687 research outputs found
Braneworld Flux Inflation
We propose a geometrical model of brane inflation where inflation is driven
by the flux generated by opposing brane charges and terminated by the collision
of the branes, with charge annihilation. We assume the collision process is
completely inelastic and the kinetic energy is transformed into the thermal
energy after collision. Thereafter the two branes coalesce together and behave
as a single brane universe with zero effective cosmological constant. In the
Einstein frame, the 4-dimensional effective theory changes abruptly at the
collision point. Therefore, our inflationary model is necessarily 5-dimensional
in nature. As the collision process has no singularity in 5-dimensional
gravity, we can follow the evolution of fluctuations during the whole history
of the universe. It turns out that the radion field fluctuations have a steeply
tilted, red spectrum, while the primordial gravitational waves have a flat
spectrum. Instead, primordial density perturbations could be generated by a
curvaton mechanism.Comment: 11 pages, 6 figures, references adde
Radionic Non-uniform Black Strings
Non-uniform black strings in the two-brane system are investigated using the
effective action approach. It is shown that the radion acts as a non-trivial
hair of the black strings. From the brane point of view, the black string
appears as the deformed dilatonic black hole which becomes dilatonic black hole
in the single brane limit and reduces to the Reissner-Nordstr\"om black hole in
the close limit of two-branes. The stability of solutions is demonstrated using
the catastrophe theory. From the bulk point of view, the black strings are
proved to be non-uniform. Nevertheless, the zeroth law of black hole
thermodynamics still holds.Comment: 9 pages, 6 figure
Black holes and a scalar field in an expanding universe
We consider a model of an inhomogeneous universe including a massless scalar
field, where the inhomogeneity is assumed to consist of many black holes. This
model can be constructed by following Lindquist and Wheeler, which has already
been investigated without including scalar field to show that an averaged scale
factor coincides with that of the Friedmann model. In this work we construct
the inhomogeneous universe with an massless scalar field, where we assume that
the averaged scale factor and scalar field are given by those of the Friedmann
model including a scalar field. All of our calculations are carried out in the
framework of Brans-Dicke gravity. In constructing the model of an inhomogeneous
universe, we define the mass of a black hole in the Brans-Dicke expanding
universe which is equivalent to ADM mass if the mass evolves adiabatically, and
obtain an equation relating our mass to the averaged scalar field and scale
factor. As the results we find that the mass has an adiabatic time dependence
in a sufficiently late stage of the expansion of the universe, and that the
time dependence is qualitatively diffenrent according to the sign of the
curvature of the universe: the mass increases decelerating in the closed
universe case, is constant in the flat case and decreases decelerating in the
open case. It is also noted that the mass in the Einstein frame depends on
time. Our results that the mass has a time dependence should be retained even
in the general scalar-tensor gravitiy with a scalar field potential.
Furthermore, we discuss the relation of our results to the uniqueness theorem
of black hole spacetime and gravitational memory effect.Comment: 16 pages, 3 tables, 5 figure
Radion and Holographic Brane Gravity
The low energy effective theory for the Randall-Sundrum two brane system is
investigated with an emphasis on the role of the non-linear radion in the brane
world. The equations of motion in the bulk is solved using a low energy
expansion method. This allows us, through the junction conditions, to deduce
the effective equations of motion for the gravity on the brane. It is shown
that the gravity on the brane world is described by a quasi-scalar-tensor
theory with a specific coupling function omega(Psi) = 3 Psi / 2(1-Psi) on the
positive tension brane and omega(Phi) = -3 Phi / 2(1+Phi) on the negative
tension brane, where Psi and Phi are non-linear realizations of the radion on
the positive and negative tension branes, respectively. In contrast to the
usual scalar-tensor gravity, the quasi-scalar-tensor gravity couples with two
kinds of matter, namely, the matters on both positive and negative tension
branes, with different effective gravitational coupling constants. In
particular, the radion disguised as the scalar fields Psi and Phi couples with
the sum of the traces of the energy momentum tensor on both branes. In the
course of the derivation, it has been revealed that the radion plays an
essential role to convert the non-local Einstein gravity with the generalized
dark radiation to the local quasi-scalar-tensor gravity. For completeness, we
also derive the effective action for our theory by substituting the bulk
solution into the original action. It is also shown that the
quasi-scalar-tensor gravity works as holograms at the low energy in the sense
that the bulk geometry can be reconstructed from the solution of the
quasi-scalar-tensor gravity.Comment: Revtex4, 18 pages, revised version, conclusions unchanged, references
adde
Brane World Effective Action at Low Energies and AdS/CFT Correspondence
A low energy iteration scheme to study nonlinear gravity in the brane world
is developed. As a result, we obtain the brane world effective action at low
energies. The relation between the geometrical approach and the approach using
the AdS/CFT correspondence is also clarified. In particular, we find
generalized dark radiation as homogeneous solutions in our iteration scheme.
