can lead to large enhancement of the spin gap.
Electron-electron interactions further enhance the spin gap, which is nonzero
for all values of the rung electron hopping and for arbitrarily small
bond-dimerization. Very large spin gaps, as are found experimentally in
quarter-filled band organic charge-transfer solids with coupled pairs of
quasi-one-dimensional stacks, however, occur within the model only for large
dimerization and rung electron hopping that are nearly equal to the hopping
along the legs. Coexistence of charge order and spin gap is also possible
within the model for not too large intersite Coulomb interaction
Gauge invariant MSSM inflaton
We argue that all the necessary ingredients for successful inflation are
present in the flat directions of the Minimally Supersymmetric Standard Model.
We show that out of many gauge invariant combinations of squarks, sleptons and
Higgses, there are two directions, , and , which are
promising candidates for the inflaton. The model predicts more than
e-foldings with an inflationary scale of GeV,
provides a tilted spectrum with an amplitude of and a
negligible tensor perturbation. The temperature of the thermalized plasma could
be as low as ~TeV. Parts of the inflaton potential
can be determined independently of cosmology by future particle physics
experiments.Comment: 4 revtex pages, some references added, stabilization of moduli and
supergravity effects are discusse
Transport and magnetic anomalies due to A-site ionic size mismatch in LaCaBa_{x}MnO
We present results of electrical resistivity, magnetoresistance and ac and dc
magnetic susceptibility on polycrystalline samples of the type
La(0.5)Ca(0.5-x)Ba(x)MnO(3) synthesized under identical heat treatment
conditions. The substitution of larger Ba ions for Ca results in a non-
monotonic variation of the curie temperature as the system evolves from a
charge ordered insulating state for x=0 to a ferromagnetic metallic state for
x=0.5. An intermediate compositino, x=0.1, interestingly exhibits
ferromagnetic. insulating behaviour with thermal hysteresis in ac chi around
the curie tem- perature (120K). The x=0.2 and 0.3 compounds exhibit
semiconducting like behavior as the temperature is lowered below 300K, with a
broad peak in rho around 80-100K: These compositions exhibit a weak increase in
rho as the temperature lowered below 30K, indicative of electron localization
effects. These compositions also undergo ferromagnetic transitions below about
200 and 235K respectively, though these are non-hysteretic; above all, for
these compositions, MR is large and conveniently measurable over the entire
tempera- ture range of measurement below Tc. This experimental finding may be
of interest from the application point of view. We infer that the A-site
ionic-size mismatch plays a crucial role in the deciding these properties.Comment: 5 pages, 6 Figures, Resubmitted with extended abstract on 26 Nov,
199
Sneutrino condensate source for density perturbations, leptogenesis and low reheat temperature
We bring together some known ingredients beyond the Standard Model physics
which can explain the hot Big Bang model with the observed baryon asymmetry and
also the fluctuations in the cosmic microwave background radiation with a
minimal set of assumptions. We propose an interesting scenario where the
inflaton energy density is dumped into an infinitely large extra dimension.
Instead of the inflaton it is the right handed sneutrino condensate, which is
acquiring non-zero vacuum expectation value during inflation, whose
fluctuations are responsible for the density perturbations seen in the cosmic
microwave background radiation with a spectral index . The decay
of the condensate is explaining the reheating of the Universe with a
temperature, GeV, and the baryon asymmetry of order one
part in with no baryon-isocurvature fluctuations.Comment: 4 pages, Title has been modified, trivial changes to match accepted
version in Phys. Rev. Let
Tests of the asymptotic large frequency separation of acoustic oscillations in solar-type and red giant stars
Asteroseismology, i.e. the study of the internal structures of stars via
their global oscillations, is a valuable tool to obtain stellar parameters such
as mass, radius, surface gravity and mean density. These parameters can be
obtained using certain scaling relations which are based on an asymptotic
approximation. Usually the observed oscillation parameters are assumed to
follow these scaling relations. Recently, it has been questioned whether this
is a valid approach, i.e., whether the order of the observed oscillation modes
are high enough to be approximated with an asymptotic theory. In this work we
use stellar models to investigate whether the differences between observable
oscillation parameters and their asymptotic estimates are indeed significant.
We compute the asymptotic values directly from the stellar models and derive
the observable values from adiabatic pulsation calculations of the same models.
We find that the extent to which the atmosphere is included in the models is a
key parameter. Considering a larger extension of the atmosphere beyond the
photosphere reduces the difference between the asymptotic and observable values
of the large frequency separation. Therefore, we conclude that the currently
suggested discrepancies in the scaling relations might have been overestimated.
Hence, based on the results presented here we believe that the suggestions of
Mosser et al. (2013) should not be followed without careful consideration.Comment: 6 pages, 4 figures, 1 table, accepted for publication by MNRAS as a
Letter to the Edito
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