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Number 551, August 8, 2001 by Phil Schewe, James Riordon, and Ben Stein

An optical atomic clock

An optical atomic clock, one based on radiation coming from a single mercury ion in a trap, has been devised by physicists at NIST Boulder. In the best atomic clock used currently in defining the universal unit of time, the emission of light from a cesium atom at a characteristic microwave frequency of about 9 GHz is observed-the cycles in the light wave are counted by an electronic circuit. The difficulty with using higher frequency light is that it has been harder to count the cycles.

In recent years, however, this counting problem has been solved by mixing high frequency waves with lesser-frequency waves (see Update 351). The new NIST device monitors visible/UV light from a mercury ion at a frequency of about 1 PHz (10¹⁵ Hz). With some further work optical atomic clocks should be able to surpass in precision the best efforts of microwave atomic clocks, about one part in 10¹⁵. (Diddams et al., Science, 3 August 2001.)

Evidence for chaos in the neocortex

Evidence for chaos in the neocortex, the most complex brain structure specific to humans and other mammals, has been obtained in a model by researchers in Australia (David Liley, Swinburne University of Technology, 011-61-3-9214-8812, dliley@swin.edu.au).

Chaos in the brain would manifest itself as unpredictable and seemingly random electrical activity in a population of nerve cells, or neurons. Chaos may have an important neurological function: it could provide, as researchers have speculated, a flexible and rapid means for the brain to discriminate between different sounds, odors, and other perceptual stimuli.

Electroencephalograms (EEGs) record electrical activity in the cerebral cortex, but they, and all other current experimental techniques, may never be able to detect clear and unequivocal signs of chaos, since the cortex also emits a very large amount of obscuring "noise" or random electrical activity.

Using realistic models of brain physiology, many researchers are trying to devise models which reproduce the output of EEGs yet also offer new insights into the brain's inner workings. However, previous models either do not allow for chaos to appear, or have been unable to demonstrate that chaos can occur under the conditions imposed by the structure of the brain.

In the present work, the researchers model the behavior of two large populations of neurons: excitatory (which bring other neurons closer to firing) and inhibitory (which make it more difficult for other neurons to fire). Specifically, they look at the "mean soma membrane potential," the electric potential between the outside and inside of the neuron's cell body (higher potential means more frequent firing).

Varying the rate of external electrical impulses to each neuron population, they found the mean electrical activity was irregular and noise-like (it looked like noise but really wasn't) for a wide range of external inputs. Quantitatively such behavior is associated with a positive Lyapunov exponent, a hallmark of chaos. The existence of chaos, the researchers say, would provide a means for the brain to change its response rapidly to even slightly different stimuli. (Dafilis et al., Chaos, September 2001.)

High temperature superconductors might be vibrational

High temperature superconductors might be vibrational after all, at least in part. Low temperature (4 K) superconductors operate according to the Bardeen-Cooper-Schrieffer (BCS) viewpoint. Electrons pair up and enter into a single unified quantum state through the agency of vibrations of the underlying lattice of atoms, an occurrence which can also be described in terms of the exchange of phonons. This BCS mechanism is inherently fragile and not expected to survive in the warmer, 100-K, regime where high temperature superconductors operate.

Therefore new tests conducted at SLAC's Stanford Synchrotron Radiation Laboratory (SSRL) and LBL's Advanced Light Source (ALS) came as a surprise. Researchers shot carefully selected photons into various cuprate superconductor samples and observed a kink in the energy spectrum of the ejected electrons, a kink which they associate with an underlying electron-phonon resonance, suggesting some kind of BCS behavior at work. (Lanzara et al., Nature, 2 August 2001.)