Spin transport and lack of quantisation in the $A\mathrm{II}$ class on the honeycomb structure

Abstract: We investigate spin transport in a class of two-dimensional $A\mathrm{II}$ insulators on the honeycomb structure, the Kane-Mele model being an emblematic example in this class. We derive the spin conductivity by the linear response `a la Kubo and show that it is well-defined and independent of the choice of the spin current. For models that do not conserve the spin, we demonstrate that the deviation of the spin conductivity from the quantised value is, at worst, quadratic in the spin-non-conserving terms, thus improving previous results. Additionally, we show that the leading-order corrections are actually quadratic for some models in the class, demonstrating that the spin conductivity is not universally quantised. Consequently, our results show that, in general, there is no direct connection between the spin conductivity and the Fu-Kane-Mele index.