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.)