Exploring new states of matter in semiconductors
Dr Alan Usher
Introduction
The study of new states of matter is of fundamental importance to physicists.
We are all familiar with the three states of matter, solid, liquid and gas,
which everyday elements and compounds form. However there are many other
states, less common but equally fascinating. For instance the liquid-crystal
state of certain compounds combines the properties of solids and liquids, and
forms the basis of electronic displays.
More exotic states can be observed when the constituent particles become
lighter: liquid helium forms a super-fluid state in which all viscosity is
lost; and the electrons in metals and certain ceramic materials undergo a
transition to a superconducting state which conducts electricity without
dissipation. It is important for us to understand these exotic states because
they often provide clues to the underlying quantum mechanical behaviour of the
particles involved. Extraordinary behaviour such as superconductivity also has
the potential for considerable technological exploitation.
It has recently become possible to observe new states of matter in an entirely
new physical system: a two-dimensional (2D) sheet of electrons. The electrons
are embedded within a piece of gallium arsenide, but behave like a virtually
pure system, free from imperfections. This is important in studying the
transitions between various states. impurities tend to smear out such
transitions (consider as an analogy the melting of butter, an impure material,
compared with the melting of ice - the latter transition is much more abrupt
and well-defined).
The prospect of studying new states of matter in 2D electron system is all the
more exciting because electrons, unlike atoms, are fundamental, indivisible
particles, one of the simplest constituents of the universe.
We know something of the condensed states of the 2D electron system already.
On lowering the temperature of the system to within one degree of absolute
zero, and applying a large magnetic field, a quantum-liquid state forms,
analogous to the superconducting state described above. In this state the
charge carriers in the system have fractional charge (compared with the charge
on an electron). At even more extreme conditions of low temperature and high
magnetic field, there is good circumstantial evidence of a further transition,
to a solid state in which the electrons form a triangular lattice.
Magnetisation measurements
One of the very few techniques to probe a thermodynamic equilibrium
property directly, magnetisation measurements provide important new
information about 2D electron systems, and potentially about their
condensed states. In collaboration with co-workers at
Cardiff
University, we have developed a unique milli-Kelvin torsion balance
magnetometer with the extremely high sensitivity necessary to measure
the tiny magnetisations of these sheets of electrons.
Optical studies
Photoluminescence has proved to be a useful method of examining the ground- and
excited states of 2D electrons in the regimes of the quantum-liquid and
electron-solid states. We are applying this technique, and that of
photoluminescence-excitation, to the study of 2D HOLE systems, whose quantum
condensed states are predicted to have an even more intricate and fascinating
phase diagram.
See also Alan Usher's publication list.
Up to
Physics Research at Exeter.
