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Caption: (a) Surface band structure of PuB4 on its (001) surface.
(b) Schematic picture of multiple fermion excitations emerging on the surface of PuB4 : 2-fold-degenerate Dirac fermion at Γ ̅, 4-fold-degenerate wallpaper fermion at M ̅, and nodal-line-type fermion at X ̅.
Wallpaper Dirac Fermion in a Nonsymmorphic Topological Kondo Insulator: PuB4
It has been recently predicted that nonsymmorphic crystalline insulators can host two exotic topological surface states (TSSs). One is “hourglass fermion" and the other is “wallpaper Dirac fermion". For the former, a few real materials were predicted and already confirmed experimentally. For the latter, however, no bulk-insulating and experimentally accessible candidate has been identified yet. Here we show that the localized 5f-electrons in PuB4, the single crystal of which was recently synthesized and was found to exhibit Kondo-insulating nature, form a closed manifold over the Brillouin zone via the Kondo coherence effect at low temperature, and host hitherto unobserved wallpaper Dirac fermions at the nonsymmorphic symmetry-preserving (001) surface. The topological nature of TSSs in PuB4 can be described by topological invariants of two Z4 indices (χx, χy) = (1, 1) of double-glide symmetries of p4g wallpaper group, and thus PuB4 is a three-dimensional nonsymmorphic topological insulator that exhibits the TSSs of peculiar fourfold surface Dirac fermions as well as twofold double-glide spin-Hall and nodal-line-type fermions. On top of interesting 5f-electron Kondo insulating nature, unique fourfold wallpaper Dirac fermions in PuB4, which are quite distinct from previously-reported nonsymmorphic Dirac-insulator or hourglass TCI fermions, would broaden our recognition on the embedded fermions in strongly-correlated Kondo systems with nonsymmorphic symmetries.