Environmental Doping-Induced Degradation of the Quantum Anomalous Hall Insulators

Abstract

The quantum anomalous Hall (QAH) insulator carries dissipation-free chiral edge current and thus provides a unique opportunity to develop energy-efficient transformative information technology. Despite promising advances, the QAH insulator has thus far eluded any practical applications. In addition to its low working temperature, the QAH state in magnetically doped topological insulators usually deteriorates with time in ambient conditions. In this work, we store three QAH devices with similar initial properties in different environments. The QAH device without a protection layer in air shows clear degradation and becomes hole-doped. The QAH device kept in an argon glovebox without a protection layer shows no measurable degradation after 560 h, and the device protected by a 3 nm AlO x protection layer in air shows minimal degradation with stable QAH properties. Our work shows a route to preserve the dissipation-free chiral edge state in QAH devices for potential applications in quantum information technology.

Low-damage photolithography for magnetically doped (Bi,Sb)2Te3 quantum anomalous Hall thin films

“…In addition to the progress in searching for new materials, developing effective sample protection scheme represents another important direction in the study of the QAH effect. [32,33] However, in real experiment, even starting with the optimized materials, it does not guarantee that the fabricated device would retain its electronic properties as the original materials. The protective layer may also react with the chemical reagents during the fabrication process.…”

Enhanced Quantum Anomalous Hall Effect with an Active Capping Layer

“…So far researchers have successfully utilized two capping layers, Te and amorphous AlO x ( a -AlO x ), for the BST system. Between these, a -AlO x is found to be more effective, presumably due to its much larger energy gap than that of Te. − Nonetheless, both Te and a -AlO x are passive capping layers, in the sense that their role is just limited to protection against reaction with air. However, we note the possibility that an amorphous CrO x ( a -CrO x ) can be utilized as an active capping layer that can not only provide protection against air reaction but also boost QAHE of the underlying CBST layer.…”

“…With the thin a -CrO x as the capping layer, the Hall conductance increases by 3 times and the stability is also enhanced by 4 times. The protection offered by a -CrO x capping against air is comparable to or slightly better than that of an AlO x passive capping layer . In order to improve the stability even further, we prepared another sample (sample #6, red circles in Figure A) and applied an additional, 7 nm thick, Se capping layer on top of the a -CrO x capping layer.…”

“…The protection offered by a-CrO x capping against air is comparable to or slightly better than that of an AlO x passive capping layer. 17 In order to improve the stability even further, we prepared another sample (sample #6, red circles in Figure 4A) and applied an additional, 7 nm thick, Se capping layer on top of the a-CrO x capping layer. Then, the degradation is further reduced by 12 times, from 19% to 1.6% over the same duration of 38 days: the inset shows the hysteresis loop of σ xy (B) for this sample, as grown (black) and after 76 days (red).…”

“…It is also notable in Figure 2a (and in Figure S1 for comparison, Supporting Information) that the ferromagnetic transition temperature (T FM ), defined as the point when σ xy starts to rise above zero, is about twice as high in the a-CrO xcapped samples than in the bare sample: 90 K vs 45 K, We will now discuss the details of how we arrived at this. Considering that magnetic proximity is known to provide magnetic coupling in AHE or QAHE systems 8,9,25,26 and that a capping layer is required for long-term stability of the QAHE samples, 17 it would be most ideal if we can find an insulating capping layer that not only protects QAHE but also provides some magnetic proximity effect to boost its ferromagnetism without introducing extra disorder. This suggests that the best capping layer should be a ferromagnetic insulator (FMI) with a perpendicular magnetic anisotropy: a FMI with in-plane anisotropy or antiferromagnetic insulator can even harm QAHE by suppressing the perpendicular magnetic anisotropy of the QAHE platform.…”