Calculations and analysis of energy gaps in YBa₂Cu₃O₇ and YBa₂Cu₃O₆

The LUMO-HOMO energy gaps in YBa₂Cu₃O₇ have been calculated with density functional method (DFT) on a cluster model of unit cell in a wide range of temperatures (5 K~300 K). The results show that the superconducting energy gap (2Δ(T)) is about 80 meV at T_c (93 K), and decreases smoothly as the temperature increasing. It reaches the minimum at T_c and exhibits pseudogap above T_c. The pseudogap value reaches the maximum at T^*(120 K) when the temperature increasing. The calculations by the cluster model of YBa₂Cu₃O₆ unit cell show that the energy gap is about 10² meV with characteristic gap of semiconductor.