Ultrafast signatures of spin and orbital orders in the spin-orbit entangled Mott insulator Sr₂CrO₄

We used ultrafast optical spectroscopy to study non-equilibrium spin and orbital order dynamics in Sr₂CrO₄, a 3d antiferromagnetic Mott insulator. Sr₂CrO₄ possesses multiple collective phases exhibiting stripe-type orbital order (T < 50 K), Néel-type spin order (T < 113 K), and antiferro-type orbital order (T < 140 K), though the details of the antiferro-type orbital order are not fully understood. In our experiments, we varied the pump photon energy to selectively drive intersite spin hopping between neighboring Cr t_2g orbitals and charge transfer-type transitions between oxygen 2p and Cr e_g orbitals. This revealed temperature-dependent anomalies in the transient reflectivity at 1.55 eV across the Néel temperature at 113 K as well as 50 K and 140 K, which can be linked to spin and orbital order. Our results reveal the distinct relaxation timescales for spin and orbital orders, respectively, and provide experimental evidence for antiferro-type orbital ordering below 140 K.