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

Abstract

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 measurements of the thermopower, $S(T)$, and electrical resistivity, $\rho (T)$. The evolution of $S(T)$ upon doping indicates drastic changes of the electronic configuration at critical values $x_{\mathrm{cr}}=0.3$ and $y_{\mathrm{cr}}=0.21$, respectively, as the spin-density-wave transition is completely suppressed and superconductivity (SC) emerges. For the case of chemical pressure, the comparison with published ARPES measurements indicates a Lifshitz transition at $y_{cr}$. The temperature dependences $S(T)/T\propto \log T$ and $\Delta \rho \propto T$ observed in the normal state above the SC transition suggest quantum criticality in both systems.

PRB accepted

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