Mid-infrared a– b plane response of YBa 2Cu 3O 7− δ as a function of doping and temperature determined by attenuated total reflection

Abstract

The mid-infrared optical response of c-axis thin films of YBa 2Cu 3O 7− δ has been studied using Otto-configuration attenuated total reflectance. The measured reflectance––angle characteristics are dominated by a strong absorption feature due to the excitation of surface plasmons, and can be modeled to determine the a– b plane dielectric function. The results show that while ε i, and therefore σ r, are temperature independent, | ε r| exhibits a moderate decrease with increasing temperature. A stronger dependence on doping is observed: the magnitudes of ε i and | ε r| are both strongly suppressed as δ increases. Critical comparison is made with the extensive body of infrared normal reflectance data. A generalized Drude analysis shows that the plasma frequency is independent of temperature, but decreases with decreasing doping. The scattering rate increases with temperature, and also increases with decreasing doping, consistent with stronger coupling in the underdoped regime. The mass-enhancement is small but increases to 30–40% at δ=0.6. Difficulties in reconciling the results with some current theories of high- T c materials are discussed. Finally, the surface plasmon propagation lengths and penetration depths are shown to vary systematically with doping.