Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors 1-3 , a central question has been whether magnetism in these materials arises from weakly correlated electrons 4,5 , as in the case of spin density wave in pure chromium 6 , requires strong electron correlations 7 , or can even be described in terms of localized electrons 8,9 such as the AF insulating state of copper oxides 10 . Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe 1.9 Ni 0.1 As 2 (T c = 20 K), which allows us to obtain the size of the fluctuating magnetic moment m 2 , and its energy distribution 11,12 . We find that superconducting BaFe 1.9 Ni 0.1 As 2 and AF BaFe 2 As 2 (ref. 13) both have fluctuating magnetic moments m 2 ≈ 3.2 µ 2 B per Fe(Ni), which are similar to those found in the AF insulating copper oxides 14,15 . The common theme in both classes of high-temperature superconductors is that magnetic excitations have partly localized character, thus showing the importance of strong correlations for high-temperature superconductivity 16 .In the undoped state, iron pnictides such as BaFe 2 As 2 form a metallic low-temperature orthorhombic phase with the antiferromagnetic (AF) structure as shown in Fig. 1a (ref. 17). Inelastic neutron scattering measurements have mapped out spin waves throughout the Brillouin zone in the AF orthorhombic and paramagnetic tetragonal phases 13 . On Co-and Ni-doping to induce optimal superconductivity via electron doping, the orthorhombic structural distortion and static AF order in BaFe 2 As 2 are suppressed and the system becomes tetragonal and paramagnetic at all temperatures 18 . In previous inelastic neutron scattering experiments on optimally electron-doped Ba(Fe, Co, Ni) 2 As 2 superconductors 11,12,[19][20][21][22] , spin excitations up to ∼120 meV were observed. However, the lack of spin excitation data at higher energies in absolute units precluded a comparison with spin waves in undoped BaFe 2 As 2 . Only the absolute intensity measurements in the entire Brillouin zone can reveal the effect of electron doping on the overall spin excitation spectra and allow a direct comparison with the results in the AF insulating copper oxides 14,15 . For the experiments, we chose to study well-characterized electron-doped BaFe 1.9 Ni 0.1 As 2 (refs 20,22) because large single crystals were available 23 and their properties are similar to Co-doped BaFe 2 As 2 (refs 11,12,19,21,24).By comparing spin excitations in BaFe 1.9 Ni 0.1 As 2 and BaFe 2 As 2 throughout the Brillouin zone, we were able to probe how electron doping and superconductivity affect the overall spin

Heterostructure based interface engineering has been proved an effective method for finding new superconducting systems and raising superconductivity transition temperature (T C ) 1-7 . In previous work on one unit-cell (UC) thick FeSe films on SrTiO 3 (STO) substrate, a superconducting-like energy gap as large as 20 meV 8 , was revealed by in situ scanning tunneling microscopy/spectroscopy (STM/STS). Angle resolved photoemission spectroscopy (ARPES) further revealed a nearly isotropic gap of above 15 meV, which closes at a temperature of 65 ± 5 K 9-11 . If this transition is indeed the superconducting transition, then the 1-UC FeSe represents the thinnest high T C superconductor discovered so far. However, up to date direct transport measurement of the 1-UC FeSe films has not been reported, mainly because growth of large scale 1-UC FeSe films ischallenging and the 1-UC FeSe films are too thin to survive in atmosphere. In this work, we successfully prepared 1-UC FeSe films on insulating STO substrates with non-superconducting FeTe protection layers. By direct transport and magnetic measurements, we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films with an onset T C above 40 K and a extremely large critical current density J C ~ 1.7×10 6 A/cm 2 at 2 K. Our work may pave the way to enhancing and tailoring superconductivity by interface engineering.The FeSe films and FeTe protection layer are grown by molecular beam epitaxy (MBE) (see Methods).

We have successfully grown large single crystals of BaFe 2−x Ni x As 2 with a series of Ni doping levels from x = 0 to x = 0.30 by the self-flux method. The resistivity and AC susceptibility measurements show that the superconductivity (SC) smoothly emerges at x ≈ 0.05, while the antiferromagnetism (AFM) survives up to x ≈ 0.092. We provide a detailed phase diagram of the BaFe 2−x Ni x As 2 system and suggest that a quantum critical point (QCP) may be located at x = 0.096 ± 0.04.

