| More than one century has passed since the superconductivity was first discovered in 1911, however, searching for new superconductors is still the hot topic and frontier of the condensed state physics by now. Through long-term effort, especially during the past three decades, one has learned two basic rational routes for new superconducting materials. The first one is to carry out chemi-cal doping in a so-called parent compound which is potentially superconducting. The discovery of BiS2 based superconductor is originating from the doping effect of the parent compounds. The oth-er is to synthesize a new compound that contains low-dimensional superconductively active layers via structural design. Prominent examples are manifested by copper-based and iron-based high-temperature superconductors, in which the superconductively active layers are 2D CuO2 planes and Fe2As2 layers, respectively.Following this guide line, we synthesized two novel BiS2 based superconductor EuBiS2F(Eu 1121) and Eu3Bi2S4F4(Eu3244). We studied the negative chemical pressure effect of layered titanium oxypnictide BaTi2Sb2O by the isovalent substation of Sb by Bi.In the undoped layered EuBiS2F(Eu1121) a CDW-like transition occurs at 280 K, following which SC emerges at 0.3 K. Unlike the other BiS2 based parent compound, which shows semicon-ductor behavior, Eu1121 is metallic because the mixed valence of Eu, the average valence of which is about+2.2. The mixed valence of Eu gives rise to self electron doping into the conduction band-s mainly consisting of the in-plane Bi6p states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by 50 times, owing to the significant hybridizations between Eu4f and Bi6p electrons. Due to the lower doping level(x～0.2) and the magnetism of Eu4f electron the The TC of Eu1121 compound is lower than the other BiS2 based superconductors. Since the maximum TC could be achieved at x-0.5, we synthesized a series compounds, Eu0.5Ln0.5BiS2F (Ln=La, Ce, Pr, Nd and Sm). The TC of Eu0.5Ln0.5BiS2F were en-hanced with the increasing ion radius of Ln. For the Eu0.5La0.5BiS2F the TC was elevated 9 times to 2.8 K, compared to the parent. The coexistence of ferromagnetic ordering at 8 K for the Ce-4f moment, superconductivity at 2.2 K and anti-ferromagnetism of Eu 4f electrons at lower temper-ature shows up in the Eu0.5Ce0.5BiS2F. Due to the lanthanum substation of Eu, the mixed valence effect were not shown in these Eu0.5Ln0.5BiS2F (Ln=La, Ce, Pr, Nd and Sm) compounds, the resistivity of normal state turn to semi-metallic because of the disorder.We synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4 (Eu3244), by solid-state reactions, which crystallizes in a tetragonal lattice (space group I4/mmm). Eu3244 is stacked by CaF2-type Eu3F4 layers and NaCl-like BiS2 bilayers alternately along the crystallographic Z axis. There are two crystallographically distinct Eu sites Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond valence sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, while Eu(2) has an average valence of-+2.64. This anomalous Eu valence state is further confirmed and supported, respectively, by Mossbauer and magnetization measurements. The material itself shows metallic conduction and superconducts at 1.5 K without extrinsic chemical doping, which could be explained by the self-dopping effect like in Eu1121. The AFM order of Eu 4f moment, and the CDW abnormal hump were smeared out because of the higher doping level(x～0.3).BaTi2Sb2O shows superconductivity at 1.5 K as well as charge- or spin-density wave (CD-W/SDW) ordering below 55 K. With the partial substitution of Sb by Bi, we studied the negative chemical pressure effect of BaTi2(Sb1-x Bix)2O. Bi doping enhanced the TC to its maximum TC= 3.7 K at x=0.17. The CDW/SDW anomaly is rapidly suppressed by Bi doping, and vanishes for x≥0.17. A metal-to-nonmetal transition takes place around x=0.3, the superconductivity is still survived at this point. We picture out the phase diagram of the relation between superconductivity and DW order in BaTi2(Sb1-x Bix)2O. |
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