Fractionalized topological insulators from frustrated spin models in three dimensions

We present the theory of a three-dimensional fractionalized topological insulator in the form of a $U(1)$ spin liquid with gapped fermionic spinons in the bulk and topologically protected gapless spinon surface states. Starting from a spin-rotation-invariant spin-$1/2$ model on a pyrochlore lattice with frustrated antiferromagnetic and ferromagnetic exchange interactions, we show that decomposition of the latter interactions, within slave-fermion representation of the spins, can naturally give rise to an emergent spin-orbit coupling for the spinons by spontaneously breaking the spin rotation symmetry. The time-reversal symmetry, however, is preserved. This stabilizes a fractionalized topological insulator which also has bulk bond spin-nematic order. We describe the low-energy properties of this state.