Quantitative Analysis of Suppressed Isotope Effect from Thermal Decoupling in High-T_c Cuprate Superconductors - Part I

Forty years have passed since the discovery of high-T_c cuprate superconductors, yet their fundamental mechanism remains unclear. Consequently, despite recent advances in low-temperature physics and the discovery of numerous unconventional superconductors, this mystery continues to deepen. Recently, we accurately reproduced the B_1g phonon anomaly in YBCO using ab initio molecular dynamics simulations and the temperature-dependent effective potential method. In this process, we discovered severe softening of A_g phonons at low temperatures due to Ba atoms. This indicates a significant weakening of bonding around Ba due to ionic bonding, leading to thermal decoupling within YBCO. We found that other alkaline earth metals and alkali metals play a role similar to Ba. Surprisingly, it appears that not only cuprate superconductors but nearly all high-temperature superconductors contain elements that induce ionic bonding within the material. We report that this thermal decoupling quantitatively explains linear-T resistivity, the Uemura relation, and the superconducting dome [1]. The formula, T_c = (A/α)(T₀²/T_F)+(α/4A)T_F obtained in this study not only clarifies the flat band and superconducting phenomena but also explains why the isotope effect diminishes in cuprate superconductors. The isotope effect exponent value calculated for the cuprate from thermal decoupling was found to be 0.022 on average, surprisingly consistent with the experimental value (~ 0.02). Therefore, we propose that thermal separation could be the key to solving the mystery of high-temperature superconductivity.

[1] Materials Today Physics 59, 101916 (2025)