Inelastic Neutron-scattering Experiments Research Articles

Molecular-dynamics simulations of the dynamical excitations in commensurate submonolayer films of nitrogen molecules on graphite.

The dynamics of commensurate submonolayer solids of ${\mathrm{N}}_{2}$ molecules adsorbed on the basal planes of graphite have been studied using molecular-dynamics simulations. The calculations yielded the temperature dependence of the Brillouin-zone-center gap in the acoustic-phonon branches, for comparison with inelastic neutron-scattering experiments on the submonolayer solid. The calculated frequency gap was the same in submonolayer and monolayer films at low temperatures. At intermediate temperatures, the diffusive molecular motion associated with the presence of vacancies caused the gap mode to be less clearly defined in the coherent scattering function. Diffusion constants are calculated at submonolayer coverages, and temperatures up to 40 K for a population of molecules identified as mobile. \textcopyright{} 1996 The American Physical Society.

Confined Phonons in Metallic Superlattices: Raman Study of Co/Ru.

The Raman-active transverse-optic (TO) phonon mode of Ru was measured in a series of Co/Ru superlattices. The changes in the frequency of the TO vibration, interpreted within the framework of confined phonons, are in excellent agreement with published dispersion curves for bulk Ru obtained from inelastic neutron-scattering experiments. Interpretations of the frequency shifts of the Ru TO mode as due to phonon-folding or lattice-strain effects are ruled out by the data. {copyright} {ital 1996 The American Physical Society.}

Inelastic neutron scattering from single crystal Zn under high pressure.

Inelastic neutron-scattering experiments have been performed for single crystals of Zn under pressures up to 8.8 GPa at 300 K. The phonon modes q/${\mathit{q}}_{\mathrm{max}}$=\ensuremath{\xi}=0.075 and \ensuremath{\xi}=0.10 were measured in the transverse acoustic branch ${\mathrm{\ensuremath{\Sigma}}}_{3}$, where q=0 corresponds with the elastic constant ${\mathit{C}}_{44}$. The phonon energy showed a substantial hardening with increasing pressure. The experimental data below 6.8 GPa for \ensuremath{\xi}=0.075 yield a constant Gr\"uneisen mode ${\ensuremath{\gamma}}_{\mathit{i}}$=-ln${\mathrm{\ensuremath{\omega}}}_{\mathit{i}}$/lnV of 2.25 in good agreement with a previous calculation [H. Ledbetter, Phys. Status Solidi B 181, 81 (1994)]. Above 6.8 GPa, there is a very rapid increase of ${\ensuremath{\gamma}}_{\mathit{i}}$ which is indicative of the presence of a giant Kohn anomaly. This rapid divergence at high pressure indicates that a phonon softening may occur at pressures higher than 8.8 GPa caused by the collapse of the giant Kohn anomaly via an electronic topological transition (ETT). In an earlier M\"ossbauer Zn study at 4 K [W. Potzel et al., Phys. Rev. Lett. 74, 1139 (1994)], a drastic drop of the Lamb-M\"ossbauer factor was observed at 6.6 GPa, which was interpreted as being due to phonon softening, indicating this ETT had occurred. This paper also compares the compressibility data for single crystal Zn and Zn powder using neutron scattering. The results were found to be similar to an earlier x-ray Zn powder experiment [O. Schulte et al., High Pressure Res. 6, 169 (1991)]. \textcopyright{} 1996 The American Physical Society.

Spin fluctuations in (Y0.97Sc0.03)Mn2: A geometrically frustrated, nearly antiferromagnetic, itinerant electron system.

Inelastic neutron-scattering experiments have been performed on a single crystal of $({\mathrm{Y}}_{0.97}{\mathrm{Sc}}_{0.03}){\mathrm{Mn}}_{2}$. The spin fluctuation spectrum is broad in energy and strongly correlated in wave vector, with maximum intensity at the Brillouin zone boundary. There is no magnetic scattering for Brillouin zones centered at the origin and at certain reciprocal lattice points, suggesting the existence of short-lived 4-site collective spin singlets. The unusual features of the dynamical susceptibility discovered in this experiment result from geometrical frustration and give new insight into the heavy-fermion-like behavior of the compound.

The temperature dependence of the phonon density of states in the amorphous Invar alloy Fe 86Co 4Zr 10

Inelastic neutron-scattering experiments on an amorphous Fe 86Co 4Zr 10 Invar alloy have been performed. It was observed that the generalized phonon density of states in the low-energy region becomes soft below the Curie temperature. This result is quite similar to the phonon softening of the crystalline Invar alloy, indicating that low-energy phonon softening is a common feature of Invar alloys. In addition, it is suggested that the electrical resistivity minimum near the Curie temperature may be attributed to the extra electron—phonon scattering contribution.

Dynamical properties of amorphous Fe 90− xCo xZr 10 ( x = 10, 40) alloys

Inelastic neutron-scattering experiments on amorphous Fe 90− x Co x Zr 10( x = 10 and 40) alloys have been performed. Generalized phonon densities of states (PDOS) were deduced from the measured time-of-flight spectra at room temperature. The results show that in the energy region below 17 meV, the generalized PDOS of the amorphous Fe 80Co 10Zr 10 Invar alloy is obviously softer than that of the amorphous Fe 50Co 40Zr 10 non-Invar alloy. It is suggested that the softening of the low-energy PDOS in Invar alloys could be attributed to an enhancement of the electron—phonon interaction.

Different Pr3+ environments in Pr1.85Ce0.15CuO4: A Raman crystal-field excitation study.

