  By applying a local Rashba spin-orbit interaction on an individual quantum
dot of a four-terminal four-quantum-dot ring and introducing a finite bias
between the longitudinal terminals, we theoretically investigate the charge and
spin currents in the transverse terminals. It is found that when the quantum
dot levels are separate from the chemical potentials of the transverse
terminals, notable pure spin currents appear in the transverse terminals with
the same amplitude and opposite polarization directions. Besides, the
polarization directions of such pure spin currents can be inverted by altering
structure parameters, i.e., the magnetic flux, the bias voltage, and the values
of quantum dot levels with respect to the chemical potentials of the transverse
terminals.
