Created Time：2022-12-13
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A new research
conducted by our group reported a topological conducting edge state in
antiferromagnetic topological insulator (AFM-TI) MnBi_{2}Te_{4}.
This was a collaboration with Prof. Yihua Wang from Fudan University, Prof.
Jinsong Zhang&Yayu Wang’s group from Tsinghua University and Prof. Chui-zhen
Chen from Soochow University. The results were published online in *Nature Communications* on December 13,
2022, entitled with “Direct visualization of edge state in even-layer MnBi_{2}Te_{4} at zero magnetic field”.

The study of
topological quantum state of matter has become an important branch of condensed
matter physics research, in which the interplay between topology and magnetism
can generate a variety of new topological phases best exemplified by quantum
anomalous Hall effect (QAHE). QAHE was first realized in magnetically doped TI
film Cr-(Bi,Sb)_{2}Te_{3}.
However, its observation temperature was restricted to below 100 mK because of
the magnetic doping induced disorders. MnBi_{2}Te_{4} is
considered to be a better candidate to realize high-temperature QAHE because
the magnetic disorder will be greatly suppressed in such an intrinsic AFM-TI
compound. The building block of MnBi_{2}Te_{4} is Te-Bi-Te-Mn-Te-Bi-Te septuple layer (SL). Within
each SL, the magnetic moments of Mn are aligned ferromagnetically along the
out-of-plane direction. They couple to each other antiferromagnetically between
adjacent SLs forming an A-type AFM (Fig. 1a). Using stope tape method, one can
easily obtain MnBi_{2}Te_{4} thin flakes with a few
atomic-layer thickness due to its van der Waals bonding character. Theory
predicts QAHE in odd-SL MnBi_{2}Te_{4} thin flakes due to its
finite net magnetization, which was soon confirmed by a transport experiment.
Another topological phase termed “axion insulator” is also predicted in even-SL
MnBi_{2}Te_{4} thin flakes considering that its layer
magnetization is perfectly cancelled out. A zero Hall plateau (ZHP) was
observed in a transport experiment of 6-SL MnBi_{2}Te_{4}, corroborating
such an axion insulator picture. An applied external magnetic field drives an
AFM to FM magnetic phase transition in 6-SL MnBi_{2}Te_{4}. At
the same time, the system undergoes a topological phase transition from an
axion insulator at zero field to a Chern insulator at high fields.

Our group employs
a scanning microwave
impedance microscopy (sMIM) to study 6-SL MnBi_{2}Te_{4}.
sMIM is a recently developed scanning near-field optical microscopy working at
microwave frequencies (~ 3 GHz). By measuring a local microwave reflectance
from the sample surface, it can characterize sample’s local conductivity or
dielectric property with a spatial resolution down to 50 nm (Fig. 1b). We first
use transport to measure 6SL-MnBi_{2}Te_{4} and observe a
quantized Hall resistance at high fields (larger than 6 T) indicating a Chern
insulator phase (Fig. 1c). At low fields, a ZHP is observed suggesting an axion
insulator phase. We then use sMIM to perform a local conductivity imaging. As
shown in Fig. 1d, at 9 T, sMIM imaging reveals an insulating bulk (small sMIM
signal) and a conducting edge (large sMIM signal) confirming that 6-SL MnBi_{2}Te_{4} is in a Chern insulator phase at high fields hosting a topological conducting
edge state. Surprisingly, sMIM imaging reveals another conducting edge state at
0 T. This observation is in a direct contradiction to the axion insulator
picture because an axion insulator requires a complete gap opening of all 2D
surface states in 3D TI. Therefore, the material is insulating from the bulk to
its surface. We perform additional theoretical calculations on 6SL-MnBi_{2}Te_{4} and suggest that, the system is now in a time reversal symmetry (TRS) broken
quantum spin Hall state (QSH). Such TRS broken QSH phase hosts a pair of
helical topological edge state with a tiny gap due to the TRS breaking.
However, such gapped helical edge state will become gapless due to disorders
offering an explanation to our experimental observations.

The interest on the
axion insulator state is largely due to its topological magnetoelectric effect
(TME). Previous experiment has reported such an axion insulator state in
“FM/TI/FM” heterostructure. Even-SL MnBi_{2}Te_{4} is now
believed to be another axion insulator system, gaining supports from the
transport observation of ZHP. However, our work points out that, ZHP is a
necessary but not sufficient condition of an axion insulator. We must be
caution enough in our continuous search for such a topological phase.

Figure. (a) The
MnBi_{2}Te_{4} crystal structure. (b) The sMIM schematic. (c)
The transport experiments of 6-SL MnBi_{2}Te_{4}. (d) The sMIM
imaging of 6-SL MnBi_{2}Te_{4} at 0 T and 9 T. Scale bar is 2 mm.

Paper link: https://doi.org/10.1038/s41467-022-35482-0