Gap and Pseudogap States in YBa₂Cu₃O₇

The origin of pseudogap and its relation to superconducting gap in cuprates are still remained as an outstanding problem in condensed matter physics. In this work, the first-principles calculation of the electronic structure of YBa₂Cu₃O₇ (YBCO) was integrated into the microscopic theory of many-body physics for superconductivity. The temperature dependent Eliashberg gap equations on the real axis for anisotropic superconductors were solved to obtain the temperature dependent gap functions. The obtained T_c for YBCO was about 89 K for µ* = 0.1, which was quite close to that of experimental observations. For T< T_c, there is a large anisotropy of superconducting gap on the Fermi surface of YBCO. Above T_c, such as 105 K, the real part of gap function is not zero at finite frequency, although for frequency near 0, the real part of gap function is zero, which indicated a pseudogap state in the material.