9,278 research outputs found
The spin gap of CaV4O9 revisited
The large-plaquette scenario of the spin gap in CaV4O9 is investigated on the
basis of extensive exact diagonalizations. We confirm the existence of a
large-plaquette phase in a wide range of parameters, and we show that the most
recent neutron scattering data actually require an intra-plaquette second
neighbor exchange integral much larger than the inter-plaquette one, thus
justifying the perturbative calculation used in the interpretation of the
neutron scattering experiments.Comment: 2 pages with 3 figure
Evolution of Cosmological Perturbations in the Long Wavelength Limit
The relation between the long wavelength limit of solutions to the
cosmological perturbation equations and the perturbations of solutions to the
exactly homogeneous background equations is investigated for scalar
perturbations on spatially flat cosmological models. It is shown that a
homogeneous perturbation coincides with the long wavelength limit of some
inhomogeneous perturbation only when the former satisfies an additional
condition corresponding to the momentum constraint if the matter consists only
of scalar fields. In contrast, no such constraint appears if the fundamental
variables describing the matter contain a vector field as in the case of a
fluid. Further, as a byproduct of this general analysis, it is shown that there
exist two universal exact solutions to the perturbation equations in the long
wavelength limit, which are expressed only in terms of the background
quantities. They represent adiabatic growing and decaying modes, and correspond
to the well-known exact solutions for perfect fluid systems and scalar field
systems.Comment: 16 pages, no figure, submitted to PR
N-soliton solutions to the DKP equation and Weyl group actions
We study soliton solutions to the DKP equation which is defined by the Hirota
bilinear form, {\begin{array}{llll} (-4D_xD_t+D_x^4+3D_y^2)
\tau_n\cdot\tau_n=24\tau_{n-1}\tau_{n+1}, (2D_t+D_x^3\mp 3D_xD_y) \tau_{n\pm
1}\cdot\tau_n=0 \end{array} \quad n=1,2,.... where . The
-functions are given by the pfaffians of certain skew-symmetric
matrix. We identify one-soliton solution as an element of the Weyl group of
D-type, and discuss a general structure of the interaction patterns among the
solitons. Soliton solutions are characterized by skew-symmetric
constant matrix which we call the -matrices. We then find that one can have
-soliton solutions with being any number from to for some of
the -matrices having only nonzero entries in the upper
triangular part (the number of solitons obtained from those -matrices was
previously expected to be just ).Comment: 22 pages, 12 figure
Field-Induced Effects of Anisotropic Magnetic Interactions in SrCu2(BO3)2
We observed a field-induced staggered magnetization in the 2D frustrated
dimer-singlet spin system SrCu2(BO3)2 by 11B NMR, from which the magnitudes of
the intradimer Dzyaloshinsky-Moriya interaction and the staggered g-tensor were
determined. These anisotropic interactions cause singlet-triplet mixing and
eliminate a quantum phase transition at the expected critical field Hc for gap
closing. They also provide a quantitative account for some puzzling phenomena
such as the onset of a uniform magnetization below the and the persistence of
the excitation gap above Hc. The gap was accurately determined from the
activation energy of the nuclear relaxation rate.Comment: 8 pages, 5 figures, published versio
Inflation and dark energy arising from geometrical tachyons
We study the motion of a BPS D3-brane in the NS5-brane ring background. The
radion field becomes tachyonic in this geometrical set up. We investigate the
potential of this geometrical tachyon in the cosmological scenario for
inflation as well as dark energy. We evaluate the spectra of scalar and tensor
perturbations generated during tachyon inflation and show that this model is
compatible with recent observations of Cosmic Microwave Background (CMB) due to
an extra freedom of the number of NS5-branes. It is not possible to explain the
origin of both inflation and dark energy by using a single tachyon field, since
the energy density at the potential minimum is not negligibly small because of
the amplitude of scalar perturbations set by CMB anisotropies. However
geometrical tachyon can account for dark energy when the number of NS5-branes
is large, provided that inflation is realized by another scalar field.Comment: 11 pages, 8 figure
Gauge-invariant Formulation of the Second-order Cosmological Perturbations
Gauge invariant treatments of the second order cosmological perturbation in a
four dimensional homogeneous isotropic universe filled with the perfect fluid
are completely formulated without any gauge fixing. We derive all components of
the Einstein equations in the case where the first order vector and tensor
modes are negligible. These equations imply that the tensor and the vector mode
of the second order metric perturbations may be generated by the scalar-scalar
mode coupling of the linear order perturbations as the result of the non-linear
effects of the Einstein equations.Comment: 5 pages, no figure. RevTeX; short letter version of gr-qc/0605108;
some details of explanations are adde
Effects of nucleus initialization on event-by-event observables
In this work we present a study of the influence of nucleus initializations
on the event-by-event elliptic flow coefficient, . In most Monte-Carlo
models, the initial positions of the nucleons in a nucleus are completely
uncorrelated, which can lead to very high density regions. In a simple, yet
more realistic model where overlapping of the nucleons is avoided, fluctuations
in the initial conditions are reduced. However, distributions are not
very sensitive to the initialization choice.Comment: 4 pages, 5 figures, to appear in the Bras. Jour. Phy
Tensor Anisotropies in an Open Universe
We calculate the anisotropies in the cosmic microwave background induced by
long-wavelength primordial gravitational waves in a universe with negative
spatial curvature, such as are produced in the ``open inflation'' scenario. The
impact of these results on the COBE normalization of open models is discussed.Comment: 5pgs, 2 figs.; also avalable at http://www.sns.ias.edu/~whu, revision
reflects ApJL published version, model dependence clarifie
Transient cosmic ray increase associated with a geomagnetic storm
On the basis of worldwide network data of cosmic ray nucleonic components, the transient cosmic ray increase due to the depression of cosmic ray cutoff rigidity during a severe geomagnetic storm was investigated in terms of the longitudinal dependence. Multiple correlation analysis among isotropic and diurnal terms of cosmic ray intensity variations and Dst term of the geomagnetic field is applied to each of various station's data. It is shown that the amplitude of the transient cosmic ray increase associated with Dst depends on the local time of the station, and that its maximum phase is found in the evening sector. This fact is consistent with the theoretical estimation based on the azimuthally asymmetric ring current model for the magnetic DS field
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