Svnthesis And Physical Property Of Novel Bi- And Sb- Based Compounds

Exploring novel states of matter, physical phenomena and their physical properties is all the time the hot issue in the condensed matter physics and material science. In the past few years, some pnictides-based compounds have substantially attracted great interests due to exhibiting exotic physical properties. The typical two examples that one is the newly discovered BiS₂-based superconductors with layered crystal structure; another is the topological semimetal materials TaP（As） with the observed Weyl points experimentally have been in particular investigated.In this dissertation, the studies mainly focus on two aspects:（1） The doping effect and their superconductivity of the newly discovered BiS₂-based compound SrFBiS₂, including Ce or Pr doping into the Sr site; and the substitution of Se for S; （2） The synthesis and transport properties of a new discovered topological semimetal TaSb2. Several innovative results in this dissertation are listed as following.（1）. Through a combination of X-ray diffraction, electrical transport, magnetic susceptibility and heat capacity measurements, we found that Sr_0.5Ce_0.5FBiS₂ undergoes a ferromagnetism transition below ～ 7.5 K, followed by a superconducting transition with the critical temperature Tc ～2.8K, implying a coexistence of both superconductivity and ferro-magnetism. Here, Ce doping not only provides carriers to the superconducting BiS₂ layers, but induces a FM ordering in the blocking SrF layers.（2）. We report a new BiS₂-based Sr_0.5Pro.sFBiS₂ superconductor with Tc of 2.7 K at ambient pressure. Upon applying pressure, Tc is abruptly enhanced, reaches 8.5 K at a critical pressure of Pc= 1.5 GPa and remains a slight increase up to 9.7 K up to 2.5 GPa. Accompanied with the enhancement of superconductivity from the low-Tc phase to the high-Tc phase, the normal state undergoes a semiconductor to metal transition under pressure. Meanwhile, the upper critical field （Bc2（0）） is substantially enhanced in excess of the Pauli paramagnetic limit. Phase diagram of electronic state is thus obtained.（3）. We first synthesized a new BiSe2-based superconductor Sr_0.5Lao.5FBiSe2 with Tc of 3.8 K. Different to other BiS₂-based compounds developed from a semiconducting-like normal state, the sample exhibits a metallic behavior down to Tc. In particular, the opposite pressure effect in the present sample shows that Tc decreases monotonously but Tmin reverses to high temperature. Moreover the absolute value of the normal state resistivity at low temperature first decreases and then increases with pressure up to 2.5 Gpa. Those results imply that the electronic structure of Fermi surface may be a difference in nature between in Sr_0.5La0.5FBiSe2 and in the other BiS₂-based systems.（4）Topological insulators and topological semimetals have attracted much attention because they exhibit exotic quantum phenomena. Here we report the discovery of a new member in TSMs, TaSb2. This compound crystallizes in a base-centered monoclinic, centrosymmetric structure, and shows metallic behavior at ambient pressure and zero field. By applying magnetic fields, it exhibits insulating behavior before appearance of a resistivity plateau below 13 K, and with a ultrahigh carrier mobility and extreme magnetoresistance （XMR） for the field perpendicular to the current. Through quantum oscillation and theoretical calculation, we believe that this system is a topological semimetal with non-trivial Berry phases. Moreover, when B//I, the negative magnetoresistance is observed and does not saturate up to 9T. Thus in the centrosymmetric structure TaSb2 system, the saturated resistivity plateau and negative magnetoresistance are observed at the same time, which also opens the new ideals and paths to further unstanding the quantum behavior of topological semimetal materials.