Density distribution of the quantum particles (excitons) in the plane of the quantum well. Yellow color corresponds to high density, red to lower, green to zero. From top left to bottom right the density is increased at constant temperature. Photo: Michael Bonitz, ITAP, CAU Kiel |
Nature
knows two opposite types of solids: one that emerges upon compression
from a liquid and a second that appears if the pressure on a liquid is
reduced. While the former is typical for substances in our everyday life
the latter occurs for example in a dense quantum liquid of electrons
(such as in metals) or ions (in exotic white dwarf or neutron stars).
Now it has been shown that there exists yet a third form of matter that
inherits both of these properties. This unusual behaviour has been
predicted to exist in crystals of excitons—hydrogen atom-like bound
states of electrons and holes—in a semiconductor quantum well placed in a
strong electric field.
A
team from Kiel University in Germany consisting of Dr. Jens Bönning,
Alexei Filinov and Prof. Michael Bonitz has performed extensive accurate
computer simulations that shed light on the mysterious properties of
this material. The results appear in the current issue of Physical Review B.
There
the authors present a simple explanation for the coexistence of the two
seemingly contradicting melting behaviors. The secret lies in the
character of the forces acting between two excitons: at low pressure
excitons repel each other via a dipole force and form a quantum liquid.
Upon compression this fluid freezes into an exciton crystal. Further
compression brings two excitons so close together that the quantum wave
nature of their constituents (electrons and holes) starts to weaken the
forces. As a consequence, further compression leads to an increasing
overlap of the exciton quantum waves that is no longer balanced by the
inter-exciton repulsion, and the crystal melts again.
The
researchers have made precise predictions where to search for this
exotic crystal of excitons (particularly well suited are zinc selenide
or gallium arsenide quantum wells)—it is now up to the experimentalists
to find this new state of matter.
