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 the "hourglass fermion", and the other is the "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; thus, PuB4 is a 3D nonsymmorphic topological insulator that exhibits the TSSs of peculiar 4-fold surface Dirac fermions as well as 2-fold double-glide spin-Hall and nodal-line-type fermions. On top of its interesting 5f-electron Kondo-insulating nature, the unique 4-fold wallpaper Dirac fermions in PuB4, which are quite distinct from previously reported nonsymmorphic Dirac insulator or hourglass TCI fermions, broaden our recognition of the embedded fermions in strongly correlated Kondo systems with nonsymmorphic symmetries.

Observation of Dirac state in half-Heusler material YPtBi.

The prediction of non-trivial topological electronic states in half-Heusler compounds makes these materials good candidates for discovering new physics and devices as half-Heusler phases harbour a variety of electronic ground states, including superconductivity, antiferromagnetism, and heavy-fermion behaviour. Here, we report a systematic studies of electronic properties of a superconducting half-Heusler compound YPtBi, in its normal state, investigated using angle-resolved photoemission spectroscopy. Our data reveal the presence of a Dirac state at the [Formula: see text] point of the Brillouin zone at 500 meV below the Fermi level. We observe the presence of multiple Fermi surface pockets, including two concentric hexagonal and six half-oval shaped pockets at the [Formula: see text] and K points of the Brillouin zone, respectively. Furthermore, our measurements show Rashba-split bands and multiple surface states crossing the Fermi level, this is also supported by the first-principles calculations. Our findings of a Dirac state in YPtBi contribute to the establishing of half-Heusler compounds as a potential platform for novel topological phases.

Electronic and Magnetic Properties of Lanthanum and Strontium Doped Bismuth Ferrite: A First-Principles Study.

While bismuth ferrite BiFeO3 (BFO) is a well studied multiferroic material, its electronic and magnetic properties in the presence of A-site dopants have not been explored widely. Here we report the results of a systematic study of the local electronic structure, spontaneous polarization, and magnetic properties of lanthanum (La) and strontium (Sr) doped rhombohedral bismuth ferrite within density functional theory. An enhanced ferroelectric polarization of 122.43 µC/cm2 is predicted in the uniformly doped BiLaFe2O6. We find that substitution of Sr in the A-site drives the system into a metallic state. The nature of magnetism arises mainly from the B-site Fe exhibiting a G-type antiferromagnetic ordering. Our study finds that upon dopant substitution, the local magnetic moment is decreased and its magnitude is dependent on the distance between the Fe and the dopant atom. The correlation between the local moment and the distance between the Fe and the dopant atom is discussed.