Elastic Neutron-scattering Measurements Research Articles

Dynamics of Water and Other Molecular Liquids Confined Within Voids and on Surface of Lignin Aggregates in Aging Bio Crude Oils.

Neutron scattering methods were employed to study the microscopic structure and dynamics of Bio Crude Oils (BCOs) and their lignin fractions. The structure of the carbonaceous aggregates was investigated using Small Angle Neutron Scattering to reveal a fractal hierarchy as well as a growth of the aggregates as the aging of the BCO proceeds. Elastic Neutron Scattering measurements indicate that BCO liquid phase, comprised of water and other hydrogenated molecular liquids, is in a state of extreme confinement. Quasi-Elastic Neutron Scattering yields information on the molecular motions, indicating that long range translational diffusion is suppressed and only localized dynamics take place on the tens of picosecond time range. The obtained results provide quantitative information on the molecular activity, as aging proceed, in these reactive materials of relevance as potential renewable energy sources.

Unconventional spin glass behavior in the cubic pyrochloreMn2Sb2O7

The new pyrochlore Mn2Sb2O7 has been synthesized via a low-temperature method to retain cubic symmetry. The resulting material has an unconventional spin glass state atTSG∼41 K, inthe absence of detectable site disorder or a structural phase transition. A peak in the DC susceptibilityat TSG, and a characteristic shift in the real part of the AC susceptibility was noted, but with a slightlymore dynamic state than predicted through the typical Mydosh spin glass parameter ofΔTSG/[TSGlog(ω)]. Time-dependent magnetization measurements show glassy behavior below 41 K. Elastic neutronscattering measurements on powder samples show significant magnetic diffuse scattering fromthe Mn spins beneath the transition, similar to the scattering observed in the spin liquidTb2Ti2O7. The dynamiccharacter of Mn2Sb2O7 is inferred to arise from the frustration inherent within the pyrochlore lattice.

Spin- and charge-density-wave orders in La 1.87Sr 0.13Cu 0.99Fe 0.01O 4

We have performed elastic neutron-scattering measurements on La 1.87Sr 0.13Cu 0.99Fe 0.01O 4 to study the magnetic impurity effects on the stripe correlations in high- T c cuprates. Both charge-density-wave (CDW) and spin-density-wave (SDW) orders are observed in the low-temperature-orthorhombic (LTO) phase for the first time. With decreasing the temperature, CDW order first appears at ∼ 60 K followed by the SDW order at lower temperature of ∼ 50 K . The incommensurability for the CDW order ( ε ) was found to be 0.224 ± 0.002 r.l.u., which is approximately twice of that for SDW order ( δ ) of 0.115 ± 0.003 r.l.u. Both ordering sequence and relation of ε ≈ 2 δ are the same for SDW and CDW orders observed in the low-temperature-tetragonal (LTT) phase of La 1.60 - x Nd 0.4 Sr x CuO 4 (LNSCO), suggesting that the similar stripe correlations exist in the cuprate oxides, irrespective of crystal structure.

Magnetic-field effects on the charge-spin stripe order in La-214 high-Tc cuprates

Magnetic-field effects on the charge-spin stripe order in La-214 high-Tc cuprates have been investigated from measurements of the in-plane electrical-resistivity, ρab. In La2−xBaxCuO4 with x = 0.10 and La2−xSrxCuO4 with x = 0.115 where the incommensurate charge peaks are weak and unobservable in zero field in elastic neutron-scattering measurements, respectively, the normal-state value of ρab at low temperatures markedly increases with increasing field up to 27 T. For La2−xBaxCuO4 with x = 0.11 and Zn-substituted La2−xSrxCu1−yZnyO4 with x = 0.115 and y = 0.02 where the charge stripe order is fairly stabilized in zero field, on the other hand, the increase in ñab with increasing field is negligibly small. In conclusion, when the charge-spin stripe order is not fully stable in zero field, magnetic field operates to stabilize the charge-spin stripe order. The value of ρab increases with increasing field depending on the stability of the charge stripe order.

