High-resolution Inelastic Neutron Research Articles

High-resolution study of the supposed fourfold Nd spin-wave degeneracy ofNd2CuO4

Model predictions for the Nd spin-wave dispersion in ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ have been tested with high-resolution inelastic neutron scattering on a single crystal at $T=0.5\mathrm{K}.$ A Heisenberg model in the random phase approximation has been previously proposed which predicts two dispersionless fourfold degenerate modes along the crystallographic AM direction. This particular feature is a consequence of the symmetry and of the type of interactions considered in the Hamiltonian. Our data unambiguously reveal additional peaks along AM which were previously unresolved. Possible origins for the lifting of the fourfold degeneracy of the modes along the AM direction are discussed.

Nonlinear spin dynamics in(ND4)2MnF5

Previously, the spin dynamics of the magnetic quasi-one-dimensional systems ${(\mathrm{N}\mathrm{H}}_{4}{)}_{2}{\mathrm{Mn}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{F}}_{5}$ and ${(\mathrm{N}\mathrm{D}}_{4}{)}_{2}{\mathrm{Mn}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{F}}_{5}$ has been studied with the aid of M\ossbauer spectroscopy. These results were interpreted on the basis of classical soliton theory. In order to confirm this concept, we have independently performed neutron scattering experiments on large single crystals (about 1.4 g). We discuss the results obtained on thermal and cold three-axis spectrometers, which probe the magnon spin-wave excitations and the existence of nonlinear excitations in the quasi-one-dimensional antiferromagnetic chains of ${(\mathrm{N}\mathrm{D}}_{4}{)}_{2}{\mathrm{MnF}}_{5},$ respectively. Additionally, we include elastic neutron diffraction and dc single-crystal susceptibility measurements to determine the magnetic structure. From the width of the quasielastic scattering signal, the temperature dependence of the inverse magnetic correlation length was derived, resulting in a soliton activation energy of ${E}_{s}/k=81(3)\mathrm{K},$ which is in good agreement with the soliton energy of ${E}_{s}/k=77(5)\mathrm{K}$ obtained by our high-resolution inelastic neutron scattering experiment. In contrast to this result, the M\ossbauer spectroscopy gives twice the value of the soliton energy, caused by soliton pair excitations.

Spin excitations and exchange couplings in the cuprate antiferromagnet La 2CuO 4

The spin excitations in La 2CuO 4, a two-dimensional spin- 1 2 Heisenberg antiferromagnet and parent compound of high- T c superconductors, have been measured throughout the Brillouin zone using high-resolution inelastic neutron scattering. The observed dispersion relations show evidence for substantial second- or third-nearest-neighbour exchange couplings in addition to the strong first nearest-neighbour antiferromagnetic exchange.

Neutron scattering studies of incommensurate systems

Since the early experimental work on the characteristic excitations of displacively modulated systems, progress has been achieved on several fronts. A model which extends the usual phe-nomenological description of the phason-amplitudon decoupling below Ti, has been used successfully to describe the low-frequency excitations in modulated betaine calcium chloride dihydrate (BCCD). In another modulated molecular compound, bis-(4-chloropheny1)-sulfone (BCPS), the close connection between the soft mode saturation frequency, as T ≤ Ti +, and the phason gap (T<i) has recently been established by means of high resolution inelastic neutron scattering. Finally, experimental results on the excitation spectrum of composite systems, such as the alkane-urea intergrowth compounds, are reviewed.

Anomalous dispersion of LO phonons in La{sub 1.85}Sr{sub 0.15}CuO{sub 4}

The dispersion of the highest energy LO phonon branch in La. 1.85 Sr 0.15 CuO 4 in the (100) direction has been reinvestigated by high resolution inelastic neutron scattering. In contrast to what has been recently reported by McQueeney et al. (Phys. Rev. Lett. 82. , 628 (1999)), we find that the dispersion is continuous throughout the Brillouin zone. These results are in conflict with the idea of charge stripe order existing on a time scale which is long compared to that of the LO frequencies. On the other hand, the strong softening and boradening of LO phonons observed on doping might be taken as a precursor phenomenon to a stripe order phase with a periodicity of about 4 lattice constants.

