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

TL;DR

This study employs femtosecond optical spectroscopy to investigate ultrafast spin and orbital order dynamics in the antiferromagnetic insulator $ ext{Sr}_2 ext{CrO}_4$, revealing temperature-dependent phase transition signatures and distinct relaxation behaviors.

Contribution

It demonstrates selective excitation of spin and orbital transitions and uncovers their different ultrafast relaxation timescales in $ ext{Sr}_2 ext{CrO}_4$, providing new insights into their interplay.

Findings

Temperature-dependent anomalies across magnetic and orbital transition temperatures.

Distinct relaxation timescales for spin and orbital orders.

Experimental evidence for phase transition related to orbital ordering.

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${\textrm{\'e}}$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 $T_\textrm{O}$, possibly related to antiferro-type orbital ordering.