The transport properties of iron-based superconductors

Abstract

There are a variety of order states in iron-based pnictides, such as electronic nematic phase, spin density wave, and so on, which leads to plenty of novel physical phenomena. The measurements of transport properties can provide extremely useful information for understanding of the low-energy excitations of iron-based superconductors. Due to the multi-band electronic structure in iron-based pnictides, the temperature dependence of resistivity and Hall coefficient varies with different systems, however, there are no evidence for the pseudo-gap opening in the normal state which is a common feature in underdoped high-<inline-formula><tex-math id="M2">\begin{document}$T_{\rm{c}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201836_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201836_M2.png"/></alternatives></inline-formula> cuprates. In the hole-doped iron-based superconductors, the Hall coefficient changes its sign in low temperatures, and meanwhile the resistivity shows a broad hump in the same temperature range. Such a behavior is proposed as a crossover from incoherent to coherent transport. The Seebeck coefficients of iron-based superconductors also show remarkable differences from the cuprates. In iron-based superconductors, the absolute value of Seebeck coefficients in the normal state becomes the largest at the optimally doping point with highest <inline-formula><tex-math id="M3">\begin{document}$T_{\rm{c}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201836_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201836_M3.png"/></alternatives></inline-formula>, which is probably related to the strong inter-band scattering. The Nernst effect in the normal state of iron-based superconductors indicates that superconducting phase fluctuations is not obvious above <inline-formula><tex-math id="M4">\begin{document}$T_{\rm{c}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201836_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20201836_M4.png"/></alternatives></inline-formula>, which is also significantly different from the cuprates. These unusual thermoelectric properties observed in iron-based superconductors have not been observed in the nickel-based pnictide superconductors with the analogous structure, i.e., LaNiAsO, and the nickel-based superconductors behave more like a usual metal. All these results above illustrate that these unusual transport properties of iron-based superconductors are inherently associated with their high temperature superconductivity, and these factors should be taken into account in the theory on its superconducting mechanism.