Single-ion excitations in (6 < x < 7)

We performed high-resolution inelastic neutron scattering experiments to examine the low-energy crystalline-electric-field (CEF) excitations of single ions in . The observed magnetic response versus oxygenation exhibits a discrete character in energy which suggests the presence of different types of cluster for different doping levels. In the undoped sample (x = 6.14) we observe a splitting of the CEF level due to the exchange interaction between the ions and the antiferromagnetic subsystem of copper spins. For all the doped compounds we observe line asymmetries for which possible origins are discussed.

Rotational dynamics of methyl groups in durene: A crystallographic, spectroscopic, and molecular mechanics investigation

Neutron powder diffraction measurements of perdeutero durene in the temperature range from 1.5 K to 290 K have been performed. The lowest temperature structure is the starting point for calculations of the methyl group tunneling and librational dynamics. Ab initio methods and atom–atom potentials are used to determine rotational single particle and coupling potentials. Tunneling splittings and librational bands are calculated by numerical solution of Schrödinger’s equation for a system of many coupled methyl groups. High-resolution inelastic neutron scattering measurements of methyl tunneling and molecular vibrations have been repeated, the tunneling results resolving an inconsistency with earlier NMR work. Quantum molecular dynamics provide a stringent test of the numerical methods and the data are ultimately well reproduced. These results are also discussed in the context of optical measurements of dye molecules in a host lattice of durene.

Vibrational dynamics in solid α-oxygen: Experimental assessment of spin-phonon couplings

The dynamics of $\ensuremath{\alpha}$-oxygen is investigated by means of high-resolution inelastic neutron scattering. The generalized frequency distribution for vibrational motions is derived from the large-angle spectra. A comparison of spectra on both sides of the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{\ensuremath{\alpha}}}$\ensuremath{\beta} magnetic ordering transition provides a clear indication of the extent of the coupling between the magnetic and lattice degrees of freedom. The specific heat and vibrational mean-square amplitudes are calculated from the spectra and compared with experimental and ab initio results. The latter also aids with the assignment of the spectral features, and to reconcile the present observations with previous measurements.

Effects of isotope disorder on energies and lifetimes of phonons in germanium

The effects of isotope disorder on phonon energies and lifetimes have been measured by means of high-resolution inelastic neutron scattering on a nearly maximally disordered ${}^{70/76}\mathrm{Ge}$ single crystal (mass variance $g=1.52\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$). Two theoretical approaches have been used to understand the observed effects: calculations based on the coherent potential approximation and on large supercells, respectively. For the mass variance of our sample, the two methods lead to rather similar results that agree with the experimental data. This agreement is marginally better for the supercell calculations. Calculations performed for larger mass variances $g$ revealed, however, that the two approaches yield significantly different results when $g$ is larger than $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}.$

Neutron scattering studies of the structure and dynamics of methane absorbed on MgO(100) surfaces

Although neutrons are not normally viewed as surface-sensitive probes, elastic and high-resolution inelastic neutron scattering (INS) techniques are surprisingly effective ways to investigate the structural and dynamical properties of films adsorbed on solid surfaces. In fact, in cases where the adsorption takes place on crystalline powder samples with large surface-to-volume ratios and with highly uniform surfaces, neutron data can yield remarkably detailed pictures of gas–solid interactions. As an illustration of the power of this approach we will describe here what has been learned with neutrons about the structure and rotational dynamics of methane films adsorbed on MgO(1 0 0) surfaces. Since the sensitivity of such investigations is largely determined by the quality of the adsorbates employed we will also briefly describe how the MgO powders used in these studies were prepared and characterized. Elastic neutron diffraction data show that at low temperatures solid methane films on MgO surfaces exhibit a layer-by-layer growth pattern essentially identical to the growth of the (1 0 0) face of the bulk solid. It is not possible, however, to determine from the diffraction data how the molecules are oriented relative to the MgO surface plane. Interestingly, questions relating to the orientational order of the molecules can be addressed by studying low-temperature rotational tunneling in monolayer methane films with high-resolution INS. Unlike the simple, two-line tunneling spectrum observed in bulk methane, the monolayer film produces a more structured spectrum which, when analyzed, is found to be only consistent with a dipod-down configuration of the molecules (C 2v axis normal to surface plane). We have been able to follow the evolution of the tunneling spectrum as the film builds progressively, layer-by-layer, from one-to-six layers. Strong indication of a crossover from 2D–3D behavior are seen.

