Heat Capacity Of Iron-based Superconductor And Transport Prop- Erties Of CeO_xF_1-xBiS₂

Superconductivity is the main area of the condensed matter since the discovery. Scientist not only pursued the higher temperature of superconducting transition, but also focus on the mechanism of superconductivity, especially for high-Tc supercon-ductors. By now the convention superconductors can be explained by BCS theory. However the mechanism of high-Tc superconductors is still unsolved, which shows much different from the convention superconductors. Heat capacity is a method to explore system excitation, which could shows the details of superconducting gap and electron-boson coupling.This thesis mainly shows the heat capacity results for iron-based superconductors and the transport measurement of CeOxF1-x BiS2.In chapter three, we introduce the condensation energy study of iron-based superconductors. We measured the specific heat for ten iron-based superconductor samples. Then we use two ways, integrating entropy and formula calculating, to obtain the condensation energy. It is found that U0∝Tc3.5±0.5 which is different from the prediction of BCS theory. Then we focus on the FeAs-122 system. The condensation energy is largest at optimal doping level, decreasing at neither low or high doping area. In our opinion the phenomenon may be caused by the quantum critical point, which could increase the effective mass.Then we introduced the low temperature specific heat measurement of FeS crys-tals. It is found that the low temperature electronic specific heat can be fitted to a linear relation in the low temperature region, but fails to be described by an exponential re-lation as expected by an s-wave gap. By fitting the electronic specific heat data, it is clear that the models with one or two nodal gaps are better than other. The field in-duced specific heat coefficient shows the Volovik relation, suggesting the presence of nodal gap in this material.Chapter five shows the transport properties of CeOxF1-x BiS2 superconductors. The resistivity result shows a bad metal instead of a band insulator. By doping electrons into the system, it is interesting to find that superconductivity appears together with a semiconducting normal state. This evolution is clearly different from the cuprate and the iron pnictide systems. We measured Hall effect and magnetoresistance for x=0,0.25,0.5, and found a low carrier density in this system. In our opinion, with more F doping, the Fermi surface approach the Lifshitz transition, from a four small pockets to a bigger one. Therefore charge density wave instability may induced this phenomenon.At last we introduce the transport properties of Ba(Fe1-x ;Rux;)2As2 single crystals. We used two different methods to calculate the anisotropy. The result is similar with other FeAs-122, which indicates the possible nodes appearing in Ru-doped samples cannot be described by the 3D warping effect of the Fermi surface.