The surface states of three-dimensional topological insulators exhibit ahelical spin texture with spin locked to momentum. To date, however, the directall-electrical detection of the helical spin texture has remained elusive owingto the lack of necessary spin-sensitive measurements. We here provide a generaltheory for spin polarized transports of helical Dirac electrons throughspin-polarized scanning tunneling microscopy (STM). It is found that differentfrom conventional magnetic materials, the tunneling conductance through the TIsurface acquires an extra component determined by the in-plane spin texture,exclusively associated with spin momentum locking. Importantly, this extraconductance unconventionally depends on the spatial azimuthal angle of themagnetized STM tip, which is never carried out in previous STM theory. Bymagnetically doping to break the symmetry of rotation and time reversal of theTI surface, we find that the measurement of the spatial resolved conductancecan reconstruct the helical structure of spin texture. Furthermore, one canextract the SML angle if the in-plane magnetization is induced purely by thespin-orbit coupling of surface Dirac elections. Our theory offers analternative way, rather than using angle resolved photoemission spectroscopy,to electrical identify the helical spin texture on TI surfaces.
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