Spintronics, propagating mode, (quantum) spin transport, and new electron liquids

The field of spintronics deals with the physics of the electron-spin in semiconductors, metals, insulators and other materials. Among these, the systems which are characterized by strong spin-orbit coupling hold a special place and exhibit a plethora of new phenomena. In classical transport in semiconductors, a propagating spin-charge collective mode appears that is qualitatively different from the diffusive charge transport. The propagating spin-charge mode is the hallmark of spin-orbit coupled systems with a Fermi surface. The Boltzmann transport equations are qualitatively different from the diffusive normal behavior. Three dimensional bulk transport will be analyzed for the first time. In semiconductors without spin-orbit coupling, an orbital Hall effect (similar to the spin-Hall effect) is present in which electrons selectively occupy different orbitals depending on their direction of motion. In quantum transport, a spatially varying spin-orbit coupling is equivalent to a Landau level problem in which electrons of opposite spin feel opposite magnetic fields, thereby exhibiting a quantum Spin-Hall effect. Time-reversal symmetry is unbroken. In these spin Hall insulators, the quantum spin transport takes place through edge states that cross the bulk gap and the Fermi level. The electron liquid on the edges is a helical liquid, in which spin is correlated with chirality, and represents a new class of one-dimensional liquids different from the Luttinger spinless, spinful or chiral liquids. I will also briefly discuss the possibility of three-dimensional quantization in systems with spin-orbit coupling. New experiments are needed and proposed to verify these predictions.