Interplay of structural and electronic phase separation in single-crystallineLa2CuO4.05studied by neutron and Raman scattering

We report a neutron- and Raman-scattering study of a single crystal of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4.05}$ prepared by high-temperature electrochemical oxidation. Elastic neutron-scattering measurements show the presence of two phases, corresponding to the two edges of the first miscibility gap, all the way up to 300 K. An additional oxygen redistribution, driven by electronic energies, is identified at 250 K in Raman scattering (RS) experiments by the simultaneous onset of two-phonon and two-magnon scattering, which are fingerprints of the insulating phase. Elastic neutron-scattering measurements show directly an antiferromagnetic ordering below a N\'eel temperature of ${T}_{N}=210\mathrm{K}.$ The opening of the superconducting gap manifests itself as a redistribution of electronic Raman scattering below the superconducting transition temperature, ${T}_{c}=24\mathrm{K}.$ A pronounced temperature-dependent suppression of the intensity of the (100) magnetic Bragg peak has been detected below ${T}_{c}.$ We ascribe this phenomenon to a change of relative volume fraction of superconducting and antiferromagnetic phases with decreasing temperature caused by a form of a superconducting proximity effect.

Uniaxial pressure dependence of the antiferromagnetic order in UPt 3

The weak antiferromagnetic order of the heavy-fermion superconductor UPt3 has been investigated by elastic neutron-scattering measurements under applied uniaxial pressure up to 6 kbar along the a and c axes of the hexagonal crystal structure. For p||c the small antiferromagnetically ordered moment of 0.02 μB/U-atom shows a non-linear decrease for increasing pressures and is still not completely suppressed at the maximum applied pressure of 6 kbar. For p||a a significant increase in the magnetic Bragg peak intensity is observed, which suggests an incomplete domain repopulation and confirms the presence of a single-k structure.

Neutron scattering studies of the cooperative paramagnet pyrochloreTb2Ti2O7

We have carried out extensive neutron-scattering studies on the pyrochlore antiferromagnet ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ in both polycrystalline and single-crystal form. This insulating material belongs to a family of rare-earth titanate pyrochlores, ${R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7},$ whose magnetic rare-earth ions reside on a network of corner sharing tetrahedra. Such a local geometry is known to give rise to geometrical frustration in the presence of antiferromagnetic interactions. Earlier studies have shown ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ to be an Ising system with large moments constrained to point along local [1, 1, 1] directions; that is, into and out of the tetrahedra. It displays a cooperative paramagnetic or spin liquid state at low temperatures, with neither long-range N\'eel order nor spin glass ordering at temperatures as low as 0.07 K. Our elastic neutron-scattering measurements show that very short-range correlations develop below \ensuremath{\sim}100 K. At 4 K a checkerboard pattern of diffuse magnetic scattering within the [h,h,l] plane in reciprocal space is observed, consistent with spin correlations over near neighbors only. Inelastic scattering measurements on both powder and single-crystal samples show three bands of magnetic excitations. At temperatures above \ensuremath{\sim}20 K, these bands are dispersionless, but at low temperature an appreciable softening in the lowest band of excitations occurs at those wave vectors which characterize the development of the very short-range magnetic order, qualitatively consistent with theoretical expectations derived from the single-mode approximation.

Diagonal static spin correlation in the low-temperature orthorhombicPccnphase ofLa1.55Nd0.4Sr0.05CuO4

Elastic neutron-scattering measurements have been performed on ${\mathrm{La}}_{1.55}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_{4},$ which exhibits a structural phase transition at ${T}_{s}\ensuremath{\sim}60 \mathrm{K}$ from the low-temperature orthorhombic $\mathrm{Bmab}$ phase (labeled LTO1) to the low-temperature orthorhombic $\mathrm{Pccn}$ phase (labeled LTO2). At low temperatures, well below ${T}_{s},$ elastic magnetic peaks are observed at the ``diagonal'' incommensurate (IC) positions $(0,1\ifmmode\pm\else\textpm\fi{}0.055,0),$ with the modulation direction only along the orthorhombic b axis, just as in Nd-free ${\mathrm{La}}_{1.95}{\mathrm{Sr}}_{0.05}{\mathrm{CuO}}_{4}.$ In the present study, the one dimensionality of the IC modulation, which is naturally explained by a stripe model, is clearly demonstrated with our ``single-domain'' crystal. The temperature dependence of the IC peak intensity suggests a substantial contribution from the ${\mathrm{Nd}}^{3+}$ spins below $\ensuremath{\sim}3 \mathrm{K}.$ Consistent with this, the L dependence of the magnetic scattering is accurately accounted for by a model in which the contribution of the ${\mathrm{Nd}}^{3+}$ spins is explicitly included.

