Gapless vortex bound states in superconducting topological semimetals.

Area of Science:

• Condensed Matter Physics
• Topological Materials Science
• Superconductivity

Background:

• Topological semimetals exhibit unique electronic properties arising from their band structure topology.
• Superconducting states in these materials can host exotic phenomena, including vortex states.
• Understanding the nature of excitations within these vortex states is crucial for exploring their potential applications.

Purpose of the Study:

• To investigate the nature of vortex bound states in superconducting topological semimetals.
• To determine if these bound states are gapped or gapless.
• To elucidate the underlying mechanisms and topological protection of these states.

Main Methods:

• Theoretical analysis of vortex bound states in various topological semimetals (Dirac, Weyl, spin-1, spin-3/2).
• Investigation of Andreev specular reflection and propagating Andreev modes in superconductor-normal metal-superconductor junctions.
• Application of topological pumping concepts to derive the properties of the bound states.

Main Results:

• Vortex bound states in superconducting topological semimetals are universally gapless.
• The gaplessness originates from topological massless excitations in the normal state of these semimetals.
• These gapless states are closely linked to Andreev reflection phenomena and are topologically protected.

Conclusions:

• The gapless nature of vortex bound states is a universal feature of superconducting topological semimetals.
• Topological massless excitations in the normal state are responsible for the gaplessness.
• The discovered gapless states are robust, topologically protected, and can be understood through topological pumping.