APS/123-QED arXiv:1001.1574v1 [cond-mat.mes-hall] 11 Jan 2010 Direct observation of spin-polarized surface states in the parent compound of a topological insulator using spin-resolved-ARPES spectroscopy in a Mott-polarimetry mode D. Hsieh, 1 L. Wray, 1 D. Qian, 1 Y. Xia, 1 Y.S. Hor, 2 R.J. Cava, 2 and M.Z. Hasan 1, 3 $^{1}$Joseph Henry Laboratories : Department of Physics, Princeton University, Princeton, NJ 08544, USA $^{2}$Department of Chemistry, Princeton University, Princeton, NJ 08544, USA $^{3}$Princeton Center for Complex Materials, Princeton University, Princeton, NJ 08544, USA (Dated: November 9, 2018) We report high-resolution spin-resolved photoemission spectroscopy (Spin-ARPES) measurements on the parent compound Sb of the recently discovered 3D topological insulator Bi 1 $_{−}$x Sb x [D. Hsieh et al., Nature 452, 970 (2008)]. By modulating the incident photon energy, we are able to map both the bulk and (111) surface band structure, from which we directly demonstrate that the surface bands are spin polarized by the spin-orbit interaction and connect the bulk valence and conduction bands in a topologically non-trivial way. A unique asymmetric Dirac surface state gives rise to a k-splitting of its spin polarized electronic channels. These results complement our previously published works on this materials class and re-confirm our discovery of topological insulator states in the Bi 1 $_{−}$x Sb x series. PACS numbers: Topological insulators are a new phase of quantum matter that are theoretically distinguished from ordinary insulators by a Z$_{2}$ topological number that describes its bulk band structure [1–3]. They are characterized by a bulk electronic excitation gap that is opened by spin- orbit coupling, and unusual metallic states that are local- ized at the boundary of the crystal. The two-dimensional (2D) version, known as the quantum spin Hall insulator [4–6], is commonly understood as two copies of the inte- ger quantum Hall effect [7] where the spin-orbit coupling acts as a magnetic field that points in a spin dependent direction, giving rise to counter propagating spin polar- ized states [8] on the 1D crystal edge. Three-dimensional topological insulators on the other hand have no quantum Hall analogue. Its surface states, which are necessarily spin polarized, realize a novel 2D metal that remains de- localized even in the presence of disorder [2, 3, 9–11]. For these reasons, they have also been proposed as a route to dissipationless spin currents which, unlike current semi- conductor heterostructure based spintronics devices, do not require an externally applied electric field. I α radiation (apart from our previous work on this), which does not allow for separation of bulk and surface states [13]. Moreover the aforementioned study, as well as ARPES experiments on Sb thin films [14], only map the band dispersion near ¯ Γ, missing the band structure near ¯ M that is critical to determining the Z$_{2}$ invariant [12]. In this work, we have performed spin- and angle-resolved photoemission experiments on single crystal Sb(111). Us- ing variable photon energies, we successfully isolate the surface from bulk electronic bands over the entire BZ and map them with spin sensitivity. We show directly that the surface states are gapless and spin split, and that they connect the bulk valence and conduction bands in a topologically non-trivial way. Spin-integrated ARPES measurements were performed with 14 to 30 eV photons on beam line 5-4 at the SSRL and at ALS BL-12 at higher photon energies. Spin re- solved ARPES measurements were performed at the SIS beam line at the SLS using the COPHEE spectrometer [15] with a single 40 kV classical Mott detector and a pho- ton energy of 20 eV. The typical energy and momentum resolution was 15 meV and 1% of the surface BZ respec- tively at beam line 5-4, and 80 meV and 3% of the surface BZ respectively at SIS using a pass energy of 3 eV. High quality single crystals of Sb and Sb$_{0}$$_{.}$$_{9}$Bi$_{0}$$_{.}$$_{1}$ were grown by methods detailed in [12]. Cleaving these samples in situ between 10 K and 55 K at chamber pressures less than 5 × 10 − 11 torr resulted in shiny flat surfaces, characterized by low energy electron diffraction to be clean and well ordered with the same symmetry as the bulk [Fig. 1(a) & (b)]. This is consistent with photoelectron diffraction measurements that show no substantial structural relax- ation of the Sb(111) surface [16]. Band calculation was performed using the full potential linearized augmented plane wave method in film geometry as implemented in the FLEUR program and local density approximation for Recent photoemission [12] and theoretical results [2, 10] suggest that single crystals of insulating Bi$_{1}$$_{−}$$_{x}$Sb$_{x}$ (0 . 07 ≤ x ≤ 0 . 22) alloys realize a 3D topological in- sulator. The non-trivial Z$_{2}$ invariant that character- izes Bi$_{1}$$_{−}$$_{x}$Sb$_{x}$ is inherited from the bulk band struc- ture of pure Sb [2, 10], therefore, although Sb is a bulk semimetal, its non-trivial bulk band topology should be manifest in its surface state spectrum. Such a study requires a separation of the Fermi surface of the sur- face states of Sb from that of its bulk states over the entire surface Brillouin zone (BZ), as well as a di- rect measurement of the spin degeneracy of the sur- face states. To date, angle-resolved photoemission spec- troscopy (ARPES) experiments on low lying states have only been performed on single crystal Sb with fixed He