Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems.

Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

Direct observation of paramagnons in palladium.

We report an inelastic neutron scattering study of the spin fluctuations in the nearly ferromagnetic element palladium. Dispersive over-damped collective magnetic excitations or "paramagnons" are observed up to 128 meV. We analyze our results in terms of a Moriya-Lonzarich-type spin-fluctuation model and estimate the contribution of the spin fluctuations to the low-temperature heat capacity. In spite of the paramagnon excitations being relatively strong, their relaxation rates are large. This leads to a small contribution to the low-temperature electronic specific heat.

Unusual relationship between magnetism and superconductivity in FeTe(0.5)Se(0.5).

We use neutron scattering to study magnetic excitations in crystals near the ideal superconducting composition of FeTe(0.5)Se(0.5). Two types of excitations are found, a resonance at (0.5,0.5,0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.

Lattice distortion and magnetic quantum phase transition in CeFeAs(1-x)P(x)O.

We use neutron diffraction to study the structural and magnetic phase diagram of CeFeAs(1-x)P(x)O. We find that replacing the larger arsenic with smaller phosphorus in CeFeAs(1-x)P(x)O simultaneously suppresses the AFM order and orthorhombic distortion near x=0.4, thus suggesting the presence of a magnetic quantum critical point. Our detailed structural analysis reveals that the pnictogen height is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity of these materials.

The structure of the high-energy spin excitations in a high-transition-temperature superconductor.

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-T(c)) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T(c). The most obvious feature in the magnetic excitations of high-T(c) superconductors such as YBa2Cu3O6+x is the so-called 'resonance'. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)(x)CuO4 family and is not universally present in Bi2Sr2CaCu2O8+delta (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T(c), and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La2-xBa(x)CuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.

Detection of the unusual magnetic orders in the pseudogap region of a high-temperature superconducting YBa2Cu3O6.6 crystal by muon-spin relaxation.

We present muon-spin relaxation (muSR) measurements on a large YBa2Cu3O6.6 single crystal in which two kinds of unusual magnetic order have been detected in the pseudogap region by neutron scattering. A comparison is made to measurements on smaller, higher quality YBa2Cu3Oy single crystals. One type of magnetic order is observed in all samples, but does not evolve significantly with hole doping. A second type of unusual magnetic order is observed only in the YBa2Cu3O6.6 single crystal. This magnetism has an ordered magnetic moment that is quantitatively consistent with the neutron experiments, but is confined to just a small volume of the sample ( approximately 3%). Our findings do not support theories that ascribe the pseudogap to a state characterized by loop-current order, but instead indicate that dilute impurity phases are the source of the unusual magnetic orders in YBa2Cu3Oy.

Neutron diffraction studies of collagen in fully mineralized bone.

Neutron diffraction measurements have been made of the equatorial and meridional spacings of collagen in fully mineralized mature bovine bone and demineralized bone collagen, in both wet and dry conditions. The collagen equatorial spacing in wet mineralized bovine bone is 1.24 nm, substantially lower than the 1.53 nm value observed in wet demineralized bovine bone collagen. Corresponding spacings for dry bone and demineralized bone collagen are 1.16 nm and 1.12 nm, respectively. The collagen meridional long spacing in mineralized bovine bone is 63.6 nm wet and 63.4 nm dry. These data indicate that collagen in fully mineralized bovine bone is considerably more closely packed than had been assumed previously, with a packing density similar to that of the relatively crystalline collagens such as wet rat tail tendon. The data also suggest that less space is available for mineral within the collagen fibrils in bovine bone than had previously been assumed, and that the major portion of the mineral in this bone must be located outside the fibrils.

Comparison of dosage-dependent effects of beta-aminopropionitrile, sodium fluoride, and hydrocortisone on selected physical properties of cortical bone.

BAPN, sodium fluoride, and hydrocortisone are reported to induce altered mineralization states. Three separate sets of experiments, one set for each agent, were performed using male New Zealand white rabbits. In each experiment the rabbits were segregated into groups, each fed a specified weight-determined dose for 13 weeks and then sacrificed. Compact bone from the left femur and tibia were tested for density, composition, sonic velocity, longitudinal elastic modulus, equatorial diffraction spacing of mineralized collagen, diaphyseal cross-sectional area, and relative load stress. beta-Aminopropionitrile (BAPN) induced monotonic degradation of all properties at all dose levels, corresponding to the decreasing density with dosage level. The elastic moduli show a decrease; the equatorial diffraction spacing of the collagen increases. The cross-sectioned diaphysis resembled woven bone. The variability in properties increased with dosage. The total cross-sectional area for a given weight increased, implying that the decreased elastic properties were compensated for by a larger area to support the weight. There was a slight increase in average density and other properties for fluoride-treated rabbits, peaking at 20 mg/kg BW/day. For higher dosages the properties are degraded and the values were much lower at high fluoride dosages than for BAPN. There was no peak for the equatorial diffraction spacing, which increased with dosage. It is inferred that the fluorosed apatite is denser than normal apatitic mineral and therefore has a smaller specific volume. A greater weight fraction of fluorosed mineral has a smaller volume fraction than the equivalent normal apatitic mineral. The bone sections look more normal, except for the porosis. The total cross-sectional area decreases when the bone density increases and then increases as the density falls, again implying that the area required to support body weight depends on the magnitude of the elastic moduli. There was a small change in some of the properties of the bones of the hydrocortisone-treated rabbits, but the effects on others were undetectable within the uncertainty of the procedures. There was no change in the cross-sectional areas of the diaphyses.

