| The discovery of iron-based high temperature superconductors has invoked intense research activities. The properties of this family seem to be dominated by various 3d-bands of the Fe-ions. The competitive interactions among the bands, on one hand, cause a complicated magnetism-superconductivity phase-diagram, yet they may offer a new route for carrier paring. Three key issues thus have been raised: A) how magnetism evolves into superconductivity with doping; B) should these superconductors be regarded as homogeneous or mesoscopic mixtures of magnetic and superconducting regions; C) what is the gap structure. This dissertation explores the three issues through the specific heat and magnetization experiments. Improved experimental and analysis procedures were developed, which enable us to better address these topics. The carrier specific heat Ce/T of Sr1-- xKxFe2As 2 deduced over 0 ≤ x ≤ 1, for example, varies with both the doping and temperature in a complex manner, which disagrees with the models associating the evolution with a competition for the Density of States at Fermi level (such as in the cuprates). Instead, shifts in interband interactions and the associated mass enhancement seem to be a better interpretation. We regard this as evidence for multiband nature of this family. Additionally, in the superconducting state of Sr1-- xKxFe2As 2, the apparent transition temperature Tc, the superconductive volume fraction as well as the penetration depth strongly depend on the characteristic length scales of the probes used. Discrepancies as large as ten-fold are observed. The results are taken as evidence for mesoscopic phase separations. It seems that, while the optimally doped Sr 0.55K0.45Fe2 As2 can still be considered homogeneous down to 10 nm, both the underdoped and the overdoped members are better regarded as Josephson-Junction-Arrays in terms of their resistivity, magnetization and specific heat properties. The gap structures of LiFeAs, Sr1-- xKxFe2As 2 and Ba1--x KxFe2As2 were also explored. A model with two-gap, s-wave pairing seems to describe the data well in each case. The data reveal that the controversial interpretation reported previously for Ba1--x KxFe2As2 with x ≈ 0.3 - 0.4 is the direct result of incorrect phonon backgrounds. Further, Ba0.6K0.4Fe 2As2 and Sr0.55 K0.45Fe2As 2 share a very similar two-gap structure, although the coupling strength is larger in Ba0.6K0.4 Fe2As2, again demonstrating the multi-band nature of the Fe-based superconductors. |
