arXiv:1308.4339v1 [cond-mat.supr-con] 20 Aug 2013 Spatial Inhomogeneity of the Superconducting Gap and Order Parameter in FeSe 0.4 Te 0.6 U. R. Singh, 1 S. C. White, 1 S. Schmaus, 1 V. Tsurkan, 2, 3 A. Loidl, 2 J. Deisenhofer, 2 and P. Wahl 1, 4, * 1 Max-Planck-Institut f¨ ur Festk¨ orperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany 2 Center for Electronic Correlations and Magnetism, Experimental Physics V, University of Augsburg, D-86159 Augsburg, Germany 3 Institute of Applied Physics, Academy of Sciences of Moldova, MD 2028, Chisinau, R. Moldova 4 SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, United Kingdom (Dated: August 21, 2013) We have performed a low temperature scanning tunneling microscopy and spectroscopy study of the iron chalcogenide superconductor FeSe0.4Te0.6 with TC ≈ 14 K. Spatially resolved measurements of the superconducting gap reveal substantial inhomogeneity on a nanometer length scale. Analysis of the structure of the gap seen in tunneling spectra by comparison with calculated spectra for different superconducting order parameters (s-wave, d-wave, and anisotropic s-wave) yields the best agreement for an order parameter with anisotropic s-wave symmetry with an anisotropy of ∼ 40%. The temperature dependence of the superconducting gap observed in places with large and small gap size indicates that it is indeed the superconducting transition temperature which is inhomogeneous. The temperature dependence of the gap size is substantially larger than would be expected from BCS theory. An analysis of the local gap size in relation with the local chemical composition shows almost no correlation with the local concentration of Se-/Te-atoms at the surface. PACS numbers: 74.55.+v, 74.70.Xa, 74.81.-g The recently discovered iron-based superconductors have sparked hope that a detailed understanding of su- perconductivity in these materials might finally help to establish an understanding of the pairing mechanism in high temperature superconductors [1–3]. The observa- tion of magnetic resonance modes at the nesting vector of different Fermi surface sheets indicates that spin fluc- tuations play an important role for superconductivity in these materials [4–6]. However despite these successes, there is still a number of open questions to be resolved. The symmetry of the superconducting order parameter has not been unambiguously determined so far, also a predictive theory of superconductivity in iron-based su- perconductors is still missing. Matters are complicated by a complex band structure with up to five bands de- rived from the Fe-3d orbitals crossing the Fermi level [2]. In the iron chalcogenide superconductor Fe 1+δ Se 1-x Te x it appears that the superconducting gap observed in tun- neling spectra near optimal doping (x ≈ 0.6) is nodeless [7] - while in MBE-grown FeSe films, it appears to have nodes [8]. An anisotropy of the superconducting gap has been observed in studies of LiFeAs by scanning tunnel- ing microscopy (STM) [9] and angle resolved photoemis- sion (ARPES) [10]. In the case of Fe 1+δ Se 1-x Te x , results from ARPES experiments have been inconclusive: both, isotropic gaps on the hole-like and electron-like sheets of the Fermi surface - though of different magnitude [11], as well as anisotropic gaps [12] have been reported. The latter is consistent with angle-resolved specific heat mea- surements which show evidence for an anisotropic gap in this sample [13]. A quasiparticle interference study by STM indicates that the order parameter reverses sign between different sheets of the Fermi surface, supporting an interpretation in terms of an s ± order parameter [7]. The superconducting gap has been found to be inhomoge- neous in iron pnictide superconductors of the 122-family [14, 15]. However, in the 122 materials cleaving usu- ally creates a disordered surface, so this inhomogeneity is likely not representative of the bulk. In this letter, we report a study of the spatial inho- mogeneity and structure of the superconducting gap in FeSe 0.4 Te 0.6 by STM. The temperature dependence of the gap shows that the inhomogeneity and spatial varia- tions of the transition temperature are closely related to each other. A comparison of the local variation of the superconducting gap size with the anion height reveals almost no correlation, indicating that interlayer coupling is not negligible. The 11 iron-chalcogenide superconductors have the simplest crystal structure of the iron-based superconduc- tors, consisting of planar iron layers with chalcogenide (Se, Te) anions above and below. The crystal structure provides a well-defined and non-polar cleavage plane be- tween the chalcogenide layers. LEED and STM studies show no indication for a surface reconstruction [16, 17]. We have carried out STM measurements on a single crys- tal of FeSe 1-x Te x with x =0.61 (determined by EDX measurements) and a superconducting transition temper- ature T C ≈ 14 K [18]. We have used a home-built low temperature STM which allows for in-situ sample trans- fer and cleavage [19]. Differential tunneling conductance dI/dV is measured through a lock-in amplifier with a modulation of 600 μV RMS . Bias voltages are applied to the sample, with the tip at virtual ground. Tunneling