Li Zheng

Superconductivity in Hole-Doped (Sr1−xKx)Fe2As2

A series of layered (Sr1-xKx)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 Tc ~ 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.

Element Substitution Effect in Transition Metal Oxypnictide Re(O1-xFx)TAs (Re = rare earth, T = transition metal)

Different element substitution effects in transition metal oxypnictide Re(O1-xFx)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 Tc to about 50 K. While in the case of Gd, Tc 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 LaO1-xFxRuAs and CeO1-xFxRuAs compounds. The metallic behaviour is observed down to 4K.

Magnetoresistance in Parent Pnictide AFe2As2(A = Sr, Ba)

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.

Superconductivity in Mg-Doped Layered Intermetallic Compound NbB2

We have performed low temperature resistivity ?(T) and specific heat C(T) measurements on a superconducting polycrystalline Nb0.75Mg0.25B2 sample. The results indicate that the superconducting transition temperature is ~ 4.6 K. The zero temperature upper critical field determined from the resistivity and specific heat is 3123 Oe. The electronic coefficient of specific heat ?n = 4.51 mJmol?1K2 and the Debye temperature ?D = 419K are obtained by fitting the zero-field specific heat data in the normal state. At low temperatures, the electronic specific heat in the superconducting state follows Ces/?nTc = 2.84exp(-1.21Tc/T). This indicates that the superconducting pairing in Nb0.75Mg0.25B2 has s-wave symmetry.