PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 822 May 1, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu
THE EFIMOV EFFECT: THREE*S COMPANY, TWO'S A CROWD. At the April APS
meeting in Jacksonville, physicists discussed the recent observations of
the Efimov effect, a purely quantum phenomenon whereby two particles
such as neutral atoms which ordinarily do not interact strongly with one
another join together with a third atom under the right conditions. The
trio can then form an infinite number of configurations, or put another
way, an infinite number of "bound states" that hold the atoms together.
The effect was first predicted around 1970 by a physicist named Vitaly
Efimov, then a Ph.D. candidate at the time, but was originally
considered "too strange to be true," according to the University of
Colorado*s Chris Greene, in part because the atoms would abruptly
switch from being standoffish to becoming stuck-together Siamese
Triplets at remarkably long distances from one another (approximately
500-10,000 times the size of a hydrogen atom in the case of neutral
atoms). For decades, experimenters tried in vain to create these
three-particle systems (which came to be known as "Efimov trimers").
In 1999, Greene and his collaborators Brett Esry and Jim Burke
predicted that gases of ultracold atoms might provide the right
conditions for creating the three-particle state. In 2005, a research
team led by Rudi Grimm of the University of Innsbruck in Austria finally
confirmed the Efimov state in an ultracold gas of cesium cooled to just
10 nanokelvin.
How do the neutral atoms attract one another in the first place? At
small distances, ordinary chemical bonding mechanisms apply, but at the
vast distances relevant to the Efimov effect, it is mainly through the
van der Waals effect, in which rearrangements of electrical charge in
one atom (forming an "electric dipole") create electric fields that can
induce dipoles in, and thereby attract, neighboring atoms. The
observation of the Efimov effect is a coup for being able to study the
rich quantum physics between three particles.
The effect can conceivably occur in nucleons or molecules (and any
object governed by quantum mechanics). However, it will likely be
harder to observe in those systems because physicists cannot alter the
strengths of interactions between the constituent particles as easily as
they can in ultracold atom gases (through their "Feshbach resonances").
But the effect could provide insights on such systems as the triton, a
nucleon with one proton and two neutrons, in addition to the BCS-BEC
crossover, in which atoms switch from forming weakly bound Cooper pairs
to entering a single collective quantum state. (See also article by
Charles Day, Physics Today, April 2006, Esry et al, Phys. Rev. Lett, 30
August 1999, and Kraemer et al., Nature, 16 March 2006).
THE PHYSICS OF UTENSILS is explained by the University of Virginia's
Lou Bloomfield in the May issue of Physics Today. Forget about cooking
classes--cutlery can provide a rich lesson in crystallography and
condensed-matter physics. Forks, knives and spoons are generally made
of steel--an alloy of iron and carbon with other elements mixed in. A
room-temperature iron crystal is soft, as it is susceptible to shear
stress. In other words, pushing layers of the iron crystal in opposite
directions causes the layers to slip, bending the iron permanently,
which, as Bloomfield points out, is "fine in a twist tie, not so good in
a knife." When the iron crystal (known as ferrite) takes in even a
small amount of carbon, the situation change
s. Dispersed throughout the
ferrite (the carbon is generally insoluble in it), the carbon makes it
more difficult for crystal impurities known as dislocations to move,
thereby frustrating shear forces and hardening the solid. Put even more
carbon into the ferrite and it distorts the crystal into the hardest
possible steel structure, suitable for the cutting edges of knives.
Making useful steel cutlery generally requires heating an iron-carbon
mixture to well over 727 degrees C, in order to facilitate structural
transformations throughout the entire steel material. What type of
utensil results depends on how quickly the steel is cooled, or
"quenched." Slowly quenched steel results in pearlite, a sturdy but
relatively soft compound often used for spoons. The most quickly cooled
steel leads to martensite, the hard steel used in cutting edges.
Reheating the martensite rearranges some crystalline structures and
"tempers" it so it becomes less brittle. Making stainless steel,
Bloomfield says, involves adding elements including chromium. When the
Cr content exceeds 11.5% by weight, a chromium oxide layer forms on the
surface to prevent rusting. More details can be found in Bloomfield's
article, which is freely accessible at
http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_60/iss_5/88_1.shtml
(Bloomfield: 434-924-6595, [EMAIL PROTECTED] )
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