Evolution of the superconducting properties inFeSe1−xSx

Abstract

We present scanning tunneling microscopy and spectroscopy measurements on FeSe1−xSx single crystals with x = 0, x=0.04 and 0.09. The S substitution into the Se site is equivalent to a positive chemical pressure, since S and Se have the same valence and S has a smaller ionic radius than Se. The subsequent changes in the electronic structure of FeSe induce a decrease of the structural transition temperature and a small increase in the superconducting critical temperature. The evolution of the gaps with increasing S concentration suggests an increase of the hole Fermi surface. Moreover, the vortex core anisotropy, that likely reflects the Fermi surface anisotropy, is strongly suppressed by the S substitution.PACS numbers: 74.70. Xa, 74.25.Jb, 74.55.+v, INTRODUCTIONFeSe has the simplest crystallographic structure among the family of iron-based superconductors and investigations into this system may yield a better understanding of the origin of superconductivity in iron pnictides/chalcogenides. FeSe undergoes a structural phase transition from tetragonal to orthorhombic at T s ≈ 90K, but differently from other Fe-based superconductors this is not accompanied by a transition to a longrange magnetic order phase [1]. Below T s a strong electronic anisotropy develops that is detected through an anisotropic Fermi surface, an anisotropic resistivity and is sensitive to external parameters such as in-plane strain [2]. The dramatic changes in the electronic properties cannot be explained in terms of the small changes in the lattice parameters of the order of 0.1%, therefore, this phase is driven by electronic degree of freedom, namely the electronic nematic order [3]. This C4 symmetry breaking, which breaks the rotational symmetry without changing the translational symmetry of the lattice may play an important role in understanding the superconducting state that sets in below T c = 8.5K [4]. In Fe-based superconductors, the nematic phase is usually regarded as the orthorombic phase sandwiched between the structural transition and the onset of the long-range magnetic order. However, in the case of FeSe no longrange magnetic order has been observed, which makes FeSe an ideal system for the study of the nematic order even in the superconducting state. Indeed, the strong Abrikosov vortex core anisotropy observed in FeSe [5] has been regarded as a manifestation of the nematic order in this material [6,7].The origin of the nematicity in Fe-based superconductors is still a subject of intense debate. It has been proposed that nematicity could be driven either by orbital ordering of the Fe d electrons or by spin fluctuations. Furthermore, orbital order with interorbital electron-electron interactions would favor a sign preserving s-wave superconducting order parameter [8,9], while spin fluctuations with strong intraorbital interaction would favor a signchanging s ± -wave or d-wave pairing [3,[10][11][12][13][14][15][16][17]. The experimental findings in Fe-based superconductors so far have supported the latter scenario [18,19]. Howev...

Elliptical vortex and oblique vortex lattice in the FeSe superconductor based on the nematicity and mixed superconducting orders

“…Chowdhury et al investigated the anisotropic interplay between the competing order and the single isotropic superconducting order 23 , and they argued that the different effective masses which are induced by the anisotropic interaction could lead to the anisotropic vortex. However, the pairing symmetry in FeSe is sign-changing s ± -wave or d-wave suggested by the recent experiments 12,13 . Meanwhile, the s + id model could suitably describe the iron-based superconductors 22 .…”

Enhanced superconductivity induced by several-unit-cells diffusion in an FeTe/FeSe bilayer heterostructure

“…Among the numerous types of Fe-based superconductors [4][5][6][7][8][9][10][11], iron selenide [11,12] (FeSe) with the simplest binary structure is an appropriate candidate for investigating the intrinsic mechanism of unconventional superconductivity. Through applying high pressure [13][14][15][16][17], elemental substitution [18][19][20][21]/intercalation [22][23][24], or a liquid-gating technique [25][26][27], the superconducting transition temperature (T c ) of FeSe crystals can be substantially raised despite the low T c of ∼8 K [11] in the original bulk FeSe. More sur-prisingly, the FeSe compound in the form of thin films or multilayers has attracted considerable attention in the past few years due to its potential for achieving significantly high T c and unique electronic properties.…”

Abrupt change of the superconducting gap structure at the nematic critical point in FeSe 1−x S x

“…Isoelectronic sulfur substitution in FeSe provides the most suitable route to study this issue. In the series of FeSe1−x Sx , the density of states at the Fermi level (or bandwidth) can be tuned significantly through chemical pressure, leading to a change of the electron correlation effect (34)(35)(36)(37)(38).…”