Ultrafast dynamics of the near-antinodal gap of Bi-based cuprates via TR-ARPES: what lies underneath the superconducting dome?

Since the discovery of high-temperature superconductivity in copper oxides (cuprates), one of the foremost challenges has been to fully comprehend the normal state from which macroscopic long-range superconductivity (SC) arises. A critical feature of this normal state is the pseudogap (PG), characterized by a partial suppression of spectral weight at the Fermi level, evident above the superconducting critical temperature (T_C) over a broad range of doping. One of the key unresolved questions include the nature of the low-energy DOS in the low-temperature normal state in the absence of SC. An experimental approach to probe the low-temperature normal state involves the selective quenching of long-range SC order while maintaining temperatures well below T_C. Here, we employ time- and angle-resolved photoemission spectroscopy [RMP 96, 015003 (2024)] with mid-infrared laser light excitation capabilities [Optics Express 32, 29549 (2024)] to quench superconductivity and expose the low-temperature normal state in optimally doped Bi₂Sr₂CaCu₂O_8+δ (Bi2212). Specifically, we show that pair fluctuations are enhanced via mid-infrared pump excitation (300 meV) while maintaining the electronic temperature below Tc. Thus, we uncover the normal state beneath the equilibrium SC phase and establish its relationship to the PG by probing directly the transient spectral function of Bi2212.