In this thesis, two kinds of superconductors Ca₃(Ir_0.91Co_0.09)₄Sn₁₃ and Ba_1-xK_xFe₂As₂ were investigated through various experiments including magnetization, specific heat, thermal expansion, thermal conductivity, resistivity and point contact Andreev reflection spectroscopy.

The specific heat and resistivity data of Ca₃(Ir_0.91Co_0.09)₄Sn₁₃ are presented, together with further analysis to study the electron-phonon interaction. We demonstrated the important role of the low-lying phonon modes in the pairing mechanism of this superconductor which points to a conventional phonon-mediated origin of superconductivity in Ca₃Ir₄Sn₁₃ and rules out a magnetic spin fluctuation mediated pairing scenario.

The critical fluctuation in the iron based superconductor Ba_1-xK_xFe₂As₂ was studied by apply...[ Read more ]

In this thesis, two kinds of superconductors Ca₃(Ir_0.91Co_0.09)₄Sn₁₃ and Ba_1-xK_xFe₂As₂ were investigated through various experiments including magnetization, specific heat, thermal expansion, thermal conductivity, resistivity and point contact Andreev reflection spectroscopy.

The specific heat and resistivity data of Ca₃(Ir_0.91Co_0.09)₄Sn₁₃ are presented, together with further analysis to study the electron-phonon interaction. We demonstrated the important role of the low-lying phonon modes in the pairing mechanism of this superconductor which points to a conventional phonon-mediated origin of superconductivity in Ca₃Ir₄Sn₁₃ and rules out a magnetic spin fluctuation mediated pairing scenario.

The critical fluctuation in the iron based superconductor Ba_1-xK_xFe₂As₂ was studied by applying the 3D XY and the 3D LLL scaling models to our specific heat and thermal expansion data. Our results indicate the weakest critical fluctuations at optimal doping and demonstrate that a strong increase of the critical fluctuations regime is responsible for the transition broadening in magnetic fields, which is a direct consequence of a magnetic-field-induced finite size effect and the confinement of quasiparticles in low Landau levels.

The re-entrant tetragonal phase of Ba_0.76K_0.24Fe₂As₂ was investigated with various experimental probes. With high precision magnetization measurements, we for the first time proved the spin reorientation in the re-entrant tetragonal phase transition in Ba_0.76K_0.24Fe₂As₂. Together with earlier investigations in Na-doped Ba122, our result reveals that the spin reorientation is a universal property in the hole doped Ba122 system.

The superconducting pairing symmetry of Ba_0.7K_0.3Fe₂As₂ was studied by field-angle resolved thermal conductivity experiment. A six-fold oscillation in the thermal conductivity was observed, which is inconsistent with the tetragonal crystal lattice structure, indicating a novel highly complex superconducting nodal order parameter symmetry rather than ±s-wave or d-wave symmetry.