Unconventional Superconductivity in Layered Iron Germanides

The intermetallic compound YFe₂Ge₂ belongs to a small group of iron-based superconductors which contain neither a pnictide nor chalcogenide element. Clean samples display a bulk superconducting transition at ~ 1.2 K. Both its normal- and superconducting states exhibit unusual properties. At low temperature, a large Sommerfeld ratio C/T ~ 100 mJ/mol K² and a T^1.5 non-Fermi liquid form of the normal-state resistivity indicate the presence of strong electronic correlations [1]. Quantum oscillation measurements revealed a nearly uniform mass enhancement over band structure values, which suggests predominantly local correlations [2]. Pronounced magnetic fluctuations probed by inelastic neutron scattering [3], the strongly detrimental effect of lattice disorder on superconductivity [1], and theoretical predictions from DFT calculations [4] all point towards an unconventional superconducting pairing mechanism. The isostructural and isoelectronic compound, LuFe₂Ge₂, exhibits physical properties that closely resemble those of YFe₂Ge₂. Despite the spin-density-wave order observed in LuFe₂Ge₂ below 9 K, the overall shape and magnitude of its resistivity and susceptibility above 10 K and the similarly enhanced Sommerfeld ratio 𝐶/𝑇 > 70 mJ/mol K² suggest similar underlying physics in the two systems [5]. This talk will review experimental and theoretical studies on YFe₂Ge₂ and LuFe₂Ge₂ and discuss them in the wider context of Fe-based superconductors.