Orbital ordering and ultrafast carrier dynamics anisotropies in orientation-engineered orthorhombic YMnO₃ films

The rich physical properties unveiled in a plethora of transition and rare-earth metal oxides have been attributed to the intricate interplays between the orbital, charge, and spin degrees of freedom. Among them, rare-earth manganites (RMnO₃) have been attracting tremendous attention owing to the ionic size-induced lattice distortion dictated by the Goldschmidt tolerance factor and the substantial Jahn-Teller distortion unique to Mn³⁺ ions, which evidently have resulted in a variety of emergent characteristics in electronic, magnetic, and orbital ordering. In this work, we deliberately engineered the orientation of a series of orthorhombic YMnO₃ (o-YMO) films grown on SrTiO₃(100) [STO(100)] and SrTiO₃(110) [STO(110)] substrates by means of pulsed laser deposition. The x-ray diffraction (XRD) and reciprocal space mapping revealed that o-YMO/STO(100) is c-axis-oriented and o-YMO/STO(110) is a-axis-oriented, respectively. The XRD ϕ-scans further indicate that both films have excellent in-plane crystallinity, allowing the exploration of anisotropies along the respective crystallographic orientations. Indeed, the x-ray absorption linear dichroism spectroscopy taken along the respective crystallographic orientations evidently exhibited substantial anisotropy. Theoretical fitting with configuration interaction cluster calculations suggests that the d 3 z 2 − r 2 orbitals are parallel to YMO[001]/(100), leading to stronger electron scattering along the c-axis. Independent polarization-dependent Δ R/R spectra obtained using the femtosecond pump-probe method exhibited substantial anisotropic behaviors in carrier relaxation dynamics when probing along different crystallographic orientations, presumably due to orbital ordering anisotropies.