Superconducting properties of sulfur-doped iron selenide

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 FeSe$_{1-x}$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 $\lambda _{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature-dependence of $\lambda _{ab}(T)$ 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 can not describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.Comment: 13 pages, 7 figure

Direct evidence of two superconducting gaps in FeSe0.5Te0.5: SnS-Andreev spectroscopy and the lower critical field

“…∆ L and ∆ S roughly scale with T C , with the determined here gap values dispose right the middle of the data point range. The 2∆ L /k B T C is similar to those obtained in PCAR [14] and specific heat [18] studies with Fe(Se,Te), and to those reported by us earlier, in IMARE measurements of both single crystal [45,46] and polycrystalline pure FeSe [43][44][45], and in specific heat measurements of S-substituted iron selenide [36]. The linear ∆(T C ) tendency favors a minor influence of (Se,Te,S) substitution on the fundamental pairing.…”

“…More detailed take on this two-band α-model can be found in our previous works [36,37] or in other works [30].…”

Evolution of the superconducting properties inFeSe1−xSx

“…It may occur with either an increase in interband scattering or a change in the Fermi surface, which alters the ratio of electron and hole states allowing for a merging of the two gaps. This interpretation seems to be consistent with specific heat measurements performed on S-substituted crystals suggesting the merging of gaps with increasing S substitution [40].The tunneling spectra cannot be adequately reproduced with two gaps models allowing different symmetries of the order parameters including the extended s ± -wave model [10]. The complex multiband structure involves different gaps on different Fermi surface sheets, and there is the possibility that some remain ungapped at temperatures well below T c [41,42].…”

Electronic structure of FeS

“…[13,24], and the phase diagram, including the Tcs for FeSe1−xTex and FeSe1−xSx were taken from Refs. [10,15,38,39]. Two regions of the SC dome marked by black dashed lines are inferred based on interpolation, in the absence of any report on these substitution.…”

“…Recently, it was found that FeS, in which Se is totally substituted by S, is still superconducting with a T c of about 4.5 K [11,12]. During this process, the nematic (orthorhombic) phase transition temperature (T N ) in FeSe is suppressed, and there is no structural or nematic transition in FeS [10,[13][14][15].…”