High-Precision Determination of the Neutron Coherent Scattering Length

The neutron coherent scattering length b_c has been determined interferometrically to an uncertainty of about 5 × 10⁻⁵ by measuring the nondispersive phase. We propose improving the uncertainty to about 10⁻⁶ by optimizing various parameters of the interferometric experiment. Any uncertainty in the b_c determination arising from possible variations in the constitution of the ambient air can be eliminated by performing the experiment in vacuum. When such uncertainty is attained, it becomes necessary to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correct b_c from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimate b_c. The refractive index for neutrons can thus be determined to a phenomenal uncertainty of about 10⁻¹².     

Virtual Excitation and Multiple Scattering Correction Terms to the Neutron Index of Refraction for Hydrogen

The neutron index of refraction is generally derived theoretically in the Fermi approximation. However, the Fermi approximation neglects the effects of the binding of the nuclei of a material as well as multiple scattering. Calculations by Nowak introduced correction terms to the neutron index of refraction that are quadratic in the scattering length and of order 10⁻³ fm for hydrogen and deuterium. These correction terms produce a small shift in the final value for the coherent scattering length of H₂ in a recent neutron interferometry experiment.     

Quantum molecular tunneling of NH4 + ions in mixed ammonium-alkali halide systems

The rotational excitations of NH₄ ⁺ ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.     

Determination of the scattering lengths of gallium isotopes by neutron interferometry with PNO-apparatus in JRR-3M

For carrying out experiments in the field of the so-called precise neutron optics (PNO), we have implemented special multi-purpose apparatus called the “PNO-apparatus” at JRR-3M. Making use of an Si triple-Laue (LLL) neutron interferometer with the PNO-apparatus, we successfully determined the coherent neutron scattering lengths of gallium isotopes, ⁶⁹Ga and ⁷¹Ga. The results are 8.053±0.013 fm for ⁶⁹Ga and 6.170±0.011 fm for ⁷¹Ga, respectively.     

A Ramsey’s Method With Pulsed Neutrons for a T-Violation Experiment

A Ramsey’s method with pulsed neutrons is discussed for neutron spin manipulation in a time reversal (T) symmetry violation experiment. The neutron spin (s_n) is aligned to the direction of a vector product of the nuclear spin (I) and the neutron momentum (k_n) for the measurement of a T-odd correlation term, which is represented as s_n · (k_n × I), during propagation through a polarized nuclear target. The phase control and amplitude modulation of separated oscillatory fields are discussed for the measurement of the T-odd correlation term.     

Magnetic excitations in iron chalcogenide superconductors

Nuclear magnetic resonance and neutron scattering experiments in iron chalcogenide superconductors are reviewed to make a survey of the magnetic excitations in FeSe, FeSe_1−xTe_x and alkali-metal-doped A_xFe_2−ySe₂ (A = K, Rb, Cs, etc). In FeSe, the intimate relationship between the spin fluctuations and superconductivity can be seen universally for the variations in the off-stoichiometry, the Co-substitution and applied pressure. The isovalent compound FeTe has a magnetic ordering with different wave vector from that of other Fe-based magnetic materials. The transition temperature T_c of FeSe increases with Te substitution in FeSe_1−xTe_x with small x, and decreases in the vicinity of the end member FeTe. The spin fluctuations are drastically modified by the Te substitution. In the vicinity of the end member FeTe, the low-energy part of the spin fluctuation is dominated by the wave vector of the ordered phase of FeTe; however, the reduction of T_c shows that it does not support superconductivity. The presence of same wave vector as that of other Fe-based superconductors in FeSe_1−xTe_x and the observation of the resonance mode demonstrate that FeSe_1−xTe_x belongs to the same group as most of other Fe-based superconductors in the entire range of x, where superconductivity is mediated by the spin fluctuations whose wave vector is the same as the nesting vector between the hole pockets and the electron pockets. On the other hand, the spin fluctuations differ for alkali-metal-doped A_xFe_2−ySe₂ and FeSe or other Fe-based superconductors in their wave vector and strength in the low-energy part, most likely because of the different Fermi surfaces. The resonance mode with different wave vector suggests that A_xFe_2−ySe₂ has an exceptional superconducting symmetry among Fe-based superconductors.     

