High-resolution Inelastic Neutron Research Articles

Experimental inelastic neutron scattering spectrum of hydrogen hexagonal clathrate-hydrate compared with rigorous quantum simulations

We have performed high-resolution inelastic neutron scattering (INS) measurements on binary hydrogen clathrate hydrates exhibiting the hexagonal structure (sH). Two samples, differing only in the ortho/para fraction of hydrogen, were prepared using heavy water and methyl tert-butyl ether as the promoter in its perdeuterated form. The INS spectrum of the translation-rotation (TR) excitations of the guest H2 molecule was obtained by subtracting the very weak signal due to the D2O lattice modes. By means of a subtraction procedure, it has been possible to obtain separately the spectra of caged p-H2 and o-H2. sH clathrates are comprised of three distinct types of cages, two of which, differing in shape and size, are each occupied by one H2 molecule only. Both contribute to the measured INS spectrum which is, therefore, rather complex and challenging to assign unambiguously. To assist with the interpretation, the INS spectra are calculated accurately utilizing the quantum methodology which incorporates the coupled five-dimensional TR energy levels and wave functions of the H2 molecule confined in each type of nanocage. The computed INS spectra are highly realistic and reflect the complexity of the coupled TR dynamics of the guest H2 in the anisotropic confining environment. The simulated INS spectra of p-H2 and o-H2 in the small and medium cages are compared with the experimental data, and are indispensable for their interpretation.

High-resolution inelastic neutron scattering and neutron powder diffraction study of the adsorption of dihydrogen by the Cu(II) metal–organic framework material HKUST-1

We present new high-resolution inelastic neutron scattering (INS) spectra (measured using the TOSCA and MARI instruments at ISIS) and powder neutron diffraction data (measured on the diffractometer WISH at ISIS) from the interaction of the prototypical metal–organic framework HKUST-1 with various dosages of dihydrogen gas. The INS spectra show direct evidence for the sequential occupation of various distinct sites for dihydrogen in the metal–organic framework, whose population is adjusted during increasing loading of the guest. The superior resolution of TOSCA reveals subtle features in the spectra, not previously reported, including evidence for split signals, while complementary spectra recorded on MARI present full information in energy and momentum transfer. The analysis of the powder neutron patterns using the Rietveld method shows a consistent picture, allowing the crystallographic indenisation of binding sites for dihydrogen, thus building a comprehensive picture of the interaction of the guest with the nanoporous host.

Inelastic neutron scattering (INS) observations of rotational tunneling within partially deuterated methane monolayers adsorbed on MgO(1 0 0) surfaces

High resolution inelastic neutron scattering (INS) measurements of the low temperature (T∼2.0K) rotational dynamics of isotopically substituted methane monolayers adsorbed on MgO(100) are presented. These spectra, obtained using BASIS at SNS, represent the most detailed measurements available for surface-adsorbed monolayer films of methane. Distinct excitations are readily observed at 15, 31, 45 and 127μeV for the CH2D2 on MgO monolayer and at 40, 51, 95 and 138μeV for CH3D/MgO. These features are attributed to tunneling transitions between sublevels within the ground librational state and are interpreted using the pocket state (PS) formalism first proposed by Hüller. This theoretical analysis employs the findings of earlier studies of CH4 on MgO(100) which suggest that molecules adsorb with their C2v axes normal to the surface plane. The comparison between theory and experiment provides direct insight into the impact of molecular versus surface symmetry on the observed tunneling spectra.

Confinement of poly(ethylene oxide) in the nanometer-scale pores of resins and carbon nanoparticles

The confinement of poly(ethylene oxide) (PEO) in porous materials derived from organic resins and carbon nanoparticles leads to the disappearance of crystallization and to a significant modification of the glass transition. Textural data on pristine and PEO-filled materials obtained by means of nitrogen physisorption link this thermal behaviour to the spatial confinement of PEO in the micro- and mesopore cavities, and it is largely dictated by the morphology and composition of the underlying substrate. High-resolution inelastic neutron scattering of PEO confined in carbon nanoparticles confirms that the chain conformations are distinctly different from those characteristic of the semicrystalline bulk, and similar to those of PEO confined in the interlayer space of graphite oxide.

Two-Dimensional Antiferromagnetic Fractons in Rb2Mn0.598Mg0.402F4

The dynamical structure factor S(q,w) for diluted two-dimensional Heisenberg antiferromagnet Rb2Mn0.598Mg0.402F4 was investigated by means of high resolution (ΔE = 17.5μeV) inelastic neutron scattering below TN. The dynamical exponents in the peak intensity (A(q) ∝ q−y) and the dispersion relation (E(q) ∝ qz) for antiferromagnetic fractons were determined as y = 2.9±0.2 and z = 1.8±0.2, in good agreement with the numerical study. Also, the validity of the scaling based on the single-length-scaling postulate (SLSP) was confirmed.

