9,419 research outputs found
A method for measuring the nonlinear response in dielectric spectroscopy through third harmonics detection
We present a high sensitivity method allowing the measurement of the non
linear dielectric susceptibility of an insulating material at finite frequency.
It has been developped for the study of dynamic heterogeneities in supercooled
liquids using dielectric spectroscopy at frequencies 0.05 Hz < f < 30000 Hz .
It relies on the measurement of the third harmonics component of the current
flowing out of a capacitor. We first show that standard laboratory electronics
(amplifiers and voltage sources) nonlinearities lead to limits on the third
harmonics measurements that preclude reaching the level needed by our physical
goal, a ratio of the third harmonics to the fundamental signal about 7 orders
of magnitude lower than 1. We show that reaching such a sensitivity needs a
method able to get rid of the nonlinear contributions both of the measuring
device (lock-in amplifier) and of the excitation voltage source. A bridge using
two sources fulfills only the first of these two requirements, but allows to
measure the nonlinearities of the sources. Our final method is based on a
bridge with two plane capacitors characterized by different dielectric layer
thicknesses. It gets rid of the source and amplifier nonlinearities because in
spite of a strong frequency dependence of the capacitors impedance, it is
equilibrated at any frequency. We present the first measurements of the
physical nonlinear response using our method. Two extensions of the method are
suggested.Comment: 25 pages, 8 figure
Correlation of hippocampal theta rhythm with changes in cutaneous temperature
A possible role for the hippocampus in alerting an animal to changes in cutaneous temperature was examined. Following local warming or cooling of the ears of unanesthetized, loosely restrained rabbits, theta waves (4-7 Hz EEG waves) were recorded from electrodes straddling the hippocampus. The onset of the hippocampal theta rhythm was correlated with changes in cutaneous temperature, an observation consistent with studies indicating that the theta rhythm is a nonspecific response evoked by stimulation of several sensory modalities. Additional data from cats and rabbits were correlated with specific neurons within the hippocampus, namely pyramidal cells. Post stimulus time histograms obtained by excitation of the dorsal fornix were interpreted in terms of excitatory and inhibitory inputs to pyramidal cells. Thus, the theta rhythm, which appears to be evoked by changes in cutaneous temperature, can be related to a specific type of hippocampal neuron which is in turn connected with other areas of the brain involved in temperature regulation
Extremal black holes as exact string solutions
We show that the leading order solution describing an extremal electrically
charged black hole in string theory is, in fact, an exact solution to all
orders in \a' when interpreted in a Kaluza-Klein fashion. This follows from
the observation that it can be obtained via dimensional reduction from a five
dimensional background which is proved to be an exact string solution.Comment: 13 pages, harvmac, Imperial/TP/93-94/51, UCSBTH-94-24 (references
added
Strings in Plane Wave Backgrounds Revisited
String theory in an exact plane wave background is explored. A new example of
singularity in the sense of string theory for nonsingular spacetime metric is
presented. The 4-tachyon scattering amplitude is constructed. The spectrum of
states found from the poles in the factorization turns out to be equivalent to
that of the theory in flat space-time. The massless vertex operator is obtained
from the residue of the first order pole.Comment: 15 pages, GTCRG-8, RevTe
When Black Holes Meet Kaluza-Klein Bubbles
We explore the physical consequences of a recently discovered class of exact
solutions to five dimensional Kaluza-Klein theory. We find a number of
surprising features including: (1) In the presence of a Kaluza-Klein bubble,
there are arbitrarily large black holes with topology S^3. (2) In the presence
of a black hole or a black string, there are expanding bubbles (with de Sitter
geometry) which never reach null infinity. (3) A bubble can hold two black
holes of arbitrary size in static equilibrium. In particular, two large black
holes can be close together without merging to form a single black hole.Comment: 23 pages, 5 figures, v2: few comments on stability modifie
A human protein required for the second step of pre-mRNA splicing is functionally related to a yeast splicing factor
We have identified a human splicing factor required for the second step of pre-mRNA splicing. This new protein, hPrp18, is 30% identical to the yeast splicing factor Prp18. In HeLa cell extracts immunodepleted of hPrp18, the second step of pre-mRNA splicing is abolished. Splicing activity is restored by the addition of recombinant hPrp18, demonstrating that hPrp18 is required for the second step. The hPrp18 protein is bound tightly to the spliceosome only during the second step of splicing. hPrp18 is required for the splicing of several pre-mRNAs, making it the first general second-step splicing factor found in humans. Splicing activity can be restored to hPrp18-depleted HeLa cell extracts by yeast Prp18, showing that important functional regions of the proteins have been conserved. A 90-amino-acid region near the carboxyl terminus of hPrp18 is strongly homologous to yeast Prp18 and is also conserved in rice and nematodes. The homology identifies one region important for the function of both proteins and may define a new protein motif. In contrast to yeast Prp18, hPrp18 is not stably associated with any of the snRNPs. A 55-kD protein that cross-reacts with antibodies against hPrp18 is a constituent of the U4/U6 and U4/U6 . U5 snRNP particles
Generic Cosmic Censorship Violation in anti de Sitter Space
We consider (four dimensional) gravity coupled to a scalar field with
potential V(\phi). The potential satisfies the positive energy theorem for
solutions that asymptotically tend to a negative local minimum. We show that
for a large class of such potentials, there is an open set of smooth initial
data that evolve to naked singularities. Hence cosmic censorship does not hold
for certain reasonable matter theories in asymptotically anti de Sitter
spacetimes. The asymptotically flat case is more subtle. We suspect that
potentials with a local Minkowski minimum may similarly lead to violations of
cosmic censorship in asymptotically flat spacetimes, but we do not have
definite results.Comment: 4 pages, v2: minor change
Strong Correlations in Actinide Redox Reactions
Reduction-oxidation (redox) reactions of the redox couples An(VI)/An(V),
An(V)/An(IV), and An(IV)/An(III), where An is an element in the family of early
actinides (U, Np, and Pu), as well as Am(VI)/Am(V) and Am(V)/Am(III), are
modeled by combining density functional theory with a generalized Anderson
impurity model that accounts for the strong correlations between the 5f
electrons. Diagonalization of the Anderson impurity model yields improved
estimates for the redox potentials and the propensity of the actinide complexes
to disproportionate.Comment: 17 pages, 10 figure, 3 tables. Corrections and clarifications; this
version has been accepted for publication in The Journal of Chemical Physic
Cosmic Censorship and the Dilaton
We investigate extremal electrically charged black holes in
Einstein-Maxwell-dilaton theory with a cosmological constant inspired by string
theory. These solutions are not static, and a timelike singularity eventually
appears which is not surrounded by an event horizon. This suggests that cosmic
censorship may be violated in this theory.Comment: 16 pages, NSF-ITP-93-9
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