Quantum criticality and nodal superconductivity in the FeAs-based superconductor KFe2As2

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The in-plane resistivity ρ and thermal conductivity κ of FeAs-based superconductor KFe₂As₂ single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior ρ(T) T¹.⁵ at Hc₂ = 5 T, and the development of a Fermi liquid state with ρ(T) T² when further increasing field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field Hc₂. In zero field there is a large residual linear term κ₀/T, and the field dependence of κ₀/T mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe₂As₂ have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.