Quantum Rotational Tunneling of Methyl Groups in Polymers

Quantum rotational tunneling of methyl groups has directly been observed for the first time in polymers by high resolution inelastic neutron scattering. The experimental results are readily explained and reproduced quantitatively, without any adjustable parameter, by considering the distribution of tunneling lines which is obtained from the distribution of energy barriers for classical hopping of methyl groups at high temperature.

Low-energy magnetic neutron scattering from α-oxygen

A high-resolution inelastic neutron scattering experiment has been carried out in the \ensuremath{\alpha} and \ensuremath{\mathrm{B}} low-temperature phases of solid oxygen. Direct comparison between the spectra of both phases as well as with previous experiments and lattice dynamics calculations, allow one to identify a feature appearing at \ensuremath{\approx} 4.3 meV in the low Q region of the spectra as arising from the antiferromagnetic excitations, the existence of which had been postulated from optical work.

Inelastic neutron scattering studies of hydrogen bonding in ices

Using high resolution inelastic neutron scattering measurements of the phonon density of states of ice, two separate molecular optic bands at about 28 and 37 meV for ice Ih and ice Ic have been observed, which is the direct result of the very high flux of pulsed neutron source and excellent energy resolution of the inelastic instruments on ISIS at the Rutherford–Appleton Laboratory. In order to reproduce the measured phonon density of states, a new lattice dynamic model has been proposed that two interaction strengths among the H bonds in ice Ih (and ice Ic) associated with the proton configurations have to be assumed. These two force constants are randomly distributed in the network of ice, having a ratio of almost 1:2. This model is capable of reproducing almost every aspect of the measured spectra, the two molecular peaks in 28 and 37 meV in particular and may provide insight into the complex nature of H bonding in ice and water. The large differences in the force constants between the strong and weak H bonds in a disordered ice structure as required by the model would affect the physical and chemical properties of ice and water and could have broader implications. In this paper, we illustrate a complete measurement for all possible recovered high-pressure forms of ice, including ice I, II, IX, V, VI, VII, VIII, and high-/low-density amorphous ice. These measurements demonstrate that the two well-separated molecular bands are associated with the local configurations of protons, which have quite different interaction strengths.

Inelastic neutron scattering from trans-polyacetylene

A high-resolution inelastic neutron scattering (INS) spectrum has been observed for trans-polyacetylene films in the range 8–4000 cm −1 at 6 K, and compared with the calculated INS spectrum based on the force field obtained in our previous study. The peak positions as well as the overall intensity profile of the observed INS spectrum are well reproduced by the present calculation, in which the effects of the Debye-Waller factors and the phonon wings are taken into account.

Neutron scattering investigations of the dynamics of thin films adsorbed on solid surfaces

While not commonly thought of as surface sensitive probes, high-resolution inelastic and quasielastic neutron scattering (INS) techniques can be used to investigate the dynamical properties of films adsorbed on solid surfaces. These studies are possible because the adsorption takes place on samples which exhibit a large surface-to-volume ratio. By combining dynamical measurements with data from structural, thermodynamic and computer simulation studies it is then possible to develop a comprehensive picture of the microscopic properties of an adsorbed film system. In order to illustrate the power of employing INS techniques we will discuss the continuous melting of monolayer ethylene and the layer-by-layer melting of multilayer argon films on the basal plane of graphite and the rotational tunneling of methane films on the MgO(100) surface.

