Superconducting properties of sulfur-doped iron selenide

The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ ($x=0$, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature ${T}_{c}$, anisotropy, upper critical field ${H}_{c2}$, and critical current density ${J}_{c}$. The upper critical field ${H}_{c2}(T)$ and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth ${\ensuremath{\lambda}}_{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature dependence of ${\ensuremath{\lambda}}_{ab}(T)$ calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with $s$-wave-like gaps. We find that a $d$ wave and single-gap BCS theory under the weak-coupling approach cannot describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.