Transport in Two-dimensional Second-order Topological Insulator Candidates Ta₂M₃Te₅ (M = Pd, Ni)

In contrast to the one-dimensional helical edge states found at the boundaries of two-dimensional topological insulator (2DTI) arising from the nontrivial topology of the insulating bulk band protected by time-reversal symmetry, 2D second-order topological insulator (SOTI) exhibits gapped edge states and degenerate localized corner states, which are pinned in the gap of bulk and edge states in the presence of chiral symmetry. Although some candidates have been proposed previously, 2D SOTI has only been explored in a few systems. Recently, Ta₂M₃Te₅ (M=Pd, Ni) has been proposed as a possible 2D SOTI candidate due to double-band inversion. Ta₂M₃Te₅ (M=Pd, Ni) also features an anisotropic in-plane crystal structure. This anisotropy may result in an anisotropic energy dispersion of phonons and electrons in momentum space. It may offer a foundation for the study of 1D physics. The materials were synthesized by chemical vapor transport and characterized by EDS, XRD, TEM, and Raman. The angle-dependent phonon vibrations have been investigated by ultra-low frequency Raman spectroscopy, which is consistent with the anisotropic crystal structure. The electrical transport properties of nanodevices made from a few atomic layers were studied. The transport results show that Ta₂M₃Te₅ devices are gate tunable and can be further turned off, which indicates that Ta₂M₃Te₅ could be suitable platforms for exploring 2D SOTI and related electronic devices.