  A scheme of spin diode is proposed that uses a step-like quantum wire with
Rashba spin-orbit interaction, connected to two leads with different width. It
is shown that a very large vertical spin-polarized current can be generated
when electrons transmit from the narrow lead to the wide lead, however, it
vanishes or becomes very weak when the transport direction is reversed. This
difference is revealed to arise from the different local density of electron
states of the quantum wire, which is dependent on the direction of bias. The
spin-polarized current in the proposed structure can be generated and
manipulated by purely electric means and with strong a robustness against
disorder, displaying the feasibility of this structure for a real application.
