London penetration depth and superfluid density in single crystals of Fe(Te,Se) and Fe(Te,S) superconductors H. Kim, 1 C. Martin, 1, ∗ R. T. Gordon, 1 M. A. Tanatar, 1 J. Hu, 2 B. Qian, 2 Z. Q. Mao, 2 Rongwei Hu, 3, † C. Petrovic, 3 N. Salovich, 4 R. Giannetta, 4 and R. Prozorov 1, ‡ $^{1}$Ames Laboratory and Department of Physics & Astronomy, Iowa State University, Ames, IA 50011 $^{2}$Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118 $^{3}$Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973 $^{4}$Loomis Laboratory of Physics, University of Illinois at Urbana-Champaign, 1110 West Green St. Urbana, IL 61801 (Dated: 12 January 2010) The in-plane London penetration depth, λ ( T ), was measured in single crystals of the iron- chalcogenide superconductors Fe 1 . 03 (Te 0 . 63 Se 0 . 37 ) and Fe 1 . 06 (Te 0 . 88 S 0 . 14 ) by using a radio-frequency tunnel diode resonator. As is also the case for the iron-pnictides, these iron-chalcogenides exhibit a nearly quadratic temperature variation of λ ( T ) at low temperatures. The absolute value of the penetration depth in the T → 0 limit was determined for Fe 1 . 03 (Te 0 . 63 Se 0 . 37 ) by using an Al coating technique, giving λ (0) ≈ 560 ± 20 nm. The superfluid density ρ s ( T ) = λ $^{2}$(0) /λ $^{2}$( T ) was fitted with a self-consistent two-gap γ − model. While two different gaps are needed to describe the full-range temperature variation of ρ s ( T ), a non-exponential behavior at low temperatures requires additional factors, such as scattering and/or significant gap anisotropy. The majority of recently found iron-based supercon- ductors are pnictides [1]. The only exception so far is binary iron-chalcogenide FeSe [2] that becomes su- perconducting with the excess Fe occupying interstitial sites of the (Te,Se) layers [3]. In these materials, gen- erally referred to as ”11” compounds, Fe forms square planar sheets whereas Se ions form distorted tetrahedra surrounding the Fe ions, which is similar to the struc- ture of the Fe-As pnictides. The electronic structure is also similar to pnictides. For ”11” system it has been suggested both theoretically [4] and experimentally [5] that superconductivity could be magnetically mediated. Furthermore, the series of iron-chalcogenides from FeS through FeTe was theoretically explored within the spin- fluctuation picture, concluding that doped FeTe could exhibit the strongest superconductivity [4]. The systems over which the doping is most controlled are FeTe$_{1}$$_{−}$$_{x}$Se$_{x}$ [6] and FeTe$_{1}$$_{−}$$_{x}$S$_{x}$ [7]. So far the highest T$_{c}$ ≈ 15 K is reported for the Fe(Te,Se) system [6, 8]. The connection between superconductivity and magnetism in the ”11” system has been demonstrated by the observation of the antiferromagnetic order in Fe$_{1+}$$_{y}$Te [3] and a spin reso- nance in Fe$_{1+}$$_{y}$(Te$_{0}$$_{.}$$_{6}$Se$_{0}$$_{.}$$_{4}$) [9]. Fe(Te,S) is a supercon- ductor with T$_{c}$ ≈ 8 . 8 K and its comprehensive character- ization is described in Ref. 10. The ”11” system exhibits many interesting phenom- ena. The transition temperature can be enhanced up to 37 K by applying modest pressures [11], which is com- parable to the T$_{c}$ of iron-arsenide superconductors. The connection between T$_{c}$ and the pressure has been sug- gested to come from the enhancement of spin fluctuations [12] and from the modulation of electronic properties due to evolution of the inter-layer Se-Fe-Se separations [11]. Several experimental works explore pairing mechanism of ”11” compounds [13–15]. The absence of a coher- Single crystals of Fe(Te,Se) were synthesized using a flux method. Mixed powders of the Fe(Te$_{0}$$_{.}$$_{6}$Se$_{0}$$_{.}$$_{4}$) com- positions were sealed in evacuated quartz tubes. The sealed ampoule was slowly heated up to 930 $^{◦}$C and slowly cooled down to 400 $^{◦}$C at a rate of 3 $^{◦}$C/hr before the furnace was shut down. Single crystals with centime- ter dimensions can easily be obtained with this method and are shown to be the pure α -phase with the P 4 /nmm space group by x-ray diffraction [8]. The actual concen- trations were analyzed using an energy dispersive x-ray spectrometer (EDXS). The measured composition for the samples used in this work is Fe$_{1}$$_{.}$$_{03}$(Te$_{0}$$_{.}$$_{63}$Se$_{0}$$_{.}$$_{37}$). Single crystals of Fe(Te,S) were grown from TeS self flux using a high temperature flux method. Elemental Fe, Te and S were sealed in quartz tubes under a partial argon at- In this work, we present an experimental study of the London penetration depth, λ ( T ), in single crystals of Fe$_{1}$$_{.}$$_{03}$(Te$_{0}$$_{.}$$_{63}$Se$_{0}$$_{.}$$_{37}$) and Fe$_{1}$$_{.}$$_{06}$(Te$_{0}$$_{.}$$_{88}$S$_{0}$$_{.}$$_{14}$). We found that at low temperatures Δ λ ( T ) ∝ T n with n ≈ 2 . 1 for Fe(Te,Se) and n ≈ 1 . 8 for Fe(Te,S). The absolute value of λ (0) ≈ 560 nm was determined for Fe(Te,Se) by measuring the total λ ( T ) of the sample coated with a thin Al film [16]. The in-plane superfluid density ρ$_{s}$ ( T ) = λ $^{2}$(0) /λ $^{2}$( T ) was analyzed in the framework of a self-consistent two-gap γ − model [17]. ence peak in NMR measurements on polycrystalline FeSe suggests unconventional superconductivity [13], while the power-law temperature dependence of the spin-relaxation rate, 1 /T$_{1}$ ∼ T $^{3}$, could be reconciled with both a nodal gap or a fully-gapped s$_{±}$ state. Muon spin rotation study of the penetration depth in FeSe$_{x}$ was consistent with ei- ther anisotropic s -wave or a two-gap extended s -wave pairing [14]. Thermal conductivity measurements con- cluded multigap nodeless superconductivity in polycrys- talline FeSe$_{x}$ [15]. arXiv:1001.2042v1 [cond-mat.supr-con] 12 Jan 2010