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.

We have investigated the effect of Fe nonstoichiometry on properties of the Fe 1+y (Te, Se) superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets, which favors a short-range magnetic order.

The atomic-scale mechanisms underlying the growth of Ag on the (√2×√2)R45°-Fe3O4(001) surface were studied using scanning tunneling microscopy and density functional theory based calculations. For coverages up to 0.5 ML, Ag adatoms populate the surface exclusively; agglomeration into nanoparticles occurs only with the lifting of the reconstruction at 720 K. Above 0.5 ML, Ag clusters nucleate spontaneously and grow at the expense of the surrounding material with mild annealing. This unusual behavior results from a kinetic barrier associated with the (√2×√2)R45° reconstruction, which prevents adatoms from transitioning to the thermodynamically favorable 3D phase. The barrier is identified as the large separation between stable adsorption sites, which prevents homogeneous cluster nucleation and the instability of the Ag dimer against decay to two adatoms. Since the system is dominated by kinetics as long as the (√2×√2)R45° reconstruction exists, the growth is not well described by the traditional growth modes. It can be understood, however, as the result of supersaturation within an adsorption template system.

While perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Srn+1RunO3n+1 (n = 1, 2) using low-temperature scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighboring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions. The OH stemming from the original molecule stays trapped at Sr-Sr bridge positions, circling the surface OH with a measured activation energy of 187±10 meV. At higher coverage dimers of dissociated water assemble into one-dimensional chains and form a percolating network where water adsorbs molecularly in the gaps. Our work shows the limitations of applying surface chemistry concepts derived for binary rocksalt oxides to perovskites.

Fe1+yTe with y < ∼ 0.05 exhibits a first-order phase transition on cooling to a state with a lowered structural symmetry, bicollinear antiferromagnetic order, and metallic conductivity, dρ/dT > 0. Here, we study samples with y = 0.09(1), where the frustration effects of the interstitial Fe decouple different orders, leading to a sequence of transitions. While the lattice distortion is closely followed by incommensurate magnetic order, the development of bicollinear order and metallic electronic coherence is uniquely associated with a separate hysteretic first-order transition, at a markedly lower temperature, to a phase with dramatically enhanced bond-order wave (BOW) order. The BOW state suggests ferro-orbital ordering, where electronic delocalization in ferromagnetic zigzag chains decreases local spin and results in metallic transport. In a pattern common with cuprates, iron pnictide and chalcogenide superconductors (FeSC) have parent phases, which, upon cooling, undergo antiferromagnetic (AFM) ordering and structural distortion(s) lowering the high-temperature tetragonal (HTT) paramagnetic lattice symmetry [1,2]. They also host strong magnetic fluctuations, a hallmark of unconventional superconductivity [3]. Recently, there has also been strong experimental evidence of broken electronic symmetry, "nematicity", accompanying, or preceding the magnetic/lattice ordered phase, reminiscent of stripes in cuprates [4]. The physics driving these phenomena, their inter-relation and relation to the superconductivity remain unclear [2].Unlike cuprates, the Fe-based materials have several unfilled 3d bands. Their parent magnetic phases have well-defined Fermi surfaces, indicating a metallic nature [4,5]. Such "weak Mott-ness" and itinerancy, combined with orbital degeneracy entangled with the magnetic and lattice degrees of freedom, leads to the proliferation of theoretical models and approaches: strong coupling where physics is spin-driven [6], weak coupling where it is determined by properties of the electronic Fermi surface [7], or mixed spin-orbital models [8][9][10]. The experimental evidence enabling one to distinguish among these models is, however, still scarce. Here we present such evidence for the case of Fe 1+y Te, the end member of the chalcogenide family of FeSC, where correlation effects are the strongest [11]. By combining the results of bulk characterization of electronic behavior and neutron diffraction data on the temperature evolution of the microscopic structure we are able to disentangle different low-temperature orders and show that the transition to the magnetically-ordered state [12,13], illustrated in Fig. 1(b), is electronically driven through ferro-orbital ordering of zigzag Fe-Fe chains.The iron-chalcogenides Fe 1+y Te 1+x Se x , with T c ≈ 14.5 K at optimal doping, consist of a continuous stacking of Fe square-lattice layers, separated by two halffilled chalcogen (Te,Se) layers [14][15][16]. Predicted by band structure calculations to be a metal [5], nonsuperconducting parent material Fe 1+y T...