Type-II Symmetry-Protected Topological Dirac Semimetals

Area of Science:

• Condensed Matter Physics
• Materials Science
• High-Energy Physics

Background:

• The type-II Weyl semimetal state has garnered significant research interest.
• Topological states of matter offer unique electronic and quantum properties.

Purpose of the Study:

• To propose and theoretically identify a new topological state: three-dimensional type-II Dirac fermions.
• To explore the realization of this state in transition-metal icosagenides (MA3).

Main Methods:

• Theoretical proposal of type-II Dirac fermions.
• Identification of these states in MA3 materials (M=V, Nb, Ta; A=Al, Ga, In).
• Symmetry analysis and electronic structure calculations.

Main Results:

• The VAl3 family hosts Lorentz-violating type-II Dirac nodes.
• Symmetry breaking can split Dirac nodes into type-II Weyl nodes with chiral charge ±1.
• Distinct Landau level spectrum predicted for type-II Dirac fermions in VAl3.
• Demonstrated topological phase transitions to quadratic Weyl semimetals or topological crystalline insulators.

Conclusions:

• Transition-metal icosagenides are promising platforms for realizing three-dimensional type-II Dirac fermions.
• The identified topological states exhibit unique properties distinct from known Dirac and Weyl semimetals.
• Tunability via symmetry breaking and crystalline distortions offers pathways to novel quantum phases.