Counter-rotating spiral order in three dimensional iridates: signature of hidden symmetry in Kitaev-$\Gamma$ model

Phys. Rev. B 98, 104401 (2018) The unconventional magnetic orderings found in Kitaev spin liquid candidates suggest frustration of spin interactions, and raise a possibility of nearby spin liquid phases. In particular, counter-rotating spiral ordering in three-dimensional (3D) iridates is striking, and understanding the microscopic mechanism of such ordering may provide routes to 3D Kitaev spin liquids. We study a minimal 3D model including Kitaev $K$ and a symmetric off-diagonal bond-dependent $\Gamma$ interaction on the hyperhoneycomb lattice using exact diagonalization. We first show that a 12-site unitary transformation unveils a Heisenberg model with hidden SU(2) symmetry when $K = \Gamma$. The magnetic ordering in the transformed basis then generically maps to the counter-rotating noncoplanar spiral order of the original spin. The moment direction depends on perturbations away from the SU(2) point. When $K$ and $\Gamma$ are negative, a positive Heisenberg interaction favors the (110)-direction reported in the neutron scattering measurement on $\beta$-Li$_2$IrO$_3$. Our findings offer a relevant set of microscopic parameters, which in turn guides a way to approach possible Kitaev spin liquids.