Physics 901 Colloquium (Internal to Department)
Metal-to-Insulator Transition in Quantum Site Percolation
Dept. of Physics, Queen's U.
Wed. April 9, 2003 11:30 AM Stirling A|
Superconductivity among spinel systems is very rare: of the 300
or so known spinels, only four of them are superconductors, only
one of these four is an oxide, and that oxide, LiTi_2O_4, has
the highest transition temperature (T_c ~ 13K) of any spinel.
The mechanism of superconductivity in this compound has not yet been
The notion that Lithium Titanate's superconductivity is due to electronic
correlations was first suggested by high-temperature superconductivity
co-discoverer Alex Muller.
As evidence supporting the hypothesis that strong electronic correlations
are important in understanding the low temperature behaviour of this system,
we have investigated the metal-to-insulator transitions of the
cation-substituted Lithium Titanate Li_(1+x)Al_yTi_(2-x-y)O_4.
In a one-electron picture, one would expect that the metal-to-insulator
transition can be modelled very accurately by an electron density
driven transition in a quantum site percolation model.
We have studied these transitions, and find that quantitative
predictions for the critical dopant concentration at which the
metal-to-insulator sets in are in disagreement with those predicted
experimentally; e.g., experimentally for the (x=0) LiAl_yTi_(2-y)O_4
compound, an Al concentration of y_c=0.33 produces a metal-to-insulator
transition, whereas a quantum site percolation model identifies a lower
bound of y_c > 0.8. One proposal that is consistent with this result is
that strong correlations are ignored in a quantum site percolation
(effectively Anderson) model of the transition, and thus y_c is
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