Higher-Order Topology in Monolayer FeSe

Abstract

Generally, the topological corner state in two-dimensional second-order topological insulator (2D SOTI) is equivalent to the well-known domain wall state, originated from the mass-inversion between two adjacent edges with phase shift of π. In this work, go beyond this conventional physical picture, we report a fractional mass-kink induced 2D SOTI in monolayer FeSe with canted checkerboard antiferromagnetic (AFM) order by analytic model and first-principles calculations. The canted spin associated in-plane Zeeman field can gap out the quantum spin Hall edge state of FeSe, forming a fractional mass-kink with phase shift of π/2 at the rectangular corner, and generating an in-gap topological corner state with fractional charge of e/4. Moreover, the topological corner state is robust to local perturbation, existing in both naturally and non-naturally cleaved corners, regardless of the edge orientation. Our results not only demonstrate a material system to realize the unique 2D AFM SOTI, but also pave a new way to design the higher-order topological states from fractional mass-kink with arbitrary phase shift, which are expected to draw immediate experimental attention.

Topological Phase Transitions and Evolution of Boundary States Induced by Zeeman Fields in Second-Order Topological Insulators

“…Thus far, the experimental studies on higher-order topological insulators and semimetals have also been mainly done in metamaterial systems owing to their great advantages in controllability in design and measurements [58]. In comparison, in spite of the accumulation in quantum material candidates for higher-order topological insulator and semimetals [59][60][61][62][63][64][65][66][67][68][69][70][71][72], related experiments on quantum materials remain relatively rare due to the much higher complexity in material growth and detecting the expected signatures of higher-order topology [73][74][75][76][77].…”

Effective Model for Fractional Topological Corner Modes in Quasicrystals

“…Alternatively, a HOTI can be viewed by gapping the (d−1)-dimensional gapless boundary (surfaces or edges) of a Z 2 TI, but the band degeneracy is locally protected at the (d − 2)dimensional boundary (hinges and corners) by spatial symmetry. For example, in a 2D HOTI, corner modes (CMs) can be viewed as topological Jackiw-Rebbi domain-wall states [25], with opposite Dirac masses between two edges enforced by a mirror symmetry [26][27][28], and a variety of 2D HOTI systems have been proposed by implementing such mirrorinvoked mass-inversion mechanism [16][17][18][19][20][21][22], including interestingly CMs in quasicrystals [29][30][31][32][33]. More generally, a CM of domain-wall state can be protected by rotation symmetry, as derived from edge network theory [34].…”