AN AMORPHOUS SOLID BEHAVING LIKE A CRYSTAL. Amorphous silicon, a solid in which atoms are arranged in a non-periodic jumble, rivals crystalline silicon for photovoltaic applications. Unfortunately, atoms in the amorphous state often have unfulfilled (dangling) bond sites which interfere with electric currents. To cure this problem hydrogen atoms are introduced into the solid to bind to these sites; but too much hydrogen itself leads to a deterioration of electrical properties. Now a collaboration of physicists from Cornell (Robert Pohl, 607-255-3303, pohl@msc.cornell.edu) and the National Renewable Energy Labs (Golden, CO) has succeeded in implanting a smaller, more judicious amount of hydrogen, greatly improving the stability of the material and, in the process, revealing something unexpected. If you shake a pure silicon crystal (chilled to low temperatures) it will ring for an hour at many different frequencies. In amorphous silicon, by contrast, the tangled-atom nature of the sample quickly (in a second or two) soaks up the vibrations at all different energies. In the NREL hydrogenated amorphous silicon, however, some vibration modes (the low-energy ones) persist for an hour, just as in crystalline silicon. This as-yet-unexplained property gives the researchers an experimental tool for exploring the role of hydrogen in these solids and for studying amorphous solids in general. For example, one can observe what happens to these low-energy excitations as impurities are added to the material. (Xiao Liu et al., upcoming article in Physical Review Letters, probably June 9.)

THE EARLY FAINT SUN PARADOX goes as follows: 4 billion years ago the sun (its fusion fire not yet having worked up to present levels) was 25-30% cooler than now. Terrestrial temperatures would have been sub-freezing, precluding liquid water. How then did life form in these early eras? Carl Sagan, in a posthumous paper co-authored by Chris Chyba (Science, 22 May) suggests a possible scenario. Ultraviolet radiation from the sun, they argue, would combine with existing methane to form solid hydrocarbons in the upper atmosphere. This in turn would shield ammonia (otherwise broken up by the UV) long enough for the ammonia to produce a greenhouse warming adequate for liquid water. Sagan and his interest in life in extreme environments was the subject of a session yesterday at the meeting of the American Geophysical Union in Baltimore. According to David Morrison of NASA Ames, there are only two places on Earth where life has not been found---on the Antarctic ice sheet and in the upper atmosphere. Everywhere else, whether in hot springs (even above boiling temperatures) or a kilometer below the surface, life seems to thrive. One speaker, Todd Stevens of the Pacific Northwest Lab, asserted that some subsurface "rock-eating" microbes constituted an ecosystem independent of photosynthesis and that their metabolism (in some cases amounting to a biomass doubling time of millennia) was perhaps the slowest of all life forms.

REMINDER ON SUBSCRIBING TO PHYSICS NEWS UPDATE: To add or subtract your email address automatically from our list, send a message to listserv@aip.org. Leave the subject line blank and specify either "add physnews" or "delete physnews."