Inelastic Neutron-scattering Measurements Research Articles

High-resolution neutron-scattering study of the roton in confined superfluid4He

We report high-resolution neutron inelastic-scattering measurements of the roton energy and linewidth of superfluid ${}^{4}\mathrm{He}$ confined in 95% porosity aerogel glass for temperatures $0.08<T<1.2\mathrm{K}.$ The confined roton line shape can be described well by a damped harmonic oscillator (DHO) function, as in the bulk. The energy of the confined roton is essentially temperature independent over this temperature range. The roton linewidth does not exhibit a finite width of greater than 0.1 \ensuremath{\mu}eV at low temperature. This result is in contrast to previous measurements carried out at lower resolution.

Selected properties of AFM-III structures — cubic MnTe and diluted magnetic semiconductors: CdMnTe and MgMnTe

One magnon excitations in cubic MBE-grown Mg 1 − x Mn x Te layers (where x ⩾ 0.7) were observed by means of Raman-scattering measurements below Néel temperature. Elastic and inelastic neutron-scattering measurements were performed on a few micrometer thick MnTe layers. MnTe lattice distortion resulting from the magnetostriction effect was confirmed and neutron scattering on magnons was demonstrated.

Orientational effects on low-energy modes in amorphous poly(ethylene terephthalate) fiber

Inelastic and quasielastic neutron-scattering measurements have been performed on an amorphous poly(ethylene terephthalate) (PET) fiber in parallel and perpendicular scattering geometries, i.e., when the fiber axis f is parallel and perpendicular to the scattering vector Q, respectively. The so-called boson peak is observed at around −1.4 meV in neutron energy transfer at low temperatures below about 150 K in both the geometries; as temperature increases, the fast process of picosecond order appears at a certain temperature below the glass-transition temperature Tg(=348 K). Although no drastic differences in the dynamics can be observed, a closer look at the spectra revealed some interesting features of the low-energy modes. From the mean-square displacements evaluated with two different energy resolutions, it was found that the fast process appears only in the direction perpendicular to the polymer chain (not the fiber axis f) in the energy region between −1 and −0.2 meV. In the inelastic scattering spectra, we found that the boson peak is stronger in intensity for the parallel direction while the intensity of the fast process is larger for the perpendicular one. The quantitative analysis based on the recent vibration–relaxation model revealed that the fast process can mainly be explained by the softening of the vibrational modes; the softening occurs at lower temperatures in the perpendicular geometry than in the parallel one. On the other hand, the conventional single-Lorentzian fit to the fast process, in which the fast process is assumed to be a relaxational process and described by a Lorentzian, showed that the onset temperature of the fast process is lower in the perpendicular geometry than in the parallel one. These have been tentatively attributed to weaker force constants, for motions perpendicular to a polymer chain, such as torsional and librational motions, than those along the chain.

Thermodynamic and near-surface correlations in uranium monophosphide

Inelastic neutron-scattering measurements of the antiferromagnetic fluctuations in uranium monophosphide above, and in the vicinity of the N\'eel temperature ${T}_{N}$ are reported. At the elastic position, with sufficiently relaxed energy resolution to integrate over the low-frequency dynamics, in the immediate neighborhood of ${T}_{N}$ a two component wave-vector profile is observed. From measurements of the inelastic linewidth as a function of temperature, it is concluded that the ``broad'' component in the scattered wave-vector profile represents the response of conventional bulk thermodynamic fluctuations. These observations, taken in conjunction with an earlier magnetic x-ray-scattering study over a similar temperature interval, strongly suggest that the ``broad'' component observed in the x-ray-scattering experiments has a distinct origin from that observed here.

