Number 849, December 5 , 2007
by Phil Schewe and Jason S. Bardi
Cooper Pairs in Insulators
Cooper pairs are the extraordinary link-up of like-charged electrons through the subtle flexings of a crystal. They act as the backbone of the superconducting phenomenon, but have also now been observed in a material that is
not only non-superconducting but actually an insulator. An
experiment at Brown University measures electrical resistance in a
Swiss-cheese-like plank of bismuth atoms made by spritzing a cloud
of atoms onto a substrate with 27-nm-wide holes spaced 100 nm
apart. Bismuth films made this way are superconducting if the
sample is many atom-layers thick but is insulating if the film is
only a few atoms thick, owing to subtle effects which arise from the
restrictive geometry.
The superconducting and insulating states are easily distinguished; as the temperature is lowered below the transition temperature (2 K) the resistance goes to zero for bismuth-as-superconductor, whereas for the insulating bismuth the resistance becomes extremely high. Cooper pairs are certainly present in the superconducting sample; they team up to form a non-resistive supercurrent. But how do the researchers know that pairs are present in the insulator too? Because of an additional test. By seeing what happens to resistance as an external magnetic field is increased.
The resistance should vary periodically, with a period proportional to the charge of the electrical objects in question. From the periodicity, proportional in this case to two times the charge of the electron, the Brown physicists could deduce that they were seeing doubly-charged objects moving through the sample. In other words, Cooper pairs are present in the insulator. This is true only at the lowest temperatures. One of the researchers, James Valles (james_valles_jr@brown.edu), says that there have been previous hints of Cooper pairs in some films related to superconductors, but that in those cases the evidence for pairs in the insulating state was ambiguous, and not as direct as the observation recorded in the Brown lab. He asserts that the realization of a boson insulator (in which the charge carriers are electron pairs) will help to further explore the odd kinship between insulators and superconductors. (Stewart et al., Science 23 November 2007)
A Moon Like Ours is Rarely Formed
Interpretations of recent infrared observations might be changing our view of the Moon. About 4.5 billion years ago, our Earth was utterly shattered-the victim of a giant impact with an object the size of Mars. The collision that was powerful enough to vaporize rock and throw a massive plume of Earth's mantle into space was not all bad, though. The impactor soon merged with the Earth giving it a fast spin, while chunks of Earth's mantle settled into a disk around our planet. Within a year or so, a large moon was formed out of this debris. The left-over rocks continued to circle around the sun over the next million years, occasionally colliding and creating a flow of dust, until it was all cleaned up by gravity and solar radiation.
Many scientists are interested in knowing how common such impacts are in other young solar systems because the heavy tidal mixing driven by the moon’s gravity may have played an important role in making conditions favorable for the origins of life on Earth. Recently Nadya Gorlova of the University of Florida and her colleagues at the Steward observatory in Tucson, Arizona and the European Southern Observatory in Santiago, Chile reported in The Astrophysical Journal that they may not be very common at all. Using the cryogenically-cooled infrared orbiting Spitzer Space Telescope, Gorlova and her colleagues surveyed the 30-million-year old star cluster NGC 2547. They selected this cluster because of its age. The planetary building process usually ends by approximately 50 million years, making the odds of a giant impact unlikely to occur outside this window.
The other advantage of NGC 2547 is that it is old enough for the material left out from the original cloud of which solar systems formed to dissipate (this takes about 3-10 million years). By focusing on radiation at a wavelength of about 8 microns, they could detect the heat they would expect from dust at a distance of about one astronomical unit (1 AU) from a solar-type star. The NGC 2547 cluster was previously surveyed spectroscopically, so they could cross-check to make sure that the emission they detected was not due to gas (which would be evident by spectral emission lines). Out of about 400 stars in the NGC 2547 cluster, they found only one that showed evidence of dust from a massive impact. From this they conclude that collisions like the one that gave rise to our moon don't happen in every system. This means that moons like ours may be rare. (The Astrophysical Journal, 20 November 2007)
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