Exploration And Study On Novel Transition Metal Dichalcogenides And Phosphide Superconductors

Since transition metal dichalcogenides?TMDs?have quasi-two-dimensional crystal structures,their electrical,magnetic,and optical properties can be regulated by changing the combination of elements,the number of stacked layers,and the type and number of intercalated atoms.The study on TMDs is not only of scientific significance,but also of important industrial application value.There is still debate about the relationship between superconductivity?SC?and charge density wave?CDW?in transition metal dichalcogenides?TMDs?superconductors.Since the transition metal phosphides have crystal structure similar to that of the widely studied iron-based arsenide superconductor,a high-temperature superconductor may also be found therein.Therefore,this subject explores the superconductivity of TMDs and transition metal phosphides through experiments and theoretical calculations.The main research results are as follows:We found that Cu intercalated 2H-Cu_x TaSe₂ significantly enhanced its superconductivity with a maximum T_c of 2.7 K?x=0.07?.The structure and superconductivity of 2H-Cu_xTaSe₂?0?x?0.12?were systematically studied.The crystal structure refinement indicates that the Cu atoms are located at the octahedral interstices.In addition,as the amount of doping increases,the CDW is gradually suppressed,and the superconductivity is gradually enhanced,indicating that there is a clear competition relationship between?CDWs?and SC.The increase in T_c in2H-Cu_xTaSe₂ is due to significant increase in the Fermi level density N?E_F?and the electron-phonon interaction parameter?introduced by Cu doping.We found that the post-transition metal Pb,Sn,and Bi intercalation can significantly enhance the superconductivity of TaSe₂.When x=0.1,the T_c is 2.6 K,2.7 K,and 2.9 K,respectively.A superconducting transition occurred only at x=0.1in the Bi_xTaSe₂?0?x?0.16?system.As the amount of doping increases,the crystal structure changes from 4H_a to 2H.We also found that substitution of Re and Ag for Ta can significantly improve the superconductivity of TaSe₂,with T_c of 2.4 K and2.75 K,respectively.Enhanced superconductivity was also found in the non-stoichiometric Ta_1+xS₂ and Ta_1+xSe₂ systems with maximum T_c of 3.5 K and 3.2 K,respectively.We found that there are two kinds of disordered structures in Ta_1+xSe₂:x?0.06,it is a mixed layer structure composed of 2H_b and 3R layers;when x?0.08,it is a random stack of single TaSe₂ layers with Ta atoms in trigonal prismatic coordination of Se.Superconductivity enhancement was observed in both of the above disordered structures.These results have important reference value for exploring new superconductors in the stacking-disorder systems in the future.We calculated the electronic structure of 60 binary transition metal phosphides using first principles.By analyzing the electronic density of states of superconducting and non-superconducting compounds,we summarize three criteria for predicting the superconductivity of transition metal phosphides:?1?Fermi level E_F should be located at the peak of density of states;?2?the Fermi level density N?E_F?is high;?3?the contribution of p electrons to N?E_F?should reach a certain proportion.Based on these criteria,we predicted eight superconducting transition metal phosphides,namely NbP,TiP₂,MoP₂,Zr₂P,Ir₂P,Hf₂P,Re₃P₄ and Re₆P₁₃.We explored the superconductivity of Ta_1-xM_xP₂?M=Ti,W?and several other ternary phosphides.We found superconductivity above 10 K in sample with a nominal composition of Ba1.04Ru2P2.08.We studied the low temperature physical properties of the Sn_xBi_1-xTe?0.0?x?0.4?system.X-ray diffraction analysis revealed that a structural transition of BiTe to Bi₄Te₃ occurred at x=0.20,and another transition of Bi₄Te₃ to Bi₂Te occurred at x=0.32.Resistance measurements show that the resistance of the doped sample is metallic when 0.04?x?0.32,and the insulator-metal transition occurs at T=220K for sample with x=0.40.