（Bi $_{0.5}$ Sb $_{0.5}$）$_2$ Te $_3$上无隙表面态对表面磁性杂质的鲁棒性研究
Robust Gapless Surface State against Surface Magnetic Impurities on (Bi$_{0.5}$Sb$_{0.5}$)$_2$Te$_3$ Evidenced by In Situ Magnetotransport Measurements
摘要：
尽管进行了大量的实验和理论研究，但表面磁性杂质对拓扑绝缘体（TI）拓扑表面态的影响这一重要问题仍未得到解决。我们阐明的（Bi $_{0.5}$ Sb $_{0.5}$）$_{2}$ Te $_{3}$：Cr吸附原子的外延薄膜上的影响是增量沉积到TI上的在低温下保持在真空中，和\textit{in situ}的磁导率和霍尔效应测量进行在每个增量与静电门控。在实验确定的表面传输制度，测得的最小电子密度显示出非单调的演变与铬密度（$n_{\mathrm{Cr}}$）：它首先增加，然后减少与$n_{\mathrm{Cr}}$。这种不寻常的行为归因于Cr作为离子化杂质和电子供体的双重作用，具有分别在低和高$n_{\mathrm{Cr}}$下增强和降低表面态中的电子不均匀性的竞争效应。不同的$n_{\mathrm{Cr}}$上的一个和相同的样品，这产生了明确的证据表明，弱的反局域化效应持续存在，表面状态保持无间隙的最高$n_{\mathrm{Cr}}$，相反的预期，沉积的Cr应该打破时间反演对称性，并诱导在狄拉克点的间隙开口。
英文摘要：
Despite extensive experimental and theoretical efforts, the important issue of the effects of surface magnetic impurities on the topological surface state of a topological insulator (TI) remains unresolved. We elucidate the effects of Cr impurities on epitaxial thin films of (Bi$_{0.5}$Sb$_{0.5}$)$_{2}$Te$_{3}$: Cr adatoms are incrementally deposited onto the TI held in ultrahigh vacuum at low temperatures, and \textit{in situ} magnetoconductivity and Hall effect measurements are performed at each increment with electrostatic gating. In the experimentally identified surface transport regime, the measured minimum electron density shows a non-monotonic evolution with the Cr density ($n_{\mathrm{Cr}}$): it first increases and then decreases with $n_{\mathrm{Cr}}$. This unusual behavior is ascribed to the dual roles of the Cr as ionized impurities and electron donors, having competing effects of enhancing and decreasing the electronic inhomogeneities in the surface state at low and high $n_{\mathrm{Cr}}$ respectively. The magnetoconductivity is obtained for different $n_{\mathrm{Cr}}$ on one and the same sample, which yields clear evidence that the weak antilocalization effect persists and the surface state remains gapless up to the highest $n_{\mathrm{Cr}}$, contrary to the expectation that the deposited Cr should break the time reversal symmetry and induce a gap opening at the Dirac point.
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