Evidence for superconducting gap nodes in the zone-centered hole bands of ${\text{KFe}}_2{\text{As}}_2$ from magnetic penetration-depth measurements

Among the iron-based pnictide superconductors the material ${\text{KFe}}_2{\text{As}}_2$ is unusual in that its Fermi surface does not consist of quasinested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio $>1200$. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that ${\text{KFe}}_2{\text{As}}_2$ is an unconventional superconductor with strong electron correlations.

Figure 1

(Color online) The Fermi surface of ${\text{KFe}}_2{\text{As}}_2$ (labels in correspond to Table ). (a) 2D cross-sectional representation of our band-structure calculations (note the pillow surface from band 1 does not appear in this cut). (b) Schematic cross-section with shapes of the various pockets determined by dHvA (Ref. 23) and ARPES (Ref. 24) experiments. (c) three-dimensional view of the DFT calculated Fermi surface with bands energies shifted to best fit the dHvA frequencies (Ref. 25).

Figure 2

(Color online) (a) Temperature dependence of in-plane resistivity $\rho \left(T\right)$ in a single crystal of ${\text{KFe}}_2{\text{As}}_2$. (b) The same data plotted against $T^2$ below 10 K.

Figure 3

(Color online) Coefficient $A$ vs Sommerfeld constant $\gamma$ [Kadowaki-Woods plot (Ref. 32)] for various superconductors showing the Fermi-liquid $A{T}^2$ dependence of $\rho \left(T\right)$. Circles are for $s$-wave superconductors while squares are for unconventional superconductors with nodes in the gap (Ref. 33). The lines represent $A=a_{\text{KW}}{\gamma }^2$ with $a_{\text{KW}}={10}^{-5}$ (solid line) and $4\times {10}^{-7}\mu \Omega \text{cm}{\left(\text{K}\text{mol}/\text{mJ}\right)}^2$ (dashed line).

Figure 4

(Color online) Change in the penetration depth $\Delta \lambda \left(T\right)$ at low temperatures in two samples. The data are shifted vertically for clarity. Inset shows the frequency shift $\Delta f$ of the oscillator containing sample 1, and relative change in the specific heat divided by temperature $\Delta C/T$ in the same sample.

Figure 5

(Color online) Normalized superfluid density at low temperatures follows a $T$-linear dependence (dashed line). We used $\lambda \left(0\right)=260\text{nm}$ estimated from the Fermi-surface parameters. The solid lines are fits to the empirical formula involving the impurity scattering in superconductors with line nodes (see text). Inset compares the normalized superfluid density data for the two samples. To account for the factor of two differences in the slope of $\Delta \lambda \left(T\right)$ at low temperatures, we used the doubled $\lambda \left(0\right)$ value for sample 2.

Figure 6

(Color online) Normalized superfluid density obtained by using representative values of $\lambda \left(0\right)$. The lines are the fits to the nodal-gap model with parameters indicated in the figure.