Evidence for two-gap nodeless superconductivity in ${\text{SmFeAsO}}_{1-x}{\text{F}}_x$ from point-contact Andreev-reflection spectroscopy

Point-contact Andreev-reflection spectroscopy measurements were performed in ${\text{SmFeAsO}}_{1-x}{\text{F}}_x$ polycrystals with $x=0.09$ $\left(T_c\simeq 42\text{K}\right)$ and $x=0.20$ $\left(T_c\simeq 52\text{K}\right)$. In all cases the experimental conductance curves reproducibly exhibit low-energy peaks and higher-energy shoulders (at 4–6 and 16–20 meV, respectively, for $x=0.20$), which indicate the presence of two nodeless superconducting gaps. While the single-band Blonder-Tinkham-Klapwijk model can only reproduce a small central portion of a given conductance curve, the two-gap one accounts remarkably well for the shape of the whole experimental $dI/dV$ vs $V$ curve. The fit of the normalized curves gives ${\Delta }_1\left(0\right)=6.15\pm 0.45\text{meV}$ and ${\Delta }_2\left(0\right)=18\pm 3\text{meV}$ for $x=0.20$, while for $x=0.09$ the values ${\Delta }_1\left(0\right)=4.9\pm 0.5\text{meV}$ and ${\Delta }_2\left(0\right)=15\pm 1\text{meV}$ are obtained. In all cases, both gaps close at the same temperature and follow a BCS-like behavior.

Figure 1

(Color online) Temperature dependence of the raw conductance curves of a point contact with $T_c^A\sim 51\text{K}$ on ${\text{SmFeAsO}}_{0.8}{\text{F}}_{0.2}$. The curve at 4.2 K (top thick line) features two peaks and two shoulders (arrows). The curve at $T_c^A$ (middle thick line) has a humplike shape, which flattens at $\sim 140\text{K}$ (bottom thick line). The inset shows the dc susceptibility of the sample.

Figure 2

(Color online) (a)–(d) Normalized conductance curves (symbols) at 4.2 K in ${\text{SmFeAsO}}_{0.8}{\text{F}}_{0.2}$. The contact resistance, $R_N$, decreases from top to bottom. The curves are reported together with their relevant one-gap (dash lines) and two-gap (solid lines) BTK fits. The gap values are indicated on the left (single-gap fit) and on the right (two-gap fit).

Figure 3

(Color online) Temperature dependence of the normalized conductance curve of Fig. 2b (symbols) together with the two-gap BTK fits of the negative-bias side (lines). The curves are vertically shifted for clarity. The raw data are those shown in Fig. 1, and the insets show the temperature dependence of ${\Delta }_1$ and ${\Delta }_2$ and of ${\Gamma }_1$ and ${\Gamma }_2$, respectively.

Figure 4

(Color online) Temperature dependence of the gaps ${\Delta }_1$ (open symbols) and ${\Delta }_2$ (solid symbols) in ${\text{SmFeAsO}}_{0.80}{\text{F}}_{0.2}$ as extracted from the two-gap fit of various conductance curves. Lines are BCS-like curves. Details are in the text.

Figure 5

(Color online) Temperature dependence of the normalized conductance curves (symbols) and of the two-gap BTK fits of their negative-bias side (lines) in a point contact on ${\text{SmFeAsO}}_{0.91}{\text{F}}_{0.09}$. The curves are vertically shifted for clarity. The insets show the temperature dependence of ${\Delta }_1$ and ${\Delta }_2$ and of ${\Gamma }_1$ and ${\Gamma }_2$, respectively.