Ultrafast Photoinduced Structural Phase Transition in Strained Ca₂RuO₄

Ultrafast time-resolved x-ray diffraction at recently developed X-ray free-electron lasers enables investigating the dynamics of photoinduced structural phase transitions. Here, we have studied the metal-insulator transition (MIT) of a strained Ca₂RuO₄ thin film. We excited the strained film with optical pulses and interrogated the structural dynamics with ultrafast time-resolved x-ray diffraction at Japanese X-FEL SACLA. Upon photoexcitation of the predominantly low-temperature phase, we observed a one picosecond rise in normalized scattering intensity corresponding to the high-temperature phase. Additionally, the Bragg peak associated with the high-temperature phase shifts in the reciprocal space, suggesting a unit cell volume expansion within one ps. Our results reveal a photoinduced volume modifying phase transition with a transition time rivaling much longer timescales observed in bulk systems. The ultrafast nature of phase transition suggests that the structural transition is non-thermal, potentially enabling the tuning of quantum material properties in ways not possible by other methods.