Pairing states are essential to understanding the underlying mechanisms of high-temperature superconductivity. Here the non-superconducting state in an optimally doped YBa_2 Cu_3 O_(7-{\delta}) film was driven out of equilibrium by an optical pump with low fluence at a temperature well below the critical temperature (T_c), and its recovery dynamics were exclusively measured using transient terahertz spectroscopy. The pump-fluence dependent experiments unveiled evidence of local pairs without superconductivity coexisting with the superconducting Cooper pairs. An energy gap opening induced by the local pairing with short-range coherence was invoked to rationalize the temperature-dependent recovery time of local pairs with a characteristic divergence at a temperature substantially below T_c. These local pairs displayed remarkable likeness to the short-range pair-density-wave state. Our finding sheds light on understanding the dynamic interplay between the coexisting superconducting and non-superconducting pairs in cuprate superconductors.
@article{arxiv.2412.06569,
title = {Observation of Local Pairs through Nonequilibrium Terahertz Spectroscopy in YBa2Cu3O7-{\delta} Below Tc},
author = {Jinzhong Zhang and Qingming Huang and Zhangqiang Yang and Zhiyuan Sun and Ye Yang},
journal= {arXiv preprint arXiv:2412.06569},
year = {2025}
}