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Number 396, October 14, 1998 by Phillip F. Schewe and Ben Stein
THE 1998 PHYSICS NOBEL PRIZE goes to Robert B. Laughlin of Stanford, Horst L. Stormer of Columbia, and Daniel C. Tsui of Princeton for their work on the fractional quantum Hall effect, a drama acted out in the two-dimensional world at the interface between two semiconductor crystals. As is the case with quantum phenomena, the act of confinement (the two-dimensional electron gas, or 2DEG, imprisoned between the semiconductors) leads to quantization. A plot of Hall conductivity versus field strength is not linear but stepwise. In other words, nature does not permit just any resistance, but only certain resistances dictated by fundamental quantum principles. The specific choice of semiconductor does not play a part. Klaus von Klitzing discovered this "quantum Hall effect" in 1980 and would win the physics Nobel Prize in 1985. So exacting is the quantization of conductivity (better than a part in many millions) that von Klitzing's experiment has since been used to define the unit of resistance. Stormer and Tsui would carry this research further. At even colder temperatures and higher magnetic fields, they discovered steps within the steps. This "fractional quantum Hall effect" (FQH) was at first hard to explain. Robert Laughlin surmised that the electrons were combining with the flux quanta of the magnetic field. One side effect of Laughlin's conjecture was that excitations of the FQH electron ensembles could have fractional charges. That is, the ensembles acted as if they were supporting particles (quasiparticles) with an electrical charge which was a non-integral multiple of the basic electron charge. This hypothesis was later experimentally verified. (Background: Updates 205 and 335; Physics Today, June 81, July 83, Dec 85, Jan 88, Jul 93, Oct 97, Nov 97; Science 19 Sep 97 and 17 Feb 1995; Nature, 11 Sep 97.)
ELECTRON-POSITRON JETS FROM A QUASAR. Accelerators in Geneva and Stanford produce electron and positron beams, and so do quasars. Physicists at Brandeis University have apprently settled the issue of whether the jets seen spewing from some quasars consist of "normal" (electron-proton) plasmas or "pair" (electron-positron) plasmas. By detecting a circular polarization in light forming the radio image of quasar 3C279, the Brandeis observers deduce that positively-charged particles must be positrons. This knowledge will help those trying to model the stupedous production of energy at the cores of quasars and active galaxies. (Wardle et al., Nature, 1 October 1998).
A PHOTONIC DOT MOLECULE, a pair of photons inside two interconnected boxes each acting as an artificial atom, has been constructed, opening new possibilities for fine-tuning the colors of light coming out of certain lasers. Having built two identical, micron-sized blocks of gallium arsenide, each with a light-producing quantum well, a German-US-Russian collaboration (Thomas Reinecke, Naval Research Laboratory, 202-767-2594) studied what happened when the two GaAs blocks were connected. In isolation, each photonic dot acted as a small confined area (a "microcavity") for light; photons with wavelengths roughly equal to the spaces between two of the cavity walls could bounce back and forth between them. When the two microcavities were connected, the researchers noticed a new, larger set of photon wavelengths, slightly different from the original ones. A highly analogous situation occurs with the wavelengths of electrons when the negatively charged particles group together in an atom or molecule. (Bayer et al., Physical Review Letters, 21 September 1998; see also Physical Review Focus, 28 September 1998, )
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