| Superconductivity is an important issue in condensed-matter physics. Since it was discovered in1911, this field attracts great attention of researchers. In this disserta-tion, we have grown BaFe2As2single crystals and Co-doped BaFe2As2, we obtain the doping dependence of physical properties:transport, magnetism etc, we get the detail electronic phase diagram and give the evidence of coexistence of superconductivity and spin density wave(SDW). We find the superconductivity in doped phenanthrene. We can realize superconductivity at5～6K by doping alkali-, alkali-earth-and rare-earth-metal. Based on the results of pressure effect and magnetic susceptibility, we believe that type superconductor possible show unconventional superconductivity.1. Brief overview of high-TC superconductors and organic superconduc-torsIn this chapter, we review the background of two types of high TC superconduc-tors:Cuprates and Iron-pnictides. We give detail description of the crystal structure, physical properties and phase diagram of these two types of high-TC superconductors. We also summarize and review the research progress of organic superconductors, some common hydrocarbon molecules are introduced.2. Anisotropy in transport properties and susceptibility of single crystals BaFe2As2In this chapter, we report that sizable single crystals of BaFe2As2have been grown with the self-flux method. Measurements and anisotropy of intrinsic transport and magnetic properties of high quality single crystal are first presented. The resistiv-ity anisotropy (pc/pab) is as large as150and independent of temperature, suggesting that transport in the ab-plane and along the c-axis direction share the same scatter-ing mechanism. In contrast to the susceptibility behavior observed in poly crystalline samples, no Curie-Weiss behavior is observed, and a linear temperature dependent sus-ceptibility occurs from the spin-density-wave (SDW) transition temperature,(Ts), to700K. This result suggests that strong antiferromagnetic correlations are present well above Ts. A twofold symmetry of susceptibility in the ab-plane indicates a stripe-like spin structure as observed by neutron scattering. The resistivity minimum is strongly dependent on the magnetic field. A log(1/T) resistivity associated with the Kondo effect, similar to that of the underdoped cuprates, is observed at low temperatures.3. Peculiar physical properties and Phase diagram of BaFe2-xCoxAs2single crystalsWe study systematically transport, susceptibility and heat capacity of single crys-tals BaFe2-xC0xAs2. In the underdoped region, SDW transition was observed in both resistivity and susceptibility. The magnetic susceptibility shows unusual T-linear de-pendence above SDW transition up to700K. With Co doping, SDW ordering is grad-ually suppressed and superconductivity emerges with a dome-like shape. Electrical transport, specific heat and magnetic susceptibility indicate that SDW and supercon-ductivity coexist in the sample BaFe2-xCoxAs2around x=0.17, being similar with that in (Ba,K)Fe2As2. The superconductivity completely disappears for the samples with x>0.34. A crossover from SDW state to Fermi-liquid state is observed with in-creasing Co doping. A detailed electronic phase diagram about evolution from SDW to superconducting state is given.4. Structure and Physical Properties for a new Layered Pnictide-Oxides: BaTi2As2OWe have successfully synthesized a new layered pnictide-oxides:BaTi2As20. It shares similar characteristics with Na2Ti2Sb2O. The crystal has a layered structure with a tetragonal P4/nmm group (a=4.047(3) A, c=7.275(4) A). The resistivity shows an anomaly at200K, which should be ascribed to a SDW or structural transition. The SDW or structural transition is confirmed by magnetic susceptibility and heat capacity measurements. These behaviors are very similar to that observed in parent compounds of high-Tc iron-based pnictides superconductors, in which the supercon-ductivity shows up when the anomaly due to the SDW or structural transition is suppressed. Therefore, the new layered pnictide-oxides:BaTi2As2O could be a poten-tial parent compound for superconductivity. It is found that Li+doping significantly suppresses the anomaly, but no superconductivity emerges so far.5. Superconductivity at5K in alkali-metal doped phenanthreneIn this chapter, we prepared the series of the samples Kxphenanthrene with differ-ent potassium content. It is found that only the sample with nominal composition of K3phenanthrene shows superconductivity, and all other samples with x deviation from3do not show superconductivity. Here, we observed only a superconducting transi-tion for the superconducting K3phenanthrene and Rb3phenanthrene, being different from the case of potassium doped picene in which there exist two superconducting phases with TC=7and18K, respectively. There exists a big family of ploycyclic aromatic compounds with an extended phenanthrene-like structural motif designated as [n]phenacens, where n is the number of fused benzene rings.[n]phenacene molecules are related to layers of graphene in the way that ribbons are related to sheets. There-fore, discovery of superconductivity in A3phenanthrene (A3C14H10, A=K and Rb), besides the superconductor Kxpicene, opens a new broad family of superconductors that consists of aromatic hydrocarbons. Searching for superconductors in such fam-ily of [n]phenacene molecules and studying their physical properties are very impor-tant to understand mechanism of superconductivity. Tc of5K is relatively low for A3Phenanthrene (A=K, Rb) with three benzene rings, compared with7K and18K for Kxpicene with five benzene rings. It suggests that such organic hydrocarbons with long benzene rings are potential superconductors with high TC.6. Unconventional superconductivity in alkali-earth and rare-earth metal doped phenanthreneIn this chapter, we found superconductivity in alkali-earth (Sr&Ba) and rare-earth (La&Sm) doped phenanthrene, superconducting transition temperature(Tc) are about5.6K and6K for alkali-earth and rare-earth metal, respectively. The shielding fraction for all the alkali-earth and rare-earth metal doped phenanthrene is much big-ger than alkali-metal doped phenanthrene and picene, this is the highest record in hydrocarbon superconductors system. Raman spectrum show7and8cm-1per elec-tron downshift, which is quite similar with A3phenanthrene and A3C6o(A=K、Rb). The pressure dependent TC shows a positive parameter:d[TC/TC(0)]/dP. For the alkali-earth and rare-earth doped phenanthrene, the larger ion radii with the lower TC, combining the pressure effect, we can draw a conclusion that these superconduc-tors are possible unconventional ones. |
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