Abstract
Using ab initio calculations based on density-functional theory and effective model analysis, we propose that the trigonal YH3 (Space Group: Pbar{{bf{3}}}c1) at ambient pressure is a node-line semimetal when spin-orbit coupling (SOC) is ignored. This trigonal YH3 has very clean electronic structure near Fermi level and its nodal lines locate very closely to the Fermi energy, which makes it a perfect system for model analysis. Symmetry analysis shows that the nodal ring in this compound is protected by the glide-plane symmetry, where the band inversion of |Y+, dxz〉 and |H1−, s〉 orbits at Γ point is responsible for the formation of the nodal lines. When SOC is included, the line nodes are prohibited by the glide-plane symmetry, and a small gap (≈5 meV) appears, which leads YH3 to be a strong topological insulator with Z2 indices (1,000). Thus the glide-plane symmetry plays an opposite role in the formation of the nodal lines in cases without and with SOC. As the SOC-induced gap is so small that can be neglected, this Pbar{{bf{3}}}c1 YH3 may be a good candidate for experimental explorations on the fundamental physics of topological node-line semimetals. We find the surface states of this Pbar{{bf{3}}}c1 phase are somehow unique and may be helpful to identify the real ground state of YH3 in the experiment.
Highlights
Topological semimetals (TSMs) have attracted great attention for both theoretical interests and experimental applications in recent years
Symmetries are important in classifying node-line semimetals (NLSMs), for instance, three types of NLSMs protected by different symmetries have been proposed: (a) mirror symmetry protected NLSMs8,14,16,40, (b) coexistence of time-reversal symmetry (TRS) and inversion symmetry (IS) protected NLSMs11–13,32 and (c) nonsymmorphic symmetry protected NLSMs15,32
Two of them are experimentally favoured with trigonal P3c1 and hexagonal P63cm symmetry[66,67,68,69,70,71], while the third candidate is in the space group of P63 which was predicted theoretically[72]
Summary
Calculations of the band structures are performed using the full-potential linearized augmented plane-wave (FP-LAPW) method[57,58] implemented in the WIEN2k59 package. We use 13 × 13 × 11 k-mesh for the BZ sampling and −7 for the plane wave cut-off parameter RMTKmax for the electronic structure calculation, where the RMT is the minimum muffin-tin radius and Kmax is the plane-wave vector cut-off parameter. SOC is taken into consideration by a second-variation method[60]. The tight-binding models are constructed with the maximally localized Wannier functions (MLWFs) method[61,62,63], the corresponding hopping parameters are determined from the projections of the bulk Bloch wave functions. The projected surface states are calculated using surface Green’s function in the semi-infinite system[64,65]
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