Ultrafast signatures of spin and orbital order in antiferromagnetic alpha-Sr2CrO4

The antiferromagnetic Mott insulator alpha-Sr2CrO4 possesses multiple spin and orbital ordered phases, but their unique interplay is still relatively unexplored. Here, we used femtosecond optical spectroscopy to study ultrafast spin and orbital ordering dynamics in alpha-Sr2CrO4 through their non-equilibrium response to photoexcitation. By varying the pump photon energy, we selectively drove inter-site spin hopping between neighboring Cr t(2g) orbitals and charge transfer-type transitions between oxygen 2p and Cr e(g) orbitals. The resulting transient reflectivity dynamics revealed temperature-dependent anomalies across the Neel temperature for spin ordering as well as the transition temperatures linked to different types of orbital order. Our results reveal distinct relaxation timescales for spin and orbital orders in alpha-Sr2CrO4 and provide experimental evidence for the phase transition at T-O, possibly related to antiferro-type orbital ordering.