We report on angle resolved photoemission spectroscopic studies on a parent topological insulator (TI), Bi2Se3. The line width of the spectral function (inverse of the quasi-particle lifetime) of the topological metallic (TM) states shows an anomalous behavior. This behavior can be reasonably accounted for by assuming decay of the quasi-particles predominantly into bulk electronic states through electron-electron interaction and defect scattering. Studies on aged surfaces reveal that topological metallic states are very much unaffected by the potentials created by adsorbed atoms or molecules on the surface, indicating that topological states could be indeed protected against weak perturbations.PACS numbers: 81.05. Uw, 63.20.kk, 73.20.At, TIs are materials with bulk gaps due to spin-orbit coupling. TIs are classified into 'weak TI' and 'strong TI' according to Z 2 topological invariants.2,3 Strong TIs have odd number of TM Dirac cones on the surface which is robust (protected) against disorder or impurities.TM states realized on the surface of a strong TI is important and could be useful.4,5 The properties of the TM are also set by the topological nature of the TI. The essential properties of TMs can be summarized as follows. First, electron spins in TM are locked into the momenta, forming spin chiral states 6 . Such spin chiral states are also well known in the field of surface science in terms of Rashba effects in surface states (for example, Sb (111) 7 , Bi (111) 8 and Au (111) 9 surfaces states). Second, back scattering is suppressed due to the spin chirality, 10 meaning relatively long quasi-particle life time. Third, metallic bands are protected against perturbations to the first order due to the topological nature. This point has yet to be experimentally observed.Experimental verification of the novel properties of TM is not only important in the fundamental aspect but also necessary for use of these materials for future applications. Due to the surface nature of the TM states, most of the experimental data thus far came from angle resolved photoemission (ARPES) 6,11-13 and to a less degree from scanning tunnelling microscopy (STM).10 By using ARPES, it was shown that there exist odd number of bands crossing the Fermi level in this class of materials.11,12 Moreover, spin resolved photoemission results show that the electron spins are indeed locked into the momentum and form spin chiral states.6 As for STM studies, a recent study shows suppression of back scattering, consistent with spin chiral states. 10Studies mentioned above are about existence and spin chiral states of TM and experimental verification of whether topological states are in general protected or not has not been discussed. Protected topological states should manifest themselves with long quasi-particle life time. In that regards, ARPES is an important tool because long quasi-particle life time should result in a sharp ARPES line-shape. In spite of the intensive recent efforts on TM electronic structure studies, photoemission lineshape issue...

We investigated the dielectric and magnetic properties of Bi2Fe4O9 single crystals. The dielectric anomalies observed at the magnetic transition temperature indicate that there is a strong coupling between the magnetic and dielectric properties. The anisotropic magnetic contribution of the dielectric constant is discussed in terms of the spin correlation function, ⟨Si⋅Sj⟩, below the antiferromagnetic transition at 240 K.

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