Abstract
Topologically protected fermionic quasiparticles appear in metals, where band degeneracies occur at the Fermi level, dictated by the band structure topology. While in some metals these quasiparticles are direct analogues of elementary fermionic particles of the relativistic quantum field theory, other metals can have symmetries that give rise to quasiparticles, fundamentally different from those known in high-energy physics. Here, we report on a new type of topological quasiparticles—triple point fermions—realized in metals with symmorphic crystal structure, which host crossings of three bands in the vicinity of the Fermi level protected by point group symmetries. We find two topologically different types of triple point fermions, both distinct from any other topological quasiparticles reported to date. We provide examples of existing materials that host triple point fermions of both types and discuss a variety of physical phenomena associated with these quasiparticles, such as the occurrence of topological surface Fermi arcs, transport anomalies, and topological Lifshitz transitions.
- Received 15 May 2016
DOI:https://doi.org/10.1103/PhysRevX.6.031003
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Topological materials can yield quasiparticles that behave in a manner similar to elementary particles that are part of the standard model of particle physics. Examples of such quasiparticles include Dirac, Weyl, and Majorana excitations, realized recently in real materials. The symmetry constraints of low-energy condensed-matter theories are weaker than those of high-energy relativistic quantum field theory, and therefore, quasiparticles that have no direct analog in the standard model can arise in solids. Here, we report on a new class of such quasiparticles—triple point fermions—which represent fermions that have mixed properties of Dirac and Weyl fermions; these quasiparticles can be thought of as an interpolation between Dirac and Weyl fermions.
We theoretically investigate the appearance of triple point fermions and show that these quasiparticles can be classified into two possible types that have to be incorporated into a yet-to-be-built standard model of crystalline materials. We also reveal that triple point fermions exhibit a variety of observable phenomena, including anomalous responses to magnetic fields that can possibly be used in future devices. A family of known materials is predicted to host these new kinds of quasiparticles, and we show that one of the types of quasiparticles resides in two-element metals of the form AB, where A = [Zr, Nb, Mo, Ta, W] and B = [C, N, P, S, Te].
We expect that our results will pave the way for futures studies that identify topological materials with technologically interesting properties.
