Microscopic study of the superconducting state of the iron pnictide ${\text{RbFe}}_2{\text{As}}_2$ via muon spin rotation

A study of the temperature and field dependence of the penetration depth $\lambda$ of the superconductor ${\text{RbFe}}_2{\text{As}}_2\left(T_c=2.52\text{K}\right)$ was carried out by means of muon-spin rotation measurements. In addition to the zero-temperature value of the penetration depth $\lambda \left(0\right)=267\left(5\right)\text{nm}$, a determination of the upper critical field $B_{c2}\left(0\right)=2.6\left(2\right)\text{T}$ was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multigap scenario. Compared to the other “122” systems which exhibit much higher Fermi level, a strong reduction in the large gap BCS ratio $2\Delta /k_{\text{B}}{T}_c$ is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed.

Figure 1

(Color online) Typical $\mu \text{SR}$ spectra recorded above and below $T_c$, in: (a) zero field and (b) transverse field.

Figure 2

(Color online) Temperature dependence of the depolarization rate due to the FLL in ${\text{RbFe}}_2{\text{As}}_2$ and obtained in fields of 1.5, 0.5, 0.1, and 0.01 T (lines are guides to the eyes). Inset: field dependence of ${\sigma }_{\text{s}}$ obtained at 1.6 K and analyzed using the Eq. (2).

Figure 3

(Color online) (a) Field dependence of the electrical resistivity. (b) Upper critical field for ${\text{RbFe}}_2{\text{As}}_2$. The open circles are obtained by analyzing the field dependence of ${\sigma }_{\text{s}}$ using Eq. (2), as explained in the text. The diamonds are the value obtained by analyzing the temperature dependence of ${\sigma }_{\text{s}}$. The stars correspond to the complete disappearance of the resistivity in field. The line is a guide to the eyes.

Figure 4

(Color online) Magnetic penetration depth as a function of temperature. Above 0.5 K, the values obtained with Eq. (2) coincides with the values measured in a field of 0.01 T, and only these latter are plotted for this temperature range. The red dashed line corresponds to a BCS $s$-wave gap symmetry whereas the solid one to represents a fit using a two-gap $s+s$ model. The inset exhibits the penetration depth as a function of ${\left(T/T_c\right)}^2$.