Press release no. 126/2005 from 2005-12-05
Crystal of holes discovered
Physicists at Kiel University discover an unusual state of matter
The existence of an unusual state of matter, a crystal that consists
entirely of holes, has been proven at Kiel University. As reported in
the latest issue of Physical Review Letters (December 2nd , 2005), an
international team led by Professor Michael Bonitz has, for the first
time, demonstrated with the help of extensive computer simulations that
this exotic phenomenon, the existence of which was hitherto only a
subject of speculation, should certainly occur. The physicists have
also been able to predict the conditions for its formation.
"We now know that this effect occurs in semiconductors with a certain
type of band structure", says Bonitz. "In normal solids, the electrons
and holes (which are formed when electrons are excited) are both far
extended inside the solid - a consequence of quantum mechanics.
Electrons and holes penetrate the material like a liquid". However,
when the mass of a hole exceeds a certain critical value - 80 times the
mass of an electron - the hole liquid undergoes a spontaneous change to
become a crystal. Furthermore, there are strong indications that in
semiconductor systems of this kind, a reduction of pressure can result
in the formation of Bose condensates of bound electron-hole pairs
(so-called excitons). Anticipating the next stage of the research, the
physicist explains that "the next exciting problem is to set up an
experiment that will confirm our prediction of the crystal of holes".
Suitable materials systems for this have already been suggested.
The scientist at the Institute of Theoretical Physics and Astrophysics
explains that the crystal of holes is also of great interest for
another reason: "We have been able to show that it has many features in
common with very different kinds of crystals, such as plasma crystals
or ionic crystals". It is especially intriguing that the crystal of
holes shows many similarities with some of the most mysterious objects
in the universe - white dwarfs and neutron stars. It has been suggested
that a crystal of nuclei exists in these exotic, very distant objects.
Bonitz hopes that "it may soon be possible to study important
properties of these systems in the laboratory by experiments on a
crystal of holes".
This extraordinary kind of crystal may also be of interest for
materials research, says Bonitz, "because it may possess properties of
importance for superconductivity". Whereas superconductivity (a flow of
electrical current with no resistance) can only be achieved at present
at very low temperatures, some scientists, in particular the 2003
Physics Nobel Prize winner Alexei Abrikosov, expect that systems
containing a crystal of holes should become superconducting at
significantly higher temperatures. This presents the Kiel scientists
and their partners with a challenge, and Bonitz comments that "an
important aim of our continuing investigations will be to test these
predictions".
In his research, Professor Michael Bonitz is collaborating with a
German-Russian team of scientists, which includes Professor Holger
Fehske (Greifswald University) and Dr. Vladimir S. Filinov (Institute
for High Energy Density, Moscow). The project forms a part of the
Transregional Collaborative Research Centre TR 24, on "Fundamentals of
Complex Plasmas", which was recently approved by the Deutsche
Forschungsgemeinschaft (German Research Foundation), and is based at
the universities of Greifswald and Kiel.
The research results haven been selected by the American Physics
Society for coverage in its online journal Physical Review Focus, see
http://focus.aps.org/story/v16/st17
Contact:
Christian-Albrechts-Universität zu Kiel
Professor Michael Bonitz
Tel. +49 (0)431/880-4122, -4117
bonitz@physik.uni-kiel.de
www.theo-physik.uni-kiel.de/~bonitz
Christian-Albrechts-Universität zu Kiel
Press and Communication Services, Leiterin: Susanne Schuck
Postanschrift: D-24098 Kiel, phone: +49 (0431) 880-2104, fax: +49 (0431) 880-1355
e-mail: presse@uv.uni-kiel.de
Christian-Albrechts-Universität zu Kiel