Measurements of the equation of state of deuterium at the fluid insulator-metal transition

A high-intensity laser was used to shock-compress liquid deuterium to pressures from 22 to 340 gigapascals. In this regime deuterium is predicted to transform from an insulating molecular fluid to an atomic metallic fluid. Shock densities and pressures, determined by radiography, revealed an increase in compressibility near 100 gigapascals indicative of such a transition. Velocity interferometry measurements, obtained by reflecting a laser probe directly off the shock front in flight, demonstrated that deuterium shocked above 55 gigapascals has an electrical conductivity characteristic of a liquid metal and independently confirmed the radiography.     

Visualization of the local insulator-metal transition in Pr0.7Ca0. 3MnO3

The light-induced insulator-metal transition in the "colossal magnetoresistance" compound Pr0.7Ca0.3MnO3 is shown to generate a well-localized conducting path while the bulk of the sample remains insulating. The path can be visualized through a change of reflectivity that accompanies the phase transition. Its visibility provides a tool for gaining insight into electronic transport in materials with strong magnetic correlations. For example, a conducting path can be generated or removed at an arbitrary position just because of the presence of another path. Such manipulation may be useful in the construction of optical switches.     

Critical behavior in the satisfiability of random boolean expressions

Determining the satisfiability of randomly generated Boolean expressions with k variables per clause is a popular test for the performance of search algorithms in artificial intelligence and computer science. It is known that for k = 2, formulas are almost always satisfiable when the ratio of clauses to variables is less than 1; for ratios larger than 1, the formulas are almost never satisfiable. Similar sharp threshold behavior is observed for higher values of k. Finite-size scaling, a method from statistical physics, can be used to characterize size-dependent effects near the threshold. A relationship can be drawn between thresholds and computational complexity.     

Critical behavior of a trapped interacting Bose gas

The phase transition of Bose-Einstein condensation was studied in the critical regime, where fluctuations extend far beyond the length scale of thermal de Broglie waves. We used matter-wave interference to measure the correlation length of these critical fluctuations as a function of temperature. Observations of the diverging behavior of the correlation length above the critical temperature enabled us to determine the critical exponent of the correlation length for a trapped, weakly interacting Bose gas to be nu = 0.67 +/- 0.13. This measurement has direct implications for the understanding of second-order phase transitions.     

Hydride Complexes: The chemistry and current significance of covalent hydrogen compounds of transition metals are discussed

Hydride complexes of transition metals were virtually unknown 12 years ago, but they have now been found to lie at the root of many interesting catalytic chemical reactions, some of which were used in industry even before the unique properties of hydride complexes, which formed their basis, had been realized. This article surveys the history, preparations, important properties, and reactions of hydride complexes, surveys their part in important catalytic industrial processes, and speculates about their possible role in the biological fixation of nitrogen.     

Critical period in the social development of dogs

Litters of puppies were isolated, with the bitch, in fenced acre fields from 2 to 14 weeks of age. They were removed indoors at different ages, played with for a week, and returned to the field. The pups manifested an increasing tendency to withdraw from human beings after 5 weeks of age and unless socialization occurred before 14 weeks of age, withdrawal reactions from humans became so intense that normal relationships could not thereafter be established     

Global azimuthal anisotropy in the transition zone

Surface wave dispersion measurements for Love wave overtones carry evidence of azimuthal anisotropy in the transition zone of Earth's mantle (400 to 660 kilometers deep). A Backus-Gilbert inversion of anisotropic phase velocity maps, with resolution kernels mainly sensitive to the transition zone, shows a robust long-wavelength azimuthally anisotropic velocity structure. This observation puts new constraints on the mineralogy and dynamics of the transition zone because this anisotropy may result from aligned minerals, tilted laminated structures, or even organized pockets of fluid inclusions.     

The mid-Pleistocene transition in the tropical Pacific

A sea surface temperature (SST) record based on planktonic foraminiferal magnesium/calcium ratios from a site in the western equatorial Pacific warm pool reveals that glacial-interglacial oscillations in SST shifted from a period of 41,000 to 100,000 years at the mid-Pleistocene transition, 950,000 years before the present. SST changes at both periodicities were synchronous with eastern Pacific cold-tongue SSTs but preceded changes in continental ice volume. The timing and nature of tropical Pacific SST changes over the mid-Pleistocene transition implicate a shift in the periodicity of radiative forcing by atmospheric carbon dioxide as the cause of the switch in climate periodicities at this time.     

Dimensionality-driven insulator-to-metal transition in the bechgaard salts

Optical experiments were conducted on a series of organic linear chain conductors with different values of the interchain single-electron transfer integral tb, which quantifies the degree of anisotropy. Electron-electron interactions together with Umklapp scattering resulted in a correlation gap and an insulating state for small tb. An insulator-to-metal transition was observed when tb exceeded a critical value, on the order of the correlation gap Egap. The absence of a plasma edge on the insulator side of the transition for polarization perpendicular to the chains suggests that the electrons are confined to the chains. The optical features of the metallic state, when contrasted with the magnetic properties, are suggestive of spin-charge separation.     

Population policy in transition in the developing world

Population growth remains rapid in the poorest countries, particularly in sub-Saharan Africa, despite substantial AIDS mortality. Voluntary family-planning programs reduce unplanned pregnancies by providing access to and information about contraception and by reducing socioeconomic obstacles to use. With sufficient political will and resources, well-run voluntary programs have been shown to bring about sustained declines in fertility and population growth across much of Asia, the Middle East, and Latin America, simply by permitting people to realize their individual reproductive goals. Such programs represent a cost-effective approach to relieving population pressures, stimulating economic development, improving health, and enhancing human freedom.