Macromolecular Structure and Vibrational Dynamics of Confined Poly(ethylene oxide): From Subnanometer 2D-Intercalation into Graphite Oxide to Surface Adsorption onto Graphene Sheets.

In this work, high-resolution inelastic neutron scattering (INS) has been used to provide novel insights into the properties of confined poly(ethylene oxide) (PEO) chains. Two limits have been explored in detail, namely, single-layer 2D-polymer intercalation into graphite oxide (GO) and surface polymer adsorption onto thermally reduced and exfoliated graphite oxide, that is, graphene (G) sheets. Careful control over the degree of GO oxidation and exfoliation reveals three distinct cases of spatial confinement: (i) subnanometer 2D-confinement; (ii) frustrated absorption; and (iii) surface immobilization. Case (i) results in drastic changes to PEO conformational (800-1000 cm-1) and collective (200-600 cm-1) vibrational modes as a consequence of a preferentially planar zigzag (trans-trans-trans) chain conformation in the confined polymer phase. These changes give rise to peculiar thermodynamic behavior, whereby confined PEO chains are unable to either crystallize or display a calorimetric glass transition. In case (ii), GO is thermally reduced resulting in a disordered pseudo-graphitic structure. As a result, we observe minimal PEO absorption owing to a dramatic reduction in the abundance of hydrophilic groups inside the distorted graphitic galleries. In case (iii), the INS data unequivocally show that PEO chains adsorb firmly onto the G sheets, with a substantial increase in the population of gauche conformers. Well-defined glass and melting transitions associated with the confined polymer phase are recovered in case (iii), albeit at significantly lower temperatures than those of the bulk.

Two-Dimensional Subnanometer Confinement of Ethylene Glycol and Poly(ethylene oxide) by Neutron Spectroscopy: Molecular Size Effects

We explore the effects of chain size on the structure and dynamics of ethylene glycol (EG) and poly(ethylene oxide) (PEO) intercalated in graphite oxide (GO). To this end, EG as well as a PEO series of increasing chain length have been studied by means of high-resolution inelastic neutron spectroscopy. The neutron experiments are complemented by X-ray diffraction, differential scanning calorimetry, Fourier-transform infrared spectroscopy, and Raman scattering. We find that EG is accommodated in a layer of thickness ∼3 Å within the GO substrate and adopts a preferential, yet largely disordered, gauche conformation. Longer PEO chains give rise to a layer thickness in the range ∼3.0–3.4 Å characterized by planar zigzag (trans–trans–trans) conformations. Moreover, we observe a strong reduction of vibrational motions for the confined EG and polymer phases, as clearly evidenced by the disappearance, shift, and/or broadening of mode-specific inelastic neutron scattering spectral features, as well as by the complete suppression of crystallization. All of these effects are surprisingly insensitive to the length of the PEO chains.

Spin dynamics in the multiferroic materials (invited)

We report high resolution inelastic neutron scattering measurements and spin dynamics calculations in two multiferroic materials: the geometrically frustrated triangular lattice CuFeO2 and mineral Hübnerite MnWO4. In un-doped CuFeO2 a low-T collinear spin structure is stabilized by long range magnetic interactions. When doped with a few percent of gallium, the spin order evolves into a complex noncollinear configuration and the system becomes multiferroic. Similarly, the ground state collinear spin order in pure MnWO4 results from delicate balance between competing magnetic interactions up to 11th nearest neighbors and can be tuned by substitution of Mn ions with magnetic or nonmagnetic impurities. The comprehensive investigation of spin dynamics in both systems help to understand the fundamental physical process and the interactions leading to the close interplay of magnetism and ferroelectricity in this type of materials.

Dynamics of azurite Cu3(CO3)2(OH)2in a magnetic field as determined by neutron scattering

Azurite, a natural mineral made up of CuO chains, is also an intriguing spin-1/2 quantum magnet. There has been much debate as to the 1-dimensional (1D) nature of this material by theorists and experimentalists alike. The focus of this debate lies in the interactions between Cu-ions within the antiferromagnetically ordered state below 1.9 K. We present high-resolution inelastic neutron scattering data which highlight the complexity of the magnetic ground state of azurite. The application of magnetic fields and temperatures were used to probe the excitations revealing important information about the dynamics in this system. From this we are able to conclude that the 1D Heisenberg antiferromagnetic spin chain model is not sufficient to describe the dynamics in azurite. Instead additional coupling including interchain interactions and an anisotropic staggered field are necessary to fully model the observed excitations.

