Thermal Decoupling: The Solution to the Mysteries of High-T_c Cuprate Superconductors - Part 1

Although many years have passed since the discovery of high-critical temperature (high-T_c) superconducting materials, the underlying mechanism is still unknown. The B_1g phonon anomaly in high-T_c cuprate superconductors has been studied for a long time and has been reported until recently, but the correlation between the B_1g phonon anomaly and the superconductivity has not yet been revealed clearly. In the present study, we reproduced successfully the B_1g phonon anomaly in YBa₂Cu₃O₇ (YBCO) using ab initio molecular dynamics (AIMD) simulation and temperature-dependent effective potential (TDEP) method. It is found that the A_g phonon by Ba atoms shows a more severe anomaly than the B_1g phonon at low temperatures. From the analysis of phonon anomaly and the temperature-dependent phonon dispersion, we found that the decoupling between thermal phenomena and electron transport at low temperatures leads to layer-by-layer thermal separation in the YBCO. Ba atoms are electronically and thermally isolated in YBCO and those are responsible for the thermal separation. Considering the thermal decoupling, we reveal that the distance between the Ba atom and the superconducting CuO₂ plane is excellently matched with the superconducting dome.