Topological bulk band structures of the hourglass and Dirac nodal-loop types in Ce Kondo systems: CeNiSn, CeRhAs, and CeRhSb

We have investigated the electronic structures of Ce Kondo systems, CeNiSn, CeRhAs, and CeRhSb, which have attracted recent attention as promising candidates of ``M\"obius Kondo insulators.'' In the conventional density functional theory calculations, all three systems are obtained to be semimetallic, while only CeRhAs opens a full gap in the band calculation incorporating the more elaborate (modified Becke-Jones) exchange-correlation potential. Intriguingly, for all three systems, we have obtained the hourglass-type bulk band crossings around $\mathbf{k}=S$, which produce Dirac nodal-loop structures enclosing S in the [100] Brillouin zone boundary. Such topologically protected hourglass-type band structures arise from the spin-orbit coupling on top of the nonsymmorphic crystal symmetry $(Pnma)$ of these systems. Our finding suggests that Ce Kondo insulators would display temperature $(T)$ -dependent topological phase transitions from a potential M\"obius Kondo insulator at low $T$ to a Dirac nodal-loop semimetal at high $T$.