[en] Magnetoresistances of SrFe₂As₂ and BaFe₂As₂ in the magnetic ordered state are studied. Positive magnetoresistance is observed in the magnetic fields H applied in the azimuthes of θ = 0° and 30° with respect to the c-axis. The magnetoresistance can reach 20% for SrFe₂As₂ and 12% for BaFe₂As₂ at H = 9 T with θ = 0° (H || c). Above the magnetic transition temperature, the magnetoresistance becomes negligible. The data in the magnetic ordered state could be described by a modified two-band galvanomagnetic model including the enhancement effect of the applied magnetic field on the spin-density-wave gap. The field enhanced spin-density-wave gaps for different types of carriers are different. Temperature dependencies of the fitting parameters are discussed

[en] Different element substitution effects in transition metal oxypnictide Re(O_1-xF_x)TAs, with Re = La, Ce, Nd, Eu, Gd, Tm, T = Fe, Ni, Ru, are studied. Similar to the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145 K, which is ascribed to the spin-density-wave instability. Electron doping by F increases T_c to about 50 K. While in the case of Gd, T_c is reduced below 10 K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO_1-xF_xRuAs and CeO_1-xF_xRuAs compounds. The metallic behaviour is observed down to 4K. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

[en] A series of layered (Sr_1-xK_x)Fe₂As₂ compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe₂As₂ shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe₂As₂, the two prototype parent compounds with ZrCuSiAs- and ThCr₂Si₂-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe₂As₂, sharp superconducting transitions at T_c ∼ 38 K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

[en] The discovery of cuprate high T_C superconductors has inspired the search for unconventional superconductors in magnetic materials. A successful recipe has been to suppress long-range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point. We report an exception to this rule in the recently discovered potassium iron selenide. The superconducting composition is identified as the iron vacancy ordered K_0.83(2)Fe_1.64(1)Se₂ with T_C above 30 K. A novel large moment 3.31 μ_B/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature T_N ≈ 559 K for a bulk superconductor. Staggeringly polarized electronic density of states is thus suspected, which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances. (condensed matter: structure, mechanical and thermal properties)

[en] High resolution angle-resolved photoemission measurements have been carried out to study the superconducting gap in the (Ba_0.6K_0.4)Fe₂As₂ superconductor with T_e = 35 K. Two hole-like Fermi surface sheets around the I' point exhibit different superconducting gaps. The inner Fermi surface sheet shows larger (10 ∼ 12 meV) and slightly momentum-dependent gap while the outer one has smaller (7 ∼ 8meV) and nearly isotropic gap. The lack of gap node in both Fermi surface sheets favours s-wave superconducting gap symmetry. Superconducting gap opening is also observed at the M(π, π) point. The two Fermi surface spots near the M point are gapped below T_e but the gap persists above T_e. The rich and detailed superconducting gap information will provide key insights and constraints in understanding pairing mechanism in the iron-based superconductors

[en] High resolution photoemission measurements are carried out on non-superconducting LaFeAsO parent compound and various superconducting RFeAs(O_1-xF_x) (R=La, Ce and Pr) compounds. It is found that the parent LaFeAsO compound shows a metallic character. By extensive measurements, several common features are identified in the electronic structure of these Fe-based compounds: (1) 0.2 eV feature in the valence band, (2) a universal 13-16 meV feature, (3) near Ef spectral weight suppression with decreasing temperature. These universal features can provide important information about band structure, superconducting gap and pseudogap in these Fe-based materials

[en] High resolution photoemission measurements are carried out on non-superconducting SmOFeAs parent compound and superconducting SmFeAs(O_1-xF_x) (x = 0.12, and 0.15) compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13meV is identified in all these samples. Spectral weight suppression near Ef with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds