Evidence of nodeless superconductivity in ${\text{FeSe}}_{0.85}$ from a muon-spin-rotation study of the in-plane magnetic penetration depth

The in-plane magnetic penetration depth ${\lambda }_{ab}$ of the iron selenide superconductor with the nominal composition ${\text{FeSe}}_{0.85}$ was studied by means of muon-spin rotation. The measurements of ${\lambda }_{ab}^{-2}\left(T\right)$ are inconsistent with a simple isotropic $s$-wave type of the order parameter but are rather in favor of two-gap $\left(s+s\right)$ and anisotropic $s$-wave order parameter symmetries, thus implying that the superconducting energy gap in ${\text{FeSe}}_{0.85}$ contains no nodes.

Figure 1

(Color online) (a) The crystal structure of $\alpha \text{-FeSe}$. (b) Temperature dependence of the ac magnetization $M_{\text{ac}}$ (${\mu }_0{H}_{\text{ac}}=0.1\text{mT}$ and $\nu =1000\text{Hz}$) of ${\text{FeSe}}_{0.85}$. (c) Dependence of dc magnetization $M_{\text{dc}}$ on magnetic field at $T=10\text{K}$ and 150 K.

Figure 2

(Color online) (a) Temperature dependence of the ZF muon depolarization rate $\Lambda$ of ${\text{FeSe}}_{0.85}$. The inset shows ZF ($T=1.5$ and 10 K) and LF ($T=10\text{K}$ and ${\mu }_{0}H=0.01\text{T}$) $\mu \text{SR}$ time spectra of ${\text{FeSe}}_{0.85}$. (b) Temperature dependence of the Gaussian depolarization rate $\sigma$ obtained in TF $\mu \text{SR}$ experiment at ${\mu }_{0}H=0.01\text{T}$. The inset shows the TF muon-time spectra above $\left(T=10\text{K}\right)$ and below $\left(T=1.5\text{K}\right)$ the superconducting transition temperature $T_c=8.26\text{K}$.

Figure 3

(Color online) Temperature dependence of ${\lambda }_{ab}^{-2}$ of ${\text{FeSe}}_{0.85}$ obtained from measured ${\sigma }_{\text{sc}}$ by means of Eq. (3). The fitting curves (solid black lines) were obtained within the following models of the gap symmetries: (a) $s$-wave, (b) $s+s$-wave, (c) anisotropic $s$-wave, and (d) $d$-wave. The corresponding angular dependences of the gaps are shown as the insets.