Proposed Measurement of the Beta-Neutrino Correlation in Neutron Decay

Currently, the beta-neutrino asymmetry has the largest uncertainty (4 %) of the neutron decay angular correlations. Without requiring polarimetry this decay parameter can be used to measure λ (g_a/g_v), test Cabibbo-Kobayashi-Maskawa (CKM) unitarity limit scalar and tensor currents, and search for Charged Vector Current (CVC) violation. We propose to measure the beta-neutrino asymmetry coeffcient, a, using time-of-flight for the recoil protons. We hope to achieve a systematic uncertainty of σ_a / a ≈ 1.0 %. After tests at Indiana University’s Low Energy Neutron Source (LENS), the apparatus will be moved to the National Institute of Standards and Technology (NIST) where the measurement can achieve a statistical uncertainty of 1 % to 2 % in about 200 beam days.     

Measurement of the Neutron Lifetime Using a Gravitational Trap and a Low-Temperature Fomblin Coating

We present a new value for the neutron lifetime of 878.5 ± 0.7_stat. ± 0.3_syst. This result differs from the world average value by 6.5 standard deviations and by 5.6 standard deviations from the previous most precise result. However, this new value for the neutron lifetime together with a β-asymmetry in neutron decay, A₀, of −0.1189(7) is in a good agreement with the Standard Model.     

Structure and cation distribution in perovskites with small cations at the A site: the case of ScCoO3

We synthesize ScCoO₃ perovskite and its solid solutions, ScCo_1−xFe_xO₃ and ScCo_1−xCr_xO₃, under high pressure (6 GPa) and high temperature (1570 K) conditions. We find noticeable shifts from the stoichiometric compositions, expressed as (Sc_1−xM_x)MO₃ with x = 0.05–0.11 and M = Co, (Co, Fe) and (Co, Cr). The crystal structure of (Sc_0.95Co_0.05)CoO₃ is refined using synchrotron x-ray powder diffraction data: space group Pnma (No. 62), Z = 4 and lattice parameters a = 5.26766(1) Å, b = 7.14027(2) Å and c = 4.92231(1) Å. (Sc_0.95Co_0.05)CoO₃ crystallizes in the GdFeO₃-type structure similar to other members of the perovskite cobaltite family, ACoO₃ (A³⁺ = Y and Pr-Lu). There is evidence that (Sc_0.95Co_0.05)CoO₃ has non-magnetic low-spin Co³⁺ ions at the B site and paramagnetic high-spin Co³⁺ ions at the A site. In the iron-doped samples (Sc_1−xM_x)MO₃ with M = (Co, Fe), Fe³⁺ ions have a strong preference to occupy the A site of such perovskites at small doping levels.     

Measurement of the Neutron Lifetime by Counting Trapped Protons

We measured the neutron decay lifetime by counting in-beam neutron decay recoil protons trapped in a quasi-Penning trap. The absolute neutron beam fluence was measured by capture in a thin ⁶LiF foil detector with known efficiency. The combination of these measurements gives the neutron lifetime: τ_n = (886.8 ± 1.2 ± 3.2) s, where the first (second) uncertainty is statistical (systematic) in nature. This is the most precise neutron lifetime determination to date using an in-beam method.     

Phase Transformations in the High-Tc Superconducting Compounds,Ba2RCu3O7?δ (R = Nd,Sm, Gd,Y, Ho,and Er)

