NEW MODEL FOR THE INPLANE PERMITTIVITY OF YBA2CU3O7 FILMS IN THE MID IR RANGE

The normal-state permittivity tensor of [100] and [001] oriented YBa2Cu3O7 films deposited on SrTiO3 substrates has been studied by means of infrared ellipsometry and effective medium theory. Drude fits to the in-plane permittivity epsilon(ab) in the interval 600 cm-1 - 4000 cm-1 were found to be unsatisfactory despite allowance for arbitrary energy dependent scattering rates GAMMA. The Im(epsilon(ab)) component goes as omega(-p) with p almost-equal-to 1.3. This behaviour and also the profile of Re(epsilon(ab)) have been interpreted as transitions to power-law distributed set of states in the interval [omega0,omega(m)]. The lower edge omega0 is in the far IR range while omega(m) is at near IR energies. The scattering rate is found to be constant and one magnitude lower (GAMMA less-than-or-equal-to 100 cm-1) than values obtained by Kramers-Kronig methods.