Magnetism breaks the time-reversal symmetry expected to open a Dirac gap in 3D topological insulators that consequently leads to the quantum anomalous Hall effect. The most common approach of inducing a ferromagnetic state is by doping magnetic 3$d$ elements into the bulk of 3D topological insulators. In ${\mathrm{Cr}}_{0.15}{({\mathrm{Bi}}_{0.1}{\mathrm{Sb}}_{0.9})}_{1.85}{\mathrm{Te}}_3$, the material where the quantum anomalous Hall effect was initially discovered at temperatures much lower than the ferromagnetic transition, $T_C$, the scanning tunneling microscopy studies have reported a large Dirac gap of $\sim 20\text{–}100$ meV. The discrepancy between the low temperature of quantum anomalous Hall effect ($\ll T_C$) and large spectroscopic Dirac gaps ($\gg T_C$) found in magnetic topological insulators remains puzzling. Here, we used angle-resolved photoemission spectroscopy to study the surface electronic structure of the pristine and potassium doped surface of ${\mathrm{Cr}}_{0.15}{({\mathrm{Bi}}_{0.1}{\mathrm{Sb}}_{0.9})}_{1.85}{\mathrm{Te}}_3$. Upon potassium deposition, the $p$-type surface state of the pristine sample was turned into an $n$-type, allowing the spectroscopic observation of Dirac point. We find a gapless surface state, with no evidence of a large Dirac gap reported in tunneling studies.

中文翻译：

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铁磁Crx（Bi0.1Sb0.9）2-xTe3中不存在Dirac间隙

磁性打破了预计会在3D拓扑绝缘体中打开Dirac间隙的时间反转对称性，从而导致量子异常霍尔效应。感应铁磁状态的最常见方法是掺杂磁性3$d$元素放入3D拓扑绝缘体的主体中。在${\mathrm{铬}}_{0.15}{（{\mathrm{双}}_{0.1}{\mathrm{锑}}_{0.9}）}_{1.85}{\mathrm{特}}_3$，最初是在比铁磁跃迁低得多的温度下发现量子异常霍尔效应的材料， ${Ť}_C$，扫描隧道显微镜研究发现Dirac间隙很大 $〜20\text{–}100$ 影片 低温量子异常霍尔效应之间的差异（$\ll {Ť}_C$）和大光谱Dirac间隙（$\gg {Ť}_C$）发现在磁性拓扑绝缘子中仍然令人困惑。在这里，我们使用角分辨光发射光谱法研究了原始和钾掺杂表面的表面电子结构。${\mathrm{铬}}_{0.15}{（{\mathrm{双}}_{0.1}{\mathrm{锑}}_{0.9}）}_{1.85}{\mathrm{特}}_3$。钾沉积后，$p$原始样品的表面型转变成 $ñ$型，允许在光谱学上观察狄拉克点。我们发现无间隙的表面状态，没有证据表明在隧道研究中有较大的狄拉克间隙。