Manipulation of antiferrodistortive order in SrTiO3 via nonlinear phononics revealed by ultrafast X-ray diffraction

Ultrafast light illumination usually suppresses the low-temperature phases via increasing the kinetic energy of electrons. Coherent phonons generated by resonant infrared absorption have been shown as an alternative way to stabilize metastable phases inaccessible through external control such as strain, chemical doping, and temperature in equilibrium. SrTiO3 is a frustrated system where polar order and antiferrodistortive (AFD) rotations compete. Here we pump the SrTiO3 below the AFD transition temperature with intense 22THz mid-infrared pulses to resonantly drive IR active phonon modes and probe with diffraction from time-delayed femtosecond X-ray pulse from the LCLS x-ray free electron laser. At 101K, the Bragg peaks associated with the oxygen rotations of the AFD is enhanced within the first picosecond after the arrival of the pump pulse, followed by subsequent suppression at later times. This suggests the coherent excitation of phonons transiently enhances the long-range AFD orders. Fluence-dependent results at 75K suggest the transient enhancement shows quadratic dependence with the pump fluence while later-on intensity suppression follows a linear fluence dependence. Further temperature dependence data shows large suppression in the pump-probe signal as temperature decreases, indicating potential fluctuation contribution to the dynamical signal. Our results demonstrate the manipulation of AFD orders in SrTiO3 via resonant phonon excitation and provide new insights into the anharmonic response in quantum materials.