Publisher's Note: Direct evidence for the magnetic ordering of Nd ions in NdFeAsO by high-resolution inelastic neutron scattering [Phys. Rev. B84, 132413 (2011)

Publisher's Note: Direct evidence for the magnetic ordering of Nd ions in NdFeAsO by high-resolution inelastic neutron scattering [Phys. Rev. B<b>84</b>, 132413 (2011)

Direct evidence for the magnetic ordering of Nd ions in NdMn2Si2 and NdMn2Ge2 by high resolution inelastic neutron scattering

We have investigated the low energy nuclear spin excitations in NdMn2Si2 and NdMn2Ge2 by high resolution inelastic neutron scattering. Previous neutron diffraction investigations gave ambiguous results about Nd magnetic ordering at low temperatures. The present element-specific technique gave direct evidence for the magnetic ordering of Nd ions. We found considerable difference in the process of the Nd magnetic ordering at low temperature in NdMn2Si2 and NdMn2Ge2. Our results are consistent with those of magnetization and recent neutron diffraction measurements.

Direct evidence for the magnetic ordering of Nd ions in NdFeAsO by high-resolution inelastic neutron scattering

We investigated the low energy excitations in the parent compound NdFeAsO of the Fe-pnictide superconductor in the $\mu$eV range by a back scattering neutron spectrometer. The energy scans on a powder NdFeAsO sample revealed inelastic peaks at E = 1.600 $ \pm 0.003 \mu$eV at T = 0.055 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at about 6 K. We interpret the inelastic peaks to be due to the transition between hyperfine-split nuclear level of the $^{143}$Nd and $^{145}$Nd isotopes with spin $I = 7/2$. The hyperfine field is produced by the ordering of the electronic magnetic moment of Nd at low temperature and thus the present investigation gives direct evidence of the ordering of the Nd magnetic sublattice of NdFeAsO at low temperature.

Antiferromagnetic Fractons in Diluted Heisenberg Systems RbMn0.4Mg0.6F3 and Rb2Mn0.598Mg0.402F4

The dynamical structure factors, S ( q ,ω), for antiferromagnetic fractons in diluted three-dimensional (3D) and two-dimensional (2D) Heisenberg systems, RbMn 0.4 Mg 0.6 F 3 and Rb 2 Mn 0.598 Mg 0.402 F 4 , respectively, with a magnetic concentration close to the percolation concentration, were observed through high-resolution (Δ E = 17.5 µeV) inelastic neutron scattering experiments well below T N . The exponents describing the peak intensity ( A ( q ) ∝ q - y ) and the dispersion relation ( E ( q ) ∝ q z ) were determined to be y = 2.9 ±0.1 and z = 2.5 ±0.1 for the 3D system and y = 2.9 ±0.2 and z = 1.8 ±0.2 for the 2D system, respectively. In addition, the validity of the single-length-scaling postulate (SLSP) was confirmed for S ( q ,ω) by using the above values of y and z . We experimentally demonstrated, for the first time, that the spectral dimension, \(\tilde{d}_{\text{AF}} = 1\), is universal and independent of the embedding Euclidean dimensions of the systems.

Lack of evidence for a singlet crystal-field ground state in the magnetic pyrochlore Tb2Ti2O7

We present high-resolution inelastic neutron scattering data on the candidate spin liquid Tb${}_{2}$Ti${}_{2}$O${}_{7}$. We find that there is no evidence for a zero-field splitting of the ground-state doublet within the $0.2$ K resolution of the instrument. This result contrasts with a pair of recent works claiming that the spin-liquid behavior of Tb${}_{2}$Ti${}_{2}$O${}_{7}$ can be attributed to a 2 K split singlet-singlet single-ion spectrum at low energies. We also reconsider the entropy argument presented by Chapuis et al. [Phys. Rev. B 82, 100420 (2010)] as further evidence of a singlet-singlet crystal-field spectrum. We argue that estimates of the low-temperature residual entropy drawn from specific heat measurements are a poor guide to the single-ion spectrum without a proper understanding of the nature of the correlations in the spin-liquid regime.

