Order induced spectral weight transfer in correlated matter.

D. van der Marel
University of Groningen, Laboratory of Solid State Physics
Nijenborgh 4, 9747 AG Groningen, the Netherlands.

The optical spectra of a number of solids are known to exhibit
a shift of spectral weight over a large energy scale when
a phase transition occurs. Often this turns out to be an
energy scale far exceeding the characteristic temperature
of the phase transition. A notorious
example of a spectral weight shift over more than 1 eV upon
opening a correlated insulating gap is FeSi. In the cuprates
spectral weight shift upon entering the superconducting state
has been associated with the condensation of Cooper-pairs. In
this talk I will discuss some of these examples,
and relate the spectral weight shift to the change in physical
state. In particular I will discuss if, and to what extent,
correlations are responsible for the spectral weight transfer.