In this thesis, we study different members of the ferropnictide superconductor family BaFe₂As₂. The main focus is on observing the vortex melting transition in a high purity single crystal of Ba_0.486K_0.514Fe₂As₂ at low pressure. For this purpose we design our own highly sensitive calorimeters, with continuous improvement throughout the process. We investigate with a variety of techniques, with the help of the AMI bottom loaded magnet systems and a SQUID VSM. In particular our data obtained from the AC calorimetry method gives us not only confirmation on the presence of the vortex melting transition, but also shows the first order nature of the transition. In addition, we study a few different members of the Ba122 family. We analysis the superconducting properties of various cobalt, pota...[ Read more ]

In this thesis, we study different members of the ferropnictide superconductor family BaFe₂As₂. The main focus is on observing the vortex melting transition in a high purity single crystal of Ba_0.486K_0.514Fe₂As₂ at low pressure. For this purpose we design our own highly sensitive calorimeters, with continuous improvement throughout the process. We investigate with a variety of techniques, with the help of the AMI bottom loaded magnet systems and a SQUID VSM. In particular our data obtained from the AC calorimetry method gives us not only confirmation on the presence of the vortex melting transition, but also shows the first order nature of the transition. In addition, we study a few different members of the Ba122 family. We analysis the superconducting properties of various cobalt, potassium and phosphorus doped samples, and in particular we compare a series of BaFe₂(P_xAs_1-x)₂ crystals of different doping. Through this investigation we provide informative results, which could improve the general understanding of superconductivity in the ferropnictides.