Terahertz Light-Induced Formation of Metastable Quantum State in a Nb₃Sn Superconductor

Exploration and characterization of metastable quantum phases hidden by superconductivity is one of the fundamental issues in high-temperature superconductivity research. We demonstrate the existence of a metastable quantum phase of prethermalized, gapless electron fluid in a Nb₃Sn superconducter, not accessible by equilibrium thermodynamics. The quantum state forms after ultrafast quenching of the superconduncting gap using an intense monocycle terahertz pulse and is absent for near-infrared optical excitation. Above a critical fluence threshold, the formed metastable state exhibits coherent transport with vanishing scattering towards low frequency together with long-lived prethermalization plateau and quantum memory that exceed the quasi-particle energy relaxation times by one order of magnitude. We present theoretical results that attribute the observations to a dynamic coexistence and interference of superconductivity with martensitic order parameter.