Transport Properties Of K_0.5CoO₂ And Li_0.9Mo₆O₁₇ Single Crystals And Phase Diagram In Ba_1-xFe₂As₂ System

The transition metal oxides have been widely studied due to their intriguing prop-erties, such as colossal magnetoresistance, abnormal Hall e?ect, large thermoelectricale?ect and multiferronics. In these systems charge, spin, orbital and lattice are cou-pled to each other, and complicated phase diagrams are introduced by the competitionamong these interactions, giving rise to some interesting phenomena such as chargeordering, magnetic ordering, glassy behavior, phase separation, metal-insulator tran-sition and spin-stat transition. These are important not only for scientific purpose,but also for the possibility of application.In this dissertation, we have studied the phase diagram of 122-systemBa1?xKxFe2As2, and transport and magnetic properties of triangular layered cobaltoxides K0.5CoO2 and Li0.9Mo6O17 single crystals.The whole dissertation is constructed as following:1. Brief overview of the physical properties of the high-Tc pnictide andtriangular cobalt oxidesIn this chapter, the author has reviewed the basic physical properties on KxCoO2system, and iron-based superconductors and Li0.9Mo6O17. For KxCoO2 system, wehave reviewed the crystalline structure and phase diagram in NaxCoO2 first, andthen the physical properties of KxCoO2. For iron-based superconductors, we haveintroduced the crystalline structure, transport properties and phase diagram. ForLi0.9Mo6O17, we have reviewed the crystalline structure and discussed the hot issueabout the origin of metal-insulator transition.2. Magnetotransport properties in K0.50CoO2 single crystalsIn this chapter, We have measured susceptibility and angle-dependent magnetore-sistance (AMR) on K0.50CoO2 single crystals. A possible magnetic structure similarto that of Na0.5CoO2, a G-type anti-ferromagnetic (AF) structure (both in-plane andinter-plane are AF), is proposed. At～20 K, a loop is observed between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility. The absolute value of magnetore-sistance (MR) at 20 K is much larger than other cases with H applied within the abplane. This could arise from the magnetic field being applied in the ab plane, leadingto spin–?op of the small magnetic moment Co3.5?δsites at 20 K to form in-planeferromagnetic ordering, as observed in Na0.52CoO2. 3. Coexistence of the spin-density-wave and superconductivity in theBa1?xKxFe2As2In this chapter, The relation between the spin-density-wave (SDW) and super-conducting order is a central topic in current research on the FeAs-based high TCsuperconductors. Con?icting results exist in the LaFeAs(O,F)-class of materials, forwhich whether the SDW and superconductivity are mutually exclusive or they cancoexist has not been settled. Here we show that for the (Ba,K)Fe2As2 system, theSDW and superconductivity can coexist in an extended range of compositions. Theavailability of single crystalline samples and high value of the energy gaps would makethe materials a model system to investigate the high TC ferropnictide superconduc-tivity..4. Magnetotransport properties in purple bronze Li0.9Mo6O17 SingleCrystalwe have studied resistivity along the a, b and c axes on the same piece of thesingle crystal, respectively. The anisotropics ofρc/ρa andρc/ρb were given, whichconfirm the quasi-one-dimensionality of the compound. The sharp decrease in theanisotropy below TM indicates dimensional crossover at this temperature. Unusuallyhuge positive MR for H c has been observed. We invoked a modified two-bandmodel to fit the MR data which had been used in CDW compounds such as quasi-two-dimensional purple bronzes A0.9Mo6O17 ( A = K, Na, Tl ) and quasi-one-dimensionalconductor NbSe3. The behavior of MR rather supports the existence of CDW or SDWinstability in Li0.9Mo6O17.