Probing the low-temperature limit of the quantum anomalous Hall effect

Abstract

Quantum anomalous Hall effect has been observed in magnetically doped topological insulators. However, full quantization, up until now, is limited within the sub–1 K temperature regime, although the material’s magnetic ordering temperature can go beyond 100 K. Here, we study the temperature limiting factors of the effect in Cr-doped (BiSb)2Te3 systems using both transport and magneto-optical methods. By deliberate control of the thin-film thickness and doping profile, we revealed that the low occurring temperature of quantum anomalous Hall effect in current material system is a combined result of weak ferromagnetism and trivial band involvement. Our findings may provide important insights into the search for high-temperature quantum anomalous Hall insulator and other topologically related phenomena.

Recent Progress in Proximity Coupling of Magnetism to Topological Insulators

“…The QAHE in a MTI was initially predicted in 2010, [ 24 ] and Chang et al., experimentally confirmed QAHE in Cr‐doped (Bi,Sb) 2 (Te) 3 in 2013. [ 37 ] However, the QAHE has so far been realized in uniformly magnetic doped TIs only at sub‐1 K temperatures, [ 39,72,121 ] 1.4 K in stiochiometric MTIs [ 90 ] and up to a few kelvins with magnetic modulation doping of TIs. [ 39 ] As discussed in the Section 1, the limitation of operating temperature for QAHI to date appears to be due to disorder in magnetically doped systems, and difficulty of simultaneously optimizing magnetic exchange coupling and T c as well as topological inverted bandgap in intrinsic TIs.…”

“…All these can result in compromised electrical transport performance, and confine the QAHE to low temperatures and low currents. [ 71,72 ] To date, the highest temperature at which the QAHE is observed in uniformly doped systems is 300 mK, [ 73 ] despite the much higher ferromagnetic T c (≈170 K), [ 58,74 ] and ferromagnetic gap (few 100s of K) [ 69 ] (though modulation doping—a form of heterostructuring—has achieved QAHI at higher temperatures [ 29 ] as discussed below). Because of all these reasons, new advances using this approach appear to have slowed.…”

Spectral signatures of the surface anomalous Hall effect in magnetic axion insulators

“…In even-layer MnBi 2 Te 4 slabs, zero Hall plateau is observed as an indirect evidence of the I -preserved axion insulator phase 23 . However, such zero Hall plateau can also be presented by a trivial case where both surface gaps are dominated by finite-size effect and thus do not contribute any surface anomalous Hall conductivity at all 33 – 35 . Therefore, despite being a fascinating theoretical concept, some key issues about the surface AHC of axion insulators, such as the locality and the device design, still remain elusive.…”

Topological insulators-based magnetic heterostructures

“…Subsequently, this tri-layer sample exhibits a more robust QAH state with a higher observable temperature [43]. Likewise, the measured QAH activation gap (~80 μeV) in a similar tri-layer structure is found to be more than four times larger than the uniform MTI counterpart at zero field, and its magnitude is almost insensitive to external magnetic field [44]. Furthermore, by applying the modulation doping method to design the TI- MTI-TI-MTI-TI penta-layer structure as displayed in Figure 3c, the embedded magnetic doping layer helps to enlarge the size of effective surface gap with improved spatial homogeneity while magnetic disorderinduced surface scattering is reduced [43].…”