| Superconductivity is remarkable in that it is a macroscopic quantum phenomenon, which bears rich physical connotation. As far as the fundamental superconductivity re-search is concerned, on one hand, researchers are committed to unravel the underlying mechanism for superconductivity, especially the exotic ones which are different from the conventional electron-phonon interaction scenario; on the other hand, much efforts are de-voted to search for new superconductors, among which those exhibiting coexistence of superconductivity and ferromagnetism are of particular interest.In this dissertation, we have studied:(1)the upper critical field, superconducting phase diagram and thermal stability of triangular lattice Naχ(H3O)zCoO2·yH2O superconductor; (2) the doping effect at Fe and As sites in ternary iron arsenide EuFe2As2. The main con-clusions of the dissertation are highlighted as follows.(i)Resistivity,59Co-NMR, specific heat measurements suggest that the zero tempera-ture upper critical field for Hc2(0) for Naχ(H3O)zCoO2·yH2O superconductor is within the Pauli paramagnetic limit Hp≈8.3T, which is much smaller than that inferred from mag-netic measurements. To resolve this controversy, we present a detailed magnetization study on aligned superconductors with magnetic field parallel to CoO2 layers up to fields of 8T. The temperature-dependent part of the normal-state magnetization is revealed to arise from the paramagnetic impurity rather than the spin fluctuations. The onset of superconductivity is characterized by the deviation of normal-state magnetization governed by Brillouin func-tion. Our result suggests that the H(?) (0) value is just within the Pauli paramagnetic limit, which is much smaller than those of the previous results by magnetic measurements and consistent with the values from other measurements. In addition, our results also support spin-singlet superconductivity in the superconductor.(ii) The superconducting phase diagrams for Naχ(H3O)zCoO2·yH2O superconductor proposed by different groups are far from consistency, which basically comes from the un- certainty in determining the Co valence(VCo) as a consequence of inevitably hydronium incorporation during the hydration process. Here we present a facile route to tune and mea-sure VCo by a topotactic postreduction using NaOH as reducing agent. The change of VCo can be precisely measured by measuring the volume of released oxygen. The remarkable advantage of this method lies in that the disturbance of hydronium incorporation can be eliminated to a great extent for the sample treated in concentrated NaOH solution, making the absolute Vco determinable. As a result, an updated superconducting phase diagram was obtained, which shows that the superconducting transition temperature increases mono-tonically with increasing Co valence in a narrow range ranging from+3.58 to+3.65.(iii)The Naχ(H3O)zCoO2·yH2O superconductor is quiet unstable at ambient condi-tion, very easily losing parts of its cyrstalline water and becoming non-superconducting. This sever chemical instability results in frequentely inconsistent experimental results in the literatures. We demonstrate that stabilization of the superconductor is feasible by treat-ing the as-prepared superconductor in concentrated NaOH solution. It is hoped that further study of the stabilized superconductor will resolve the continuing controversy in physical property measurements and enrich our knowledge of this fascinating material.(iv)We have performed the transport and thermodynamical measurements of the ternary iron arsenide EuFe2As2 for the first time. It is found that EuFe2As2 undergoes two mag-netic transitions at 200K and 19K, respectively. The former is accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribu-tion), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like (SDW) antiferromagnetic transition. The latter is proposed to be associated with an A-type antiferromagnetic ordering of Eu2+ moments, which also undergoes a field-induced metamagnetic transition. Comparing with the physical properties of the iso-structural com-pounds BaFe2As2 and SrFe2As2, we expect that EuFe2As2 is a possible parent compound for superconductors, as has been soon verified by subsequent experiments.(v)We have systematically studied the transport and magnetic properties on a series of EuFe2-χNiχAs2 polycrystalline samples with 0 |
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