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, XB₆ (X = Ca, Sr, and Ba). It was revealed that XB₆ 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 XB₆ become strong topological insulators with Z₂ 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.