Spin gap in a quasi-one-dimensionalS=12antiferromagnet:Cu2(1,4−diazacycloheptane)2Cl4

${\mathrm{Cu}}_{2}$(1,4-diazacycloheptane)${}_{2}{\mathrm{Cl}}_{4}$ contains double chains of spin-$\frac{1}{2}$ ${\mathrm{Cu}}^{2+}$ ions. We report ac susceptibility, specific heat, and inelastic neutron-scattering measurements on this material. The magnetic susceptibility $\ensuremath{\chi}(T)$ shows a rounded maximum at $T=8$ K indicative of a low-dimensional antiferromagnet with no zero-field magnetic phase transition. We compare the $\ensuremath{\chi}(T)$ data with exact diagonalization results for various one-dimensional spin Hamiltonians and find excellent agreement for a spin ladder with intrarung coupling ${J}_{1}=1.143(3)$ meV and two mutually frustrating interrung interactions ${J}_{2}=0.21(3)$ meV and ${J}_{3}=0.09(5)$ meV. The specific heat in zero field is exponentially activated with an activation energy $\ensuremath{\Delta}=0.87(1)$ meV. A spin gap is also found through inelastic neutron scattering on powder samples that identify a band of magnetic excitations for $0.8<\ensuremath{\Elzxh}\ensuremath{\omega}<1.5$ meV. Using sum rules we derive an expression for the dynamic spin-correlation function associated with noninteracting propagating triplets in a spin ladder. The Van Hove singularities of such a model are not observed in our scattering data, indicating that magnetic excitations in ${\mathrm{Cu}}_{2}$(1,4-diazacycloheptane)${}_{2}{\mathrm{Cl}}_{4}$ are more complicated. For magnetic fields above ${H}_{c1}\ensuremath{\simeq}7.2$ T specific-heat data versus temperature show anomalies indicating a phase transition to an ordered state below $T=1$ K.

Phonon dispersion relation in rhodium:Ab initiocalculations and neutron- scattering investigations

The phonon dispersion relation in face-centered-cubic rhodium has been investigated by ab initio local-density functional (LDF) calculations and inelastic neutron-scattering measurements. The LDF calculations have been performed both using ultrasoft pseudopotentials and a plane-wave basis and norm-conserving pseudopotentials and a mixed basis set and include also all-electron calculations at a few high-symmetry points. Theory predicts the existence of Kohn anomalies that can be interpreted in terms of the calculated Fermi surfaces. The neutron-scattering experiments confirm that the ab initio calculations are accurate to within 3% (including the position and amplitude of the anomalies).

Magnetic excitations of bandwidth-controlled Mn perovskite (La 1- x- yY y)Sr xMnO 3

Inelastic neutron-scattering measurements were carried out to study the spin dynamics of colossal magnetoresistance material (La 1- x- y Y y )Sr x MnO 3, where the electronic bandwidth can be controlled by the average rare-earth ionic radius, i.e., the Mn-O-Mn bond angle. The normalized spin-wave stiffness D/T c and energy width of spin waves, Γ, at various temperatures were measured, which are discussed in comparison with the double-exchange model.

Low-energy excitation in dehydrated gel

A low-energy excitation of dehydrated PAAm gel has been observed in the Raman and inelastic neutron-scattering measurements. The low-lying mode in the dehydrated gel could be distinguished even far above the glass-transition temperature with remarkable intensity decrease.

A local dynamic correlation function from inelastic neutron scattering

Information about local and dynamic atomic correlations can be obtained from inelastic neutron-scattering measurements by Fourier transform of the Q-dependent intensity oscillations at a particular frequency. A local dynamic structure function, S( r, ω), is defined from the dynamic scattering function, S( Q, ω), such that the elastic and frequency-integrated limits correspond to the average and instantaneous pair-distribution functions, respectively. As an example, S( r, ω) is calculated for polycrystalline aluminum in a model where atomic motions are entirely due to harmonic phonons.

Vibrational density of states and structural origin of the heat capacity anomalies in Ca 3Al 2Si 3O 12 glasses

The low-temperature heat capacity, C p, of the crystalline and glassy forms of the mineral grossular Ca 3Al 2Si 3O 12 are distinctly different. The effect has been linked to differences in coordination of Al and the resulting structural roles of the ion in each phase. Inelastic neutron-scattering measurements for the glassy phase reveals an excess density of states below 30 meV and C p, calculated from the vibrational density of states agree with thermodynamic measurements. Static neutron-scattering data confirm that Al is tetrahedrally coordinated in the glass as distinct from octahedral coordination in the crystal.

