Direct visualization of edge state in even-layer MnBi2Te4 at zero magnetic field

Abstract

Being the first intrinsic antiferromagnetic (AFM) topological insulator, MnBi2Te4 is argued to be a topological axion state in its even-layer form due to the antiparallel magnetization between the top and bottom layers. Here we combine both transport and scanning microwave impedance microscopy (sMIM) to investigate such axion state in atomically thin MnBi2Te4 with even-layer thickness at zero magnetic field. While transport measurements show a zero Hall plateau signaturing the axion state, sMIM uncovers an unexpected edge state raising questions regarding the nature of the “axion state”. Based on our model calculation, we propose that the even-layer MnBi2Te4 at zero field is in an AFM quantum spin Hall (QSH) state hosting a pair of helical edge states. Such novel AFM QSH is originated from the combination of half translation symmetry and time-reversal symmetry in MnBi2Te4. Our finding thus signifies the richness of topological phases in MnB2Te4 that has yet to be fully explored.

Helical Luttinger liquid on the edge of a 2-dimensional topological antiferromagnet

“…Having shown the existence of helical edge states in the AFM ground state of 6-SL MnBi2Te4, we investigate the dimensionality and emergent behavior of the edge excitations. Previous microwave impedance imaging found enhanced conductivity in a micron-wide stripe along the edge of an evenlayer sample [37]. This width from the local impedance is technique-specific and therefore is not a true reflection of the intrinsic width of the edge channel [44].…”

“…1c) or other extrinsic disorder breaking PT symmetry, an axion insulator is predicted [25,26] , [35]. However, if only the top and bottom surfaces are gapped by the out-of-plane magnetization, while the AFM side surfaces preserve the PT symmetry and retain the gapless surface states [26,[36][37][38], a TRS-broken AFM TI phase with odd HLL on its edge may occur. In this work, we show clear evidence of the existence of HLL by imaging the helical edge current in the AFM ground state of a 6-SL sample.…”

“…2h). In the charge neutral region (15 V < 𝑉 " < 35 V), the large 𝑅 $$ at 0 T in contrast to the diminishing 𝑅 $$ at ±9 T suggests the edge channel in the AFM state is characteristically different from the chiral edge state of the Chern insulator [36,37].…”

“…Having shown the existence of helical edge states in the AFM ground state of 6-SL MnBi2Te4, we investigate the dimensionality and emergent behavior of the edge excitations. A wide stripe of enhanced local microwave conductivity on the edge of an even-layer sample [37] is technique-specific and is not a true reflection of the intrinsic width of the edge channel [44]. We resort to a particular transport configuration (Fig.…”

Helical Luttinger Liquid on the Edge of a Two-Dimensional Topological Antiferromagnet

“…For the even layers, on the other hand, two distinctive topological phases with C = 0 are possible. If all the surface states, including those on the sides, are fully gapped by either the magnetization normal to the surface (Figure c) or other extrinsic disorder breaking PT symmetry, an axion insulator is predicted. ,, However, if only the top and bottom surfaces are gapped by the out-of-plane magnetization, while the AFM side surfaces preserve the PT symmetry and retain the gapless surface states, ,− a TRS-broken AFM TI phase with odd HLL on its edge may occur. In this work, we show evidence for HLL by imaging the helical edge current in the AFM ground state of 6-SL samples.…”