Ultrafast signatures of spin and orbital order in antiferromagnetic $\alpha$-Sr${}_2$CrO${}_4$

Abstract

We used femtosecond optical spectroscopy to study ultrafast spin and orbital ordering dynamics in the antiferromagnetic Mott insulator $\alpha$-Sr${}_2$CrO${}_4$. This chromate system possesses multiple spin and orbital ordered phases, and therefore could enable us to study the unique interplay between these collective phases through their non-equilibrium response to photoexcitation. Here, 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 2$p$ and Cr $e_g$ orbitals. The resulting transient reflectivity dynamics revealed temperature-dependent anomalies across the N$\text{eˊ}$el 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 $\alpha$-Sr${}_2$CrO${}_4$ and provide experimental evidence for the phase transition at $T_{\text{O}}$, possibly related to antiferro-type orbital ordering.

6 pages, 4 figures