Magnetic excitations in NpBi

We report inelastic neutron-scattering measurements on a single crystal of NpBi. The magnetic excitation spectrum consists of two branches with longitudinal and transverse polarization, respectively. The excitations are interpreted as spin waves in an antiferromagnetic triple-k type-$I$ structure. With increasing temperature, the anisotropy gap decreases monotonously and disappears at ${T}_{N}.$ As the ordered magnetic moment in NpBi has the free-ion ${\mathrm{Np}}^{3+}$ moment value, the anisotropy gap cannot originate from crystal-electric-field interactions, and the triple-k structure of NpBi is anticipated to be stabilized by two-ion quadrupolar terms.

Quantum effects in the single-particle kinetic energy of high-density fluid4He

Deep inelastic neutron-scattering measurements have been performed in dense fluid ${}^{4}\mathrm{He}$ along three iso- chores at number densities $33$, $38$, and $45 {\mathrm{nm}}^{\ensuremath{-}3}$ and temperatures ranging from $4.35$ to $56 \mathrm{K}$. Data have been analyzed in the plane-wave impulse-approximation framework to obtain the root-mean-square values of the single-particle momentum along a given direction ${p}_{x}.$ The density and temperature behavior of the derived mean kinetic energies is discussed in comparison with a simple harmonic model for the fluid and with quantum simulations performed using a path-integral Monte Carlo code. The comparison with the harmonic models sheds light on the density dependence of the zero-point kinetic energy of the particles and points out the inadequacy of a single-frequency Einstein-oscillator model in describing its temperature dependence. The quantum simulation results are in substantial agreement with the experimental data although systematic deviations are found as the density is increased.

Anomalous temperature dependence of the low-frequency vibrational density of states in vitreous B 2O 3

Inelastic neutron-scattering measurements of v-B 2O 3 reveal an upshift in energy of the first moment of the vibrational density of states for frequencies below ≈ 15 meV as temperature increases from 20 to 300 K. Such a stiffening is at odds with predictions based on some current approaches. Instead, the present result and related anomalies in specific heat and other microscopic and transport properties can be understood in terms of anharmonic motions at low frequencies. The atomic origin of the anharmonicity, and its consequence for the thermodynamical behavior are examined by molecular-dynamics simulations.

Intermultiplet transitions and crystal field in mixed valence Sm 3Te 4

Time-of-flight inelastic neutron-scattering measurements have been carried out on polycrystalline samples of 154Sm 3Te 4 and La 3Te 4 with incoming neutron energies of 67 and 300 meV. The results are, in general, consistent with the assumption that both Sm valence states, Sm 2+ and Sm 3+, contribute to the magnetic scattering function as well-defined local-moment states subjected to the crystal field from the surroundings.

Inelastic neutron-scattering measurements on the C15 Laves phase compounds HfV 2H x

The C15 Laves phase compounds have been the subject of several investigations due to their ability to absorb large amount of hydrogen, which make them a good candidates as the negative electrode material for metal hydride batteries. The inelastic neutron scattering technique was used on the TFXA spectrometer at RAL, to measure the vibrational frequency of hydrogen in the Laves phase compound HfV 2H x . The measurements were made for different hydrogen concentration and different temperatures. The data obtained indicate that the site occupancy depends on the hydrogen concentration, first the 2Hf-2V site is filled and then when the concentration is increased the 1Hf-3V is also occupied, which is consistent with previous diffraction data.

