Cs2CoBr4

A "lost" quantum magnet from Cs2BX4 generic family, Cs2CoBr4, was re-discovered in our group. Like its relatives, the primary exchange patterns are on the so-called distorted triangular lattice. Yet, this particular material has a twist: cobalt ions have S=3/2 that are subject to extreme easy plane ionic anisotropy, such that only the pseudospin-1/2 degrees of freedom are active at low temperature. The orientation of the easy plane is alternating between the sites and thus the effective S=1/2 Hamiltonian is full of strongly anisotropic direction-dependent interactions. This leads to the very complicated quantum phase diagram in this material, featuring several magnetization plateaux and purely quantum exotic phases.

Particularly, we observed for the first time a longitudinal spin-density wave in the triangular-lattice model. We comprehensively mapped its ground state and excitations. We discovered that its incommensurability is field-dependent, and how it is related to bulk magnetization. At higher field, the spin-density wave ordering vector locks at 1/3 value, and the magnetic structure is replaced by an up-up-down 1/3 magnetization plateau state.