Classification of accidental band crossings and emergent semimetals in two-dimensional noncentrosymmetric systems

Abstract

We classify all possible gap-closing procedures which can be achieved in two-dimensional time-reversal invariant noncentrosymmetric systems. For exhaustive classification, we examine the space group symmetries of all 49 layer groups lacking inversion taking into account spin-orbit coupling. Although a direct transition between two insulators is generally predicted to occur when a band crossing happens at a general point in the Brillouin zone, we find that a variety of stable semimetal phases with point or line nodes can also arise due to the band crossing in the presence of additional crystalline symmetries. Through our theoretical study, we provide, for the first time, the complete list of nodal semimetals created by a band inversion in two-dimensional noncentrosymmetric systems with time-reversal invariance. The transition from an insulator to a nodal semimetal can be grouped into three classes depending on the crystalline symmetry. Firstly, in systems with a two-fold rotation about the z-axis (normal to the system), a band inversion at a generic point generates a two-dimensional Weyl semimetal with point nodes. Secondly, when the band crossing happens on the line invariant under a two-fold rotation (mirror) symmetry with the rotation (normal) axis lying in the two-dimensional plane, a Weyl semimetal with point nodes can also be obtained. Finally, when the system has a mirror symmetry about the plane embracing the whole system, a semimetal with nodal lines can be created. Applying our theoretical framework, we identify various two-dimensional materials as candidate systems in which stable nodal semimetal phases can be induced via doping, applying electric field, or strain-engineering, etc.

14 pages, 6 figures, 3 tables

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• Layer group classification of two-dimensional materials
• Encyclopedia of emergent particles in 528 magnetic layer groups and 394 magnetic rod groups
• Two-dimensional Topological Semimetals Protected by Symmorphic Symmetries
• Spin-orbit-stable type-II nodal line band crossing in n-doped monolayer MoX2 (X=S,Se,Te)
• Robustness of spin-polarized edge states in a two-dimensional topological semimetal without inversion symmetry
• Unified low-energy effective Hamiltonian and the band topology of $p$-block square-net layer derivatives
• Fully linear band crossings at high symmetry points in layers: classification and role of spin-orbit coupling and time reversal
• Laser induced surface magnetization in Floquet-Weyl semimetals
• Floquet-Weyl semimetals generated by an optically resonant interband-transition