Number 335 (Story #1), September 5, 1997 by Phillip F. Schewe and Ben Stein

FRACTIONALLY CHARGED CARRIERS have been detected experimentally for the first time. Charge carriers come in a variety of forms, such as electrons in copper wires, pairs of electrons in superconductors, and even holes (the absence of an electron) in certain semiconductors and high-temperature superconductors. More precisely, a hole is a "quasiparticle," an excitation of a physical system (e.g., a chunk of silicon) as a whole. Quasiparticles are important (some of the data in your computer is encoded in the form of holes) but quasiparticles can't exist independently of the lattice through which they move; they arise from the collective behavior of many electrons. Such a collective behavior is at the heart of the quantum Hall effect, a phenomenon in which, at conditions of low temperature and high magnetic field, the electrons at the boundary between two semiconductors form a two-dimensional electron liquid possessing discrete energy states and exhibiting a quantized electrical resistance. Theorists predicted more than a decade ago that excitations in some of the collective electron states could have a charge equal to a fraction of the basic electron charge e, but only now have scientists been able to confirm this view in the lab. Using the latest techniques for making very small electrical contacts (100-300 nm) and for detecting minuscule currents, researchers at the Condensed Matter Lab at CEA/Saclay (Christian Glattli, cglattl@spec.saclay.cea.fr, 33-169-087243) and the Lab for Microstructures and Microelectronics in Bagneux, France, have studied the "shot noise" emerging from a tiny GaAs sample. This form of noise represents the fluctuation in the current owing to the random way (governed by quantum mechanics) in which carriers tunnel from one side of an electrical junction to the other (reminiscent of the discrete fall of raindrops on a roof). What the French researchers found in probing the "granularity" of the quasiparticle carriers in the sample was that their charge equaled e/3, demonstrating that fractional charges could carry the current in a conductor. The French results (L. Saminadayar et al., upcoming article in Physical Review Letters) were obtained by measuring current fluctuations at kHz frequencies, while a competing group (Nature, 11 September 1997) at the Weizmann Institute in Israel, taking a comparable approach, worked in the MHz range. (Please note also the work of Goldman and Su, Science, 17 February 1995)