Single-layer FeSe film on SrTiO3 (001) was recently found to be the champion of interfacial superconducting systems, with a much enhanced superconductivity than the bulk iron-based superconductors. Its superconducting mechanism is of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unsettled. Here by using low-temperature scanning tunneling microscopy (STM), we systematically investigated the superconductivity of single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling spectrum and magnetic vortex lattice in the film. Quasi-particle interference (QPI) patterns reveal scatterings between and within the electron pockets, and put constraints on possible pairing symmetries. By introducing impurity atoms onto the sample, we show that the magnetic impurities (Cr, Mn) can locally suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K) cannot. Our results indicate that single-layer FeSe/ SrTiO3 has a plain s-wave paring symmetry whose order parameter has the same phase on all Fermi surface sections.Recently the discovery of enhanced superconductivity in single-layer FeSe on SrTiO3(001) has attracted tremendous interest [1][2][3][4][5][6][7][8][9], not only for the new possible superconducting transition temperature records of Fe-based superconductors and interfacial superconductors (65K [3,4] or even higher [9]), but also its intriguing mechanism that enhances the paring. Thus it is of great importance to understand the pairing symmetry and underlying electron structure of single-layer FeSe/SrTiO3(001). Angle-resolved photoemission spectroscopy (ARPES) revealed that such films have only electron Fermi surfaces, similar to that of the alkali metal intercalated iron selenides (AxFe2-ySe2, A=K, Cs…) [3][4][5]. This seriously challenges the original s±-pairing scenario proposed for the iron pnictides that relies on the coupling between the electron pockets and the hole pockets at the Brillouin zone center [10,11]. Meanwhile both ARPES and previous STM studies found fully gapped superconducting state in single-layer FeSe, indicative of the absence of gap nodes [1,[3][4][5]. Various possible paring symmetries have been proposed for such systems with only electron pockets [12][13][14][15][16][17][18][19], such as plain s-wave paring [12][13][14], "quasi-nodeless" d-wave paring [15,16], and several new types of s± paring that involve the "folding" of Brillouin zone and band hybridization [17], orbital dependent pairing [18], or mixing of the even and odd-parity pairing [19]. Except the plain s-wave paring, all the other proposed pairing symmetries involve sign changing of the order parameter on different sections of the Fermi surface. To distinguish these scenarios, phase sensitive measurements are required, plus the detailed knowledge on the superconducting gap.STM has been shown to be able to provide information on the pairing symmetry by measuring local response of superconductivity to impurities (in-gap impurity states) [20][21][22] and throu...

Superconductivity in FeSe is greatly enhanced in films grown on SrTiO3 substrates, although the mechanism behind remains unclear. Recently, surface potassium (K) doping has also proven able to enhance the superconductivity of FeSe. Here, by using scanning tunneling microscopy, we compare the K doping dependence of the superconductivity in FeSe films grown on two substrates: SrTiO3 (001) and graphitized SiC (0001). For thick films (20 unit cells (UC)), the optimized superconducting (SC) gaps are of similar size (∼9 meV) regardless of the substrate. However, when the thickness is reduced to a few UC, the optimized SC gap is increased up to ∼15 meV for films on SrTiO3, whereas it remains unchanged for films on SiC. This clearly indicates that the FeSe/SrTiO3 interface can further enhance the superconductivity, beyond merely doping electrons. Intriguingly, we found that this interface enhancement decays exponentially as the thickness increases, with a decay length of 2.4 UC, which is much shorter than the length scale for relaxation of the lattice strain, pointing to interfacial electron-phonon coupling as the likely origin.

