Science > Physics > Physics News Update - Number 652, September 4, 2003
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Science > Physics |
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"Sam Wormley" |
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05 Sep 2003 05:45:54 PM |
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Physics News Update - Number 652, September 4, 2003 |
Physics News Update - Number 652, September 4, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
A Spinless BEC
A spinless BEC, a Bose-Einstein condensate that is insensitive to
any external magnetic field, has been created by researchers at
Kyoto University (contact Yosuke Takasu or Yoshiro Takahashi),
potentially offering a route to improved atomic clocks, more
precise atom interferometry, and more highly controlled means of
depositing atoms on surfaces. In all previous Bose-Einstein
condensates, the raw ingredients have either been alkali metals
(such as rubidium and cesium) or helium, all of which have been
sensitive to magnetic fields. In contrast, the researchers decided
to make a BEC of ytterbium (Yb), a rare-earth element that has two
outer (valence) electrons, whose "spins" determine the atom's
response to a magnetic field. When the spins of Yb's two electrons
are in opposite directions, the total spin is zero and the atom
assumes a "singlet" state, in which it is unresponsive to a
magnetic field. In their setup, the researchers trap approximately
1 million Yb atoms in the singlet state with light beams. The
hotter atoms evaporate away, leaving a chilly gas cloud of about
5000 atoms that form a BEC at temperatures of below 790
nanokelvins. Since the Yb BEC is insensitive to stray magnetic
fields in its surroundings, it may allow for more precise atomic
deposition and atom interferometry. Moreover, the very heavy mass
of Yb compared to other BEC atoms means that certain fundamental
physics effects, such as atomic parity violation and time symmetry
violation, are more pronounced, making a Yb BEC desirable for such
studies. Furthermore, lasers interacting with the Yb atoms can be
tuned to a very narrow frequency range, potentially enabling a Yb
BEC to be the basis of an atomic clock with unprecedented
precision. Finally, the many stable isotopes of Yb (5 are bosons, 2
are fermions) facilitates the possibility of creating a BEC and a
Fermi degenerate gas in the same cloud. (Takasu et al., Physical
Review Letters, 25 July 2003)
Pressing Forward from Teeth to Superconductors
Found in teeth and bones as well as fertilizers and DNA, phosphorus
is an insulator at room temperature. However, exerting a large
amount of pressure on a stable specimen of phosphorus changes its
crystalline structure, enabling it to superconduct at temperatures
of around 10 K. Exerting even more pressure (2.5 Mbar, about 30,000
times greater than the pressure of clenching your teeth) can
transform it again, to a body-centered-cubic (bcc) crystal
structure (Akahama et al., Phys Rev B, 1 Feb 2000). Now, Sergey
Ostanin of the University of Warwick in the UK and his colleagues
have shown that bcc phosphorus crystals achieve superconductivity
at higher temperatures, somewhere between 14-22 K. This is still
much lower than the temperature of your mouth, even after an
ice-cream headache. But such phosphorus superconductors might be
very useful in spintronics. For example, they could be help in the
construction of a superconducting spin switch, specifically one in
which the phosphorus layer would lie in between a pair of
ferromagnets, an arrangement that could alter its identity from
superconductor to regular conductor (L. R. Tagirov, Phys. Rev.
Lett, 6 September 1999). Furthermore, high pressures might not even
be needed to make bcc phosphorus crystals: they could possibly be
grown by depositing the atoms onto a substrate of iron, which
itself organizes into a bcc structure. (Ostanin et al., Physical
Review Letters, 22 August 2003)
Non-Contact Friction
Non-contact friction can be artificially enhanced. Usually for two
bodies in relative motion to feel friction the respective surface
atoms have to be in contact. There is a type of friction, however,
which can act between two surfaces not actually in contact. This
dilute friction is attributed to the van der Waals force, a common
but weak attractive force which arises when an atom or molecule
spontaneously develops a dipole moment (that is, although it is
neutral, a small region of net negative charge can develop, offset
slightly from a comparable positive region) owing to a thermal
fluctuation (related to the random motion of the electrons and
ions) or a quantum fluctuation (the very positions of the particles
varies from moment to moment owing to the uncertainty relations
built into quantum reality). This short-lived polarity can in turn
induce a dipole moment in a neighboring atom or molecule, some
distance away. A new study of van der Waals friction by Alexander
Volokitin and Bo Persson at the Institut fur Festkorperforschung
(Julich, Germany) accounts for recent odd friction experiments
conducted with STM probes. The theory holds that van der Waals
friction can be greatly enhanced (by up to a factor of ten million
at a separation of 10 angstroms in comparison with the case of good
conductors with clean surfaces) by adsorbing certain molecules onto
one or both of the surfaces. This increases the resonant
electromagnetic force (which can be viewed as the tunneling of
photons) between the objects, especially if they are made of the
same material. The adsorbate atoms can be thought of as tiny
antennas, one acting as an emitter and one as a receiver; when the
two antennas are in tune the electromagnetic interaction between
them will be greatly enhanced (see figure).
A better understanding of this kind of non-contact friction will,
at the fundamental level, help physicists to study the quantum
behavior of atoms at surfaces and, at the level of applications, to
prepare "brakes" for micromachines where large friction is not
needed. (Volokitin and Persson, Physical Review Letters, 5
September 2003)
Physics News Update is a digest of physics news items arising from
physics meetings, physics journals, newspapers and magazines, and other
news sources. Subscriptions are free as a way of broadly disseminating
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others can read it; please credit the American Institute of Physics.
Physics News Update appears approximately once a week. Questions?
Contact the editors at physnews@aip.org.
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