Catalytic Electron-Driven Non-Equilibrium Phase Transition in Quantum Electronic Heterostructures.

Thermodynamic phases of matter are defined by temperature, pressure, and particle number while their changes in solids are known to induce phase transitions. It is demonstrated that in heterostructures with an extra degree of freedom-two different electron pathways across the interface-electron flow creates new thermodynamic phases. Topological insulators (TIs) are used with two distinct electronic states: topological surface and confined insulating bulk states. The electrons can flow from a surface state to a bulk band (Path I) across the interface and vice versa (Path II). By manipulating electron density with light pulses and monitoring electron flow with ultrafast terahertz probes, a new thermodynamic phase of TI is identified with interfacial excitons (Phase II: excitonic TI), distinct from the equilibrium TI state (Phase I: TI). The results indicate that ≈3.5 × 1012 electrons (16% of the total) act as catalysts facilitating the transition to Phase II, while 84% (18.5 × 1012 electrons) occupy the new Phase II (excitonic TI). While the study focuses on ultrafast, pathway-selective electron dynamics-beyond the conventional photodoping paradigm-the underlying principle of electron-flow-mediated phase control through interface can be generalized to diverse quantum heterostructures for realizing emergent quantum states and for advancing quantum technologies.