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