Robust Second-Order Topological Insulators with Giant Valley Polarization in Two-Dimensional Honeycomb Ferromagnets

Abstract

Magnetic topological states have attracted great attention that provide exciting platforms for exploring prominent physical phenomena and applications of topological spintronics.Here, using a tight-binding model and first-principles calculations, we put forward that, in contrast to previously reported magnetic second-order topological insulators (SOTIs), robust SOTIs can emerge in two-dimensional ferromagnets regardless of magnetization directions. Remarkably, we identify intrinsic ferromagnetic 2H-RuCl 2 and Janus VSSe monolayers as experimentally feasible candidates of predicted robust SOTIs with the emergence of nontrivial corner states along different magnetization directions. Moreover, under out-of-plane magnetization, we unexpectedly point out that the valley polarization of SOTIs can be huge and much larger than that of the known ferrovalley materials, opening up a technological avenue to bridge the valleytronics and higher-order topology with high possibility of innovative applications in topological spintronics and valleytronics.

Ferrovalley and topological phase transition behavior in monolayer Ru(OH)2

“…3(c), the valley polarization emerges at the K point of the VBM and reverses to 201 meV. This is close to the values of monolayer RuCl 2 52 and monolayer 2H-MBr 2 (M = Ru and O S ). 53 Moreover, Fig.…”

A mixed Weyl semimetal in a two-dimensional ferromagnetic BaCrSe2 monolayer

“…24,25 On the other hand, while 2D ferromagnets are a focus of intensive research, 26,27 the combining of 2D magnetism and nontrivial topology holds great opportunities for exploring fundamental quantum physics and innovative design of topotronic devices. [28][29][30][31][32] Remarkably, including the magnetization direction y into the topological analysis allows for a natural classification of the topologically nontrivial semimetallic states, i.e., the emergence of mixed WSMs (MWSMs), as a result of the interplay between spin-orbit and exchange interactions. 33,34 Indeed, the MWSMs in 2D ferromagnets have been observed experimentally.…”

Tunable valley polarization effect and second-order topological state in monolayer FeClSH

“…4 Experimentally, valley splitting has been observed by magnetic field in some 2D materials, such as WSe 2 , VS 2 and MoS 2 . [5][6][7] Recently, some 2H phase Fe-group compounds were found to generate a large spontaneous VP, such as FeX 2 (X = Cl, Br, I), [8][9][10][11][12] FeClF, 13 FeClBr, 14 RuBr 2 , [15][16][17] RuCl 2 , 18 RuClBr, 19 Ru(OH) 2 , 20 OsBr 2 , 21 and OsClBr. 22 Here, the VBM and CBM are both located at the K/K 0 point, where the VP can be observed in the valence band (VB), different from other 2D FV materials.…”