The Thermoelectric Effect And Superconducting Property Of Iron-based Superconductors LaTAsO(T=Fe,Ni) And BaFe_2-xNi_xAs₂

The recently discovered iron-based superconductors belong to a new kind of unconventional superconductors and this family of superconductors is the second family whose superconducting transition temperature (Tc) beyond the McMillan limit predicted by the BCS theory. To investigate the properties of this family of superconductors will be very helpful to understand the mechanism of high-Tc cuprates and to encourage people to explore the new superconducting materials with higher Tc.Base on the review of recent research progress on the iron-based superconductors, we have studied the transport properties and thermoelectric effects of typical "1111" type LaTAsO(T=Fe,Ni) parent compounds and "122" type Ni-doped BaFe2-xNixAs2 systems. The main results and conclusions are summarized as following.For the "1111" type parent compounds, we found that the thermoelectric effects of LaFeAsO which contains the FeAs layer exhibit anomalous behavior. There is a significant increase in both thermopower and Nernst coefficient when the system is colded down to become the spin-density wave (SDW) state. Meanwhile, the thermopower and Nernst coefficient are very small for LaNiAsO, consistent with conventional metals. We proposed that these anomalies in thermoelectric effects of LaFeAsO should be related to the inter-band scattering and/or the electron correlation effect. The LaNiAsO system with NiAs layer behaves like conventional metals, implying the Fermi liquid behavior, and its superconducting properties are also consistent with conventional superconductors. These results suggest that there is a close relationship between high-Tc superconductivity and anomalous thermoelectric properties in iron-based superconductors containing the FeAs layer.For the "122" type superconductors, we systematically studied the transport properties and Nernst effect of BaFe2-xNixAs2 single crystals with different Ni doping lever. The anisotropy of superconducting properties (including the upper critical field and the critical current) for the optimal doped sample (x=0.1) was studied. For the parent and underdoped samples, we observed the two-fold symmetry of magneto-resistance in the SDW state, consistent with the stripe-type of AFM order. Meanwhile, we found that the temperature dependence of Hall coefficient and the absolute value of thermopower increase with doping and show a maximum at the optimal doping level, which similar to the "1111" type materials. The superconducting properties of BaFe2-xNixAs2 are similar to the Co-doped "122" type BaFe2-xCoxAs2, but the anisotropy is even stronger.The Nernst effect of "122" type parent compound BaFe2As2 exhibits a huge enhancement when it enters SDW state. The normal state Nernst effect decreases gradually with increasing Ni doping, and it finally behaves like conventional metal in the overdoped region. The Nernst effect of the optimal doped sample (x=0.1) indicated that there exists a broad vortex liquid region, larger than that of conventional superconductors but smaller than high-Tc cuprates. This vortex liquid region is obviously smaller than that of "1111" type superconductors like LaFeAsO1-xFx, implying that the three-dimension nature of "122" type materials can help to form the vortex lattice solid. The anomalous Nernst signal associated with preformed pairs above Tc is not observed, absolutely different to high-Tc cuprates. However, the normal state Nernst signal is quite large. We proposed that the large Nernst signal may result from the unusual inter-band scattering between electron and hole bands and/or the electron correlation effect.