  We study quantum disordered ground states of the two dimensional
Heisenberg-Kitaev model on the triangular lattice using a Schwinger boson
approach. Our aim is to identify and characterize potential gapped quantum spin
liquid phases that are stabilized by anisotropic Kitaev interactions. For
antiferromagnetic Heisenberg- and Kitaev couplings and sufficiently small spin
$S$ we find three different symmetric $Z_2$ spin liquid phases, separated by
two continuous quantum phase transitions. Interestingly, the gap of elementary
excitations remains finite throughout the transitions. The first spin liquid
phase corresponds to the well known zero-flux state in the Heisenberg limit,
which is stable with respect to small Kitaev couplings and develops $120^\circ$
order in the semi-classical limit at large $S$. In the opposite Kitaev limit we
find a different spin liquid ground-state, which is a quantum disordered
version of a magnetically ordered state with antiferromagnetic chains, in
accordance with results in the classical limit. Finally, at intermediate
couplings we find a spin liquid state with unconventional spin correlations.
Upon spinon condensation this state develops Bragg peaks at incommensurate
momenta in close analogy to the magnetically ordered Z2 vortex crystal phase,
which has been analyzed in recent theoretical works.

Strongly Correlated Electrons