Figure 1

(a) Crystal growth setup with the liquid-Sn-melt sealing technique. (b) Top view of a KFe${}_2$As${}_2$ ingot with a 15-mm diameter, revealing easy-to-distinguish shiny pieces and lamellar structure of crystals. (c) Single crystals of KFe${}_2$As${}_2$, cleaved out of the ingot, with sizes up to $10\times 5\times 0.2$ mm${}^3$.Reuse & Permissions

Figure 2

The dc SQUID magnetization measurements of the KFe${}_2$As${}_2$ crystals in ZFC and FC protocols in a magnetic field of 10 Oe applied parallel to the $ab$ plane.Reuse & Permissions

Figure 3

The dc SQUID measurements of magnetization loops at 2 K, the base temperature of our SQUID apparatus, in magnetic fields applied (a) parallel to the $ab$ plane and (b) parallel to the tetragonal $c$ axis.Reuse & Permissions

Figure 4

Temperature-dependent electrical resistivity of KFe${}_2$As${}_2$ for current direction along the plane in samples 1 and 2 (this study) in comparison with published data with the highest residual resistivity ratio by Reid et al.[7] The left panel shows data over a broad temperature range, and the right panel shows the same data plotted vs $T^2$, allowing for linear fits through data above the onset of the resistive transition and extrapolation of the $\rho \left(T\right)$ to $T=0$ to determine $\rho \left(0\right)$. Samples 1 and 2 show lower residual resistivity values at $T_c$ and in $\rho \left(0\right)$ extrapolation than the best samples reported so far by Reid et al., giving $\rho \left(\text{300}\text{K}\right)/\rho \left(0\right)$ in the 2500 to 3000 range, significantly higher than in all previous reports.[7, 9, 10, 17, 27] For reference in the left panel we show ${\rho }_c\left(T\right)$ as measured in sample 3 in this study and reported by Kimata et al.[35]Reuse & Permissions

Figure 5

Temperature-dependent in-plane electrical resistivity of KFe${}_2$As${}_2$ sample 1 in magnetic fields aligned parallel to the $c$ axis. The left panel shows ${\rho }_a/{\rho }_a\left(\text{300}\text{K}\right)$ plotted vs. $T$, and the right panel shows the same data plotted vs $T^2$. Field values increase from 0 to 1.5 T. Lines and symbols at the highest field of 1.5 T show resistivity for two reversed directions of magnetic field, revealing no significant contribution of the Hall voltage to resistivity measurements. Note that at 1.5 T and 1.8 K, the base temperature of our apparatus, ${\rho }_a\left(T\right)$ does not show any signature of saturation. The actually measured $r_R$ is about 2000.Reuse & Permissions

Figure 6

Temperature-dependent in-plane electrical resistivity of KFe${}_2$As${}_2$ sample 1 in magnetic fields aligned in plane transverse to the current parallel to the $b$ axis. The left panel shows ${\rho }_a/{\rho }_a\left(\text{300}\text{K}\right)$ plotted vs $T$. The right panel shows the same data plotted vs $T^2$. Field values increase from 0 to 9 T. Lines and symbols at the highest field of 9 T show resistivity for the two reversed directions of magnetic field, revealing the negligible contribution of the Hall voltage to the resistivity measurements. Notable magnetoresistance is observed in this configuration, which leads to clear downward deviations from perfect $T^2$ dependence observed at low fields.Reuse & Permissions

Figure 7

The upper critical field as determined from the midpoint (black curves) and offset (green curves) of resistive transition in KFe${}_2$As${}_2$ sample 1. Open symbols show data for $H\parallel c$, and solid symbols are for $H\parallel b$. Blue curves show data determined from the resistive transition midpoint in Ref. 17, from measurements on lower-quality samples. Green curves with downward triangles show data determined from specific-heat measurements by Abdel-Hafiez et al.;[37] the magenta curve is from specific-heat measurements in the magnetic field parallel to the plane by Kim et al.[38]Reuse & Permissions

Figure 8

The upper critical field in the configuration $H\parallel c$ as determined from the midpoint (black curve) and offset point (green curve) of resistive transition in KFe${}_2$As${}_2$ sample 1. The blue curve shows data determined from the resistive transition midpoint by Terashima et al.[17] from measurements on lower-quality samples. The cyan curve shows the data from Ref. 17 with $H_{c2,a}$, $H_{c2,c}$, and $T_c$ multiplied by a constant factor to match $T_c$ of high-quality samples. For reference, we show data determined from specific-heat measurements by Abdel-Hafiez et al.[37] Note that irrespective of the measurements type or criteria used, the slope of the curves does not depend on sample $T_c$.Reuse & Permissions