High Frequency Behavior of the Infrared Conductivity of Cuprates
  We analyze recent infrared conductivity data in the normal state of the cuprates.   We find that the high frequency behavior, which has been suggested as evidence for quantum critical scaling, is generally characteristic of electrons interacting with a broad spectrum of bosons.   From explicit calculations, we find a frequency exponent for the modulus of the conductivity, and a phase angle, in good agreement with experiment. The data indicate an upper cut-off of the boson spectrum of order 300 meV. This implies that the bosons are of electronic origin rather than phonons. 
M. R. Norman, A. V. Chubukov