Muon-spin-spectroscopy study of the penetration depth of ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$

Muon-spin-spectroscopy measurements have been used to study the superconducting state of ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$. The temperature dependence of the in-plane magnetic penetration depth, ${\lambda }_{ab}\left(T\right)$, is found to be compatible with either a two-gap $s+s$-wave or an anisotropic $s$-wave model. The value for ${\lambda }_{ab}\left(T\right)$ at $T=0\text{K}$ is estimated to be ${\lambda }_{ab}\left(0\right)=534\left(2\right)\text{nm}$.

Figure 1

(Color online) (a) Temperature dependence of the magnetic susceptibility of ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ measured using zero-field-cooled warming and field-cooled cooling protocols. The diamagnetic susceptibility corresponds to complete diamagnetic screening with a $T_c$ onset of 14.4 K. (b) Magnetization versus applied field curves for ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ collected above $T_c$ at 20, 150, and 350 K.

Figure 2

(Color online) Zero-field ($T=20$ and 1.2 K) and longitudinal-field ($T=20\text{K}$ and ${\mu }_{0}H=30\text{mT}$) $\mu \text{SR}$ time spectra for a sample of ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$.

Figure 3

(Color online) One component of the transverse-field muon-time spectra for ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ collected in a magnetic field ${\mu }_{0}H=30\text{mT}$ at temperatures above $\left(T=20\text{K}\right)$ and below $\left(T=1.2\text{K}\right)$ the superconducting transition temperature $T_c=14.4\text{K}$.

Figure 4

(Color online) The temperature dependence of the Gaussian depolarization rate $\sigma$ extracted from the TF muon-time spectra collected in an applied magnetic field ${\mu }_{0}H=30\text{mT}$. The inset demonstrates the magnetic field independence of the parameter $\Lambda$ at 20 K.

Figure 5

(Color online) Temperature dependence of ${\lambda }^{-2}$ for ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$. The curve (black line) is a fit to the data using two $s$-wave components, each with an isotropic gap.