Gap and Pseudogap States in YBa₂Cu₃O₇ from First-Principles

For high Tc cuprate superconductors, there exists an energy gap well above Tc, over a wide region of compositions and temperatures, which is called pseudogap. The origin of this pseudogap and its relation to the superconducting gap are still remained as an outstanding problems in physics. In order to shed some light on the problem, we integrated the first-principles calculation of electronic structure of YBa₂Cu₃O₇ (YBCO) into the theory of many-body physics for superconductivity, and further studied gap and pseudogap states from first-principles. We solved the four-dimensional Eliashberg gap equation in strong coupling theory utilizing first-principles electronic structure of YBCO. The results showed that there is a large anisotropy of superconducting gap on the Fermi surface of YBCO for T< Tc. Above Tc, the real part of gap function is not zero at finite frequency on some parts of the Fermi surface, although for frequency near 0, the real part of gap function is zero. The work was funded in part by NSF (Award # HRD 1736136 ) and ARO (Award # W911NF-15-1-0483).