In recent years theoretical predictions [1-3] as well as experimental confirmations bring on new class of materials called topological insulators (TI). These materials are characterized by inverted electronic bands with an odd number of Dirac cones of metallic surface states with specific spin texture. Both spin-orbit coupling and time reversal symmetry protect spins against local non-magnetic perturbations and guarantee preservation of spin features of surface states. Lately the novel class – topological crystalline insulators (TCI) – was anticipated theoretically [4-5]. The exploration of TCIs has been directed onto SnTe, lead salts and related alloys from IV-VI group of narrow gap semiconductors, which was shortly confirmed experimentally [6-8]. Differently to TIs, in these materials the crucial role plays crystalline mirror symmetry originating from the rock-salt crystal structure.

In contrast to SnTe the ternary compounds provide the ability to manipulate both spin-orbit interaction and band gap. The increasing tin content in Pb_1-xSn_xSe and Pb_1-xSn_xTe leads to closing the bulk band gap at some specific composition x_c. For higher values of x the band structure undergoes inversion, band gap opens and distinct Dirac-like surface states appear [6]. The same effect can be achieved by changing the temperature, i.e. high temperatures correspond to topologically trivial state while at low temperatures TCI state with inverted bands is established [6].

To check spin phenomena in Pb_1-xSn_xSe we used angle-resolved as well as spin-resolved photoelectron spectroscopy at the MAX III synchrotron at MAX-lab, Lund University (Sweden). We studied (001) surface of n-type monocrystalline Pb_0.73Sn_0.27Se grown by self-selected vapour method. From the evolution of the band gap the temperature of transition into closed gap state for this crystal composition was estimated to be approximately 250 K. The formation of metallic surface states revealed by the birth of Dirac-cones was clearly observed at low temperatures TCI state. The spin polarization was investigated in both trivial (at T=300 K) and TCI (at T=80 K) states at 2D Brillouin zone in vicinity of X (the projection of the L(111) points in the bulk crystal onto the (001) surface). The experimental results revealed that both trivial and TCI states posses spin polarization with the chiral spin textures. These findings are consistent with our using tight-binding calculations for high-symmetry directions of (001) surface Brillouin zone.

[1] C.L. Kane and E.J. Mele, Phys. Rev. Lett. 95 (2005) 146802.

[2] L. Fu and C.L. Kane, Phys. Rev. B 76 (2007) 045302.

[3] M. Hasan and C.L. Kane, Rev. Mod. Phys. 82 (2010) 3045.

[4] L. Fu, Phys. Rev. Lett. 106 (2011) 106802.

[5] T.H. Hsieh et al., Nature Communications 3 (2012) 982.

[6] P. Dziawa et al., Nat. Mater. 11 (2012) 1023.

[7] S.-Y. Xu et al., Nat. Commun. 3 (2012) 1192.

[8] Y. Tanaka et al., Nat. Phys. 8 (2012) 800.

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