Rotational tunneling studies of methane films adsorbed on MgO: Crossover from two-to-three dimensions?

High-resolution inelastic neutron scattering techniques have been used to investigate the rotational tunneling properties of methane films adsorbed on the (1 0 0) face of MgO surfaces. The monolayer solid forms a square-√2 × √2 R45° commensurate lattice. Methane vapor pressure isotherms and elastic diffraction studies indicate that four or five discrete layering transitions occur. Results will be presented which illustrate the tunneling properties of monolayer and bilayer CH 4. The possibility of observing a crossover from two-dimensional to three-dimensional behavior will also be discussed.

High resolution neutron scattering study of low‐energy magnetic excitations in Nd2‐xCexCuO4

AbstractThe low‐energy magnetic excitations (ħω ≤ 2 meV) of polycrystalline samples of Nd2‐xCexCuO4 with x = 0, 0.1 and 0.15 have been measured with high resolution inelastic neutron scattering using time‐of‐flight technique at temperatures below 0.3 K. All observed scattering originates from transitions within the ground state doublet of Nd3+. For Nd2CuO4 the response is inelastic, and shows at least three different modes, of which one is almost dispersionless. For x = 0.1 the excitation spectrum shifts towards lower energies, but the main features of the undoped compound are still visible, whereas for x = 0.15 the response is mainly quasi‐elastic but remains localised in Q‐space around the first magnetic Bragg point (0.5 0.5 1). A comparison with data obtained on a triple‐axis‐spectrometer on single crystals with x = 0.0 and x = 0.15 reveals that the response of the doped sample in the [0 0 1] direction is still inelastic.

Multiexcitation scattering by superfluid4He

High-resolution inelastic neutron scattering results for the multiphonon component of S(q,ω) for superfluid4He at T=1.35 K at saturated vapour pressure (SVP), 10 bars and 20 bars are presented. The results exhibit marked structure, with numerous sharp features whose intensities and positions depend strongly on momentum transfer and change appreciably with pressure. Some of these features clearly correspond to two-roton, roton-plus-maxon and two-maxon processes.

Exchange mixing and soliton dynamics in the quantum spin chain CsCoCl3.

The lowest-lying excited states in the S=1/2 one-dimensional Ising-like antiferromagnet ${\mathrm{CsCoCl}}_{3}$ consist of domain-wall (soliton) pair states. Although the dynamical response function S(Q,\ensuremath{\omega}) has been calculated for these states, it has not proved possible to explain the results of neutron scattering and Raman experiments without recourse to the introduction of extra terms in the spin Hamiltonian. We argue against the two modifications to the Hamiltonian of ${\mathrm{CsCoCl}}_{3}$ proposed in previous papers, a staggered field term arising from a mean field approach to exchange mixing, and next-nearest-neighbor intrachain coupling, as being unphysical. Instead we derive a nearest-neighbor effective Hamiltonian, which takes account of the mixing of higher ${\mathrm{Co}}^{2+}$ ion crystal-field levels in a self-consistent manner. We also present a high-resolution inelastic neutron scattering experiment on ${\mathrm{CsCoCl}}_{3}$, which has allowed the dispersion of the excited states across the Brillouin zone to be studied more carefully than before. These results give a direct measure of S(Q,\ensuremath{\omega}) in the one-dimensional phase where there is a continuum of excited states, and in the three-dimensionally ordered phases, where weak interchain interactions split the continuum into a ``Zeeman ladder'' of discrete states. The predictions of our theory are found to be in quantitative agreement with experiment.

Neutron scattering study of K 1− x(NH 4) xH 2PO 4 mixed crystals

K 1− x (NH 4) x H 2PO 4 (KADP) mixed crystals exhibit proton-glass behaviour over a certain composition range. The results of neutron-diffraction, high-resolution inelastic and quasielastic neutron scattering studies of KADP are presented. The anomalous behaviour of the proton glass system (0.2 ⩽ × ⩽ 0.8) is discussed.