The phase transformation between the orthorhombic and tetragonal structures of six high-T_c superconductors, Ba₂RCu₃O_7−δ, where R = Nd, Sm, Gd, Y, Ho, and Er, and δ = 0 to 1, has been investigated using techniques of x-ray diffraction, differential thermal analysis/thermogravimetric analysis (DTA/TGA) and electron diffraction. The transformation from the oxygen-rich orthorhombic phase to the oxygen-deficient tetragonal phase involves two orthorhombic phases. A superlattice cell caused by oxygen ordering, with a′ = 2a, was observed for materials with smaller ionic radius (Y, Ho, and Er). For the larger lanthanide samples (Nd, Sm, and Gd), the a′ = 2a type superlattice cell was not observed.The structural phase transition temperatures, oxygen stoichiometry and characteristics of the T_c plateaus appear to correlate with the ionic radius, which varies based on the number of f electrons. Lanthanide elements with a smaller ionic radius stabilize the orthorhombic phase to higher temperatures and lower oxygen content. Also, the superconducting temperature is less sensitive to the oxygen content for materials with smaller ionic radius. The trend of dependence of the phase transformation temperature on ionic radius across the lanthanide series can be explained using a quasi-chemical approximation (QCA) whereby the strain effect plays an important role on the order-disorder transition due to the effect of oxygen content on the CuO chain sites.     

Powder X-Ray Reference Patterns of Sr2RGaCu2Oy (R = Pr,Nd, Sm,Eu, Gd,Dy, Ho,Er, Tm,and Y)

X-Ray Rietveld refinements were conducted on a series of eleven lanthanide phases, Sr₂RGaCu₂O_y (2112 phase, R = Pr, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, and Yb) that are structurally related to the high T_c superconductor Ba₂YCu₃O₇ (213). In the 2112 structure, instead of square planar Cu-O chains, tetrahedral GaO4 chains were found to run in a zig-zag fashion along the diagonal of the basal 213 ab-direction. Reference powder patterns for these compounds were prepared by using the Rietveld decomposition technique. The unit cell volume of these compounds follows the expected trend of the lanthanide contraction. The lattice parameters range from a = 22.9694(3) Å, b = 5.5587(2) Å, and c = 5.44743(7) Å for R = Pr, to a = 22.8059(2) Å, b = 5.46031(5) Å, and c = 5.37773(5) Å for R = Yb. An electon diffraction study of the Sm- and Er-analogs showed characteristic diffuse streaks along the b-axis, suggesting some disorder within the GaO₄ chains.     

Microstructure and mechanical properties of the superalloy ATI Allvac 718Plus™

ATI Allvac^® 718Plus™ is a novel nickel-based superalloy, which was designed for heavy-duty applications in aerospace turbines. In the present study the high-resolution investigation techniques, atom probe tomography, electron microscopy and in situ high-temperature small-angle neutron scattering were used for a comprehensive microstructural characterization. The alloy contains nanometer-sized spherical γ′ phase precipitates (Ni₃(Al,Ti)) and plate-shaped δ phase precipitates (Ni₃Nb) of micrometer size. The precipitation kinetics of the γ′ phase can be described by a classical model for coarsening. The precipitation strongly influences the mechanical properties and is of high scientific and technological interest.     

The microscopic network structure of mussel (Mytilus) adhesive plaques

Marine mussels of the genus Mytilus live in the hostile intertidal zone, attached to rocks, bio-fouled surfaces and each other via collagen-rich threads ending in adhesive pads, the plaques. Plaques adhere in salty, alkaline seawater, withstanding waves and tidal currents. Each plaque requires a force of several newtons to detach. Although the molecular composition of the plaques has been well studied, a complete understanding of supra-molecular plaque architecture and its role in maintaining adhesive strength remains elusive. Here, electron microscopy and neutron scattering studies of plaques harvested from Mytilus californianus and Mytilus galloprovincialis reveal a complex network structure reminiscent of structural foams. Two characteristic length scales are observed characterizing a dense meshwork (approx. 100 nm) with large interpenetrating pores (approx. 1 µm). The network withstands chemical denaturation, indicating significant cross-linking. Plaques formed at lower temperatures have finer network struts, from which we hypothesize a kinetically controlled formation mechanism. When mussels are induced to create plaques, the resulting structure lacks a well-defined network architecture, showcasing the importance of processing over self-assembly. Together, these new data provide essential insight into plaque structure and formation and set the foundation to understand the role of plaque structure in stress distribution and toughening in natural and biomimetic materials.     

