Evidence of Strong Correlations and Coherence-Incoherence Crossover in the Iron Pnictide Superconductor ${\mathrm{KFe}}_2{\mathrm{As}}_2$

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of ${\mathrm{KFe}}_2{\mathrm{As}}_2$. Both the Sommerfeld coefficient ($\gamma \approx 103\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$) and the Pauli susceptibility ($\chi \approx 4\times {10}^{-4}$) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas–van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund’s coupling between orbitals.

Figure 1

Low-temperature electrical resistivity of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ as a function of $T^2$. The solid line is a fit to the usual Fermi liquid behavior, ${\rho }_0+A{T}^2$. Units of ${\rho }_0$ and A are $\mu \Omega \mathrm{cm}$ and $\mu \Omega \mathrm{cm}{\mathrm{K}}^{-2}$, respectively. The inset shows the temperature dependence of $\rho (T)$ to 300 K.

Figure 2

Temperature dependence of the heat capacity of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ in 0 and 5.5 T. The green line represents a fit to the normal-state heat capacity. The Sommerfeld coefficient $\gamma$, the Debye term $B_3$, and $B_5$ are given in $\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$, $\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-4}$, and $\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-6}$, respectively. The inset shows the normal- and superconducting-state electronic entropies.

Figure 3

(a) Measured magnetic susceptibility $M/H$ of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ for $H\perp c$ and $H\parallel c$ for several magnetic fields (solid lines). The symbols shows the temperature dependence of the intrinsic susceptibilities ${\chi }_c(T)$ and ${\chi }_{ab}(T)$ of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ derived from magnetic isotherms. (b),(c) $M(H){|}_T$ isotherm measurements performed at various temperatures, shown in a Honda-Owen representation.

Figure 4

(a) Temperature dependence of the thermal expansion of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ and $\mathrm{Ba}({\mathrm{Fe}}_{0.67}{\mathrm{Co}}_{0.33}{)}_2{\mathrm{As}}_2$ measured along the $a$ axis. The dashed and dotted lines are low-temperature fits to the Fermi liquid ($aT$) and Debye ($b_3{T}^3$) models, respectively. The blue line shows the electron thermal expansion of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ obtained by subtracting the expansivity of $\mathrm{Ba}({\mathrm{Fe}}_{0.67}{\mathrm{Co}}_{0.33}{)}_2{\mathrm{As}}_2$ from that of ${\mathrm{KFe}}_2{\mathrm{As}}_2$. The inset shows a magnified view of the low-temperature regime. (b) Temperature dependence of ${\alpha }_e/T$.

Figure 5

Gutzwiller slave-boson mean-field calculations of the orbitally resolved mass-enhancement factor $z_{\alpha }$ for ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{KFe}}_2{\mathrm{As}}_2$, for two different values of the intraorbital Coulomb repulsion $U$.