Vibrational dynamics of the filled skutteruditesM1−xFe4Sb12(M=Ca, Sr, Ba, and Yb): Temperature response, dispersion relation, and material properties

The vibrational dynamics of the ternary Ca${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$, Ba${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$, Sr${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$, and Yb${}_{1\ensuremath{-}x}$Fe${}_{4}$Sb${}_{12}$ compounds is studied for its temperature dependence, dispersion relations, and material properties by experiments and ab initio powder-averaged lattice-dynamics (PALD) calculations. The experimental techniques used are high-resolution inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS). Vibrational properties of polycrystalline material have been mapped out in an extensive energy-momentum phase space facilitating the study of powder-averaged phonon-dispersion relations within the first and second Brillouin zones. Qualitatively, the experimental dynamic structure factor $S(Q,\ensuremath{\omega})$ matches, to a high degree, PALD calculations allowing different schemes of collective vibrational eigenmodes to be evaluated. From the PALD calculations, quantitative values of velocity of sound, Debye temperatures, and elastic, bulk, and Young's moduli are computed. The variation of the generalized density of states $G(\ensuremath{\omega})$ with temperature is studied experimentally in the range from 2 to 600 K. No particular anomalies in the collective dynamics are observed apart from a global softening of $G(\ensuremath{\omega})$ upon heating.

Dispersion relation of low-energy excitations in Zr50Be50 metallic glass

A high-resolution (1.5 meV) inelastic neutron scattering experiment is carried out to investigate in detail the low momentum (0.6–1.5 A−1) dynamic response in Zr50Be50 metallic glass. The results are obtained on the hot neutron three-axis spectrometer IN1 in the Laue-Langevin Institute (Grenoble, France). A comparison with recent neutron scattering results for the momentum transfer region above 1.4 A−1 shows a minimum in the dispersion relation of low energy excitations near the prepeak of the static structure factor. These excitations appear to be due to the short-range order in this amorphous system.

Long-range magnetic interactions in the multiferroic antiferromagnet MnWO4

The spin-wave excitations of the multiferroic $\rm MnWO_4$ have been measured in its low-temperature collinear commensurate phase using high-resolution inelastic neutron scattering. These excitations can be well described by a Heisenberg model with competing long-range exchange interactions and a single-ion anisotropy term. The magnetic interactions are strongly frustrated within the zigzag spin chain along c axis and between chains along the a axis, while the coupling between spin along the b axis is much weaker. The balance of these interactions results in the noncollinear incommensurate spin structure associated with the magnetoelectric effect, and the perturbation of the magnetic interactions leads to the observed rich phase diagrams of the chemically-doped materials. This delicate balance can also be tuned by the application of external electric or magnetic fields to achieve magnetoelectric control of this type of materials.

Polymers under extreme two-dimensional confinement: Poly(ethylene oxide) in graphite oxide

The confinement of poly(ethylene oxide) in graphite oxide is studied using a combination of diffraction, calorimetric, and spectroscopic methods. Polymer intercalation into subnanometer graphite oxide layers leads to the complete suppression of crystallization phenomena and dielectric α-relaxation processes, as well as a slowdown of β-relaxation modes. For the first time, high-resolution inelastic neutron scattering shows that poly(ethylene oxide) under these extreme confinement conditions adopts a planar zig-zag conformation, in no way resembling the characteristic 72 helical structure of the bulk crystal. The neutron data also shows a strong suppression of the COC and OCC bending modes and a distinct broadening of C–O torsional bands.

Superconductivity-induced distortions of phonon lineshapes in niobium

Superconductivity-induced changes of phonon lineshapes in niobium have been re-investigated by high-resolution inelastic neutron scattering. We show that the changes go beyond a simple change in lifetime and frequency when the phonon frequency is close to the superconducting energy gap $2\Delta$. The observed lineshapes in elemental niobium are qualitatively similar to those found previously in borocarbide superconductors and agree very well with those predicted by the theory of Allen et al. [PRB 56, 5552 (1997)]. Our results indicate that the peculiar phonon lineshapes in the superconducting state predicted by the theory of Allen et al. are a general phenomenon and not restricted to a particular class of compounds.

High-resolution neutron scattering study ofTb2Mo2O7: A geometrically frustrated spin glass

The low-temperature magnetic properties of ${\text{Tb}}_{2}{\text{Mo}}_{2}{\text{O}}_{7}$ have been studied with bulk susceptibility measurements and with elastic and high-resolution inelastic neutron scattering. This system is a spin glass with a freezing temperature ${T}_{g}\ensuremath{\sim}25\text{ }\text{K}$. A reverse Monte Carlo simulation of the neutron diffraction data shows weak ferromagnetic near-neighbor spatial correlations that do not extend beyond $\ensuremath{\lesssim}10\text{ }\text{\AA{}}$. Neutron measurements of the spin dynamics reveal a slowing down with decreasing temperature without an anomaly at the glass transition. A low-lying $Q$-independent mode is seen at $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}\ensuremath{\sim}0.28\text{ }\text{meV}$. This dispersionless crystal electric field transition is measurable up to 60 K.