We carried out high resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of K0.68Fe1.79Se2 (Tc=32 K) and (Tl0.45K0.34)Fe1.84Se2 (Tc=28 K) superconductors. In addition to the electron-like Fermi surface near M(π,π), two electronlike Fermi pockets are revealed around the zone center Γ(0,0) in K0.68Fe1.79Se2. This observation makes the Fermi surface topology of K0.68Fe1.79Se2 consistent with that of (Tl,Rb)xFe2−ySe2 and (Tl,K)xFe2−ySe2 compounds. A nearly isotropic superconducting gap (∆) is observed along the electron-like Fermi pocket near the M point in K0.68Fe1.79Se2 (∆∼ 9 meV) and (Tl0.45K0.34)Fe1.84Se2 (∆∼ 8 meV). The establishment of a universal picture on the Fermi surface topology and superconducting gap in the AxFe2−ySe2 (A=K, Tl, Cs, Rb and etc.) superconductors will provide important information in understanding the superconductivity mechanism of the iron-based superconductors.

We report the effects of the Mg doping on the dielectric properties of CaCu3Ti4O12 in the frequency range of 200 Hz–200 kHz and in the temperature range of 58–473 K. It is found that the incorporations of Mg2+ result in increases in the dielectric constant by 12%–20% and decreases in the dielectric loss by 41%–64% (with the minimum of 0.042 for CaCu2.7Mg0.3Ti4O12) at room temperature and at 1 kHz. The x-ray photoemission spectroscopy measurements reveal that the content of Cu3+ increases with the increasing concentration of Mg2+. The enhanced dielectric response may be related to the influence of Cu3+ and/or Mg2+. In other words, Mg2+ is an effective ion to optimize the dielectric properties of CaCu3Ti4O12.

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