Li0.8Fe0.2)OHFeSe is a newly-discovered intercalated iron-selenide superconductor with a Tc above 40 K, which is much higher than the Tc of bulk FeSe (8 K). Here we report a systematic study of (Li0.8Fe0.2)OHFeSe by low temperature scanning tunneling microscopy (STM). We observed two kinds of surface terminations, namely FeSe and (Li0.8Fe0.2)OH surfaces. On the FeSe surface, the superconducting state is fully gapped with double coherence peaks, and a vortex core state with split peaks near EF is observed. Through quasiparticle interference (QPI) measurements, we clearly observed intra-and interpocket scatterings in between the electron pockets at the M point, as well as some evidence of scattering that connects Г and M points. Upon applying magnetic field, the QPI intensity of all the scattering channels are found to behave similarly. Furthermore, we studied impurity effects on the superconductivity by investigating intentionally introduced impurities and intrinsic defects. We observed that magnetic impurities such as Cr adatoms can induce in-gap states and suppress superconductivity. However, nonmagnetic impurities such as Zn adatoms do not induce visible in-gap states. Meanwhile, we show that Zn adatoms can induce in-gap states in thick FeSe films, which is believed to have an s±-wave pairing symmetry. Our experimental results suggest it is likely that (Li0.8Fe0.2)OHFeSe is a plain s-wave superconductor, whose order parameter has the same sign on all Fermi surface sections.

Ta 4 Pd 3 Te 16 is a newly discovered layered superconductor with quasi-one-dimensional (1D) structure. Recent thermal transport measurements show the possible existence of nodes in the superconducting gap. Here we report low-temperature scanning tunneling microscopy/spectroscopy study on Ta 4 Pd 3 Te 16 single crystals. We observed stripelike structure composed of atom chains on the cleaved (103) surface. There exists charge-density-wave (CDW)-like modulations along stripes with commensurate periods. Meanwhile, the tunneling conductance shows an s-wave-like superconducting gap. The magnetic vortex mapped at low field is highly anisotropic with a bound state in the core. At increased field, strong vortex overlapping is directly observed and the bound state is suppressed, indicating the delocalization of the superconducting quasiparticles. Our observations suggest that Ta 4 Pd 3 Te 16 is of multiband superconductivity with strong 1D characters, which possibly coexist with CDW transition. PACS number(s): 74.55.+v, 74.25.Jb, 74.25.Uv, Superconductivity in low-dimensional systems has long been a fascinating topic in condensed matter physics. The increased phase fluctuation is an obstacle of electron paring, and unique orders such as charge/spin density waves (CDW/SDW) may also compete with superconductivity. However, for last decades, many superconductors were found at low dimensions. For examples, cuprates [1], iron-based material [2], and heavy fermion materials like CeCoIn 5 [3], all have quasi-two-dimensional (2D) structure. They are also believed to be unconventional superconductors. But for the quasione-dimensional (1D) case, represented by the compounds (TMTSF) 2 X (X = PF 6 , ClO 4 ) [4,5] and Li 0.9 Mo 6 O 17 [6], how the superconductivity would exist and behave is still unclear.Recently, Jiao et al. reported superconductivity in a layered materials Ta 4 Pd 3 Te 16 with quasi-1D chains, which has a superconducting transition temperature T c of 4.6 K [7]. The specific heat measurement suggests that there might be strong electron-electron interactions in this system. Soon after, Pan et al. reported thermal conductivity of Ta 4 Pd 3 Te 16 measured at temperature down to 80 mK [8], which shows significant residual linear term at zero field and its field dependence resembles that of cuprate superconductors. The band structure calculations show that Ta 4 Pd 3 Te 16 is a multiband system, whose low energy electronic structure near the Fermi energy (E F ) is mainly contributed by the Te 5p states with little correlation [9,10]. Thus the thermal transport data might be interpreted by its multiband nature, where the gap of certain band could be very small, rather than by a nodal superconducting gap structure. To clarify these two scenarios, more direct measurements of the superconducting state are required.In this paper, we report low-temperature scanning tunneling microscope (STM) and spectroscopy (STS) measurements on * tzhang18@fudan.edu.cn Ta 4 Pd 3 Te 16 single crystals. The surface atomic structure, superconduct...

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