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Register a new userObservation of Ultrahigh Mobility Surface States in a Topological Crystalline Insulator by Infrared Spectroscopy
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Topological crystalline insulators (TCIs) possess metallic surface states protected by crystalline symmetry, which are a versatile platform for exploring topological phenomena and potential applications. However, progress in this field has been hindered by the challenge to probe optical and transport properties of the surface states owing to the presence of bulk carriers. Here we report infrared (IR) reflectance measurements of a TCI, (001) oriented $Pb_{1-x}Sn_{x}Se$ in zero and high magnetic fields. We demonstrate that the far-IR conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small IR penetration depth. Moreover, our experiments yield a surface mobility of 40000 $cm^{2}/(Vs)$, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin TCI crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for TCIs.
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The topological crystalline insulator tin telluride is known to host superconductivity when doped with indium (Sn$_{1-x}$In$_{x}$Te), and for low indium contents ($x=0.04$) it is known that the topological surface states are preserved. Here we present the growth, characterization and angle resolved photoemission spectroscopy analysis of samples with much heavier In doping (up to $xapprox0.4$), a regime where the superconducting temperature is increased nearly fourfold. We demonstrate that despite strong p-type doping, Dirac-like surface states persist.
Confining two dimensional Dirac fermions on the surface of topological insulators has remained an outstanding conceptual challenge. Here we show that Dirac fermion confinement is achievable in topological crystalline insulators (TCI), which host multiple surface Dirac cones depending on the surface termination and the symmetries it preserves. This confinement is most dramatically reflected in the flux dependence of these Dirac states in the nanowire geometry, where different facets connect to form a closed surface. Using SnTe as a case study, we show how wires with all four facets of the <100> type display pronounced and unique Aharonov-Bohm oscillations, while nanowires with the four facets of the <110> type such oscillations are absent due to a strong confinement of the Dirac states to each facet separately. Our results place TCI nanowires as versatile platform for confining and manipulating Dirac surface states.
Recently, the topological classification of electronic states has been extended to a new class of matter known as topological crystalline insulators. Similar to topological insulators, topological crystalline insulators also have spin-momentum locked surface states; but they only exist on specific crystal planes that are protected by crystal reflection symmetry. Here, we report an ultra-low temperature scanning tunneling microscopy and spectroscopy study on topological crystalline insulator SnTe nanoplates grown by molecular beam epitaxy. We observed quasiparticle interference patterns on the SnTe (001) surface that can be interpreted in terms of electron scattering from the four Fermi pockets of the topological crystalline insulator surface states in the first surface Brillouin zone. A quantitative analysis of the energy dispersion of the quasiparticle interference intensity shows two high energy features related to the crossing point beyond the Lifshitz transition when the two neighboring low energy surface bands near the point merge. A comparison between the experimental and computed quasiparticle interference patterns reveals possible spin texture of the surface states.
We present angle resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb$_{1-x}$Sn$_{x}$Se, a three dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at $bar{Gamma}$ and $bar{M}$ in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality and alternative surface orientations in (Pb,Sn)Se compounds.
We report on an infrared magneto-spectroscopy study of Pb$_{1-x}$Sn$_x$Se, a topological crystalline insulator. We have examined a set of samples, all in the inverted regime of electronic bands, with the tin composition varying from $x=0.2$ to $0.33$. Our analysis shows that the observed response, composed of a series of interband inter-Landau level excitations, can be interpreted and modelled using the relativistic-like Hamiltonian for three-dimensional massive Dirac electrons, expanded to include diagonal quadratic terms that impose band inversion. In our data, we have not found any clear signature of massless electron states that are present on the surface of Pb$_{1-x}$Sn$_x$Se crystals in the inverted regime. Reasons for this unexpected result are discussed.
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