Number 364, March 27, 1998 by Phillip F. Schewe and Ben Stein
WEIRD BEHAVIOR IN QUANTUM DOTS. Interesting things happen when particles are confined in a tiny box. Researchers at MIT led by Raymond Ashoori (617-253-5585) make themselves such a box, a quantum dot, out of semiconductors---a layer of gallium arsenide between layers of aluminum gallium arsenide. On top of this sandwich sits a metal gate electrode which attracts electrons into the dot and controls the arrival or departure of electrons one at a time. Building up from just one electron, the MIT physicists collect a puddle of electrons and observe how the arrival of each newcomer must overcome (with the help of an increasing gate voltage) the mutual repulsion ("Coulomb blockade") of those already in place. For small dots (0.2 microns across) a graph of charge-vs-voltage would look like a staircase. Such an effect is at the heart of single-electron transistors (SET), which act as sensitive detectors of electrical charge (just as superconducting quantum interference devices---SQUIDS--- are sensitive detectors of magnetic flux). For larger dots (0.5-1.2 microns across), containing fewer than several hundred electrons, the MIT scientists were astonished to observe an unexpected and mysterious pairing: for each stepwise voltage increase not one but two electrons were able to join the puddle. The pairing has not yet been explained but the data strongly suggest that it arises from a novel quantum effect that develops whenever electrons are localized into spatially isolated regions within the dots. For medium-sized dots (0.5 microns) the physics gets even weirder: the pairing occurs only for every fourth or fifth electron. The goal now is to understand the underlying pairing mechanism. (Talk at last week's APS meeting in Los Angeles.)
THE FLATTEST SURFACE IN THE SOLAR SYSTEM is on Mars' northern lowlands. Over a 2000-km-around belt between 50 and 80 degrees north latitude, the surface tilts up only about 0.05 degrees to the south. This data and a number of other results from the Mars Global Surveyor---such as information on the Martian atmosphere, magnetism, plasma fields, topography, and mineralogy---are reported in the 13 March issue of Science.
ENERGETIC COSMIC RAYS MAY BE IRON NUCLEI. Cosmic rays are particles that come from far away (many likely to be of extra-galactic origin) and strike our atmosphere, where they engender a shower of new particles that show up in detectors on the ground. One of the biggest puzzles in cosmic ray physics is why there should be so many events with total energies above 100 EeV (1020 eV). The main reason for this was the notion that if the primary particle were journeying from a distant galaxy, the particle's energy would be sapped by interactions with cosmic microwave background photons or with infrared background radiation (IBR). There is the additional problem of how cosmic rays with energies as high as 300 EeV could have been accelerated to such a degree if the first place (see Update 243). Floyd Stecker of the Goddard Space Flight Center has determined that a 200 EeV nucleus (starting out as an iron nucleus) could negotiate a 300-million-light- year journey through the IBR. The nucleus would partially disintegrate en route, but would still arrive at Earth with a potent energy. (Physical Review Letters, 2 March.)
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