Atomic Clusters, INT, Seattle, WA, July - August, 1998
Program at the Institute for Nuclear Theory,
June 29 - August 28, 1998
Joint INT/ITAMP Workshop,
July 6 - 8, 1998
The purpose of the
INT program
is bring together researchers from
various disciplines (condensed matter, chemistry, nuclear
physics...) who have common interests in atomic clusters - their
properties and their theoretical description.
The program will start with
a
three-day INT/ITAMP workshop,
as a concentrated forum to examine the
issues and guide the direction of the summer program. The
program itself will provide an informal setting for researchers
to interact over an extended time period. Ideally, this will
foster collaborations that would not otherwise have occurred.
Attendance at the program and at the workshop is limited, but
all researchers interested in the scientific themes, including
graduate students, are invited to apply. Program participants
receive office space, computer access and other office support
at the INT, as well as financial support for their local
expenses. For an application form,
click here .
Program Organizers:
Program Coordinator:
Nancy DRAGUN -
dragun@phys.washington.edu, FAX: (206)685-3730
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DYNAMICS OF CLUSTERS
July 6 - July 8, 1998, INT/ITAMP Workshop
The enormous advances in experimental techniques for studying clusters
creates a need for improved theory, both of understanding the
experimental findings and for guiding future experiments. The workshop
will be focussed on questions of cluster dynamics. The three days
will be devoted to the structural, the electronic, and the electronic-
structural aspects, respectively. A convenor has been designated to
lead each day's discussion and organize the presentations on the topic.
(Not all speakers have been confirmed so far.)
- July 6, Phase transitions
Convenor:
T. Patrick MARTIN -
martin@vaxff3.mpi-stuttgart.mpg.de
Some questions to be considered: what determines the structure
of clusters as a functtion of size? When is bulk behavior reached?
How sharp are thermodynamic phase transitions in finite clusters,
and how do the transition temperatures and latent heats depend on
size?
Speakers:
P. Alivistatos (Berkeley),
R.S. Berry (Chicago),
C. Borgs (Microsoft)
C. Brechignac (Orsay),
D. Gross (Berlin)
H. Haberland (Freiburg),
M. Jarrold (Evanston),
U. Landman (Atlanta),
M. Manninen (Jyvaskyla),
D. Wales (Cambridge).
- July 7 Electronic properties
Co-convenors:
Steve G. LOUIE -
louie@jungle.berkeley.edu
and
James R. CHELIKOWSKY -
jrc@msi.umn.edu
How do electrons attach and detach, or otherwise get transported
by clusters? What determines the electronic response, and can it
be used to determine structural properties? Is there a finite-system
signature for the metal-insulator transition? The relevance of the
electron-phonon coupling in clusters.
Speakers:
K.M. Ho (Iowa State),
M. Pederson (Naval Research Lab.),
D. Salahub (Montreal),
J. Jellinek (Argonne),
J. Grossman (Berkeley),
A. Rubio (Valladolid),
M. Rohlfing (Berkeley),
W. Andreoni (Zurich),
G. Bertsch (Seattle),
J.R. Chelikowsky (Minneapolis).
- July 8 Picosecond dynamics
Convenor: Gustav GERBER -
gerber@physik.uni-wuerzburg.de
Large amplitude electron dynamics on the femtosecond scale,
pulse-probe experiments, how does a
cluster respond to a strong ultra-short laser pulse.
Speakers:
M. Zanni (Berkeley),
L. Woeste (Berlin),
W. Eberhardt (Julich),
B. v. Issendorff (Freiburg)
G. v. Plessen (Munich),
F. Traeger (Kassel),
C. Rose-Petruck (San Diego),
G. Gerber (Wuerzburg),
J.-Y. Bigot (Strasbourg).
The workshop is sponsored jointly by the Institute for Nuclear Theory
at University
of Washington in Seattle, WA and the Institute for Theory of Atomic and
Molecular Physics at Harvard University in Cambridge, MA.
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Description of the Program
The physics of atomic clusters in interesting from many points of view.
These systems bridge the domains of atomic and molecular physics on one
side and condensed matter physics on the other. Their properties may be
dominated by their large surface-to-volume ratio, giving a unique
opportunity to study the interplay between surface and volume effects. They
may exhibit a discrete spectroscopy because of their finite size.
Nuclei are also finite quantum systems with a discrete spectroscopy, and
this common feature has brought many nuclear physicists into atomic cluster
research. Phenomena that are familiar to nuclear physicists have reappeared
under a new guise on the nanometer scale. Theoretical techniques that are
well-known for problems on the femtometer scale have also shown value on
the nanometer scale. We also feel that there may be cross-fertilization in
the other direction. While much of nuclear phsyics is moving into the
high-energy direction, the problem of ``the classical nucleus'' is still
largely unsolved. The study of atomic clusters should give us much insight
into at least some of the required technology; the availability of
experiments on both metallic and atomic clusters should help us pin down
many of the pertinent questions in an often cleaner environment than in the
nucleus. For these reasons, we feel it is appropriate for the INT to hold a
program on this subject.
We list some of the specific questions which have been under study, from
the above point of view. This should be taken with a grain of salt, since
we are making here a rather bold projection for problems which should be
interesting to study in two years from now. In such a rapidly evolving
field as atomic clusters this is rather chancy. This list is merely meant
to illustrate the richness of the field and its tremendous potential.
