First direct observation of Spin-textures in Topological Insulators : Spin-resolved ARPES as a probe of topological quantum spin Hall effect and Berry's phase

摘要

A topologically ordered material is characterized by a rare quantumorganization of electrons that evades the conventional spontaneously brokensymmetry based classification of condensed matter. Exotic spin transportphenomena such as the dissipationless quantum spin Hall effect have beenspeculated to originate from a novel topological order whose identificationrequires a spin sensitive measurement, which does not exist to this date in anysystem (neither in Hg(Cd)Te quantum wells nor in the topological insulatorBiSb). Using Mott polarimetry, we probe the spin degrees of freedom of thesequantum spin Hall states and demonstrate that topological quantum numbers areuniquely determined from spin texture imaging measurements. Applying thismethod to the Bi{1-x}Sb{x} series, we identify the origin of its novel orderand unusual chiral properties. These results taken together constitute thefirst observation of surface electrons collectively carrying a geometricalquantum (Berry's) phase and definite chirality (mirror Chern number, n_M =-1),which are the key electronic properties for realizing topological computingbits with intrinsic spin Hall-like topological phenomena. Our spin-resolvedresults not only provides the first clear proof of a topological insulatingstate in nature but also demonstrate the utility of spin-resolved ARPEStechnique in measuring the quantum spin Hall phases of matter.