Simulations Help to Understand the Origin of High-Temperature Superconductivity
Superconductivity, i.e. the phenomenon where a
current is flowing without any resistivity,
is caused by the effective interaction
between two electrons as shown above:
The negatively charged electron deforms the
oppositely charged ionic lattice (left).
The consequence is an effective positive charge
around the electron. A second electron will be
attracted by this positive charge to form the so
called Cooper-pair (middle and right), which gives
rise to the superconductivity.
High-temperature superconductivity (HTSC)
is even more complex:
Adding electrons or holes to copper-oxygen crystals
by doping breaks the antiferromagnetic Neel order
(left in the generic phase diagram of a
high-temperature superconductor below).
This may lead to high-temperature
superconductivity by flipping the magnetic moments of
the crystal ions which can then form a so called
"spin-bag" (this corresponds to the "deformation"
(left) in the top figure).
When "spin-bags" of two electrons
overlap each other, like in top figure, right,
a resulting attractive interaction
and thus superconductivity can occur.
Nevertheless, it is not yet understood how the
coherent motion of about
1023 Cooper- pairs
comes about, which builds up the
supercontucting current.
Therefore, one tries to simulate the dynamics of
larger and larger systems of
electrons with complex computer
algorithms. The corresponding
supercomputer-simulations have shown that the
Hubbard model, a simplified model for HTSC,
exhibits superconductivity.
This gives strong hope that the so-far mostly
empirical search for optimized
(highest transition temperature)
HTSC can soon be
replaced by a systematic guiding principle.
(Werner Hanke,
Institut für Theoretische Physik und Astrophysik,
Universität Würzburg)
Ludger Palm