  We consider spin-polarized transport in a quantum spin Hall antidot system
coupled to normal leads. Due to the helical nature of the conducting edge
states, the screening potential at the dot region becomes spin dependent
without external magnetic fields nor ferromagnetic contacts. Therefore, the
electric current due to voltage or temperature differences becomes spin
polarized, its degree of polarization being tuned with the dot level position
or the base temperature. This spin-filter effect arises in the nonlinear
transport regime only and has a purely interaction origin. Likewise, we find a
spin polarization of the heat current which is asymmetric with respect to the
bias direction. Interestingly, our results show that a pure spin current can be
generated by thermoelectric means: when a temperature gradient is applied, the
created thermovoltage (Seebeck effect) induces a spin-polarized current for
vanishingly small charge current. An analogous effect can be observed for the
heat transport: a pure spin heat flows in response to a voltage shift even if
the thermal current is zero.
