Fragility of the Dirac Cone Splitting in Topological Crystalline Insulator Heterostructures

Abstract

The 'double Dirac cone' 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb1−xSnxSe feature degeneracies located away from time reversal invariant momenta, and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultra-high vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy. We examine bulk Pb1−xSnxSe(001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction that splits the two Dirac cones at eachX is extremely sensitive to atomic-scale details of the surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, possibly reflecting a symmetry confinement mechanism at the buried topological interface.The recent experimental realization of three dimensional topological insulators has triggered an expansive program of research, driven largely by the accessibility and rich physics of the 2D electronic states hosted on their surfaces. 1-5 These states are often said to be protected, referencing two distinct concepts. Firstly, the electrical connection of a band-inverted and normal material is not possible without the existence of interface states that thread the bulk bandgap. Secondly, in many band-inverted materials some kind of symmetry exists that forbids hybridization between these cross-gap interface states, thereby ensuring that they remain metallic. These basic characteristics of topological interface states are dictated solely by considerations of symmetry and bulk bandstructure, and in the sense that these are not easily affected by perturbations at the surface, can be considered robust.However it does not follow that the interface states are immune to surface perturbations. For example, adsorption of non-magnetic impurities on the topological insulator Bi 2 Se 3 surface can dope the surface states. [6][7][8] Similarly, the topological crystalline insulator (TCI) Pb 1−x Sn x Se can be surface doped with Rb or Sn, 9,10 while lattice distortions that disrupt mirror symmetry open a bandgap. 11,12 For both classes of materials, the surface termination is predicted to play an important role in the topological band dispersions. [13][14][15][16] Details of the interface are also important in special cases where two bulk band inversions project to the same point in the surface Brillouin zone (Fig.1a), i.e...

Experimental search for the origin of low-energy modes in topological materials

“…1(b,c) and 2(c) were actually taken for these two surfaces, respectively. We claim that a collective low-temperature phase of the carrier liquid occupying 1D topological step states [19][20][21][22][23] may account for the differential conductance spectra reported here. According to AFM micrographs, a single Ag grain extends over a dozen of steps.…”

“…As demonstrated by BWH [19], the 1D states adjacent to the surface atomic step show much more abundant spectrum than anticipated previously [20][21][22][23]. Within this insight, the effects of magnetic instabilities have been considered and the nature of low-energy excitations proposed [19].…”

“…The above interpretation requires the Fermi level of our p-type samples to reside within the 1D states, whereas tight-binding computations place the 1D band in the gap [19][20][21][22][23]. We note, however, that our experimental method implies the formation of Schottky's metalsemiconductor junction, which leads usually to the Fermi level pinning in the band gap region at the semiconduc- tor surface [50].…”

“…We find such a behavior of differential conductance for topological surfaces of non-magnetic and magnetic Pb1−y−xSnyMnxTe. We examine a possible contribution from superconducting nanoparticles, and show to what extend our data are consistent with Brzezicki's et al theory [arXiv:1812.02168] assigning the observations to a collective state adjacent to atomic steps at topological surfaces.Second, a possible origin of a local collective phase at topological surfaces has recently been proposed by Brzezicki, Wysokiński, and Hyart (BWH) [19], who noted that the electronic structure of one-dimensional (1D) states at atomic steps in TCIs, revealed by scanning tunneling microscopy [20,21], is significantly richer than anticipated previously [20][21][22][23]. According to BWH, these 1D states may show a low-temperature Peierls-like instability leading to the appearance of low-energy excitations associated with topological states at the domains walls of the collective phase.…”

“…Second, a possible origin of a local collective phase at topological surfaces has recently been proposed by Brzezicki, Wysokiński, and Hyart (BWH) [19], who noted that the electronic structure of one-dimensional (1D) states at atomic steps in TCIs, revealed by scanning tunneling microscopy [20,21], is significantly richer than anticipated previously [20][21][22][23]. According to BWH, these 1D states may show a low-temperature Peierls-like instability leading to the appearance of low-energy excitations associated with topological states at the domains walls of the collective phase.…”