Abstract
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 (, 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 , anisotropy, upper critical field , and critical current density . The upper critical field and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth and specific heat, we show clear evidence for strong coupling two-gap -wave superconductivity. The temperature dependence of calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with -wave-like gaps. We find that a 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.
- Received 28 January 2015
- Revised 20 March 2015
DOI:https://doi.org/10.1103/PhysRevB.91.165109
©2015 American Physical Society