Dirac metals (gapless semiconductors) are believed to turn into Weyl metals when perturbations, which break either time reversal symmetry or inversion symmetry, are employed. However, no experimental evidence has been reported for the existence of Weyl fermions in three dimensions. Applying magnetic fields near the topological phase transition from a topological insulator to a band insulator in Bi1-xSbx we observe not only the weak antilocalization phenomenon in magnetoconductivity near zero magnetic fields (B<0.4  T), but also its upturn above 0.4 T only for E//B. This "incompatible" coexistence between weak antilocalization and "negative" magnetoresistivity is attributed to the Adler-Bell-Jackiw anomaly ("topological" E·B term) in the presence of weak antilocalization corrections.

Polarized reflectivity spectra of single crystals of ␣-PbF 2 , ␤-PbF 2 , PbCl 2 , and PbBr 2 have been measured at 10 K in the energy range up to 30 eV using synchrotron radiation as a light source. The spectra are well resolved in comparison with those reported previously. Optical constants have been derived through the Kramers-Kronig analysis. The effect of phase transition on the optical spectra of PbF 2 is also examined. With use of the discrete variational X␣ method, the electronic structures of the clusters consisting of a Pb 2ϩ ion and neighboring halogen ions have been calculated for better understanding of the optical properties of Pb halides. The calculation indicates that the lowest excitons in all lead halides are attributed to the cationic transitions. The electronic structures of lead halides are discussed on the basis of a comparison of experiment and calculation.

We propose a phase diagram for Fe x Bi 2 Te 3 (0≤x≤0.1) single crystals, which belong to a class of magnetically bulk-doped topological insulators. The evolution of magnetic correlations from "ferromagnetic"-to "antiferromagnetic"-gives rise to topological phase transitions, where the paramagnetic topological insulator of Bi 2 Te 3 turns into a band insulator with ferromagnetic-cluster glassy behaviours around x~0.025, and it further evolves to a topological insulator with valence-bond glassy behaviours, which spans over the region between x~0.03 up to x~0.1. This phase diagram is verified by measuring magnetization, magnetotransport, and angle-resolved photoemission spectra with theoretical discussions.

No abstract

Photoluminescence (PL) and thermally stimulated luminescence (TSL) properties of Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) crystals have been studied in the temperature range of 10–300 K under electron excitation from the 4f level to the 5d levels of Ce3+ ions. PL and TSL spectra commonly exhibit a prominent band around 2.31 eV, due to electron transitions from the lowest 5d (5d1) level to the 4f levels. The intensity of the Ce3+ PL band is significantly changed with temperature under excitation from the lowest 4f level to a higher-lying 5d (5d2) level, whereas it is scarcely changed under excitation from the 4f level to the 5d1 level. Such change correlates well with the appearance of TSL glow peaks at 84 and 175 K. The luminescence properties are apparently influenced by the thermal behavior of electrons trapped at some lattice site, but not by nonradiative processes of electrons starting from the 5d1 level. The phosphorescence process will be discussed using a phenomenological model based on the results obtained.