Number 263, March 22, 1996 by Phillip F. Schewe and Ben Stein METALLIC HYDROGEN HAS BEEN ACHIEVED at Livermore in a sample of fluid hydrogen. Hydrogen atoms constitute the bulk of the universe's ordinary matter, so scientists have long sought to understand the properties and phases of this simplest of elements. Squeezing hydrogen atoms until they surrender their electrons has been tried ever since Eugene Wigner predicted in 1935 that hydrogen would metallize at sufficiently high pressure. It was long thought that the road to metallic hydrogen lay with crystalline hydrogen rather than with the disordered fluid phase. Indeed, solid hydrogen has been crushed in diamond anvil presses up to pressures at 2.5 Mbar, but without making hydrogen metallic. Therefore William Nellis at Livermore (510-422-7200) was somewhat surprised when he succeeded at lesser pressures with fluid hydrogen. He used a gas gun to compress samples of liquid H2 and D2. Able to make direct electrical measurements on his 1- inch-wide sample (unlike the anvil experiments---with their micron- sized samples---which can only use indirect optical probes), he observed that the sample's resistivity fell with increasing pressure, leveling off at a low value (comparable to that of the fluid alkali metals Cs and Rb under similar conditions) at pressures above 1.4 Mbar, about a million times Earth's atmospheric pressure. In studying the span from insulator to conductor, physicists look at the energy gap, the difference between the highest filled electron energy level and the next available energy level, a level at which the electron is free to flow as part of an electrical current. In hydrogen at ambient pressures, the gap is 15 eV, big enough to qualify hydrogen as an insulator. In his shock-compression experiment, Nellis lowers the gap to only 0.3 eV, which is comparable to the thermal energy of the fluid. Besides being of interest to condensed matter physicists studying the insulator/metal transition, the formation of metallic hydrogen is of interest to fusion scientists who need to know what hydrogen does at high pressures, and to astronomers who model the interiors of gas giants like Jupiter and Saturn, which are expected to harbor vast reservoirs of metallic fluid hydrogen. Nellis reported his result this week at the APS March meeting in St. Louis (see also Physical Review Letters, 11 March 1996). SINGLE-MOLECULE BIOSENSOR. Atomic force microscopes (AFM) can directly measure forces at the nanoscopic level. For instance, scientists at the Naval Research Lab have measured the force between two complementary strands of DNA. The NRL researchers (Richard Colton (202-767-0801), Gil Lee, and David Baselt) now hope to use a device based on AFM technology to detect biomolecules. They have developed a "force amplified biological sensor" which will soon be capable of detecting atto- molar (10**-18 M) amounts of various biological species such as cells, proteins, viruses, and bacteria. Employed in a working device, an array of such biosensors would be able to perform immunoassays (the process by which the presence of antigens is detected) in about 10 minutes, much faster that other methods at these small concentrations. Currently, the prototype device works in this way: an antibody is attached to a sensitive cantilever beam. Next an antigen in solution binds to the antibody. A second antibody, mounted on a micron-sized magnetic bead, also binds to the antigen, forming an antibody-antigen-antibody-bead sandwich. What is measured is the deflection of the cantilever when a magnetic force is applied to the bead. By counting the beads one arrives at the antigen concentration in the solution. Colton and his colleagues reported on the sensor at the APS meeting.