Enhanced Nernst effect above Tc in the quasi-two-dimensional iron pnictide superconductor CsCa2Fe4As4F2

Abstract

As a new type of iron pnictide superconductors, ${\mathrm{CsCa}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$, featured by its unique FeAs bilayer, presents a quasi-two-dimensional characteristic which distinguishes itself from other iron pnictides. Here we systematically investigated its transport properties and thermoelectric effects, including the Seebeck and Nernst effect, and NMR. An unusual hump in the Nernst coefficient is observed as temperature decreases below 90 K, far above its ${T}_{c}$ of 28.5 K. Simultaneously there is a suppression in the Hall coefficient as well as the nuclear spin-lattice relaxation rate in the same temperature range. To clarify this phenomenon, we comparatively investigated two other similar iron-based pnictide superconductors, ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$. Our work indicates there could exist an incoherence-coherence crossover or a kind of Cooper pairing precursor in ${\mathrm{CsCa}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$ before achieving its superconductivity, which is different from other iron-based high-${T}_{c}$ superconductors. Such a comparative study may help us to understand the effect of dimensional evolution from three dimensional to two dimensional on high-${T}_{c}$ superconductivity.