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Spin-based electronics promises a radical alternative to charge-based electronics, namely the possibility of logic operations with much lower power consumption than equivalent charge-based logic operations. In this paper we review three potential means of dissipationless spin transport in semiconductors with and without spin-orbit coupling: the use of spin currents, propagating modes, and orbital currents. Spin and orbital currents induced by electric fields obey a fundamentally different law than charge transport, which is dissipative. Dissipationless spin currents occur in materials with strong spin-orbit coupling, such as GaAs, while orbital currents occur in materials with weak spin-orbit coupling, such as Si, but with degenerate bands characterized by an atomic orbital index. Spin currents have recently been observed experimentally. Propagating modes are the coupled spin-charge movement that occurs in semiconductors with spin-orbit coupling. In contrast to normal charge transport, which is diffusive, the spin-charge mode can exhibit propagating transport, with low energy loss over relatively large distances (>100 µm), by funneling energy between the spin and the charge component through the spin-orbit coupling channel. This opens the possibility for spin-based transport without either spin injection or spin detection. The schemes discussed in this paper are analyzed in comparison with schemes based on molecular electronics phenomena, dilute magnetic semiconductors, etc.
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