| This paper mainly includes two parts. In the first part, a new nickel-based superconductor CsNi2Se2 was synthesized successfully. The structure, transport properties and magnetic properties were systematically studied. The results shown that the structure of CsNi2Se2 is similar with that of 122-BaFe2As2. At low temperatures, a superconducting transition with Tc?2.7 K was observed, with a large sommerfeld coefficient ?n(?77.9mJImol·K-2). In the superconducting state, we found that the electronic heat of zero field data,Ces(T)(0.5K<T<2.7K), can be well fitted with a two-gap BCS model, indicating the multi-gap feature of CsNi2Se2. In the end, the comparison of the first principles suggested that the large ?n in these nickel-selenide superconductors may be related to the large density of States at the fermi surface.In the second part, a series of TlNi2Se2-xSx (0.0?x?2.0) single crystals were synthesized successfully. Measurements of resistivity, specific heat, and susceptibility were carried out on these crystals. The results shown that the cell parameters a and c of T1Ni2Se2-xSx decrease monotonously with the increase of S concentration x, indicating the existence of positive chemical pressure. In all the samples, there was a superconducting transition at low temperature and the electronic heat coefficient is much larger than that of the other nickel arsenic superconductors, suggesting a heavy electronic behavior in these compounds. Under magnetic fields, a novel change of the field dependence of the residual specific heat coefficient, ?n(H), occurs in T1Ni2Se2-xSx with increasing S content. At the end, we studied the superconductivity in all of the T1Ni2Se2-xSx samples, and it was found that the superconducting transition temperature Tc is closely related to the disorder degree, which is characterized by the residual resistivity ratio (RRR). Thus, the T1Ni2Se2-xSx system provides a platform to study the effect of disorder on the multi-band superconductors.In the third part, single crystal BaMnBi2 was synthesized successfully. The structure, transport properties and magnetic properties were systematically studied. At low temperatures, BaMnBi2 exhibits a significantly SdH oscillation. The combining analysis of quantum oscillations and first principles band structure calculations indicate that there are two distinct Dirac cones near the Fermi level. However, compared with CaMnBi2 and SrMnBi2, the yielding two berry phases are trivial, which may be attributed to the existence of the energy gap near the Dirac cones, due to the strong spin orbital coupling effect in BaMnBi2. On the other hand, angular-dependent magnetoresistance and quantum oscillations suggest dominant two-dimensional Fermi surfaces.At last, we report pressure-induced superconductivity in the antiferromagnetic Dirac material BaMnBi2 (Tn-288 K). Resistive superconducting transition is observed above 2.14 GPa, while zero resistivity is only achieved at the highest pressure investigated,2.58 GPa. At this pressure, the superconducting transition temperature Tc reaches 4 K and the corresponding zero temperature upper critical field is estimated to be T 7 Tesla. Moreover, a large linear magnetoresistivity is observed after the superconductivity is suppressed at T= 2 K, indicating a nontrivial connection between Dirac Fermions and superconductivity. Thus, BaMnBi2 may provide a new platform to study the nontrivial connection between superconductivity and dirac fermions... |
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