Probing the pathway of an ultrafast structural phase transition to illuminate the transition mechanism in Cu₂S

Disentangling the primary order parameter from secondary order parameters in phase transitions is critical to the interpretation of transition mechanisms in strongly correlated systems and quantum materials. Here, we present a study of structural phase transition pathways in superionic Cu₂S nanocrystals that exhibit intriguing properties. Utilizing ultrafast electron diffraction techniques sensitive to both the momentum-space and the time-domain, we distinguish the dynamics of crystal symmetry breaking and lattice expansion in this system. We are able to follow the transient states along the transition pathway, and so observe the dynamics of both the primary and secondary order parameters. Based on these observations, we argue that the mechanism of structural phase transition in Cu₂S is dominated by the electron-phonon coupling. This mechanism advances the understanding from previous results, where the focus was solely on dynamic observations of the lattice expansion.