Magnetic order ofUPt3in high magnetic fields

The weak magnetic order of the heavy-fermion superconductor ${\mathrm{UPt}}_{3}$ has been investigated by elastic neutron-scattering measurements in magnetic fields up to 12 T along the a and c axes of the hexagonal crystal structure. The small antiferromagnetically ordered moment of $0.02{\ensuremath{\mu}}_{B}/(\mathrm{U}\mathrm{ }\mathrm{atom})$ shows only a weak dependence on the applied magnetic field and no sign of a domain repopulation for $B\ensuremath{\parallel}a.$ In high magnetic fields an increase in the magnetic correlation length is observed for magnetic fields along the c axis.

Intrinsic spin-density-wave magnetism in Cu-Mn alloys.

Elastic neutron-scattering measurements on two samples of Cu alloyed with 1.3% Mn and 0.55% Mn show that the spin-density-wave (SDW) features found in more concentrated alloys persist in the limit of very dilute alloys. These features consist of temperature-dependent incommensurate peaks in magnetic neutron scattering, with positions and strengths which are fully consistent with those in the concentrated alloys. The implications of these measurements are twofold. First, it is clear from our data that SDW magnetic ordering occurs across the entire range of Cu-Mn alloys which have typically been interpreted as spin glasses. Second, the more fundamental significance of this work is the suggestion via extrapolation that a peak in the magnetic susceptibility \ensuremath{\chi}(q) occurs in pure copper, at a value of q given by the Fermi-surface diameter 2${\mathit{k}}_{\mathit{F}}$.

Magnetic phase transitions in CsCoBr3

Elastic neutron-scattering measurements have been performed on CsCoBr3, an Ising-like antiferromagnet on a stacked triangular lattice. Three magnetic phase transitions are seen in both zero field and a magnetic field of 2.2 T applied along the stacking direction c. The field does not qualitatively change the behavior near the phase transitions and the transition temperatures are affected only slightly. The high-temperature ordered phase is known to be partially paramagnetic and the transition from the completely paramagnetic phase to this ordered phase was carefully studied. The transition is found to be continuous and characterized by the critical exponent β=0.22±0.01, a result that disagrees with previous measurement. The disagreement can be attributed to a large amount of temperature-dependent paramagnetic scattering that exists in both the paramagnetic and the partially paramagnetic ordered phase.

Magnetic phases of EuxSr1-xAs3 investigated by specific-heat, susceptibility, and elastic neutron-scattering measurements.

We report on measurements of the specific heat C, the magnetic susceptibility \ensuremath{\chi}, and elastic neutron scattering on single-crystalline monoclinic ${\mathrm{Eu}}_{\mathrm{x}}$${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{As}}_{3}$. The two successive phase transitions into incommensurate (IC) and antiferromagnetic (AF1) order with decreasing temperature T, observed previously for ${\mathrm{EuAs}}_{3}$, persist upon dilution with Sr down to x\ensuremath{\sim}0.5. For a sample with x=0.47, the IC order dominates over the AF1 order and only (long-range) IC order is found. At still lower concentrations long-range magnetic order vanishes and the system shows anisotropic spin-glass behavior. With measurements of the specific heat and of the magnetocaloric effect in magnetic fields up to B=6 T, we establish the (B-T) phase diagram for x=0.6 and compare the field-induced phases with those of pure ${\mathrm{EuAs}}_{3}$.

Long-range solute-host interactions in dilute Al-Mg alloys

The scattering structure factor S(Q) describing distortions induced by a substitutional defect in a simple metal has been calculated from lattice statics and Shaw model potentials; interactions of arbitrary range can be included via reciprocal lattice sums. Diffuse elastic neutron-scattering measurements on Al single crystals with a few at.% Mg confirm the dependence on the momentum transfer hQ predicted for a Mg impurity in Al. Phenomenological fits assuming forces acting upon the next-nearest neighbours of a solute atom cannot explain that dependence.

Elastic neutron-scattering study of the three low-temperature transitions in tetrathiafulvalenium-tetracyanoquinodimethanide

Elastic neutron-scattering measurements of the satellite structure of tetrathiafulvalenium-tetracyanoquinodimethanide show the existence of phase transition points at 38, 48.5, and 54 K. All of the low-temperature phases are incommensurate with a 0.295 component along ${b}^{*}$ but differ as to their modulation along the ${a}^{*}$ axis. These results are interpreted within the framework of a theory by Bak and Emery. An unusual hysteresis behavior in the intermediate phase between 38.5 and 48.5 K is reported. Measurements on satellites with a component of 0.59 along ${b}^{*}$ for the lowest temperature phase are also discussed.