Number 564, November 7, 2001
by Phil Schewe, James Riordon, and Ben Stein
The Thinnest Superconducting Wires
The thinnest superconducting wires ever made, only 10 nm wide, have
been used in an experiment showing how the superconducting state gets
extinguished as the wire narrows.
Just as traffic becomes more problematic as you reduce flow on an interstate
from four lanes down to three and then down to two and finally to one
lane, so electron pairs (or Cooper pairs, which constitute the supercurrent)
moving through very thin passages are sensitive to quantum effects not
noticeable in larger wires.
A quantum phase slip (QPS) is one such effect. It is a quantum fluctuation
in which the superconducting wavefunction spontaneously tunnels from
one state into another, a process which results in a momentary voltage,
and therefore a nonzero electrical resistance, even if the temperature
could somehow be reduced to absolute zero.
Armed with thin wires (10-20 nm) consisting of molybdenum-germanium
deposited onto carbon nanotubes, Michael Tinkham (Tinkham@RSJ.Harvard.edu)
and his colleagues at Harvard have conducted the most thorough study
yet made of this phenomenon and have definitely shown that resistance
goes up as the wire gets thinner.
The quantum resistance effect only becomes noticeable for wires below
about 30 nm in size, far smaller than most wires used in today's computers,
so there is no bottleneck yet. Future advanced superconducting computers,
however, might have trouble; by going to lower temperatures you can
eliminate resistivity arising from thermal fluctuations, but not from
quantum fluctuations. (Lau
et al., Physical Review Letters, 19 November 2001.)
Pyroelectric Accelerator
In a pyroelectric crystal held below a critical temperature (the Curie
temperature) heating or cooling causes distortions in the lattice of
atoms which in turn creates strong electric fields at the surface of
the crystal. James Brownridge of the State University of New York at
Binghamton (jdbjdb@binghamton.edu) and Stephen Shafroth of the University
of North Carolina (919-962-3015, shafroth@physics.unc.edu) have used
these electric fields to create stable, self-focused electron beams
with energies as high as 170 keV.
The energy conversion is not especially efficient: inputting watts
of heating energy produces only microwatts of output electron beam energy,
but this might not be important. Pyroelectric crystals (such as those
made of LiNbO3) are widely used as detectors of infrared
and THz radiation, but the discovery by Brownridge that they can also
be used to produce energetic electron beams if heated or cooled in dilute
gas atmospheres means that they can be used to produce x-ray fluorescence
for elemental analysis of complex materials, such as tree leaves, rocks,
air filters, blood samples, etc. Portable economical x-ray fluorescence
is now a real possibility. (Applied
Physics Letters, 12 Nov. 2001; also see Brownridge's website.)
Sound Waves Make Filters Finer
Generally, filters that remove particulates from fluids are limited
by their pore sizes. That is, a filter with millimeter-sized pores isn't
likely to catch many micron-sized particles. On the other hand, a filter
with tiny pores can trap small particles at the expense of inhibiting
fluid flow.
Donald Feke (Case Western Reserve University, dlf4@po.cwru.edu, 216-368-2750),
however, has found a way to reduce the effective pore size in highly
porous media without significantly hindering fluid flow. By applying
a low power acoustic signal to a filter, Feke can trap particles as
much as a hundred times smaller than the nominal filter pore size. An
acoustically aided filter provides relatively little resistance to the
fluid that passes through it, and yet collects particles as efficiently
as a much finer filter. And once the filter has done its job, the trapped
particles can be released at the flip of a switch that cuts off the
acoustical signal (see figure).
The trapping arises because acoustic signals traveling through a porous
material create patterns of standing waves that focus particulate matter
toward certain positions on the walls of the pores. Rather than wending
their way through the filter, particles headed for the focal points
line up to form intricate, stable filaments. In other locations, groups
of particles collect in regions of stability within the pores, where
they orbit for as long as the signal persists.
In addition to novel filter designs, Feke proposes that acoustic manipulation
may lead to efficient material sorting technologies or methods that
aid in assembling microscopic structures. Feke presented
his work at the 73rd Annual Society
of Rheology meeting in Bethesda, Maryland.
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