Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals

Based on an accurate first principles description of the energetics in H-bonded KDP, we conduct a first study of nuclear quantum effects and of the changes brought about by deuteration. Cluster tunneling involving also heavy ions is allowed, the main effect of deuteration being a depletion of the proton probability density at the O-H-O bridge center, which in turn weakens its proton-mediated covalent bonding. The ensuing lattice expansion couples selfconsistently with the proton off-centering, thus explaining both the giant isotope effect, and its close connection with geometrical effects.Comment: 4 two-column pages, 4 figure

An Investigation of the Neural Mechanism by which the Prefrontal Cortex Facilitates Anti-saccade Task Performance

Cognitive control enables us to guide our behaviour in an appropriate manner, such as rapid eye movements (saccades) toward a location or object of interest. A well-established test of cognitive control is the anti-saccade task, which instructs subjects to look away from a suddenly-appearing stimulus. The dorsolateral prefrontal cortex (dlPFC) and anterior cingulate cortex (ACC) are part of a cortical saccade control network that influences the superior colliculus (SC), which sends saccade commands to the brainstem saccade generator. To compare and contrast the roles of the dlPFC and ACC in saccade control, the cryoloop method of reversible cryogenic deactivation was used to identify the effects of dlPFC and ACC deactivation on pro-saccades and anti-saccades. Both dlPFC and ACC deactivation increased the incidence of ipsilateral saccades, but only dlPFC deactivation impaired contralateral saccades. An inhibitory model of prefrontal function has been proposed by which the prefrontal cortex suppresses the activity of SC saccade neurons on anti-saccade trials, to inhibit an unwanted saccade toward the stimulus. A direct test of this inhibitory model was performed by deactivating the dlPFC and recording the activity of SC saccade neurons. Unilateral dlPFC deactivation delayed the onset of saccade-related activity in the SC ipsilateral to deactivation, which suggests that the dlPFC has an excitatory influence on SC saccade neurons. There was also an increase of activity in the contralateral SC, which suggests that unilateral dlPFC deactivation caused a neural imbalance at the SC. Bilateral dlPFC deactivation, on the other hand, should not cause a neural imbalance, and thus was used to identify the effects of dlPFC deactivation that were caused by cognitive control impairments. Bilateral dlPFC deactivation increased the stimulus-related activity, and decreased the saccade-related activity, of SC saccade neurons. An increase of anti-saccade errors was more substantial in a “rule memorized” condition, which suggests that the dlPFC plays an important role in rule maintenance. Given an excitatory influence of the dlPFC on SC saccade neurons, I propose that the dlPFC facilitates anti-saccade task performance by first maintaining the relevant rule in working memory, then implementing the rule by enhancing the saccade-generating signal at the SC

Deuteron Momentum Distribution in KD2HPO4

The momentum distribution in KD2PO4(DKDP) has been measured using neutron Compton scattering above and below the weakly first order paraelectric-ferroelectric phase transition(T=229K). There is very litte difference between the two distributions, and no sign of the coherence over two locations for the proton observed in the paraelectric phase, as in KH2PO4(KDP). We conclude that the tunnel splitting must be much less than 20mev. The width of the distribution indicates that the effective potential for DKDP is significantly softer than that for KDP. As electronic structure calculations indicate that the stiffness of the potential increases with the size of the coherent region locally undergoing soft mode fluctuations, we conclude that there is a mass dependent quantum coherence length in both systems.Comment: 6 pages 5 figure

Enhanced Macroscopic Quantum Tunneling in Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} Intrinsic Josephson Junction Stacks

We have investigated macroscopic quantum tunneling in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ intrinsic Josephson junctions at millikelvin temperatures using microwave irradiation. Measurements show that the escape rate for uniformly switching stacks of N junctions is about $N^2$ times higher than that of a single junction having the same plasma frequency. We argue that this gigantic enhancement of macroscopic quantum tunneling rate in stacks is boosted by current fluctuations which occur in the series array of junctions loaded by the impedance of the environment.Comment: 4 pages and 5 figure

Prefrontal Cortex Deactivation in Macaques Alters Activity in the Superior Colliculus and Impairs Voluntary Control of Saccades

The cognitive control of action requires both the suppression of automatic responses to sudden stimuli and the generation of behavior specified by abstract instructions. Though patient, functional imaging and neurophysiological studies have implicated the dorsolateral prefrontal cortex (dlPFC) in these abilities, the mechanism by which the dlPFC exerts this control remains unknown. Here we examined the functional interaction of the dlPFC with the saccade circuitry by deactivating area 46 of the dlPFC and measuring its effects on the activity of single superior colliculus neurons in monkeys performing a cognitive saccade task. Deactivation of the dlPFC reduced preparatory activity and increased stimulus-related activity in these neurons. These changes in neural activity were accompanied by marked decreases in task performance as evidenced by longer reaction times and more task errors. The results suggest that the dlPFC participates in the cognitive control of gaze by suppressing stimulus-evoked automatic saccade programs