Experimental evidence for topological surface states wrapping around a bulk SnTe crystal

Abstract

We demonstrate that the metallic topological surface states wrap on all sides the 3D topological crystalline insulator SnTe. This is achieved by studying oscillatory quantum magneto-transport and magnetization at tilted magnetic fields which enables us to observe simultaneous contributions from neighbouring sample sides. Taking into account pinning of the Fermi energy by the SnTe reservoir we successfully describe theoretically the de Haas-van Alphen oscillations of magnetization. The determined π-Berry phase of surface states confirms their Dirac fermion character. We independently observe oscillatory contributions of magneto-transport and magnetization originating from the bulk SnTe reservoir of high hole density. It is concluded that the bulk and surface Landau states exist in parallel. Our main result that the bulk reservoir is surrounded on all sides by the topological surface states has an universal character.Topological insulators (TI) and topological crystalline insulators (TCI) are new phases of quantum matter with topologically protected gapless boundary states. The topological protection is ensured by time reversal symmetry (TI) or specific crystalline symmetry (TCI), respectively. It was predicted theoretically for a nontrivial band ordering of a bulk semiconductor that the twodimensional (2D) topologically protected surface states appear on all surfaces of the bulk material [1,2]. For two opposite surfaces this property was demonstrated experimentally for thin films of strained HgTe [3] and unstrained BiSbTeSe 2 [4]. A separation of the Shubnikovde Haas oscillations originating from the two surfaces was achieved because they had different 2D electron densities [3]. The IV-VI compound SnTe, having the rock-salt crystal symmetry and the nontrivial band ordering at the L points of the Brillouin zone, is known to generate the topological states of the TCI at the (001), (110) and (111) surfaces. This has been first demonstrated in the angle resolved photoemission spectroscopy (ARPES) studies [5] and by scanning tunneling spectroscopy [6,7].In the present work we show that the topological surface states (TSS) appear equally well on neighbouring surfaces thus wrapping bulk SnTe sample. As compared to the strained HgTe, our system has two distinct novel properties. First, we deal with topological crystalline insulators whose topological protection is assured by the proper order and symmetry of energy bands in bulk SnTe, see [5,[8][9][10]. Second, we deal with a large reservoir of holes in the bulk SnTe which pins the Fermi level of the entire system and determines properties of TSS. We study the quantized Hall regime, the Shubnikov-de Haas effect and the de Haas-van Alphen effect. In order to separate contributions of the two investigated surfaces we rotate our sample with respect to the direction of magnetic field. In the studies of TI one usually tries to suppress the effect of the bulk by reducing its volume, carrier density etc. Very high quality of our samples al-lows us to separate contributions of the...

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