Two-dimensional resonant magnetic excitation in BaFe1.84Co0.16As2.

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wave vectors (1/2 1/2 L) in tetragonal notation. On cooling below T_{C}, a clear resonance peak is observed at this wave vector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) k_{B}T_{C}. This is in good agreement with the canonical value of 5 k_{B}T_{C} observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c axis, indicating that the magnetic fluctuations are two dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.

Low energy spin waves and magnetic interactions in SrFe2As2.

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe2As2 (T_{N}=200-220 K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the magnetic spectrum S(Q,Planck's omega) consists of a Bragg peak at the elastic position (Planck's omega=0 meV), a spin gap (Delta< or =6.5 meV), and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across T_{N}, the low-temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe2As2 is therefore consistent with a first order phase transition, similar to the structural lattice distortion.

High-energy spin excitations in the electron-doped superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) with T(c) = 21 K.

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of the electron-doping superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) (T(c) = 21 +/- 1 K) over a wide energy range (4 meV < or = homega < or = 330 meV). The effect of electron doping is to cause a wave vector (Q) broadening in the low-energy (homega < or = 80 meV) commensurate spin fluctuations at (0.5, 0.5) and to suppress the intensity of spin-wave-like excitations at high energies (homega > or = 100 meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility chi''(omega), and reveals a new energy scale similar to that of the lightly hole-doped YB2Cu3O(6.353) (T(c) = 18 K).

Equatorial diffraction spacing as a function of water content in fully mineralized cow bone determined by neutron diffraction.

Variation of the equatorial diffraction spacing of soft type I collagen tissues with water content using X-rays has been known for many years. Recently, a generalized model for collagen molecule packing within fibrils was deduced from this information for different collagenous tissues. It is now known that the eq. diff. sp. of mineralized tissues can be less than for soft tissues and is inversely dependent on the wet density. A determination of the eq. dif. sp. dependence on water content using neutron diffraction of fully mineralized cow bone was undertaken for comparison. Specimens with various partial water content between 0 and 100% were tested. Data show collagen molecules pack more closely together as water content decreases, just as for soft tissues.

Charge and spin structure in of YBa(2)Cu(3)O(6.35).

Neutron scattering has been used to measure the charge and spin structure in the highly underdoped superconductor YBa(2)Cu(3)O(6.35). Incommensurate static charge ordering is found that remains at high temperatures. The magnetic pattern is complex with a resonance and incommensurate structure observed at low temperatures. The results clarify the role of striped phases in YBa(2)Cu(3)O(6+x) superconductors.

Effect of sodium chloride solutions on mineralized and unmineralized turkey leg tendon.

Previous studies showed that the equatorial diffraction spacing of the collagen molecules in mineralized tissues decreases when the tissue is dried and that the spacing in totally dried tissue is about the same (1.1 nm) whether mineralized or not. Here we report that spacing decreases were observed in both mineralized and unmineralized turkey leg tendon after soaking in various sodium chloride solutions up to 4.0 M concentration. The effect was seen by X-ray diffraction as well as by neutron diffraction. No effect was seen in turkey leg tendon soaked in 3.0 M ethylene glycol solution. The spacing in unmineralized tissue decreased from 1.459 +/- 0.011 nm in 0.15 M saline to 1.403 +/- 0.025 nm in 1.5 M saline, a change of 0.056 +/- 0.03 nm or 3.84%. In mineralized turkey leg tendon the corresponding spacings were 1.387 +/- 0.012 and 1.321 +/- 0.019 nm, a change of 0.046 +/- 0.02 nm or 3.4%. No significant dimensional change was noted in the thickness even though the equatorial diffraction spacing decreased by 3.4%. Electron microscopy showed the collagen fibrils within the mineralized turkey leg tendon to be surrounded by highly mineralized material. Presumably the composition of the extrafibrillar material is different from the intrafibrillar and therefore the extrafibrillar material is a different kind of composite. If it is assumed that the extrafibrillar material does not change dimensions significantly, then the collagen molecules in the fibrils can be mobile within the dimensionally stable cage-like structure.