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Topological Insulators

Recently, a new class of topological insulators has been proposed in materials with strong spin-orbit coupling. These topological insulators are characterized by a bulk insulating gap, but have topologically protected edge states due to time-reversal symmetry. In two dimensional insulators (2D HgTe/CdTe quantum wells), band inversion and strong spin-orbit interaction give rise to the quantum spin Hall (QSH) phase wherein gapless modes with opposite spins counter-propagate at a given edge. This phase is analogous to the Quantum Hall Effect, but it appears without any time-breaking magnetic field. Some basic signatures of the QSH effect have been observed experimentally in HgTe quantum wells. In three dimensions, the surface states of topological insulators  are two dimensional conducting systems consisting in an odd number of Dirac cones. The existence of such surface states in candidate materials (BiSb, Bi2Te3 and Bi2Se3) has been confirmed by recent photo-emission experiments.