Incommensurate fluctuations in Bi2Sr2CaCu2O8

A special neutron scattering technique has been used to discover an incommensurate fluctuation in Bi₂Sr₂CaCu₂O₈ that appears below T_c. The fluctuation is identified as a dynamic charge density wave since its scattering intensity appears to increase with increasing momentum transfer. The fluctuation is found at a wavevector near 2k_F and could be associated with a dynamic stripe phase.     

Precision Neutron Polarimetry for Neutron Beta Decay

The abBA collaboration is developing a new type of field-expansion spectrometer for a measurement of the three correlation coefficients a, A, and B and the shape parameter b. The measurement of A and B requires precision neutron polarimetry. We will polarize a pulsed cold neutron beam from the SNS using a ³He neutron spin filter. The well-known polarizing cross section for n-³He has a 1/v dependence, where v is the neutron velocity, which is used to determine the absolute beam polarization through a time-of-flight (TOF) measurement. We show that by measuring the TOF dependence of A and B, the coefficients and the neutron polarization can be determined with a small loss of the statistical precision and with negligible systematic error. We conclude that it is possible to determine the neutron polarization averaged over a long run in the neutron beta decay experiment with a statistical error less than 10⁻⁴. We discuss various sources of systematic uncertainty in the measurement of A and B and conclude that the fractional systematic errors are less than 2 × 10⁻⁴.     

Spatial modulation of low-frequency spin fluctuations in hole-doped La2CuO4

Systematic neutron scattering measurements have been performed on the Sr-doped La_{2-x}Sr_xCuO₄ to study the doping dependence of spatially modulated dynamical spin correlations or so-called incommensurate spin fluctuations. The modulated spin correlations appear beyond x ≈ 0.05 which is close to the lower boundary of the superconducting phase. First evidence was observed for the linear relation between the degree of spatial modulation or the incommensurability Σ(x) and the maximumT _c atx. We present a universal curve for Σ(x) by adding data from other La₂CuO₂ systems such as oxygen-doped superconductors, oxygen-reduced or Zn-substituted nonsuperconductors, and La_1.6-xNd_0.4Sr_xCuO₄.     

Evidence of Electron–Phonon Interaction in Al-Substituted Mg1?xAlxB2

Phonon density of states in Al substituted Mg_1–x Al _x B₂ has been studied by means of inelastic neutron scattering experiments with pulsed neutron time-of-flight technique. Phonon dynamics have a significant role for realizing superconductivity in the recent discovered superconductor MgB₂. We present a systematic investigation on generalized phonon density of states (PDOS) up to E = 120 meV on Al content of x = 0.1, 0.25, and 0.5. We have clearly observed a large effect on the PDOS by Al substitution; a large energy shift occurs around at 70 meV and 89 meV. We have also observed a visible increase of PDOS at a particular energy range below T _c. Those phonon states are related with E _2g mode to be expected to have a strong coupling with the electron states at the –A region.     

Static Ferromagnetic Modulation Revealed by Neutron Scattering in La1?xCaxMnO3

Mn perovskites La_1–x Ca _x MnO₃ (0x0.1) have been studied by small angle neutron scattering (SANS) and elastic neutron scattering (ESANS). At low temperature, both experiments reveal a broad modulation centered at the same q _m whatever the q direction. This scattering pattern is typical of a liquid-like distribution of similar magnetic droplets, or large magnetic polarons.     

Magnetic Properties of the Frustrated fcc – Antiferromagnet HoB12 Above and Below T N

We have investigated the magnetic ordering of the frustrated fcc – antiferromagnet HoB₁₂. Below T_N= 7.4 K antiferromagnetic order and a complex phase diagram is observed. Above T_N neutron scattering experiments show strong diffuse scattering. The diffuse signal indicates strong correlations between rare earth moments along the [111] direction well above T_N. The behavior of this component resembles low dimensional magnets which are known to show long range order only at T = 0. Close to T_N correlations perpendicular to the [111] direction get relevant, they diverge toward T_N. Thus we observe a complex ordering process where the frustration is lifted in steps. The experimental data and their interpretation are presented, some of the possible microscopic origins are discussed.