Iron Oxypnictide Research Articles

On superconducting and magnetic properties of iron oxypnictides

Pairing symmetry in oxypnictides, a new family of multiband high-Tc superconductors, is partially imposed by the positions of multiple Fermi pockets, which itself can give rise to new order parameters, such as s+,- states or the state of dx^2-y^2 symmetry. Other pairing states may appear on small pockets for long range interactions, but they are expected to be sensitive to defects. We identify the competing antiferromagnetic order with the triplet exciton transition in the semi- metallic background and discuss whether its coexistence with superconductivity explains the doping dependence of Tc.

Electronic structure of the iron-based superconductor LaOFeP

The recent discovery of superconductivity in the iron oxypnictide family of compounds has generated intense interest. The layered crystal structure with transition-metal ions in planar square-lattice form and the discovery of spin-density-wave order near 130 K (refs 10, 11) seem to hint at a strong similarity with the copper oxide superconductors. An important current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories, distinguished by the underlying band structure, have been put forward: a local-moment antiferromagnetic ground state in the strong-coupling approach, and an itinerant ground state in the weak-coupling approach. The first approach stresses on-site correlations, proximity to a Mott-insulating state and, thus, the resemblance to the high-transition-temperature copper oxides, whereas the second approach emphasizes the itinerant-electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. The debate over the two approaches is partly due to the lack of conclusive experimental information on the electronic structures. Here we report angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (superconducting transition temperature, T(c) = 5.9 K), the first-reported iron-based superconductor. Our results favour the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper-based superconductors.

Magnetic excitations in the high-Tciron pnictides

We calculate the expected finite frequency neutron-scattering intensity based on the two-sublattice collinear antiferromagnet found by recent neutron-scattering experiments as well as by theoretical analysis on the iron oxypnictide LaOFeAs. We consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling ${J}_{1}$ and next-nearest-neighbor coupling ${J}_{2}$. We show how to distinguish experimentally between ferromagnetic and antiferromagnetic ${J}_{1}$. Whereas magnetic excitations in the cuprates display a so-called resonance peak at $(\ensuremath{\pi},\ensuremath{\pi})$ (corresponding to a saddlepoint in the magnetic spectrum), which is at a wave vector that is at least close to nesting Fermi-surface-like structures---no such corresponding excitations exist in the iron pnictides. Rather, we find saddlepoints near $(\ensuremath{\pi},\ensuremath{\pi}/2)$ and $(0,\ensuremath{\pi}/2)$ (and symmetry related points), which are not close to nesting the Fermi surfaces.

Magnetic order of the iron spins in NdFeAsO

Polarized and unpolarized neutron-diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdFeAsO. Antiferromagnetic order is observed below 141(6) K, which is in close proximity to the structural distortion observed in this material. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the $c$ axis are antiparallel like LaFeAsO. The ordered moment is $0.25(7)\phantom{\rule{0.2em}{0ex}}{\ensuremath{\mu}}_{B}$, which is the smallest moment found so far in these systems.

Evidence for two distinct scales of current flow in polycrystalline Sm and Nd ironoxypnictides

Early studies have found quasi-reversible magnetization curves in polycrystallinebulk rare-earth iron oxypnictides that suggest either widely spread obstacles tointergranular current or very weak vortex pinning. In the present study of polycrystallinesamarium and neodymium iron oxypnictide samples made by high pressure synthesis,the hysteretic magnetization is significantly enhanced. Magneto-optical imagingand study of the field dependence of the remanent magnetization as a functionof particle size both show that global currents over the whole sample do existbut that the intergranular and intragranular current densities have distinctivelydifferent temperature dependences and differ in magnitude by about 1000. Ifthe highest current density loops lie only within grains, their magnitude is∼5 × 106 A cm−2 at 5 K and self-field. Whole sample current densities, though two orders of magnitude lower at1000–10 000 A cm−2, are some two orders of magnitude higher than in random polycrystalline cuprates. Wecannot yet be certain whether this large difference in global and intragrain current densityis intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because thesamples contain a significant second phase, some of which wets the grain boundaries andmay cause a superconducting–normal–superconducting proximity effect in the wholesample critical current.

