Signatures of quantum criticality in hole-doped and chemically pressurized EuFe_2As_2 single crystals

摘要

We study the effect of hole doping and chemical pressure (isovalent doping) in single crystals of K_x Eu_(1-x) Fe_2 As_2 and EuFe_2(As_(1-y)P_y)_2, respectively, by measuring the thermopower, S(T) and electrical resistivity, ρ(T). The evolution of S(T) upon doping indicates drastic changes in the electronic configuration at critical values x_(cr) = 0.3 and y_(cr) = 0.21, respectively, as the spin-density-wave transition is completely suppressed and superconductivity (SC) emerges. For the case of chemical pressure, a comparison with published ARPES measurements indicates a Lifshitz transition at y_(cr). The temperature dependences S(T)/T ∝ log T and △ρ ∝ T observed in the normal state above the SC transition suggest quantum criticality in both systems.