Thermal and electrical transport in the spin density wave antiferromagnet CaFe4As3

Abstract

We present here measurements of the thermopower, thermal conductivity, and electrical resistivity of the newly reported compound CaFe${}_{4}$As${}_{3}$. Evidence is presented from specific heat and electrical resistivity measurements that a substantial fraction of the Fermi surface survives the onset of spin density wave (SDW) order at the N\'eel temperature ${T}_{\mathrm{N}}=88$ K and its subsequent commensurate lock-in transition at ${T}_{2}=26.4$ K. The specific heat below ${T}_{2}$ consists of a normal metallic component from the ungapped parts of the Fermi surface and a Bardeen-Cooper-Schrieffer (BCS) component that represents the SDW gapping of the Fermi surface. A large Kadowaki-Woods ratio is found at low temperatures, showing that the ground state of CaFe${}_{4}$As${}_{3}$ is a strongly interacting Fermi liquid. The thermal conductivity $\ensuremath{\kappa}$ of CaFe${}_{4}$As${}_{3}$ is an order of magnitude smaller than those of conventional metals at all temperatures, due to a strong phonon scattering. The thermoelectric power $S$ displays a sign change from positive to negative indicating that a partial gap forms at the Fermi level with the onset of commensurate spin density wave order at ${T}_{2}=26.4$ K. The small value of the thermopower $S$ and the enhancements of the resistivity due to gap formation and strong quasiparticle interactions offset the low value of the thermal conductivity $\ensuremath{\kappa}$, yielding only a modest value for the thermoelectric figure of merit $\mathcal{Z}\ensuremath{\leqslant}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in CaFe${}_{4}$As${}_{3}$. The results of ab initio electronic structure calculations are reported, confirming that the sign change in the thermopower at ${T}_{2}$ is reflected by a sign change in the slope of the density of states at the Fermi level. Values for the quasiparticle renormalization $Z$ are derived from measurements of the specific heat and thermopower, indicating that as $T\ensuremath{\rightarrow}0$, ${\mathrm{CaFe}}_{4}{\mathrm{As}}_{3}$ is among the most strongly correlated of the known Fe-based pnictide and chalcogenide systems.