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Abstract
The breaking of time-reversal symmetry (TRS) in topological insulators is a prerequisite for unlocking their exotic properties and for observing the quantum anomalous Hall effect (QAHE). The incorporation of dopants which exhibit magnetic long-range order is the most promising approach for TRS-breaking. REBiTe3, wherein 50% of the Bi is substitutionally replaced by a RE atom (RE = Gd, Dy, and Ho), is a predicted QAHE system. Despite the low solubility of REs in bulk crystals of a few %, highly doped thin films have been demonstrated, which are free of secondary phases and of high crystalline quality. Here we study the effects of exposure to atmosphere of rare earth-doped Bi2(Se, Te)3 thin films using x-ray absorption spectroscopy. We demonstrate that these RE dopants are all trivalent and effectively substitute for Bi3+ in the Bi2(Se, Te)3 matrix. We find an unexpected high degree of sample oxidation for the most highly doped samples, which is not restricted to the surface of the films. In the low-doping limit, the RE-doped films mostly show surface oxidation, which can be prevented by surface passivation, encapsulation, or in-situ cleaving to recover the topological surface state.
Highlights
A recent breakthrough in quantum anomalous Hall effect (QAHE) research has been the realization of perfectly quantized transport in the absence of an applied magnetic field at temperatures that can be reached without the need for a dilution refrigerator[17]
Changes in the electronic state are better evaluated from the first derivative of the X-ray absorption near edge structure (XANES) spectrum, shown in the bottom panel of Fig. 3 for all RE-doped samples
These XANES results indicate that there is a strong change in the electronic structure of the Gd and Dy atoms in the samples when going from 6.4 to 10.6 at.% Gd, and from 4.5 to 14.2 at.% Dy doping, respectively
Summary
A recent breakthrough in QAHE research has been the realization of perfectly quantized transport in the absence of an applied magnetic field at temperatures that can be reached without the need for a dilution refrigerator[17]. For the exposure of Bi2Se3 films to ambient conditions it has been shown that a continuous growth of a native oxide leads to additional bulk carriers, overshadowing the relative contribution of the TSS24 and leading to a modification of the surface band structures[22]. For Bi2Se3 and Bi2Te3 films, Se25 and Te26 caps have shown to protect the as-grown surface well, and the TSS is recovered after ambient exposure followed by removal of the cap in ultra-high vacuum by heating. Another approach, most appropriate for surface-sensitive spectroscopic studies, is the in-situ cleaving of the TI films[27]. We use the element-specific technique of x-ray absorption fine structure (XAFS) to discern the location of the dopant in the
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