Ideal Nodal-Sphere Semimetal in the Three-Dimensional Boron Allotrope CT-B$_{24}$

TL;DR

This paper predicts a new three-dimensional boron allotrope, CT-B$_{24}$, as an ideal nodal-sphere semimetal with nearly gapless band crossings, robust surface states, and tunable topological phases, providing a promising platform for topological materials research.

Contribution

The study introduces CT-B$_{24}$ as the first nearly ideal NSSM with minimal energy gap and demonstrates its stability, surface states, and strain-tunable topological phase transitions.

Findings

Maximum energy gap of 0.008 meV, much smaller than previous pseudo-NSSMs.

Presence of drumhead-like surface states outside the projected nodal sphere.

Strain induces topological phase transitions from NSSM to Dirac semimetal and trivial insulator.

Abstract

Nodal-sphere semimetals (NSSMs), featuring spherical band degeneracies in momentum space, constitute a fascinating class of topological materials. However, their realization in real materials is severely hampered by discrete crystallographic symmetry constraints, often resulting in gapped ``pseudo'' nodal spheres. Here, combining first-principles calculations and symmetry analysis, we predict a new three-dimensional boron allotrope, CT-B$_{24}$, as a nearly ideal NSSM. Its structural stability is systematically confirmed by phonon calculations, \textit{ab initio} molecular dynamics simulations at 600~K, and elastic constant analysis. Notably, the electronic structure of CT-B$_{24}$ exhibits two bands crossing linearly near the Fermi level, forming a quasi-nodal sphere around the $\Gamma$ point. The maximum energy gap is merely 0.008~meV, which is two orders of magnitude smaller than the gaps reported in previous pseudo-NSSMs. Furthermore, the (001) surface hosts pronounced drumhead-like surface states located outside the projected nodal sphere, providing distinct signatures detectable by angle-resolved photoemission spectroscopy (ARPES). The nodal sphere also demonstrates remarkable robustness and tunability under external strain, driving a topological phase transition from an NSSM to a Dirac semimetal and finally to a trivial insulator. Our work not only presents a superior material platform for exploring nodal-sphere physics but also suggests potential for strain-tunable topological devices.