Phononic Role of Alkaline-earth Metal in High T_c Cuprate Superconductors

Although many years have passed since the discovery of high T_c superconductors, the underlying mechanism is still unknown. The B_1g phonon anomaly in high T_c cuprate has been studied for a long time, but the correlation between the B_1g anomaly and the superconductivity has not yet been revealed clearly. In here, we reproduced successfully B_1g anomaly in YBa₂Cu₃O₇ (YBCO) using ab initio molecular dynamics simulation and temperature-dependent effective potential method. Beside B_1g anomaly, it is found that A_g mode by Ba atoms showed a more severe anomaly than B_1g mode and most out-of-plane modes were suppressed at low temperatures. We also found that Ba atoms are electronically and thermally isolated in YBCO at low temperatures. Ba atoms play an important phononic role in heat phenomena in YBCO. We also reveal that the distance between Ba atom and CuO₂ plane is closely related to the superconducting dome. Our findings could present a new paradigm for the understanding of high T_c superconductivity.