The record superconducting transition temperature (T(c)) for the iron-based high-temperature superconductors (Fe-HTS) has long been 56 K. Recently, in single-layer FeSe films grown on SrTiO3 substrates, indications of a new record of 65 K have been reported. Using in situ photoemission measurements, we substantiate the presence of spin density waves (SDWs) in FeSe films--a key ingredient of Fe-HTS that was missed in FeSe before--and we find that this weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate suppress the otherwise pronounced SDWs in single-layer FeSe. Beyond providing a comprehensive understanding of FeSe films and directions to further enhance its T(c), we map out the phase diagram of FeSe as a function of lattice constant, which contains all the essential physics of Fe-HTS. With the simplest structure, cleanest composition and single tuning parameter, monolayer FeSe is an ideal system for testing theories of Fe-HTS.

The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention . How to further increase the superconducting transition temperature ( T c ) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T c superconductivity in a single-layer FeSe is therefore both surprising and signifi cant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T c ~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T c superconductivity in iron-based superconductors.

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+ySexTe1−x. Fe1.068Te exhibits a first order phase transition near 67 K with a tetragonal to monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of FeSe1−xTex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density functional calculations, where FeSe1−xTex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density-wave AF order.

The iron chalcogenide Fe(1+y)(Te(1-x)Se(x)) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe(1+y)Te exhibits antiferromagnetic order with an in-plane magnetic wave vector (pi,0) (ref. 6). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (pi,pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (pi,pi) (refs 9, 10, 11). A central question in this burgeoning field is therefore how (pi,pi) superconductivity can emerge from a (pi,0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (pi,0) are suppressed and the mode at (pi, pi) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.

Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.

We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe1.9Ni0.1As2 above and below its superconducting transition temperature Tc = 20 K. In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wavevector (1, 0, 0), the effect of superconductivity is to form a three dimensional resonance with clear dispersion along the c-axis direction. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at (1, 0, 0) and (1, 0, 1). If the resonance energy is directly associated with the superconducting gap energy ∆, then ∆ is dependent on the wavevector transfers along the c-axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.PACS numbers: 74.25. Ha, 78.70.Nx Understanding the interplay between spin fluctuations and superconductivity in high-transition-temperature (high-T c ) superconductors is important because spin fluctuations may mediate electron pairing for superconductivity [1,2]. In the case of high-T c copper oxides, it is now well documented that the spin fluctuation spectrum is dominated by a collective excitation known as the resonance mode centered at the antiferromagnetic (AF) ordering wavevector Q = (1/2, 1/2) [3,4,5,6,7,8]. Although the intensity of the mode behaves like an order parameter below T c , the energy of the mode is dispersionless for wavevector transfers along the c-axis and directly tracks T c [4,5,6,7,8], thus suggesting that the mode is an intrinsic property of the two-dimensional (2D) CuO 2 planes and intimately associated with superconductivity. For FeAs-based superconductors [9,10,11,12], the presence of static AF ordering in their parent compounds (with spin structure of Fig. 1a) [13,14,15,16,17,18] and the remarkable similar doping dependent phase diagram to that of the high-T c copper oxides [15] suggest that AF spin fluctuations may also play an important role in the superconductivity of these materials. Indeed, recent neutron scattering measurements on spin fluctuations of powder samples of superconducting Ba 0.6 K 0.4 Fe 2 As 2 (T c = 38 K) [19] and crystalline electric field (CEF) excitations of Ce in CeFeAsO 0.84 F 0.16 (T c = 41 K) [20] found clear evidence for resonant-like magnetic intensity gain below T c athω ∼ 14 and 18.7 meV, respectively. However, the Ce CEF measurements give no information on the Q-dependence of the scattering [20]. Although the resonant-like scattering in Ba 0.6 K 0.4 Fe 2 As 2 occurs near the AF ordering wavevector, the powder nature of the experiment impedes to distinguish whether the resonant scattering is centered at the three-dimensional (3D) AF wavevector Q = (1, 0, 1) of its parent compound [16,17,18] or simply at a 2D AF in-plane wavevector Q = (1, 0, 0) [19].In this Letter, we report the results of inelastic neutron scattering studies of s...

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe0.56Se0.44, monolayer FeSe grown on SrTiO3 and K0.76Fe1.72Se2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.

The in-plane London penetration depth, λ(T ), was measured in single crystals of the ironchalcogenide superconductors Fe1.03(Te0.63Se0.37) and Fe1.06(Te0.88S0.14) by using a radio-frequency tunnel diode resonator. As is also the case for the iron-pnictides, these iron-chalcogenides exhibit a nearly quadratic temperature variation of λ(T ) at low temperatures. The absolute value of the penetration depth in the T → 0 limit was determined for Fe1.03(Te0.63Se0.37) by using an Al coating technique, giving λ(0) ≈ 560 ± 20 nm. The superfluid density ρs(T ) = λ 2 (0)/λ 2 (T ) was fitted with a self-consistent two-gap γ−model. While two different gaps are needed to describe the full-range temperature variation of ρs(T ), a non-exponential behavior at low temperatures requires additional factors, such as scattering and/or significant gap anisotropy.