Pressure-induced decoupling of the pseudogap and superconductivity in a bilayer cuprate
The physical origin of the pseudogap in cuprate superconductors, whether a precursor to superconductivity or a distinct intertwined order, remains an open question. Although hydrostatic pressure provides a clean tuning parameter, probing the relevant energy scales under compression has remained experimentally elusive. Here, we employ ultrafast optical spectroscopy to track the non-equilibrium quasiparticle dynamics of slightly underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ up to 37 GPa. By disentangling the pseudogap and superconducting relaxation channels in the time domain, we uncover a contrasting pressure evolution: despite an increase in carrier density, the pseudogap thermal onset (T$^*$) rises monotonically, whereas its energy gap ($Δ_{PG}$) is continuously suppressed. Conversely, the critical temperature (T$_C$) and the superconducting gap ($Δ_{SC}$) exhibit a synchronized dome-like trajectory. The effective coupling ratio 2$Δ_{SC}$/k$_B$T$_C$ collapses abruptly from ~10 toward the weak-coupling d-wave limit, signifying a crossover from a phase-fluctuating two-dimensional regime to a globally coherent three-dimensional state. Upon compression to 37 GPa, the superconducting condensate is completely quenched into an insulating-like state, yet the pseudogap persists. The extended phase diagram reveals an unambiguous decoupling between the pseudogap and superconductivity, imposing stringent constraints on microscopic theories of high-temperature superconductivity.