Anomalous Wilson Ratio in AFe2As2 with A = K, Rb, Cs

In Fermi liquids, both the electronic specific heat 𝐶𝑒𝑙 and the paramagnetic susceptibility 𝜒 are proportional to the effective quasiparticle mass. Their so-called Wilson ratio 𝑅𝑤=(4𝜋2𝑘𝐵2 𝑇/3𝜇0𝑔2𝜇𝐵2)∙𝜒/𝐶𝑒𝑙 is expected to be close to unit and, therefore, considered as a hallmark for the presence of quasiparticles in strongly correlated electron systems. While higher 𝑅𝑤 values are common for ferromagnetic exchange interactions, due to the Stoner enhancement, smaller values have hardly been reported, so far.
In the iron pnictide superconductors AFe2As2, measurements of the specific heat, elastoresistivity, and quantum oscillations reveal a diverging quasiparticle mass with increasing alkali-metal ion radius 𝑟𝐴, suggesting the proximity to a quantum phase transition at negative pressures [1,2]. In sharp contrast to this behavior, the paramagnetic susceptibility remains approximately constant. This results in a strong reduction of the Wilson ratio and values of 𝑅𝑤<1. We have performed thermal expansion and magnetostriction measurements to determine the stress and strain dependence of 𝑅𝑤. We will compare the evolution of 𝑅𝑤 with pressure and K doping with the change of the superconducting properties and the electronic structure. ... mehrA possible origin of the reduction will be discussed in context of nematic fluctuations and orbital-selective Mott physics.