Topological crystalline insulators 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 reflectance measurements of a topological crystalline insulator, (001)-oriented Pb1−xSnxSe in zero and high magnetic fields. We demonstrate that the far-infrared conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small infrared penetration depth. Moreover, our experiments yield a surface mobility of 40,000 cm2 V−1 s−1, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin topological crystalline insulator crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for topological crystalline insulators.

We calculate the electronic band structures and topological properties of twisted homobilayer transition metal dichalcogenides(t-TMDs), in particular, bilayer MoTe2 and WSe2 based on a low-energy effective continuum model. We systematically show how the twist angle, vertical electric field and pressure modify the band structures of t-TMDs, often accompanied by topological transitions.We find the variation of topological transitions mainly take place in a limited range of parameters. The electric field can efficiently tune the energy of the topmost second valence band to motify the Chern numbers of the topmost three valance bands. The topological property of the topmost first valance band can be modified by electric field and pressure, but doesn’t depend on twist angle. We show the band gap between the topmost second and third valance bands that both change from non-trivial to trivial closes at κ − -point of the moiré Brillouin zone.

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Polaritons - material excitations coupled with light - are thought to hold the potential for the extreme control of light down to the atomic length-scale because of their high field...

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We theoretically study the interference and propagation of phonon polaritons in hexagonal boron nitride (hBN) in van der Waals heterostructures composed of hBN and twisted bilayer graphene (TBG) with different interlayer spacing in TBG. We show that varying the interlayer spacing and, hence, the interlayer coupling strength results in dramatic modifications of the local optical conductivity at the domain walls (DWs) in TBG, which leads to significant changes in the polariton interference profile near DWs. Moreover, our simulation reveals that the two-dimensional near-field interference pattern generated by polariton propagation in hBN/TBG heterostructures can be dramatically changed by interlayer spacing and the superlattice period. Our study demonstrates that combining interlayer spacing modification with moiré superlattices is a valuable route to control light at the nanoscale and design nanophotonic devices with tunable functionalities.

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