  Electron transport properties in a triple-quantum-dot ring with three
terminals are theoretically studied. By introducing local Rashba spin-orbit
interaction on an individual quantum dot, we calculate the charge and spin
currents in one lead. We find that a pure spin current appears in the absence
of a magnetic field. The polarization direction of the spin current can be
inverted by altering the bias voltage. In addition, by tuning the magnetic
field strength, the charge and spin currents reach their respective peaks
alternately.
