Number 217, March 10, 1995 by Phillip F. Schewe and Ben Stein
HIGHER CURRENT DENSITIES can now be achieved in high-temperature superconductors.
Arunava Gupta and his colleagues at IBM (Yorktown Heights, NY) have measured
current densities as high as 10**5 amps/sq cm in mercury-cuprate (Hg-Ba-Ca-Cu-O)
films at a temperature of 110 K. This is ten times the density achieved
with thallium or bismuth-based superconducting films at such an elevated
temperature. These measurements were made with a magnetic field perpendicular
to the film sample. When the field lines are oriented in the plane of the
film, an even higher current density, 10**7 to 10**8 amp/sq cm, can be
sustained without the film losing its superconducting property. Gupta's
samples can bear the high currents only in low perpendicular magnetic fields
(high fields would disrupt the superconducting state), but this would still
make the mercury-based compound potentially valuable in such applications
as microwave transmitters and filters, or in the fabrication of superconducting
quantum interference devices (SQUIDs), which are highly sensitive magnetometers.
(L. Krusin-Elbaum, C.C. Tsuei, and A. Gupta, Nature, 23 February 1995.)
EUROPA HAS AN OXYGEN ATMOSPHERE . Roughly the size of Earth's moon, Jupiter's
moon Europa has been known for some time to possess exposed surface water
ice. New observations by the Hubble Space Telescope (at ultraviolet wavelengths)
show that Europa also has atmospheric molecular oxygen (O2), but only at
a pressure of about 10**-11 that of Earth's sea-level atmosphere. The Hubble
astronomers believe that the chief source of the gaseous oxygen is a process
in which water molecules are broken up by incoming charged particles from
Jupiter's magnetosphere. Last year oxygen was also found on the Jovian
moon Ganymede, but only in surface ice form and not in gaseous form. (D.T.
Hall et al., Nature, 23 February 1995.)
VERTICAL CAVITY SURFACE EMITTING LASERS (VCSELs) are now able to work
efficiently at visible and far-infrared wavelengths. VCSELs emit light
vertically out of the plane of the laser's active medium rather than horizontally
from one of its edges. Also, because one of the laser's reflective mirrors
consists of disk-shaped AlAs/GaAs layers, the emergent laser beam has a
circular profile, lessening the need for focusing elements. Arrays of such
lasers could be used in displays, short-distance communications between
microchips, and possibly in long- distance transmissions over fibers. Previously
successful at getting VCSELs to emit at near- infrared wavelengths (a few
months ago Motorola started selling the first commercial devices employing
VCSELs), researchers at Sandia have only recently overcome obstacles (such
as a mismatch between neighboring semiconductor layers) to produce VCSELs
that emit in the visible (red) range with nearly the efficiency as near-infrared
devices. At far-infrared wavelengths (important for transmission in fibers)
a Santa Barbara group has produced a VCSEL that emits at 1.52 microns.
Furthermore, this device has set a record for the lowest room- temperature
threshold current density for a laser (Dubravko Babic et al., Applied Physics
Letters, 27 February 1995.) This device does not yet operate in a continuous
mode, which is desirable for commercial use. Finally, a company in Minnesota,
APA Optics, has made the first VCSEL to operate at ultraviolet wavelengths.
(Science, 24 February 1995.)
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