PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 643  June 26, 2003   by Phillip F. Schewe, Ben Stein, and
James Riordon

THE MESON Ds(2317), discovered a couple of months ago in high energy
electron-positron collisions at SLAC, possesses a mass of 2.317 GeV,
some 170 MeV lighter than expected, at least according to prevalent
theories of quark interactions. Hence physicists need a new
explanation of how a charm quark attached to an antistrange quark
should have this particular mass. In general, Ds and D mesons are a
class of particles, each consisting of a charm quark attached to a
light antiquark. (The subscript "s" pertains to all those D's
containing a strange antiquark; "ordinary" D mesons consist of a
charm quark and a down antiquark.) The Babar detection group at SLAC
responsible for the experimental discovery (Aubert et al., Physical
Review Letters, 20 June 2003, text at www.aip.org/physnews/select;
also see press release at
http://www.slac.stanford.edu/slac/media-info/20030428/index.html )
suggests that the Ds(2317) might be a novel particle made of 4
quarks.  But a pair of physicists in Portugal claim that in their
model, assuming that the meson is indeed a charm/antistrange
combination, the mass comes out in the right range if the
strong-nuclear-force interactions responsible for the creation and
annihilation of extra quark-antiquark pairs are taken into account.
Using this model, Eef van Beveren (University of Coimbra) and George
Rupp (CFIF Lab, IST, Lisbon) have successfully predicted meson
masses in the past (such as the kappa meson, discovered at Fermilab
(E791) at a mass of 800 MeV), while in the case of Ds mesons they
predict a mass very near the Ds(2317) found already, and another at
about 2.9 GeV (yet to be found). As to D mesons, they predict the
equivalent of the Ds(2317) at a mass range of 2.1-2.3 GeV (for which
preliminary evidence exists), and a heavier one at about 2.8 GeV
(still undetected). According to van Beveren and Rupp, both pairs of
Ds and D mesons are, in some sense, different aspects of the same
underlying quark-antiquark state. (Physical Review Letters, upcoming
article, see website http://cft.fis.uc.pt/eef/default.htm or contact
George Rupp at george{at}ist.utl.pt, +351-21-841-9103)

MOUNTAIN-CLIMBING ATOMS.  Atoms that are deposited on crystal
surfaces, through a method known as molecular beam epitaxy, often
form surfaces covered with numerous small mounds rather than smooth
layers, if the substrate temperature is sufficiently low. For higher
temperatures, an atom near the top of a mound can often move about
and diffuse down toward the crystal surface. Conventional wisdom
holds that upward diffusion, on the other hand, is essentially
negligible. Recently, however, a collaboration of researchers at the
INFM-Universit… di Genova in Italy, the Chinese Academy of Sciences,
and Oak Ridge National Laboratory has found that deposited atoms may
sometimes diffuse upward spontaneously, forming faceted mountains
that tower over the surrounding crystal plane. Although the
formation of faceted nanocrystals has been observed before, these
were generally thought to be due to a mismatch between a crystal
substrate and the crystal structure of the deposited atoms (for
example, when germanium atoms are deposited on silicon, differences
in the spacing of the two types of crystals lead to a strain that
encourages the growth of large, hut-shaped crystals). The new
research, by contrast, reveals for the first time that the
crystalline mountains (see image soon to be posted at
www.aip.org/mgr/png ) can form even when the deposited atoms and the
substrate crystal consist of the same element, and no strain energy
is involved. Specifically, aluminum atoms deposited on an aluminum
crystal substrate may diffuse upward into crystal structures that
rise upward as much as ten times higher than the thickness of the
surrounding planes.
Computer simulations seem to indicate that the growth may be caused
by processes thought to be insignificant in previous deposition
studies. In particular, an atom sitting at the inner corner near the
base of a crystal protrusion may jump out of place and onto the
crystal facet, or a pair of atoms can conspire to exchange positions
as they leapfrog up a crystal slope.   The counterintuitive
formation of tall nanocrystals via upward diffusion of aluminum
atoms only occurs within a temperature window of about 330 K to 500
K, when the total crystal surface coverage exceeds critical values
of about 10 or more deposited layers, depending on the specific
temperature. The researchers (Francesco Buatier de Mongeot,
buatier{at}fisica.unige.it, +39-10-3536324, and Zhenyu Zhang,
zhangz{at}ornl.gov) predict that the often neglected processes leading
to upward atom diffusion are likely to be important for other
crystals grown via molecular beam epitaxy, leading to much richer
dynamics in the growth of thin films than previously suspected. (F.
Buatier de Mongeot et al., Physical Review Letters, upcoming
article, probably 4 July)

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