3,398 research outputs found
Periodic Oscillations of Josephson-Vortex Flow Resistance in Oxygen-Deficient Y1Ba2Cu3Ox
We measured the Josephson vortex flow resistance as a function of magnetic
field applied parallel to the ab-planes using annealed Y1Ba2Cu3Ox intrinsic
Josephson junctions having high anisotropy (around 40) by oxygen content
reduction. Periodic oscillations were observed in magnetic fields above 45-58
kOe, corresponding to dense-dilute boundary for Josephson vortex lattice. The
observed period of oscillations, agrees well with the increase of one fluxon
per two junctions (\textit{=}\textit{/2Ls}), may correspond
to formation of a triangular lattice of Josephson vortices as has been reported
by Ooi et al. for highly anisotropic (larger than 200) Bi-2212 intrinsic
Josephson junctions.Comment: 5 pages, 4 figure
Accelerating Universes in String Theory via Field Redefinition
We study cosmological solutions in the effective heterotic string theory with
-correction terms in string frame. It is pointed out that the
effective theory has an ambiguity via field redefinition and we analyze
generalized effective theories due to this ambiguity. We restrict our analysis
to the effective theories which give equations of motion of second order in the
derivatives, just as "Galileon" field theory. This class of effective actions
contains two free coupling constants. We find de Sitter solutions as well as
the power-law expanding universes in our four-dimensional Einstein frame. The
accelerated expanding universes are always the attractors in the present
dynamical system.Comment: 22 pages, 3 figures, some additional formulae adde
Effect of Void Network on CMB Anisotropy
We study the effect of a void network on the CMB anisotropy in the
Einstein-de Sitter background using Thompson &Vishniac's model. We consider
comprehensively the Sacks-Wolfe effect, the Rees-Sciama effect and the
gravitational lensing effect. Our analysis includes the model of primordial
voids existing at recombination, which is realized in some inflationary models
associated with a first-order phase transition. If there exist primordial voids
whose comoving radius is larger than Mpc at recombination, not
only the Sachs-Wolfe effect but also the Rees-Sciama effect is appreciable even
for multipoles l\lsim1000 of the anisotropy spectrum. The gravitational
lensing effect, on the other hand, slightly smoothes the primary anisotropy;
quantitatively, our results for the void model are similar to the previous
results for a CDM model. All the effects, together, would give some constraints
on the configuration or origin of voids with high-resolution data of the CMB
anisotropy.Comment: 23 pages, latex, 12 eps figures, some calculations and discussions
are added, to appear in ApJ 510 (1999
Effective String Theory of Vortices and Regge Trajectories
Starting from a field theory containing classical vortex solutions, we obtain
an effective string theory of these vortices as a path integral over the two
transverse degrees of freedom of the string. We carry out a semiclassical
expansion of this effective theory, and use it to obtain corrections to Regge
trajectories due to string fluctuations.Comment: 27 pages, revtex, 3 figures, corrected an error with the cutoff in
appendix E (was previously D), added more discussion of Fig. 3, moved some
material in section 9 to a new appendi
Pure nematic state in iron-based superconductor
Lattice and electronic states of thin FeSe films on LaAlO substrates are
investigated in the vicinity of the nematic phase transition. No evidence of
structural phase transition is found by x-ray diffraction below K, while results obtained from resistivity measurement and angle-resolved
photoemission spectroscopy clearly show the appearance of a nematic state.
These results indicate formation of a pure nematic state in the iron-based
superconductor and provide conclusive evidence that the nematic state
originates from the electronic degrees of freedom. This pure nematicity in the
thin film implies difference in the electron-lattice interaction from bulk FeSe
crystals. FeSe films provide valuable playgrounds for observing the pure
response of "bare" electron systems free from the electron-lattice interaction,
and should make important contribution to investigate nematicity and its
relationship with superconductivity
Normal-state conductivity in underdoped La_{2-x}Sr_xCuO_4 thin films: Search for nonlinear effects related to collective stripe motion
We report a detailed study of the electric-field dependence of the
normal-state conductivity in La_{2-x}Sr_xCuO_4 thin films for two
concentrations of doped holes, x=0.01 and 0.06, where formation of diagonal and
vertical charged stripes was recently suggested. In order to elucidate whether
high electric fields are capable of depinning the charged stripes and inducing
their collective motion, we have measured current-voltage characteristics for
various orientations of the electric field with respect to the crystallographic
axes. However, even for the highest possible fields (~1000 V/cm for x=0.01 and
\~300 V/cm for x=0.06) we observed no non-linear-conductivity features except
for those related to the conventional Joule heating of the films. Our analysis
indicates that Joule heating, rather than collective electron motion, may also
be responsible for the non-linear conductivity observed in some other 2D
transition-metal oxides as well. We discuss that a possible reason why moderate
electric fields fail to induce a collective stripe motion in layered oxides is
that fairly flexible and compressible charged stripes can adjust themselves to
the crystal lattice and individual impurities, which makes their pinning much
stronger than in the case of conventional rigid charge-density waves.Comment: 10 pages, 10 figures, accepted for publication in Phys. Rev.
Numerical Evolution of General Relativistic Voids
In this paper, we study the evolution of a relativistic, superhorizon-sized
void embedded in a Friedmann-Robertson-Walker universe. We numerically solve
the spherically symmetric general relativistic equations in comoving,
synchronous coordinates. Initially, the fluid inside the void is taken to be
homogeneous and nonexpanding. In a radiation- dominated universe, we find that
radiation diffuses into the void at approximately the speed of light as a
strong shock---the void collapses. We also find the surprising result that the
cosmic collapse time (the -crossing time) is much smaller than
previously thought, because it depends not only on the radius of the void, but
also on the ratio of the temperature inside the void to that outside. If the
ratio of the initial void radius to the outside Hubble radius is less than the
ratio of the outside temperature to that inside, then the collapse occurs in
less than the outside Hubble time. Thus, superhorizon-sized relativistic void
may thermalize and homogenize relatively quickly. These new simulations revise
the current picture of superhorizon-sized void evolution after first-order
inflation.Comment: 37 pages plus 12 figures (upon request-- [email protected])
LaTeX, FNAL-PUB-93/005-
Clumps into Voids
We consider a spherically symmetric distribution of dust and show that it is
possible, under general physically reasonable conditions, for an overdensity to
evolve to an underdensity (and vice versa). We find the conditions under which
this occurs and illustrate it on a class of regular Lemaitre-Tolman-Bondi
solutions. The existence of this phenomenon, if verified, would have the result
that the topology of density contours, assumed fixed in standard structure
formation theories, would have to change and that luminous matter would not
trace the dark matter distribution so well.Comment: LaTeX, 17 pages, 4 figures. Submitted to GRG 20/4/200
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