Surface states of 3D topological insulators

In summary, three-dimensional topological insulators have Dirac-like states at their surfaces which are immune to scattering from non-magnetic impurities. To image these surface states, angle-resolved photoemission spectroscopy (ARPES) is used under ultra-high vacuum conditions. However, even with this high vacuum, adsorption of molecules (mostly water) on the cleaved surface can alter the surface electronic states, leading to blurred spectra over time. This suggests that the topologically-protected states may still survive, but cannot be easily imaged due to the effects of adsorption. More research is needed to understand the mechanism behind this blurring.
  • #1

EdB

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I have a question (more like a curiosity) related to three-dimensional topological insulators, which support Dirac-like states at their surfaces. From the theory, it is well known that these states are immune to scattering from non-magnetic impurities, i.e. impurities that do not break time-reversal symmetry. Therefore, they are topologically protected surface states.

Now, when one performs an experiment to image these surface states, the best and clearest signature is provided by angle-resolved photoemission spectroscopy (ARPES). This technique is performed under ultra-high vacuum, which means under a vacuum of < 10-10 torr. This is done to minimize the collisions between the photoemitted electrons and the remaining particles in the ARPES chamber environment.
To image the surface states, a high-quality single crystal is cleaved in situ, which means that the material is cracked inside the ARPES chamber to expose a clean and fresh surface that can be imaged by the spectrometer. Who has performed this technique, knows that the surface states of 3D topological insulators can survive only up to a few hours/days and eventually the spectra will get blurred over time. So, after some amount of time, the spectra degrade because of the dirty environment in the ARPES chamber. What is the mechanism behind this blurring? If the surface states are immune to non-magnetic impurities and in the absence of any non-magnetic impurity, these states should survive over time. What am I missing here?
 
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  • #2
EdB said:
What am I missing here?
Adsorption!. Even at such high vacuum, there will be adsorption of molecules (mostly water) on the cleaved surface. The adsorbed molecules alter surface electronic states.
 
  • #3
Thanks for the answer, Henryk. I do agree with you in the case of trivial surface states in a band semiconductor, while I find more difficult to imagine how adsorbed non-magnetic molecules can microscopically interact to "destroy" topologically-protected electronic states. Should I interpret your answer as "adsorption masks any surface state in the photoemission process"? Does this imply that the topologically-protected states survive but they cannot be simply imaged?
 
  • #4
it is my understanding that adsorbed molecules alter the surface states. I am not quite sure about the mechanism, it could as simple as an electric field of an adsorbed molecule shifts the energy of the state. I do not thing that a monolayer of any species is actually capable of screening high energy electron beam.
 

1. What are 3D topological insulators?

3D topological insulators are materials that are insulators in their bulk form, but have conducting surface states. These surface states are topologically protected, meaning they cannot be scattered or localized by impurities or defects. This makes 3D topological insulators potential candidates for applications in spintronics, quantum computing, and other fields.

2. How are surface states of 3D topological insulators different from conventional surface states?

Conventional surface states arise due to the lack of translational symmetry at the surface of a material, and can be easily scattered or localized by impurities or defects. On the other hand, surface states of 3D topological insulators are topologically protected and cannot be scattered or localized, making them more robust and potentially useful for applications.

3. How are surface states of 3D topological insulators experimentally observed?

Surface states of 3D topological insulators can be observed through various experimental techniques such as angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and quantum oscillation measurements. These techniques can provide information about the energy-momentum dispersion of the surface states and their topological properties.

4. What are the potential applications of surface states of 3D topological insulators?

The topologically protected surface states of 3D topological insulators have potential applications in spintronics, quantum computing, and energy harvesting. These materials can also be used as efficient photocatalysts and sensors due to their unique surface properties.

5. Are there any challenges in studying and utilizing surface states of 3D topological insulators?

Yes, there are still challenges in understanding the properties and behaviors of surface states of 3D topological insulators. These include the identification of suitable materials, controlling the surface states without affecting the bulk properties, and realizing their potential applications in real-world devices. Further research and development in this field are needed to overcome these challenges.

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