Vortex and Surface Phase Transitions in Superconducting Higher-order Topological Insulators.

Area of Science:

• Condensed Matter Physics
• Topological Quantum Matter
• Superconductivity

Background:

• Topological insulators with superconductivity host Majorana zero modes (MZMs) at vortex lines.
• MZMs in topological insulators are typically annihilated by vortex phase transitions at critical doping levels.

Purpose of the Study:

• To investigate the distinct phenomenology of MZMs in higher-order topological insulators (HOTIs).
• To explore the impact of vortex placement and doping on MZM existence in HOTIs.

Main Methods:

• Theoretical analysis of vortices on the gapped surfaces of HOTIs.
• Numerical simulations to study critical doping variations with vortex position relative to sample hinges.

Main Results:

• Vortices on HOTI surfaces exhibit two phase transitions: a surface transition enabling MZMs, followed by a vortex transition.
• The surface transition arises from competition between superconducting and local time-breaking gaps.
• Critical doping for the vortex transition significantly varies as the vortex center approaches or recedes from sample hinges.

Conclusions:

• HOTIs present novel phenomenology for MZMs, differing from conventional topological insulators.
• The observed phenomena offer potential signatures for identifying HOTIs.
• HOTIs represent a promising platform for the realization and control of MZMs.