Nature of the pseudogap in the optical conductivity of oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$

The pseudogap in the c-axis optical conductivity of oxygen-deficient ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ is explained on the basis of the resonant tunneling through the barrier between the ${\mathrm{CuO}}_2$ planes with the use of localized centers formed on place of the CuO chains. The conductivity as a function of frequency has a maximum, and its location and magnitude depend only on the spectrum of resonant levels, and hence do not depend on temperature. In the case of a fully oxygenated substance the chains are complete, and no resonance centers are formed. These features were observed in experiment. Comparison is done with existing data. Other possible mechanisms of the pseudogap formation are briefly discussed. © 1996 The American Physical Society.