- How are cluster properties influenced by electron delocalization? The
quantum mechanical single-particle degrees of freedom give rise to shell
structure and magic numbers in both clusters and nuclei. The
insulator-metal transition is another aspect which can be and is studied
in such systems. While this transition occurs at essentially the same point
for all relevant properties when it occurs in bulk matter, the various
properties of small systems do not all make their changes at the same size,
temperature, field, etc., which makes for a much richer set of
ramifications of delocalization and how it sets in. In fact, in clusters
one sees a richer behavior, with the shells organized into
supershells.
Moreover in metal and semiconductor clusters the electronic shell
structure and the geometric shell structure compete to determine patterns
of shape and stability. The electronic properties of systems in two
dimensions constitutes one more extremely exciting area of
investigations. Shell effects may be pronounced in the ionization
potentials of metallic clusters. Is there a connection between the lowering
of work functions on rough surfaces and the lowered ionization potentials
of nonmagic metal clusters? The role of the shape fluctuations induced by
finite temperatures and/or packing of the atoms awaits also its
resolution.
- What are the collective degrees of freedom, the elementary
excitations of atomic clusters? In systems with delocalized electrons,
at some frequencies the electrons respond to an external field
collectively as a plasmon-like resonance. The form of the electron-electron
interaction is still a topic which needs further refinement. The LDA
traditionally used has well known limitations, one of the most glaring ones
is the treatment of the gradient corrections, which needs further study.
The position and line shape of the plasmon carry valuable information about
the geometry, shape fluctuations, geometry of the packing and temperature
of the cluster and about the electron-ion coupling.
- What is the electron-ion interaction? The interaction of electronic
degrees of freedom with the geometric degrees - the vibrations or phonons
- is still a subject in its infancy. The alkali-doped fullerenes may
provide the best examples of BCS superconductors with the pairing driven by
the intra-molecular vibrational coupling. The observed superparamagnetism
of small magnetic clusters provides a new impetus to try to understand the
coupling between spin and other degrees of freedom. The magnetic properties
of clusters reveal a series of interesting and unexpected features such as
nonmonotonic dependence of magnetization on particle number or external
field, strong magnetism in normally nonmagnetic elements. Another
particularly interesting aspect: in very small systems, e.g. around 4-10
particles, the extensions of geometric phase to the multicomponent wave
functions of the non-Abelian gauge groups become a fine challenge to work
out.
- What remains of phase transitions, which are bulk phenomena, when one
shrinks the system to cluster sizes? Finite-sized objects may also melt,
or in the case of magnetic systems have a Curie temperature. In theory,
clusters can exhibit such effects of phase transitions &&& finite bands of
temperature and pressure within which solid and liquid coexist. &&& What
is the analogue in finite systems of what is a second-order transition in
the bulk? There are also questions of whether fluctuations would hide the
sharp temperature limits of the stable branches of the spinodals.
Experimentally, there is only indirect evidence so far for some of these
interesting apects. Atomic and molecular clusters provide a vehicle that
may lead to tools for studying potential landscapes, the microscopic
factors that lead to glass formation or to "structure-seeking," meaning to
crystal formation or to protein folding.
- What determines cluster geometry? The existence of spherical shells
in alkali metal clusters proves that electron delocalization can be
crucial, forcing near-spherical geometric shapes at specific electron
numbers. There are many other possibilities as well. For Lennard-Jones
systems such as the noble gases (e.g. Ar, Xe), one see polyhedral shapes
having the highest symmetry possible in finite systems: icosahedra. Also is
of interest in the study of phase transition, the possibility of amorphous
phase of clusters and their thermal stablity and ways of identifying
critical parameters that uniquely signify the phase change. Clusters made
of carbon have an especially rich set of shapes, including chains, rings,
buckyballs, tubes and onions.
- Atom-cluster, cluster-cluster, electron-cluster and cluster-surface
collisions are themes which emerge both experimentally and theoretically.
Are there specific phase transitions in finite systems, e.g. fragmentation
transition as function of the surface to volume ratio. Evaporation,
fragmentation and fission are still largely unexplored problems in cluster
physics; people who have done the key studies in these problems will be
natural participants in this Program.
- Effects of strong external laser fields, where the nonlinear
response of the system has to be evaluated.
- The study of time dependent phenomena at femtosecond scale.
Experimentally it has been demonstrated demonstrated that using two well
correlated laser pulses one can probe the time evolution of such small
systems in ``real time''.
- Quantum confinement as witnessed in quantum dots presents a plethora
of very exciting phenomena: exciton confinement, collective excitations,
conductance fluctuations, etc. The overlap between methods and phenomena
familiar to nuclear physicists is obvious. It seems that the studies of
Coulomb clusters confined by an external potential are also interesting for
both theory and applications. For the classic Coulomb cluster examples are
electrons and ions on the helium surface, ions in the Penning and
radio-frequency traps, the dust clusters and crystals in the
radio-frequence discharge plasma etc. The quantum systems with the Coulomb
interaction are electrons and holes in the quantum dots and quantum wires.
In many cases the systems under consideration are two-dimensional. The low
dimensionality leads to a lot of new effects and allows to treat the
transition from cluster to bulk and cluster melting with larger assurance.
R.S. Berry ,
G.F. Bertsch
and
A. Bulgac
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