Quadrupole topological insulators in Ta2M3Te5 (M = Ni, Pd) monolayers
Zhaopeng Guo, Junze Deng, Yue Xie, Zhijun Wang
Abstract
Abstract Higher-order topological insulators have been introduced in the precursory Benalcazar-Bernevig-Hughes quadrupole model, but no electronic compound has been proposed to be a quadrupole topological insulator (QTI) yet. In this work, we predict that Ta 2 M 3 Te 5 ( M = Pd, Ni) monolayers can be 2D QTIs with second-order topology due to the double-band inversion. A time-reversal-invariant system with two mirror reflections (M x and M y ) can be classified by Stiefel-Whitney numbers ( w 1 , w 2 ) due to the combined symmetry T C 2 z . Using the Wilson loop method, we compute w 1 = 0 and w 2 = 1 for Ta 2 Ni 3 Te 5 , indicating a QTI with q x y = e /2. Thus, gapped edge states and localized corner states are obtained. By analyzing atomic band representations, we demonstrate that its unconventional nature with an essential band representation at an empty site, i.e., A g @ 4 e , is due to the remarkable double-band inversion on Y–Γ. Then, we construct an eight-band quadrupole model with M x and M y successfully for electronic materials. These transition-metal compounds of A 2 M 1,3 X 5 ( A = Ta, Nb; M = Pd, Ni; X = Se, Te) family provide a good platform for realizing the QTI and exploring the interplay between topology and interactions.