Quenched Disorder in the Triangular Lattice Antiferromagnet YbZn${}_2$GaO${}_5$

Abstract

We investigate the crystal electric field (CEF) excitations of Yb${}^{3+}$ ions in powder samples of the triangular-lattice rare-earth-based antiferromagnet YbZn${}_2$GaO${}_5$ using inelastic neutron scattering (INS). Three CEF excitations from the ground-state Kramers doublet were observed, each exhibiting significant broadening beyond instrumental resolution. Combining temperature-dependent INS and neutron powder diffraction, we identify a significant static contribution to this broadening and attribute it to heterogeneous coordination of Yb${}^{3+}$ ions due to Ga${}^{3+}$/Zn${}^{2+}$ site mixing. Rietveld refinement of neutron powder diffraction indicates that 35% of Ga occupies the Zn site and 60% of Zn occupies the Ga site. We show with a point charge model for the CEF Hamiltonian that heterogeneous coordination of Yb${}^{3+}$ ions leads to broadened CEF peaks. First-principles calculations demonstrate that the random Ga${}^{3+}$/Zn${}^{2+}$ distribution can produce the distortions of the YbO${}_6$ octahedra observed from neutron diffraction. Because the documented heterogeneity will extend to exchange interactions, our results suggest that disorder is a significant factor in the unusual magnetism previously reported in YbZn${}_2$GaO${}_5$, including broad low-energy magnetic excitations and the absence of magnetic ordering down to 0.3K.