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...

Sr 2 IrO 4 was predicted to be a high-temperature superconductor upon electron doping since it highly resembles the cuprates in crystal structure, electronic structure, and magnetic coupling constants. Here, we report a scanning tunneling microscopy/spectroscopy (STM/STS) study of Sr 2 IrO 4 with surface electron doping by depositing potassium (K) atoms. We find that as the electron doping increases, the system gradually evolves from an insulating state to a normal metallic state, via a pseudogaplike phase, and a phase with a sharp, V-shaped low-energy gap with about 95% loss of density of state (DOS) at E F . At certain K coverage (0.5-0.6 monolayer), the magnitude of the low-energy gap is 25-30 meV, and it closes at around 50 K. Our observations show that the electron-doped Sr 2 IrO 4 remarkably resembles hole-doped cuprate superconductors.

Alkali-fulleride superconductors with a maximum critical temperature Tc  40 K exhibit similar electronic phase diagram with unconventional high-Tc superconductors [1][2][3][4][5] where the superconductivity resides proximate to a magnetic Mott-insulating state 3-6 . However, distinct from cuprate compounds, which superconduct through two-dimensional (2D) CuO2 planes 7 , alkali fullerides are attributed to the three-dimensional (3D) members of high-Tc family 8,9 . Here, we employ scanning tunneling microscopy (STM) to show that trilayer K3C60 displays fully gapped strong coupling s-wave superconductivity that coexists spatially with a cuprate-like pseudogap state above Tc  22 K and within vortices. A precise control of electronic correlations and doping reveals that superconductivity occurs near a superconductor-Mott insulator transition (SMIT) and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of superconductivity below half-filling, demonstrates that alkali fullerides are predominantly phonon-mediated superconductors, although the multiorbital electronic correlations also come into play.

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.

Alkali-doped fullerides exhibit a wealth of unusual phases that remain controversial by nature.Here we report a cryogenic scanning tunneling microscopy study of the sub-molecular structural and electronic properties of expanded fullerene C n− 60 films with various cesium (Cs) doping. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of C n− 60 anions, yet the tunneling conductance spectra are all robustly characteristic of energy gaps, hallmarks of Jahn-Teller instability and electronic correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of n = 1, 2, 3 and 4, apart from which it moves toward band edges with concomitant electronic states within the energy gap. Our findings establish the universality of Jahn-Teller instability, and clarify the relationship among the doping, structural and electronic structures in CsnC60 fullerides.