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)
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