Emergence of topological nodal loops in alkaline-earth hexaborides XB₆(X = Ca, Sr, and Ba) under pressure

Abstract

Based on first-principles calculations, we report that external pressure can induce a topological phase transition in alkaline-earth hexaborides, XB (X = Ca, Sr, and Ba). It was revealed that XB are transformed from trivial semiconductors to topological node-line semimetals under moderate pressure when the spin-orbit coupling (SOC) is ignored. The band inversion between the B p-orbitals at the X point is responsible for the formation of node-line semimetals. Three node-line rings around the X point are protected by the combination of the time-reversal and spatial inversion symmetries, and drumhead surface bands are obtained in the interiors of the projected node-line rings. When the SOC is included, tiny gaps (<4.8 meV) open at the crossing lines and the XB become strong topological insulators with Z indices (1 : 111). As the SOC-induced gap opening is negligible, our findings thus suggest ideal real systems for experimental exploration of the fundamental physics of topological node-line semimetals.

Nonsymmorphic-symmetry-protected hourglass Dirac loop, nodal line, and Dirac point in bulk and monolayer X3SiTe6 ( X = Ta, Nb)

“…For a threedimensional (3D) system, besides the 0D nodal points, the nontrivial band-crossings may also take the form of 1D nodal lines [21]or even 2D nodal surfaces [3,[22][23][24]. The nodal-line materials have been intensively studied recently [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Interesting properties such as the presence of drumhead-like surface states [44,45], the anisotropic electron transport [27], the possible surface magnetism/superconductivity [34,[46][47][48], the anomalous Landau level spectrum [49,50], and the unusual optical response [51][52][53] have been proposed for nodal-line materials.…”

Almost ideal nodal-loop semimetal in monoclinic CuTeO3 material

“…Several interesting properties have been predicted for nodalloop semimetals, such as the anisotropic electron transport [23], the unusual optical response and circular dichorism [24][25][26], the possible surface magnetism and superconductivity [27][28][29][30][31], the anomalous Landau level spectrum [32,33], and density fluctuation plasmons and Friedel oscillations [34]. Quite a few realistic materials have been proposed as nodal-loop semimetals [23,29,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49], and interestingly, Dirac and Weyl loops have also been proposed in cold-atom optical lattices [50]. However, to unambiguously identify the predicted features due to the nodal loops in these systems still remains challenging, partly because the low-energy band structures of the realistic materials suffer from various drawbacks.…”

Topological quantum catalyst: Dirac nodal line states and a potential electrocatalyst of hydrogen evolution in the TiSi family

“…Topological semimetals [1], which have been classified into topological Dirac semimetal (TDs) [2][3][4][5][6][7][8][9], topological Weyl semimetals (TWs) [10][11][12][13][14][15][16][17][18][19][20][21][22], and topological nodal line semimetals (DNLs) [23][24][25][26][27][28][29][30][31][32][33][34] and beyond [35], have currently attracting extensively interest in condensed matter physics and materials science. In difference from both TDs and TWs which exhibit isolated Dirac cones and Weyl nodes in its bulk phase, the class of DNLs [27][28][29][30][31][32][33][34][36][37][38] shows a fully closed line nearly at the Fermi level in its bulk phase. The projection of the DNLs into a certain surface would result in a closed ring...…”