Heisenberg-Kitaev model on the hyperhoneycomb lattice

Abstract

Motivated by recent experiments on $\ensuremath{\beta}$-Li${}_{2}$IrO${}_{3}$, we study the phase diagram of the Heisenberg-Kitaev model on a three-dimensional lattice of tricoordinated Ir${}^{4+}$, dubbed the hyperhoneycomb lattice. The lattice geometry of this material, along with Ir${}^{4+}$ ions carrying ${J}_{\mathrm{eff}}=1/2$ moments, suggests that the Heisenberg-Kitaev model may effectively capture the low-energy spin-physics of the system in the strong-coupling limit. Using a combination of semiclassical analysis, exact solution, and slave-fermion mean-field theory, we find, in addition to the spin liquid, four different magnetically ordered phases depending on the parameter regime. All four magnetic phases---the N\'eel, the polarized ferromagnet, the skew-stripy, and the skew-zig-zag---have collinear spin ordering. The three-dimensional ${Z}_{2}$ spin liquid, which extends over an extended parameter regime around the exactly solvable Kitaev point, has a gapless Majorana mode with a deformed Fermi circle (codimensions, ${d}_{c}=2$). We discuss the effect of the magnetic field and finite temperature on different phases that may be relevant for future experiments.