Topological Surface States with Persistent High Spin Polarization across the Dirac Point inBi2Te2SeandBi2Se2

Abstract

Helical spin textures with the marked spin polarizations of topological surface states have been firstly unveiled by the state-of-the-art spin-and angle-resolved photoemission spectroscopy for two promising topological insulators Bi2Te2Se and Bi2Se2Te. The highly spin-polarized natures are found to be persistent across the Dirac point in both compounds. This novel finding paves a pathway to extending their utilization of topological surface state for future spintronic applications. Three-dimensional topological insulators (3D TIs) with massless helical Dirac fermions at the surface in a bulk energy gap induced by a strong spin-orbit coupling have attracted a great attention as key materials to revolutionize current electronic devices [1][2][3][4]. A spin helical texture of topological surface state (TSS), where the electron spin is locked to its momentum, is a manifestation of 3D TI, which is sufficiently distinguished from the real-spin degenerate Dirac cone in graphene. This situation promises an effective spin polarized current under an electric field as well as a substantial suppression of backscattering in the presence of non-magnetic impurities. These new states of matter are also expected to provide fertile ground to realize new phenomena in condensed matter physics, such as a magnetic monopole arising from the topological magnetoelectric effect and Majorana fermions hosted by hybrids with superconductors [5,6].A number of 3D TIs, such as Bi 2 Te 3 [7], Bi 2 Se 3 [8-10], and thallium-based compounds [11][12][13][14][15][16], were predicted and experimentally realized. Among the established 3D TIs, the binary tetradymite compounds, Bi 2 Se 3 and Bi 2 Te 3 have been mostly studied because of their rel- * Electronic address: kmiyamoto@hiroshima-u.ac.jp atively large energy gap and the simplest TSS. The spinmomentum locking feature is experimentally proved at least for the upper-lying TSS by spin-and angle-resolved photoemission spectroscopy with widely spread values of raw spin polarizations (20-80 %) [9,[17][18][19][20][21], but clear spin polarizations are obscured near and below the Dirac point (E D ). The scanning tunneling spectroscopy for Bi 2 Se 3 under a perpendicular magnetic field has revealed the Landau levels (LL) with energy distance being proportional to |n|B, where n and B denote the LL index and the magnetic field. It evidently signifies the existence of the surface Dirac cone. However, such characteristic LLs are missing below E D [22,23]. These features obviously tell us that topological nature of the material survives only in the upper part of TSS but is no longer available below E D in Bi 2 Se 3 and Bi 2 Te 3 . The absence of such a topological nature at TSS below E D restricts its variety of spintoronic applications. Futhermore, in spite of significant efforts to realize the surface isolated transport, the progress has thus been hampered by too small surface contribution in the total conductance [24-27] because the uncontrolled bulk carrier doping takes place in Bi 2 Se 3 and Bi 2 Te 3 d...

Electronic and spin structure of the topological insulator Bi2Te2.4Se0.6

“…3(b)] using p-polarized light and a photon energy of 52 eV. One can observe the topological surface state with the Dirac point located at a binding energy of about 0.28-0.33 eV which is comparable to the one obtained for Bi 2 Se 2 Te and Bi 2 Te 2 Se compounds [20]. Similarly, in Ref.…”

“…Some of these compounds possess extraordinary wide fundamental energy gaps and nearly ideal Dirac cones [8,15]. It was also shown that the modification of such features of the electronic structure as the position of the Dirac point relative to the Fermi level and to the bulk valence band edge, the absence of bulk conduction band states at the Fermi level, etc., can be efficiently controlled by means of a variation of the third component concentration [1,[5][6][7]9,20] or by diluted metal atom doping [7].…”

“…The ternary topological compounds Bi 2 Se 2 Te and Bi 2 Te 2 Se were studied by SARPES [20], ARPES with circular light polarization [21], and angle-resolved two-photon photoemission [18] with analysis of their spin texture. In contrast to the binary compounds, Bi 2 Te 2 Se is characterized by a higher insulating gap resistivity [11,22].…”

Shubnikov–de Haas oscillations from topological surface states of metallicBi2Se2.1Te0.9

“…[12][13][14] The controlled removal of excess electrons was achieved by Sn doping and Bi excess in Bi 2 Te 2 Se. 15 Bi 2 Se 2 Te was theoretically predicted as a topological insulator 16 and recent experiments have shown the characteristic topological features in the band structure and in transport measurements 17,18 In these experimental studies, the Fermi energy was located in the bulk band gap, well above the Dirac point, implying negative charge carriers (electrons) confirmed by Hall measurements. In this study, we have observed quantum oscillations from the topological surface states of p-type single crystals of Bi 2 Se 2.1 Te 0.9 despite their bulk metallic temperature dependence.…”

Interband absorption edge in the topological insulators Bi2(Te1−xSex)3

“…Bi 2 (Te 1−x Se x ) 3 compounds, originally employed for thermoelectric devices due to their large Seebeck constant 5 , belong to this category, thanks to the large spin orbit coupling due to heavy bismuth atoms. The linearly dispersing surface states have been observed by angle resolved photoemission (ARPES) 3,[6][7][8][9] . Samples are usually of ntype with the Fermi level located in the conduction band because of the antisite doping related to Se vacancies 10 .…”