Strongly anisotropic electronic and magnetic structures in oxide dichlorides RuOCl$_2$ and OsOCl$_2$

Abstract

The van der Waals oxide dichlorides M OX2 (M = V, Ta, Nb, Ru, and Os; X = halogen element), with different electronic densities, are attracting considerable attention. Ferroelectricity, spin-singlet formation, and orbital-selective Peierls phases were reported in this family with d 1 or d 2 electronic configurations, all believed to be caused by the strongly anisotropic electronic orbital degree of freedom. Here, using density functional theory and density matrix renormalization group methods, we investigate the electronic and magnetic properties of RuOCl2 and OsOCl2 with d 4 electronic configurations. Different from a previous study using VOI2 with d 1 configuration, these systems with 4d 4 or 5d 4 do not exhibit a ferroelectric instability along the a-axis. Due to the fully-occupied dxy orbital in RuOCl2 and OsOCl2, the Peierls instability distortion disappears along the b-axis, leading to an undistorted Immm phase (No. 71). Furthermore, we observe strongly anisotropic electronic and magnetic structures along the a-axis. The large crystal-field splitting energy (between d xz/yz and dxy orbitals) and large hopping between nearest-neighbor Ru and Os atoms suppresses the spin-orbital effect in M OCl2 (M = Ru or Os) with electronic density n = 4, resulting in a spin-1 system instead of a J = 0 singlet ground state. Moreover, we find staggered antiferromagnetic order with π wavevector along the M -O chain direction (a-axis) while the magnetic coupling along the b-axis is weak. Based on Wannier functions from first-principles calculations, we calculated the relevant hopping amplitudes and crystal-field splitting energies of the t2g orbitals for the Os atoms to construct a multi-orbital Hubbard model for the M -O chains. Staggered AFM with ↑-↓-↑-↓ spin structure dominates in our DMRG calculations, in agreement with DFT calculations. Our results for RuOCl2 and OsOCl2 provide guidance to experimentalists and theorists working on this interesting family of oxide dichlorides.