Néel and stripe ordering from spin-orbital entanglement in $\hat{I}\pm$-Sr${}_2$CrO${}_4$

Abstract

The rich phenomenology engendered by the coupling between the spin and orbital degrees of freedom has become appreciated as a key feature of many strongly-correlated electron systems. The resulting emergent physics is particularly prominent in a number of materials, from Fe-based unconventional superconductors to transition metal oxides, including manganites and vanadates. Here, we investigate the electronic ground states of $\hat{I}\pm$-Sr${}_2$CrO${}_4$, a compound that is a rare embodiment of the spin-1 Kugel-Khomskii model on the square lattice -- a paradigmatic platform to capture the physics of coupled magnetic and orbital electronic orders. We have used resonant X-ray diffraction at the Cr-$K$ edge to reveal Néel magnetic order at the in-plane wavevector ${\mathbf{Q}}_N=(1/2,1/2)$ below $T_N=112$ K, as well as an additional electronic order at the 'stripe' wavevector ${\mathbf{Q}}_s=(1/2,0)$ below T${}_s$ $\sim 50$ K. These findings are examined within the framework of the Kugel-Khomskii model by a combination of mean-field and Monte-Carlo approaches, which supports the stability of the spin Néel phase with subsequent lower-temperature stripe orbital ordering, revealing a candidate mechanism for the experimentally observed peak at ${\mathbf{Q}}_s$. On the basis of these findings, we propose that $\hat{I}\pm$-Sr${}_2$CrO${}_4$ serves as a new platform in which to investigate multi-orbital physics and its role in the low-temperature phases of Mott insulators.

6 pages, 4 figures