Relation between magnetism, non-Fermi liquid behavior and superconductivity in Fe-based superconductors

Infrared spectroscopy is a powerful tool to study the charge dynamics in complex materials [1]. We investigated the detailed temperature dependence of the optical conductivity in Ba1-xKxFe2As2 and LiFe1-xCoxAs with different dopings. (i) In underdoped Ba1-xKxFe2As2, a pseudogap, as well as gaps associated with the spin-density-wave (SDW) and superconducting (SC) gaps, is observed in our optical conductivity spectra. An optical spectral weight analysis indicates that while the SDW gap competes with SC, the pseudogap is likely to originate from a precursor of superconductivity [2]. (ii) In optimally doped Ba1-xKxFe2As2, our optical study reveals a hidden T-linear scattering rate (non-Fermi liquid behavior) for a narrow Drude component, which contributes to the transport properties in parallel with a broad Drude component, leading to a hump-like feature in the resistivity curve [3]. (iii) In LiFe1-xCoxAs, we observed a doping-induced Fermi liquid to non-Fermi liquid to Fermi liquid (FL-NFL-FL) crossover. In combination with transport, ARPES and NMR techniques, we attribute this anomalous FL-NFL-FL crossover to low-energy spin fluctuations arising from Fermi surface nesting [4]. [1] Basov and Timusk, RMP 77, 721 (2005) [2] Dai et al. PRB 86, 100501(R) (2012) [3] Dai et al. PRL 111, 117001 (2013) [4] Dai et al. PRX 5, 031035 (2015)