On the Multi-_q Characteristics of Magnetic Ground States of Honeycomb Cobalt Oxides

The Kitaev honeycomb model has received significant attention due to its exactly solvable quantum spin liquid ground states and fractionalized excitations. Layered cobalt oxides have been considered as a promising platform for realizing this model. However, in contrast to the conventional wisdom regarding the single-\bm q zigzag magnetic order inferred from previous studies of the candidate materials Na_2IrO_3 and \alpha-RuCl_3, recent experiments on two representative honeycomb cobalt oxides, hexagonal Na_2Co_2TeO_6 and monoclinic Na_3Co_2SbO_6, have uncovered evidence for more complex multi-\bm q zigzag order variants. This review surveys the experimental strategies used to distinguish between single- and multi-\bm q orders, along with the crystallographic symmetries of cobalt oxides, in comparison with previously studied systems. The general formation mechanism of multi-\bm q order is also briefly discussed. The goal is to provide a solid ground for examining the relevance of multi-\bm q order in honeycomb cobalt oxides and discuss its implications for the microscopic model of these intriguing quantum magnets.