Resolving the topological classification of bismuth with topological defects

Area of Science:

• Condensed Matter Physics
• Materials Science
• Topological Matter

Background:

• Topological insulators exhibit unique boundary phenomena, leading to classification ambiguities.
• Bulk bismuth has been controversially classified as a strong or higher-order topological insulator.
• Both classifications predict helical boundary modes, complicating differentiation.

Purpose of the Study:

• To resolve the topological classification of bulk bismuth.
• To differentiate between strong and higher-order topological insulator classifications.
• To investigate the response of boundary modes to screw-dislocations.

Main Methods:

• Spectroscopic mapping of boundary modes in bismuth.
• Analysis of boundary mode response to screw-dislocation perturbations.
• Characterization of energy gaps at time-reversal invariant momenta.

Main Results:

• A one-dimensional mode on step-edges was observed to extend over a wide energy range.
• This mode did not open a gap near screw-dislocations, indicating binding to the dislocation.
• A small energy gap was found at the time-reversal invariant momentum L.

Conclusions:

• The observed boundary mode behavior confirms nonzero weak indices in bismuth.
• Bismuth is positioned in a critical region, indicating a topological phase transition.
• The findings suggest a transition between a higher-order and a strong topological insulator with nonzero weak indices.