Thermal and electrical transport in the spin density wave antiferromagnet CaFe$_{4}$As$_{3}$

TL;DR

This study investigates the thermal and electrical transport properties of CaFe4As3, revealing a complex interplay of Fermi surface gapping, strong correlations, and low thermoelectric efficiency associated with spin density wave order.

Contribution

It provides the first comprehensive measurement of thermopower, thermal conductivity, and resistivity in CaFe4As3, and links these to electronic structure and correlation effects.

Findings

Fermi surface partially survives SDW transition at 88 K

Thermal conductivity is significantly lower than in conventional metals

Thermopower changes sign at the SDW transition, indicating Fermi level gap formation

Abstract

We present here measurements of the thermopower, thermal conductivity, and electrical resistivity of the newly reported compound CaFe4As3. 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 Neel temperature TN=88 K, and its subsequent commensurate lockin transition at T2=26.4 K. The specific heat below T2 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 CaFe4As3 is a strongly interacting Fermi liquid. The thermal conductivity of CaFe4As3 is an order of magnitude smaller than those of conventional metals at all temperatures, due to a strong phonon scattering. The thermoelectric power 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 T2=26.4 K. The small value of the thermopower and the enhancements of the resistivity due to gap formation and strong quasiparticle interactions offset the low value of the thermal conductivity, yielding only a modest value for the thermoelectric figure of merit Z < 5x10^-6 1/K in CaFe4As3. The results of ab initio electronic structure calculations are reported, confirming that the sign change in the thermopower at T2 is reflected by a sign change in the slope of the density of states at the Fermi level. Values for the quasiparticle renormalization are derived from measurements of the specific heat and thermopower, indicating that as T->0, CaFe4As3 is among the most strongly correlated of the known Fe-based pnictide and chalcogenide systems.