Two Superconducting States In The LaFeAs_1-xP_xO System

Since the discovery of superconductivity in iron pnictides in2008, the superconducting mech-anism in the new superconducting materials has been a hot topic. There are still debates on the pairing mechanism and pairing symmetry, competition or coexistence with other ordering states. More experimental evidences are necessary to address these issues. In this thesis, superconductiv-ity induced by chemical pressure (P-for-As doping) is revisited in the1111-type LaFeAs1-xPxO system, and two distinct superconducting states are discovered around x=0.3and x=0.7. The two supercondonducting states are characterized by many measurements.The thesis consists of three chapters. The first chapter is an overview of iron pnictides su-perconductors, including the discovery of this material, the six types of crystallographic structures and superconductivity induced by electron, hole and chemical doping in iron pnictides. Our exper-imental methods are summarized in the second chapter, where the powder x-ray diffraction (XRD) and transport property measurements such as resistivity, Hall, and thermopower as well as specific heat measurements will be introduced in detail.The third chapter is the key chapter, in which the physical properties of LaFeAs1-xPxO poly-crystalline samples are investigated systematically. A new superconducting state around x=0.7is discovered beyond the usual optimally doped level of x=0.3. Two distinct superconducting states with the optimally doped levels around x=0.3and x=0.7respectively are characterized by re-sistivity, Hall effect, thermopower, and magneto-resistance measurements. Furthermore, the upper critical field (Hc2) is obtained by magneto-resistance measurements and Hc2of the x=0.3sample is24.8T, much higher than the Pauli limit of18T, while Hc2of the x=0.7sample is only13.7T, suggesting the different superconducting mechanisms in the two doping regimes. Hall and specific heat measurements are also performed in order to explore the difference between the two states. It is found that the charge carrier is electron type in the whole doping range. The Hall coefficient at the low P-content side (x～0.3) is strongly temperature dependent, indicating a residual spin-density wave type antiferromagnetic ordering, while it exhibits a weak temperature dependence at high P-content side. There are also significant difference in the γ value, the Sommerfeld coeffi-cient, derived from the specific heat data measured under magnetic field. The study of Zn impurity effect found that superconductivity is significantly suppressed by Zn impurity at the low P-content side but it seems quite robust against the Zn impurity at high P-content side. All these results to a conclusion that the two superconducting states are distinctly different.