Terahertz Light-Quantum-Tuning of a Metastable Correlated Phase Hidden by Superconductivity*

“Sudden” quantum quench and prethermalization have emerged as a cross-cutting theme
for discovering emergent states of matter, such as quantum criticality and generalized Gibbs
ensembles of cold atoms, quark-gluon plasmas, or phase transitions in the early universe.
Yet this remains challenging in electron matter, especially superconductors (SC), despite
recent progress. The grand open question of what is hidden underneath the “SC dome”
occurring in several quantum materials appears to be universal, yet the new, even thermodynamically
forbidden states have been unexplored by quantum quench of the SC. Here we reveal a hidden quantum phase of prethermalized, gapless electron fluid, which evolves
following single-cycle, resonant terahertz quench of the SC gap above a critical fluence.
Its “non-Fermi-liquid” conductivity is characterized by a sharp coherent peak and a vanishing
scattering rate, most pronounced around full depletion of the SC condensate, which
is absent for high frequency pump and decreases almost linearly at low frequency. Above
threshold, such quantum behaviors with memory persist as an unusual prethermalization
plateau, without relaxation to normal metallic or SC states for order of magnitude longer
than the quasi-particle energy relaxation times. Switching to such a metastable quantum
fluid signals a dynamic coexistence of SC and hidden electronic orders, and
implies novel organization principles beneath superconductivity.