We use inelastic neutron scattering to systematically investigate the Ni-doping evolution of the low-energy spin excitations in BaFe2−xNixAs2 spanning from underdoped antiferromagnet to overdoped superconductor (0.03 ≤ x ≤ 0.18). In the undoped state, BaFe2As2 changes from paramagnetic tetragonal phase to orthorhombic antiferromagnetic (AF) phase below about 138 K, where the low-energy (≤∼ 80) meV spin waves form transversely elongated ellipses in the [H, K] plane of the reciprocal space. Upon Ni-doping to suppress the static AF order and induce superconductivity, the c-axis magnetic exchange coupling is rapidly suppressed and the momentum distribution of spin excitations in the [H, K] plane is enlarged in both the transverse and longitudinal directions with respect to the in-plane AF ordering wave vector of the parent compound. As a function of increasing Ni-doping x, the spin excitation widths increase linearly but with a larger rate along the transverse direction. These results are in general agreement with calculations of dynamic susceptibility based on the random phase approximation (RPA) in an itinerant electron picture. For samples near optimal superconductivity at x ≈ 0.1, a neutron spin resonance appears in the superconducting state. Upon further increasing the electron-doping to decrease the superconducting transition temperature Tc, the intensity of the low-energy magnetic scattering decreases and vanishes concurrently with vanishing superconductivity in the overdoped side of the superconducting dome. Comparing with the low-energy spin excitations centered at commensurate AF positions for underdoped and optimally doped materials (x ≤ 0.1), spin excitations in the over-doped side (x = 0.15) form transversely incommensurate spin excitations, consistent with the RPA calculation. Therefore, the itinerant electron approach provides a reasonable description to the low-energy AF spin excitations in BaFe2−xNixAs2.

We use polarized neutron scattering to demonstrate that in-plane spin excitations in electron doped superconducting BaFe1.904Ni0.096As2 (Tc=19.8  K) change from isotropic to anisotropic in the tetragonal phase well above the antiferromagnetic (AFM) ordering and tetragonal-to-orthorhombic lattice distortion temperatures (TN≈Ts=33±2  K) without an uniaxial pressure. While the anisotropic spin excitations are not sensitive to the AFM order and tetragonal-to-orthorhombic lattice distortion, superconductivity induces further anisotropy for spin excitations along the [110] and [110] directions. These results indicate that the spin excitation anisotropy is a probe of the electronic anisotropy or orbital ordering in the tetragonal phase of iron pnictides.

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We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brioullion zone in electron-doped iron pnictide superconductors BaFe2−xNixAs2 with x = 0.096, 0.15, 0.18. While the x = 0.096 sample is near optimal superconductivity with Tc = 20 K and has coexisting static incommensurate magnetic order, the x = 0.15, 0.18 samples are electronoverdoped with reduced Tc of 14 K and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped (x = 0) and electron optimally doped (x = 0.1) samples, the effect of electron-doping was found to modify spin waves in the parent compound BaFe2As2 below ∼100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the x = 0.096 sample confirms the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its Q-width below Tc similar to that of the optimally hole-doped iron pnictides Ba0.67K0.33Fe2As2. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron-doping to overdoped iron pnictides with x = 0.15 and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above ∼100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for x = 0.096, 0.15, 0.18 confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent.

We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally holedoped superconductor Ba0.67K0.33Fe2As2 (Tc = 38 K). In the normal state, the imaginary part of the dynamic susceptibility, χ ′′ (Q, ω), shows magnetic anisotropy for energies below ∼7 meV with c-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at ∼5 and 0.75 meV open at wave vectors Q = (0.5, 0.5, 0) and (0.5, 0.5, 1), respectively, with a broad neutron spin resonance at Er = 15 meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the c-axis and in-plane polarized spin excitations, resulting resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both opitmally electron and hole-doped BaFe2As2 is due to their proximity to the AF ordered BaFe2As2 where spin anisotropy exists below TN .