Invited Paper MI-MoM5
Probing Spin Textures of Topological Surface States in Ternary Chalcogenides

Monday, October 28, 2013, 9:40 am, Room 202 A

Three-dimensional topological insulators (3D TIs) with a gapless topological surface state (TSS) in a bulk energy gap induced by a strong spin-orbit coupling have attracted much attention as key materials to revolutionize current electronic devices. A spin helical texture of a TSS, where the electron spin is locked to its momentum, is a manifestation of a 3D TI.

A number of well-known thermoelectric and phase-change materials have so far been predicted to be 3D TIs. In order to experimentally confirm their topological natures, spin- and angle- resolved photoemission spectroscopy (SARPES) is one of the most powerful tools and it has actually been playing major roles in finding some real 3D TIs [1, 2]. Among the established 3D TIs, Bi₂Se₃ has been most extensively studied because of its relatively large energy gap and the simplest TSS. However, the topological surface state is energetically obscured by bulk continuum near and below the Dirac point, which is disadvantageous for spintoronic applications.

SARPES experiments were performed at the ESPRESSO end station attached to the APPLE-II type variable polarization undulator beam line (BL-9B) at Hiroshima Synchrotron Radiation Center (HSRC) [3]. The VLEED-type spin detector utilized in the ESPRESSO machine achieves a 100 times higher efficiency compared to that of conventional Mott-type spin detectors [2]. Photoelectron spin polarizations are measured by switching the direction of in-plane target magnetizations, thereby simultaneously eliminating the instrumental asymmetry, which is a great advantage for a quantitative spin analysis of nonmagnetic systems such as 3D TIs.

In this talk, some of the ternary 3D TIs such as TlBiSe₂ [4], GeBi₂Te₄ [5], Bi₂Te₂Se, and Bi₂Se₂Te [6] are shown to possess TSSs with marked spin polarizations. It has been revealed for GeBi₂Te₄ that a disorder in the crystal has a minor effect on the surface-state spin polarization, which is ~70% near the Dirac point in the bulk energy gap region (~180 meV). Highly spin-polarized features are also found for Bi₂Te₂Se and Bi₂Se₂Te, which are persistent across the Dirac point. The availability of both upper and lower TSSs promises to extend the variety of spintoronic application, for instance, to dual gate TI devices and topological p-n junctions.

[1] M. Z. Hasan et al., Rev. Mod. Phys. 82, 3045 (2010).

[2] T. Okuda and A. Kimura, J. Phys. Soc. Jpn. 82, 021002 (2013).

[3] T. Okuda et al., Rev. Sci. Instrum. 82, 103302 (2011).

[4] K. Kuroda et al., submitted.

[5] K. Okamoto et al., Phys. Rev. B 86, 195304 (2012).

[6] K. Miyamoto et al., Phys. Rev. Lett. 109, 166802 (2012).