Experimental realization of a topological crystalline insulator in SnTe (1206.5399v1)

Abstract: Topological insulators materialize a topological quantum state of matter where unusual gapless metallic state protected by time-reversal symmetry appears at the edge or surface. Their discovery stimulated the search for new topological states protected by other symmetries, and a recent theory predicted the existence of "topological crystalline insulators" (TCIs) in which the metallic surface states are protected by mirror symmetry of the crystal. However, its experimental verification has not yet been reported. Here we show the first and definitive experimental evidence for the TCI phase in tin telluride (SnTe) which was recently predicted to be a TCI. Our angle-resolved photoemission spectroscopy shows clear signature of a metallic Dirac-cone surface band with its Dirac point slightly away from the edge of the surface Brillouin zone in SnTe. On the other hand, such a gapless surface state is absent in a cousin material lead telluride (PbTe), in line with the theoretical prediction. Our result establishes the presence of a TCI phase, and opens new avenues for exotic topological phenomena.

• The paper experimentally verifies a topological crystalline insulator phase in SnTe using ARPES.
• The study employs high-resolution ARPES to reveal robust Dirac-cone surface states safeguarded by crystal mirror symmetry.
• The paper shows that an intrinsic band inversion in SnTe distinguishes it from PbTe, paving the way for novel quantum material applications.

Experimental Realization of a Topological Crystalline Insulator in SnTe

This paper presents the experimental demonstration of a Topological Crystalline Insulator (TCI) phase in tin telluride (SnTe), leveraging angle-resolved photoemission spectroscopy (ARPES) to establish the presence of a metallic Dirac-cone surface state. Such surface states are protected by the mirror symmetry of the crystal, distinct from the time-reversal symmetry protection characteristic of traditional Topological Insulators (TIs). This distinction broadens the taxonomy of topological phases and suggests the potential for novel quantum phenomena.

The paper makes a substantial contribution to the field by verifying the TCI phase predicted theoretically for narrow-gap IV-VI semiconductors, particularly within the face-centered cubic (f.c.c.) lattice of SnTe. This experimental verification is critical because previously, the theoretical anticipation of TCIs lacked tangible empirical evidence. In SnTe, the TCI phase arises due to an intrinsically inverted band structure, a feature absent in its structural cousin, lead telluride (PbTe), which lacks the TCI phase due to the absence of band inversion.

The ARPES data highlight a distinct Dirac-cone surface state in SnTe, with a Dirac point located slightly away from the X point of the surface Brillouin zone. The surface origin of this Dirac-like band is confirmed by the stationary energy position observed across varying photon energies. The consistency of the band dispersion across different temperatures, as shown in their experiments, further supports the robustness of the TCI surface states against small rhombohedral distortions.

Quantitative outcomes reinforce the paper's findings. For instance, the Dirac band velocity is calculated as 4.5 eVÅ and 3.0 eVÅ for the left- and right-hand-side branches, respectively. Moreover, the experimental setup refined sample quality by minimizing Sn vacancies to reduce excess hole carriers, overcoming earlier challenges faced in revealing the Dirac-like band.

The paper's observations are consistent with a topological phase transition in a Pb$_{1-x}$Sn$_x$Te solid-solution system, deduced from the lack of Dirac-like bands in PbTe under similar measurement conditions. The transition is theoretically ascribed to differing mirror Chern numbers and a predicted band inversion at critical values, providing a basis for future investigations into phase transitions in related materials.

Theoretically, the implications of this work lie in substantiating the concept of TCIs and demonstrating its occurrence in a material that can be experimentally fabricated and investigated. Practically, the realization of TCIs in SnTe opens avenues for leveraging these states in applications where control over electronic states via crystallographic symmetry is advantageous, potentially impacting areas such as spintronics and quantum computing.

In reflecting upon future advancements, the work set forth in this paper beckons further explorations into other potential TCIs, potentially within other crystalline structures or composite materials. Given the promising foundation laid by this research, subsequent studies might explore the manipulation of TCI properties or the engineering of broader categories of topological materials beyond SnTe and its immediate derivatives. Consequently, this paves the way for innovative applications drawing on the unique properties engendered by TCIs, highlighting the ongoing evolution of the quantum material landscape.