Minimal two-band model of the superconducting iron oxypnictides

Following the discovery of the Fe-pnictide superconductors, local-density approximation (LDA) band structure calculations showed that the dominant contributions to the spectral weight near the Fermi energy came from the $\text{Fe}\text{ }3d$ orbitals. The Fermi surface is characterized by two hole surfaces around the $\ensuremath{\Gamma}$ point and two electron surfaces around the $M$ point of the two Fe/cell Brillouin zone. Here, we describe a two-band model that reproduces the topology of the LDA Fermi surface and exhibits both ferromagnetic and $q=(\ensuremath{\pi},0)$ spin-density wave fluctuations. We argue that this minimal model contains the essential low energy physics of these materials.

Superconductivity at 43 K in an iron-based layered compound LaO1-xFxFeAs

The iron- and nickel-based layered compounds LaOFeP (refs 1, 2) and LaONiP (ref. 3) have recently been reported to exhibit low-temperature superconducting phases with transition temperatures T(c) of 3 and 5 K, respectively. Furthermore, a large increase in the midpoint T(c) of up to approximately 26 K has been realized in the isocrystalline compound LaOFeAs on doping of fluoride ions at the O2- sites (LaO(1-x)F(x)FeAs). Experimental observations and theoretical studies suggest that these transitions are related to a magnetic instability, as is the case for most superconductors based on transition metals. In the copper-based high-temperature superconductors, as well as in LaOFeAs, an increase in T(c) is often observed as a result of carrier doping in the two-dimensional electronic structure through ion substitution in the surrounding insulating layers, suggesting that the application of external pressure should further increase T(c) by enhancing charge transfer between the insulating and conducting layers. The effects of pressure on these iron oxypnictide superconductors may be more prominent than those in the copper-based systems, because the As ion has a greater electronic polarizability, owing to the covalency of the Fe-As chemical bond, and, thus, is more compressible than the divalent O2- ion. Here we report that increasing the pressure causes a steep increase in the onset T(c) of F-doped LaOFeAs, to a maximum of approximately 43 K at approximately 4 GPa. With the exception of the copper-based high-T(c) superconductors, this is the highest T(c) reported to date. The present result, together with the great freedom available in selecting the constituents of isocrystalline materials with the general formula LnOTMPn (Ln, Y or rare-earth metal; TM, transition metal; Pn, group-V, 'pnicogen', element), indicates that the layered iron oxypnictides are promising as a new material platform for further exploration of high-temperature superconductivity.

Synthesis and Superconducting Properties of the Iron Oxyarsenide TbFeAsO0.85

The iron oxyarsenide TbFeAsO 0.85 was synthesized by a high-pressure method. A high-quality polycrystalline sample was obtained and characterized by measurements of magnetic susceptibility and electrical resistivity. Bulk superconductivity with T c of 42 K was clearly established without an F doping usually conducted to tune on superconductivity in the iron oxypnictide system.

Enhancement of an Iron-Based Layered Compound Superconducting Properties by High Pressure Synthesis Technique

Here we report the high pressure synthesis technique, physical properties and effect of Y substitution on the La-based iron oxypnictide LaFeAsO 0.6 superconductor. Replacement of La 3+ by Y 3+ in La 1- x Y x FeAsO 0.6 results in the decrease of lattice parameters, due to internal chemical pressure effect into the system. Superconducting transition temperature ( T c ) increases up to 43.1 K for x =0.5. The present results provide an alternative, much simpler way to achieve higher- T c in the La-based oxypnictide superconductors, as well as revealing that the lattice parameters are the dominating parameter to determine T c of these new high- T c superconductors. Also we report the new discovery of Ca 1- x Na x Fe 2 As 2 with T c =26 K.

Unusual Pseudogap Features Observed in Iron Oxypnictide Superconductors

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical transition temperatures (Tc's) of 26 K and 5 K, respectively. We find that high-Tc LaFeAsO:F exhibits a temperature-dependent pseudogap-like feature extending over ~0.1 eV about the Fermi level at 250 K, whereas such a feature is absent in low-Tc LaFePO:F. We also find ~20-meV pseudogap-like features and signatures of superconducting gaps both in LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogap-like features through comparison with the high-Tc cuprates.