The topological insulator ${\mathrm{BiSbTeSe}}_2$ corresponds to a compensated semiconductor in which strong Coulomb disorder gives rise to the formation of charge puddles, i.e., local accumulations of charge carriers, both in the bulk and on the surface. Bulk puddles are formed if the fluctuations of the Coulomb potential are as large as half of the band gap. The gapless surface, in contrast, is sensitive to small fluctuations but the potential is strongly suppressed due to the additional screening channel provided by metallic surface carriers. To study the quantitative relationship between the properties of bulk puddles and surface puddles, we performed infrared transmittance measurements as well as scanning tunneling microscopy measurements on the same sample of ${\mathrm{BiSbTeSe}}_2$, which is close to perfect compensation. At 5.5 K, we find surface potential fluctuations occurring on a length scale $r_s=40–50$ nm with amplitude $\mathrm{\Gamma }=8–14$ meV, which is much smaller than in the bulk, where optical measurements detect the formation of bulk puddles. In this nominally undoped compound, the value of $\mathrm{\Gamma }$ is smaller than expected for pure screening by surface carriers, and we argue that this arises most likely from a cooperative effect of bulk screening and surface screening.

中文翻译：

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通过扫描隧道显微镜和光谱学研究了拓扑绝缘子BiSbTeSe2的本体和表面上的电荷坑

拓扑绝缘子 ${\mathrm{BiSbTeSe}}_{\mathrm{2个}}$对应于补偿的半导体，其中强的库仑紊乱会引起电荷坑的形成，即电荷载流子在体中和表面上的局部积累。如果库仑电势的波动大到带隙的一半，就会形成大块的水坑。相反，无间隙的表面对微小的波动敏感，但是由于金属表面载体提供了额外的屏蔽通道，因此可以极大地抑制电势。为了研究散装水坑和表面水坑的性能之间的定量关系，我们对同一样品进行了红外透射率测量以及扫描隧道显微镜测量。${\mathrm{BiSbTeSe}}_{\mathrm{2个}}$，接近完美的补偿。在5.5 K下，我们发现表面电势波动发生在长度范围内${\mathrm{[R}}_s=40–50$ 振幅为nm $\mathrm{\Gamma }=8–14$meV，远小于体积测量中的体积，在光学测量中，meV可以检测到体积较大的水坑的形成。在这种名义上未掺杂的化合物中，$\mathrm{\Gamma }$ 小于表面载体纯筛选的预期值，我们认为这很可能是由于批量筛选和表面筛选的协同作用而引起的。