Low-temperature specific heat was measured on the BaFe_(1.9)Ni_(0.1)As_(2) single crystals with critical transition temperature Tc=20.1 K.A clear specific heat jump with the valueΔC/T|_(Tc)≈23 mJ/mol K^(2) was observed.In addition,a roughly linear magnetic field dependence of the electronic specific heat coefficientΔγ(H)was found in the zero-temperature limit,suggesting that at least one Fermi pocket,probably the hole derivative one,was fully gapped with a small anisotropy in the present sample.A slight curvature of the curveΔγ(H)may suggest a complex gap structure(anisotropic gap or nodes)at other Fermi surfaces.
Superconductivity was achieved in Ti-doped iron-arsenide compound Sr4Cr0.8Ti1.2O6Fe2As2 (abbreviated as Cr-FeAs-42622). The X-ray diffraction measurement shows that this material has a layered structure with the space group of P4/nmm,and with the lattice constants a = b = 3.9003 and c = 15.8376 . Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K,confirming the occurrence of bulk superconductivity. Meanwhile we observed a supercon-ducting transition in the resistive data with the onset transition temperature at 29.2 K,which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase.
ZHU XiYu,HAN Fei,MU Gang,CHENG Peng,SHEN Bing,ZENG Bin & WEN HaiHu National Laboratory for Superconductivity,Institute of Physics,Chinese Academy of Sciences,Beijing 100190,China