Magnetic order of the heavy-fermion URu 2Si 2 in a field of 12 T

We have performed elastic and inelastic neutron-scattering measurements on the heavy-fermion superconductor URu 2Si 2 in a magnetic field of 12 T along the c-axis of the tetragonal crystal structure. The small ordered moment and the energy gap in the magnetic excitation spectrum show a comparable transition temperature in a magnetic field of 12 T. This is in disagreement with recent predictions of a decoupling of the ordered moment and the energy gap in high magnetic fields.

Phonons and spin waves in the magnetic semiconductor chalcopyriteCuFeS2

We present the results of inelastic neutron-scattering measurements of the spin wave and phonon dispersion in the antiferromagnetic semiconductor chalcopyrite ${\mathrm{CuFeS}}_{2}$. The spin waves are steeply dispersive, and are described well by a simple local-moment model that considers only nearest-neighbor interactions. The nearest-neighbor exchange energy is determined as 2.27(2) THz, and there is an anisotropy gap of 1.28(1) THz.

Crystal field splitting and anomalous thermal expansion in YbVO4

We have investigated the Yb3+ crystal-field-level structure in YbVO4 using inelastic magnetic neutron-scattering measurements and crystal-field model calculations. We determined the temperature dependence of the a and c lattice parameters of the tetragonal unit cell by neutron diffraction and observed a minimum at ca. 120K in the c lattice parameter. This anomaly in the thermal expansion is interpreted as arising from a coupling of the anisotropic low-lying crystal-field states of the Yb3+ ions and the crystal lattice at low temperatures.

Neutron study of the frustrated spinel compound Mg1+tFe2-2tTitO4 (t=0.55).

We have performed neutron-diffraction and inelastic neutron-scattering measurements in the frustrated ferrimagnetic spinels ${\mathrm{Mg}}_{1+\mathit{t}}$${\mathrm{Fe}}_{2\mathrm{\ensuremath{-}}2\mathit{t}}$${\mathrm{Ti}}_{\mathit{t}}$${\mathrm{O}}_{4}$. The concentration studied here (t=0.55), is close to the percolation threshold for the dominant interaction. We observe spin fluctuations of short-range correlated spins, which persist well above the N\'eel temperature ${\mathit{T}}_{\mathit{N}}$. Below ${\mathit{T}}_{\mathit{N}}$, these dynamical correlations coexist and compete with static domains, ordered on a longer scale. In this inhomogeneous system, we measure several energy levels for the spin excitations, corresponding to different length scales for the spin correlations. The neutron results are discussed in comparison with magnetic measurements, performed on the same sample. \textcopyright{} 1996 The American Physical Society.

Effect of pressure on spin-gap behavior in Pr-doped and oxygen-deficient YBa2Cu3O7 thin films.

The influence of an external pressure on the behavior of the spin-gap temperature observed in the dc resistivity is measured on Y 12xPr xBa 2Cu 3O 7 films with x50.2, 0.3, and 0.4 and an oxygen-deficient YBa xCu 3O y film withy56.6. It is found that the spin-gap temperature shifts with pressure in a sense opposite to the shift of the critical temperature, consistent with the resonating valence bond mean-field phase diagram. The modified carrier density induced by the pressure is the major effect, while no significant effect of enhanced interplane-coupling of the Cu 21 spins due to a decrease in the distance between the CuO 2 planes is observed. @S0163-1829~96!05930-9# In many experiments on high-Tc superconductors a strong pressure dependence of the critical temperature ( Tc) is observed. It is generally believed that the applied pressure causes extra holes to be pushed into the CuO 2 planes. This is experimentally confirmed by, e.g., Furrer et al. 1 with inelastic neutron-scattering measurements. These extra holes lead to an increase of Tc with pressure. In the case of YBaCuO, the charge transfer from the CuO chains to the CuO 2 planes is influenced by a rearrangement of the ions in the chains resulting from the modified pressure. For oxygen-depleted films~YBa 2Cu 3O 72d) the effect of increasing pressure on the critical temperature is always positive ( dTc / dP .0) up to a certain pressure. 2 The influence of pressure on the Y 12xPr xBa 2Cu 3O 7 compound is