Properties Of Binary Layered Chalcogenides/Pnictides

Chalcogenides/pnictides have many novel physical properties,such as charge density wave?CDW?,unconventional superconductivity,topological state and so on,which have attracted extensive attention in condensed matter physics.Such materials have great application value in photoelectron,sensors,energy storage materials,etc.Binary layered compounds provide an ideal platform for the study of novel physical phenomena on experiment and theory.In this thesis,we focus on the basic physical properties of several binary layered chalcogenides/pnictides.The main four chapters are listed as follows:In chapter one,we introduce the research progresses of transition metal chalcogenides,including the crystal structure,CDW,superconductivity and application properties.Then the CDW is briefly introduced.Finally,the method of in-situ resistance measurement under high pressure is presented.The piston-cylinder device used in testing the transport properties of samples under hydrostatic pressure is highlighted.In chapter two,we study the anisotropy of CDW material HfTe₃ in detail.Single crystals and polycrystals of HfTe₃ were grown using a chemical transport reaction and solid phase reaction in an extremely narrow temperature range,respectively.The resistivity measured on polycrystalline samples shows a broad hump and clear drop at80 and 1.7 K,which correspond to the formation of the CDW and superconducting transition,respectively.In contrast to the polycrystalline samples,the resistivity of single crystals has CDW transition at T_CDW=93 K,and the superconductivity is absent.With the current flowing along the a and b directions,the coincidence of the linear temperature dependence of??T?/??300 K?above T_CDW strongly implies that the electron-electron scattering mechanism dominates the transport properties in a quasi-one-dimensional?Q1D?chain.A metal-semiconductor-like transition is confirmed below T_CDW in?_c.For the single crystal with more defects,the drop observed at 4.3 K in?_b?T?provides direct evidence of a disorder-related superconducting fluctuation in the CDW system.With temperature decreasing,the carrier density exhibits a similar and rapid decrease below T_CDW for flowing current in both the a and b directions,whereas an obvious enhancement of carrier mobility appears as I?b.In addition,an analysis of x-ray photoelectron spectroscopy spectra?XPS?suggests that the mixed-valence states of Hf and Te could be related to the CDW formation in the multichain system of HfTe₃.In chapter three,we try the study the single crystals of HfTe₃ upon intercalation of Cu and Ni,the substitution of Te with Se,and applying pressure.HfTe₃ has a similar crystal structure to ZrTe₃,but the results show that Cu and Pd are difficult to be inserted into single crystals of HfTe₃,which is very different from that in ZrTe₃.We observe that both T_CDW decreases by about 10 K in HfTe_3-xSe_x?x=0.02,0.05?single crystal samples.However,due to the very small Se content and the absence of appreciable change in the lattice parameter c,this could suggest strong sensitivity of CDW to substitutions on Te site and trace Se might lead to disorder.Under hydrostatic pressure,T_CDW gradually increases with increasing pressure up to 1.2 GPa and the maximum value of T_CDW is 127 K.As the pressure continues to increase,the T_CDW begins to decrease slowly.It needs higher pressure to further complete the exploration of the correlation between CDW and possible superconductivity in HfTe₃.In chapter four,we report the transport properties of the CaAs₃ single crystal under atmospheric and hydrostatic pressure,which has been predicted to be a candidate for topological nodal-line semimetal.CaAs₃ has a two-dimensional?2D?crystal structure and exhibits n-type semiconducting behavior with a small gap at ambient pressure.While in some crystals containing tiny defects or impurities,a large“hump”in the resistivity is observed around 230 K,at which the dominated carriers change from hole-type to electron-type.By applying hydrostatic pressure,the samples appear to a tendency towards metallic behavior,but not fully metallized up to 2 GPa.A possible superconducting transition is observed in some samples containing tiny defects or impurities above 1.5 GPa.Further higher pressure studies are needed to explore the topological characteristics of CaAs₃.In summary,we prepared single crystals of HfTe₃ and investigated the intercalation,doping and pressure effect on single crystals,which provided a new platform for exploring the coexistence and competition between CDW and superconducting states in low-dimension systems.In addition,we also explored the topological material CaAs₃,which establishes the foundation for further study of its topological properties in the future.