Topological nature of step-edge states on the surface of the topological crystalline insulator Pb0.7Sn0.3Se

Abstract

In addition to novel surface states, topological insulators can also exhibit robust gapless states at crystalline defects.Step edges constitute a class of common defects on the surface of crystals. In this work we establish the topological nature of one-dimensional (1D) bound states localized at step edges of the [001] surface of a topological crystalline insulator (TCI) Pb0.7Sn0.3Se, both theoretically and experimentally. We show that the topological stability of the step edge states arises from an emergent particle-hole symmetry of the surface low-energy physics, and demonstrate the experimental signatures of the particle-hole symmetry breaking. We also reveal the effects of an external magnetic field on the 1D bound states. Our work suggests the possibility of similar topological step edge modes in other topological materials with a rocks-salt structure. PACS numbers:arXiv:1809.10689v3 [cond-mat.str-el]

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.…”

Proximity-induced superconductivity in a topological crystalline insulator

“…For example, a single unit cell thick film of SnTe is shown to exhibit room-temperature ferroelectricity 14 , while a bulk single crystal of (Pb,Sn)Se is reported to host 1D edge modes propagating along step edges 15,17 . Theory predicts that if a TCI undergoes a superconducting transition, a new topological superconducting phase could emerge [18][19][20][21] distinct from that in proximitized Z2 topological insulators Bi2(Se,Te)3 [22][23][24][25][26][27][28] .…”