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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 FeSe1−xSx (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
Tc, anisotropy, upper critical field Hc2, and critical current density Jc. The upper critical field Hc2(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 λab(T ) and specific heat, we show clear evidence
for strong coupling two-gap s-wave superconductivity. The temperature dependence of λab(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.