Sputtering induced re-emergence of the topological surface state in Bi$_2$Se$_3$

TL;DR

This study investigates how different types of surface disorder, caused by neon sputtering and adsorbates, affect the topological surface states of Bi$_2$Se$_3$, revealing that certain disorder can actually restore the visibility of these states.

Contribution

It demonstrates that neon sputtering-induced unitary disorder can re-emerge the topological surface state by pushing it inward, contrasting with the smearing effect of Gaussian disorder from adsorbates.

Findings

Unitary disorder shifts the Dirac state inward, making it less visible but retaining quasiparticle peaks.

Gaussian disorder from adsorbates smears out the surface state, destroying the quasiparticle peak.

Neon sputtering can sharpen the ARPES signal of the surface state by removing adsorbates, despite introducing unitary scatterers.

Abstract

We study the fate of the surface states of Bi$_2$Se$_3$ under disorder with strength larger than the bulk gap, caused by neon sputtering and nonmagnetic adsorbates. We find that neon sputtering introduces strong but dilute defects, which can be modeled by a unitary impurity distribution, whereas adsorbates, such as water vapor or carbon monoxide, are best described by Gaussian disorder. Remarkably, these two disorder types have a dramatically different effect on the surface states. Our soft x-ray ARPES measurements combined with numerical simulations show that unitary surface disorder pushes the Dirac state to inward quintuplet layers, burying it below an insulating surface layer. As a consequence, the surface spectral function becomes weaker, but retains its quasiparticle peak. This is in contrast to Gaussian disorder, which smears out the quasiparticle peak completely. At the surface of Bi$_2$Se$_3$, the effects of Gaussian disorder can be reduced by removing surface adsorbates using neon sputtering, which, however, introduces unitary scatterers. Since unitary disorder has a weaker effect than Gaussian disorder, the ARPES signal of the Dirac surface state becomes sharper upon sputtering.