Moreover, the precise correspondence between the bulk geometry and the brane
effective action is established, which gives a holographic view of the brane
world.Comment: Revtex4, 12 pages, references added. Version accepted for publicaton
in Phys. Rev.
59Co-NMR Knight Shift of Aligned Crystals and Polycrystalline Samples of Superconducting Na0.3CoO2.1.3H2O
Temperature (T) dependence of 59Co-NMR Knight shifts K of Na0.3CoO2.1.3H2O
has been studied, where samples of randomly oriented powder and aligned
crystals have been used for the applied magnetic fields H // ab plane and H
//c-axis, respectively. For both directions of H, the shift K decreases below
the superconducting transition temperature Tc(H) with decreasing T, indicating
that the superconducting electron pairs are in the singlet state. The upper
critical fields Hc2(T) determined from the K(H)-T curves are found to be
consistent with the values reported by the resistivity measurements for both
directions of H.Comment: 8 pages, 6 figures, J.Phys. Soc. Jpn 74 (2005) No.
Influence of Mg, Ag and Al substitutions on the magnetic excitations in the triangular-lattice antiferromagnet CuCrO2
Magnetic excitations in CuCrO, CuCrMgO,
CuAgCrO, and CuCrAlO have been
studied by powder inelastic neutron scattering to elucidate the element
substitution effects on the spin dynamics in the Heisenberg triangular-lattice
antiferromagnet CuCrO. The magnetic excitations in
CuCrMgO consist of a dispersive component and a flat
component. Though this feature is apparently similar to CuCrO, the energy
structure of the excitation spectrum shows some difference from that in
CuCrO. On the other hand, in CuAgCrO and
CuCrAlO the flat components are much reduced, the
low-energy parts of the excitation spectra become intense, and additional
low-energy diffusive spin fluctuations are induced. We argued the origins of
these changes in the magnetic excitations are ascribed to effects of the doped
holes or change of the dimensionality in the magnetic correlations.Comment: 7 pages, 5 figure
Numerical Renormalization Group Approach to a Quantum Dot Coupled to Normal and Superconducting Leads
We study transport through a quantum dot coupled to normal and
superconducting leads using the numerical renormalization group method. We show
that the low-energy properties of the system are described by the local Fermi
liquid theory despite of the superconducting correlations penetrated into the
dot due to a proximity effect. We calculate the linear conductance due to the
Andreev reflection in the presence of the Coulomb interaction. It is
demonstrated that the maximum structure appearing in the conductance clearly
characterizes a crossover between two distinct spin-singlet ground states, i.e.
the superconducting singlet state and the Kondo singlet state. It is further
elucidated that the gate-voltage dependence of the conductance shows different
behavior in the superconducting singlet region from that in the Kondo singlet
region.Comment: 10 pages, 6 figures; a typo in eq. (B.5) corrected, which does not
affect any other results of the pape
Distinguishing d-wave from highly anisotropic s-wave superconductors
Systematic impurity doping in the Cu-O plane of the hole-doped cuprate
superconductors may allow one to decide between unconvention al ("d-wave") and
anisotropic conventional ("s-wave") states as possible candidates for the order
parameter in these materials. We show that potential scattering of any strength
always increases the gap minima of such s-wave states, leading to activated
behavior in temperature with characteristic impurity concentration dependence
in observable quantities such as the penetration depth. A magnetic component to
the scattering may destroy the energy gap and give rise to conventional gapless
behavior, or lead to a nonmonotonic dependence of the gap on impurity
concentration. We discuss how experiments constrain this analysis.Comment: 5 page
Magnetic properties of superconducting cobalt oxides NaxCoO2.yH2O
Studies of the NMR Knight shift K of Na0.3CoO2.yH2O have been carried out in
detail. The suppression of K by the occurrence of the superconductivity
reported previously by the present authors in both magnetic field directions
perpendicular and parallel to the c axis has been confirmed, indicating that
the Cooper pairs are in the singlet state. The anisotropy of the suppression
amplitudes is consistent with the anisotropy of the hyperfine coupling constant
Aspin estimated by the K-kai plot. It has also been found that even samples,
which do not exhibit a significant amount of the Curie-Weiss-like increase of
the uniform magnetic susceptibility kai with decreasing temperature T, exhibit
the superconducting transition, which indicates that the superconducting
Na0.3CoO2.1.3H2O is not necessarily be in the proximity region of the
ferromagnetic phase. It has also been confirmed that the superconducting
transition temperature Tc of the samples prepared by mixing Na0.7CoO2, H2O (or
D2O) and bromine for 4 h, and separating the resultant powder from the solution
by filtration, depends on the elapsed time t after filtration. For these
samples, the Curie constant of kai estimated at low temperatures increases with
t, supporting the idea that the number of the lattice imperfection possibly due
to the oxygen-vacancy-formation increases with t. Even for such samples, the
superconductivity appears, which seems to exclude the possibility of the
anisotropic superconducting order parameter. These results are favorable for
the full-gapped superconductivity.Comment: 5 pages, 8 figures, submitted to JPS
- …