Physics News Update
The American Institute of Physics Bulletin of Physics News
Number 320, May 6, 1997 by Phillip F. Schewe and Ben Stein
SINGLE-PHONON CALORIMETRY, including the measurement of heat flow at
the 10-27 joule level, now appears possible through a combination
of nano-engineering and thermometry in the milli-kelvin realm. Michael
Roukes and his colleagues at Caltech begin by making a tiny GaAs plate
1-3 microns on a side and only 100 nm thick, supported by 4 bridges each
about 100 nm wide. On top of this monocrystalline structure are separate
sets of GaAs fingers for gently adding heat to the plate and measuring
the subsequent flow of heat. With this setup the group has made the first
direct thermal conductance measurements on a nanostructure (to appear in
Applied Physics Letters,
19 May). Their present efforts are directed towards measuring the extremely
small heat capacity (the energy needed to raise the temperature of an object
by 1 K) of their sample, estimated to be about 10-22 J/K at
a temperature of 10 mK. Roukes is in the process of switching from currently-
used resistance-based thermometry (a method which itself adds heat to samples
at low temperatures) to much less-intrusive measurements based on the thermal
noise induced in a SQUID detector. Speaking at the March APS meeting in
Kansas City, Roukes said that with this scheme he expects to track the
movement of single phonons---single pulses of thermal energy-- -in parcels
as small as 10-26 joules. Physics has not yet seen fit to give
a name to a unit as tiny as this. In this single-phonon regime, Roukes
points out, many analogies exist between the thermal transport of phonons
and the quantum optics of photons.
AN EXCITED ATOMIC STATE WITH A 10-YEAR LIFETIME has been discovered
in the ytterbium atom, raising hopes for atomic clocks 1000 times more
accurate than now possible. The Heisenberg uncertainty principle states
that the longer a system can be observed, the smaller the uncertainty in
its energy can be; therefore, it is extremely desirable to tune an atomic
clock to a long-lived high-energy (excited) state. Researchers at the National
Physical Laboratory in the UK laser cool and trap a single ytterbium ion.
They then use a laser photon to boost the atom's outermost electron to
the long-lived state. With additional laser light, the researchers subsequently
induce the electron to return to its lowest-energy (ground) state. By noting
the characteristics of the laser light interacting with the electron, the
researchers determine a 3700-day lifetime for the state. In addition to
being the longest living excited energy state yet detected in an atom,
it is the first observed "octupole" transition, a very rare transition
in which the electron changes its angular momentum by a relatively large
amount of three units. Once in this state, the electron (in the absence
of external perturbations) can only decay via the octupole transition,
which is why the state lasts so long. An atomic clock based on the transition
would be very precise but requires much additional development. (M. Roberts
et al., Physical Review
Letters, 10 March 1997; see also Nature,
20 March 1997.)
A GIANT PIEZOELECTRIC EFFECT has been observed in strontium titanate
at low temperatures. The piezoelectric process, by which mechanical energy
in a crystal is converted into electricity (and vice versa), generally
gets worse below 50 K, but in the case of SrTiO3, it gets better.
At 1.6 K, in fact, STO competes with the best room-temperature piezoelectrics.
The ultralow-temperature manifestation of this effect might result in new
forms of microscopy or thermometry. (Science,
18 April 1997.)
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