Abstract

In a heavy fermion metal, Fermi liquid theory is thought to hold, with strongly renormalized effective masses for the quasiparticles. The interactions which renormalize the mass also renormalize the scattering rate, however, so that these effects remain unseen to dc conductivity measurements:σ_dc= ne²τ^*/m^* = ne²τ/m. Independent values of m^* and τ^* from conductivity measurements must rely on the frequency dependence, σ(ω) = ne² m^*(1/τ^* – iω)).^1,2 Some compounds, notably the cuprate superconductors, cannot be understood in this simple way, because their low frequency conductivity does not follow a Drude form, and it has been proposed that such deviations are indicative of a more general and fundamental violation of Fermi liquid theory (FLT) itself.³ While cuprate superconductors represent the most visible challenge to FLT, the superconducting state frequently interferes with our attempts to understand the normal state at low temperatures, where FLT has the most validity.

© 1999 Optical Society of America