  We show that double-quantum spin vortices, which are characterized by doubly
quantized circulating spin currents and unmagnetized filled cores, can exist in
the ground states of SU(3) spin-orbit coupled Bose gases. It is found that the
SU(3) spin-orbit coupling and spin-exchange interaction play important roles in
determining the ground-state phase diagram. In the case of effective
ferromagnetic spin interaction, the SU(3) spin-orbit coupling induces a
three-fold degeneracy to the magnetized ground state, while in the
antiferromagnetic spin interaction case, the SU(3) spin-orbit coupling breaks
the ordinary phase rule of spinor Bose gases, and allows the spontaneous
emergence of double-quantum spin vortices. This exotic topological defect is in
stark contrast to the singly quantized spin vortices observed in existing
experiments, and can be readily observed by the current magnetization-sensitive
phase-contrast imaging technique.
