QUANTUM BOXES FOR COOPER PAIRS. Quantum dots, comparable in size to the wavelength of electrons, are used to study how spatial confinement alters allowed electron energies. Recently a team of scientists from Holland, Russia, Ukraine, and Belgium (contact Andre Geim, geim@sci.kun.nl) has done the same thing for Cooper pairs, the doublets of electrons which form in superconductors. Essentially studying the size-dependence of superconductivity, these researchers use a sensitive detector to monitor the magnetization (the response to an applied magnetic field) of superconducting aluminum disks ranging in size from .1 up to 2.4 microns for a variety of temperature and field conditions. The result is an unexpected diversity of superconducting behavior for the different disk sizes, probably arising from the disks being nearly the same size as the Cooper pairs.(Apparatus, including a new form of micro-magnetometry, described in Applied Physics Letters, 20 Oct.; experimental results, Nature, 20 Nov.; numerical simulations, Physical Review Letters, 8 Dec; see image at Physics News Graphics.)

CAN ELECTRONS BEHAVE LIKE PLANETS? According to quantum mechanics, an electron in an atom forms a hazy cloud of possible positions around the nucleus. Recently, physicists have pondered the possibility of creating a "Trojan state" in which an electron would conform to quantum mechanical principles yet occupy a small region in space and orbit the nucleus like a little planet. The name comes from Trojan asteroids which revolve around the sun in the same orbit as Jupiter, some in advance of the planet and some behind. To bring about a Trojan state, lasers would first put the electron into a "circular Rydberg state" in which the electron exists in a thin donut of possible positions. A microwave beam would subsequently cause the donut to coalesce into a small sausage-shaped region which then revolves around the nucleus. Two potential obstacles to the creation of Trojan states are the possibility that the electrons would be ionized by the microwave fields (a fear since discounted) or that they would fall to a lower-energy state by spontaneously emitting a photon. Now, researchers in Poland have calculated that such spontaneous emission is millions of times less likely than ionization, paving the way for experimental realization of Trojan electrons. (Zofia Bialynicka-Birula, Institute of Physics, Warsaw, and Iwo Bialynicki-Birula (birula@theta1.ifpan.edu.pl), Center for Theoretical Physics, Warsaw, in Physical Review A, November 1997; illustration at Physics News Graphics.)

PHOTONIC-CRYSTAL FILTERS. Just as semiconductors exclude the movement of electrons in certain energy bands (with important implications for useful devices), photonic crystals (also called photonic bandgap materials) exclude the passage of photons in certain wavelength bands. Developed first for microwaves, photonic crystals for infrared light have also become available. In fact, MIT scientists have now fashioned a silicon structure (a strip of silicon .5 microns wide drilled with .4 micron holes) that forbids light over a wide range---roughly 1300 to 1700 nm. Light at those wavelengths is defeated by multiple reflections from the holes. The photonic crystal has one additional feature, made possible by deliberately staggering slightly the spacing of the holes in the middle of the strip: in the middle of the forbidden band is a small island of wavelength (at 1.54 microns) where light is actually encouraged. In other words, the crystal acts as a filter allowing the transmission of 1.54 micron light (a crucial wavelength favored by fiber optics) but cutting out light at surrounding wavelengths. The tiny zone of silicon in the middle of the lightguide is in effect a tiny optical microcavity with a volume of only 0.055 cubic microns. (Foresi et al., Nature, 13 Nov.)