Scanning tunneling microscopy studies of a tunable topological insulator and magnetic Weyl semimetal

Howard, Sean T

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https://hdl.handle.net/2142/108119 Copy

Description

Title

Scanning tunneling microscopy studies of a tunable topological insulator and magnetic Weyl semimetal

Author(s)

Howard, Sean T

Issue Date

2020-04-22

Director of Research (if dissertation) or Advisor (if thesis)

Madhavan, Vidya

Doctoral Committee Chair(s)

Mahmood, Fahad

Committee Member(s)

• Hughes, Taylor
• Gadway, Bryce

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Scanning Tunneling Microscopy
• Topological Insulator
• Weyl Semimetal

Abstract

The discovery of topological order with material systems, and the unique electronic states necessitated by non-trivial topology, has been a major thrust of condensed matter research for the past two decades. As exotic topological electronic states often manifest themselves at reduced dimensional interfaces, local experimental probes play an important role in the understanding and development of topological materials. In this thesis, I describe the construction of a Scanning Tunneling Microscope (STM) for the study of topological materials. After construction, the tunable topological insulator system Ge(Bix,Sb1-x)2Te4 and the magnetic Weyl semimetal Co3Sn2S2 were studied. In Ge(Bix,Sb1-x)2Te4, increasing Sb substitution effectively electron dopes the sample and moves the Dirac point from 300 meV below the Fermi energy to energies above the Fermi energy while remaining topological. As the topological surface states in this material are well isolated from the bulk bands, Dirac point tuning in this material system has interesting consequences for potential spintronic applications, as well as existing thermoelectric and phase change applications. In Co3Sn2S2, the seminal model of a magnetic Weyl semimetal as a stacking of Chern insulators is extended to step edges exposed upon cleavage. We find that for a large region of parameter space within this model, the chiral edge states present in the 2D limit can be recovered on surfaces where the inter-layer coupling is sufficiently reduced. Our subsequent STM study finds an edge state on the kagome Co3Sn surface, which is predicted to be a Chern insulator in the 2D limit. STM density of states maps on thin terraces of the Co3Sn surface show quantum well like bound states that have a linear dependence on the nth bound state and energy. By constructing a tight binding model of linearly dispersing states that can hybridize, we reproduce our results on the terrace. These results suggest that elusive chiral edge states could be studied by local probe measurements within bulk materials. Additionally, our experiments provide evidence that Co3Sn2S2 could host the quantum anomalous Hall effect at elevated temperatures in the 2D limit.

Graduation Semester

2020-05

Type of Resource

Thesis

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http://hdl.handle.net/2142/108119

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Copyright 2020 Sean Howard

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