Exploration And Study On Transition Metal Chalcogenide Superconductors And Related Materials

It is an important subject in condensed matter physics to study the relationship between charge order and superconductivity in transition metal chalcogenides and to regulate their novel quantum states.In order to deeply understand the crystal structure,superconductivity and charge density wave of the intercalated transition metal dichalcogenides,we have studied the superconducting Ca-intercalated 2H-MoS₂ and Pd-intercalated 2H-TaS₂ systems and several related non-superconducting systems?Pd_xTiS₂,Ca_x WS₂ and Cr_xTaS₂ systems?.In addition,we synthesized the quaternary sulfide mineral kiddcreekite Cu₆WSnS₈ and its homologue Cu₆WSnSe₈ for the first time,and studied their crystal structure and physical properties,which laid a foundation for further exploring the novel physical properties of the quaternary chalcogenide systems and developing their potential applications.We synthesized a series of Ca-intercalated 2H-MoS₂ samples by liquid ammonia method,and studied the evolution of superconductivity with composition.We have found that the system has a dome-like superconducting phase diagram with a maximum T_c of 8.6 K(observed in Ca_0.20?NH₃?_0.17MoS₂)and is the highest of the chemically doped 2H-MoS₂ superconductors to date.The crystal structure of Ca_0.50?NH₃?_0.40MoS₂ was determined by X-ray powder diffraction method?space group:P-3m1,lattice parameter:a?28?3.1984?,c?28?18.5439??.Ca and N occupy the2d and 2c Wyckoff positions,respectively,and S occupies two sets of 2d Wyckoff positions in the unit cell.We determined the superconductivity-related physical parameters?uppercriticalfield,coherencelength,Debyetemperature,electron-phonon coupling parameter and density of states?of the optimally doped samples,and analyzed the possible causes of higher T_c in the sample.We synthesized a series of Pd_xTaS₂ samples and found that Pd doping can effectively suppress the charge density wave in TaS₂ and significantly improve its superconductivity.Compared with pure 2H-TaS₂?T_c?28?0.8 K?,T_c can be increased to4.2 K in Pd_0.04TaS₂.The crystal structure of Pd_0.08TaS₂ was determined by X-ray powder diffraction method?space group:P31c,a?28?3.3151?1??,c?28?12.1497?9???,and it was found that Pd occupies 2a Wyckoff positions?interlayer octahedral interstices?,and S occupies two sets of 2b Wyckoff positions in the unit cell.We have found that doping-induced structural distortion can suppress the charge density wave of the system and enhance its superconductivity.We give a physical explanation of the kink appearing on the susceptibility-temperature curve caused by the charge density wave,and thus define a physical quantity?35??_e that characterizes the intensity of the charge density wave.Based on the electronic phase diagram of the system,we conclude that there is a competition between superconductivity and charge density wave.Kiddcreekite is a rare quaternary sulfide mineral.We synthesized kiddcreekite Cu₆WSnS₈ and its homologue Cu₆WSnSe₈ in the laboratory and found that they have the same cubic crystal structure?space group F-43m?.The lattice parameters are10.8299?7??and 11.28785?9??,respectively.In the unit cell,Cu,W and Sn occupy the 24f,4a and 4c Wyckoff positions,respectively,while S or Se atoms occupy two sets of 16e Wyckoff positions.We verified the rationality of the two structures according to the bond valence theory.We studied the electrical,magnetic and thermal properties of Cu₆WSnS₈ and Cu₆WSnSe₈.It was found that the?-T curves above 30K exhibit metal behavior,while the?-T curves below 30 K could be explained by the variable range hopping mechanism.Both compounds exhibit diamagnetism at room temperature and Curie-Weiss paramagnetism at low temperature.The magnetic susceptibility analysis shows that the effective magnetic moments in Cu₆WSnS₈ and Cu₆WSnSe₈ are most likely due to the contribution of a small amount of Cu^2?10?ions.The Cu^2?10?ions introduce hole carriers into the samples,which makes them show metallicity above 30 K.The Debye temperatures??48?_D?of Cu₆WSnS₈ and Cu₆WSnSe₈are 206.6 K and 191.2 K,respectively,determined from the low-temperature specific heat measurements.According to the above experimental results,it can be considered that both compounds belong to diamagnetic metals.We have synthesized intercalated compounds Pd_xTiS₂,Ca_xWS₂ and Cr_xTaS₂,and studied their crystal structures and physical properties.Pd_xTiS₂ samples have 1T structure?space group:P-3m1?,and both experiments and theoretical calculations demonstrate that they are paramagnetic semi-metals.All Ca_x WS₂ samples have 2H structure?space group:P6₃/mmc?and exhibit paramagnetic semiconducting behaviour.In Cr_xTaS₂ system,the samples with x?0.14 have a 2H structure and those with x?29?0.14 have a superstructure with a?28??3 a₀?where a₀ is the a axis lattice parameter of2H-TaS₂?.We found that Cr_0.16TaS₂ undergoes ferromagnetic transition near 126 K.We have tried to prepare doped graphene by various methods?including solvothermal method,electrochemical method,high temperature pyrolysis method,etc.?,and found that the high-temperature pyrolysis method has the advantages of high yield and uniform doping.We prepared N and B doped graphene samples by this method and studied their structure and low temperature physical properties.X-ray diffraction and Raman spectroscopy indicate that both N and B atoms are doped into the graphene lattice.All doped samples exhibit paramagnetism at low temperatures,and no signs of superconductivity were found therein.These findings have certain reference value for exploring new superconductors in graphene-related materials in the future.