Low-temperature Raman scattering of crystal-field (CF) excitations within the ${\mathrm{Pr}}^{3+}$ ground-state $^{3}\mathrm{H}_{4}$ multiplet of the compound ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ suggests the presence of more than one CF environment for the ${\mathrm{Pr}}^{3+}$ ions. The Raman spectra were similar to spectra obtained in inelastic neutron-scattering experiments. The analysis of the Raman spectra for ${\mathrm{Pr}}_{1.85}$${\mathrm{R}}_{0.15}$${\mathrm{CuO}}_{4}$ samples with R=Pr, La, Y, Ce, and Th indicates three different environments for ${\mathrm{Pr}}^{3+}$. One is associated with charge transfer caused by the replacement of ${\mathrm{Pr}}^{3+}$ by the tetravalent ions ${\mathrm{Ce}}^{4+}$ or ${\mathrm{Th}}^{4+}$. A second is almost identical to that observed in ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$. The third is attributed to either an orthorhombic distortion or to sites of smaller lattice volume. The Nd- and Nd(Ce)-based compounds are also discussed.

Influence of hole doping on spin dynamics in lightly doped copper oxide superconductors.

The spin dynamics in the doped antiferromagnetic (AF) state of high-${\mathit{T}}_{\mathit{c}}$ copper oxide superconductors has been investigated within the t-t'-J model. We have shown that a small number of doped holes affects the spin dynamics in the ${\mathrm{CuO}}_{2}$ planes so strongly that above a very small hole concentration, ${\mathit{n}}_{\mathit{c}}$=0.027, the two-dimensional (2D) AF ground state becomes unstable. We have also predicted a great change in the spin excitation spectrum for the doped AF state. For doping close to ${\mathit{n}}_{\mathit{c}}$, a hydrodynamic regime of well-defined magnons collapses, and almost the whole region of long-wavelength spin waves corresponds to the overdamped regime of softened magnons in agreement with inelastic neutron-scattering experiments in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$+x reported by Rossat-Mignod et al. Analyzing the imaginary part of the spin susceptibility \ensuremath{\chi}''(q,\ensuremath{\omega}) we have discovered, in addition to the spin-wave part, also an incoherent band associated with electron-hole excitations. For the overdamped regime the energy ranges of coherent excitations and the incoherent band are overlapped and so it is impossible to separate them. Even for smallest q, when the magnon peak and the band are separated, the relative contribution of the band increases with increasing n and dominates near the critical point.

Dynamics of hydrogen in alpha -LaNi5 hydride investigated by neutron scattering.

Quasielastic and inelastic neutron-scattering experiments have been performed in order to determine the structural and dynamical properties of \ensuremath{\alpha}-${\mathrm{LaNi}}_{5}$ hydride. A microscopic-jump model on the basis of the well-known ${\mathrm{CaCu}}_{5}$ structure of the compound (extended Chudley-Elliott model) has been used. It starts from the assumption of the hydogen atoms occupying two energetically different interstitial sites. This model has been applied to the long-range and localized diffusion of hydrogen in a single crystalline sample. To enhance the contrast for the incoherent scattering from the H nuclei, natural Ni was substituted with its $^{60}\mathrm{Ni}$ isotope. An appreciable anisotropy of the long-range diffusion is not observable at 423 K. A fast and localized diffusive process was found which is confined to a hexagonal arrangement of six sites in the (001) lattice plane. The local hydrogen vibrational modes are resolved and assigned to specific vibrations taking into account the crystallographic directions in which they appear.

Softening of phonon density of states in Fe-based amorphous Invar alloys

Inelastic neutron-scattering experiments and thermal expansion measurements on amorphous Fe 90− x Co x Zr 10 ( x = 10 and 40) and Fe 80− y Cr y P 13C 7 ( y = 4 and 8) alloys have been performed. Generalized phonon densities of states (PDOS) were deduced from the measured time-of-flight spectra at room temperature. The results show that the generalized PDOS in the energy region below 17 meV become soft at the Invar concentrations for both amorphous Fe 90− x Co x Zr 10 and Fe 80− y Cr y P 13C 7 alloys. It is suggested that the softening of the low energy PDOS in Fe-based amorphous Invar alloys could be attributed to the enhancement of electron-phonon interaction.

Theoretical study of dynamical properties of binary metallic glasses in the harmonic approximation

A theory is developed to calculate the vibrational frequency spectrum of binary amorphous solids in the harmonic approximation. It is shown that the spectrum of isotropically disordered material can be separated into longitudinal acoustic (LA), transverse acoustic (TA), longitudinal optical (LO) and transverse optical (TO) phonons in a strict sense. Calculations are performed both analytically and numerically for each mode of Mg 70Zn 30 metallic glass. It is clarified that the roton-like dispersion curve of the longitudinal vibrations, determined by an inelastic neutron-scattering experiment, is well described as consisting of LA and LO phonons.

Structure and lattice dynamics of rational approximants to icosahedral Al-Cu-Li.

We present a detailed theoretical investigation of the structure and the dynamical properties of icosahedral Al-Cu-Li alloys. Structural models of the quasicrystalline structure have been constructed on the basis of three-dimensional Penrose- and canonical-cell tilings. We show that the periodic approximants (up to \ensuremath{\tau}\ensuremath{\sim}5/3) are stable under molecular-dynamics (MD) annealing with realistic pair forces and that the partial-pair-correlation functions, powder- and single-crystal and neutron- and x-ray diffraction data are in good agreement with experiment, with a slight preference for the canonical-cell models. The vibrational spectrum (spectral functions, densities of state) and inelastic neutron-scattering intensities have been calculated using a real-space recursion technique and the same interatomic forces as used in the MD annealing. Well-defined propagating longitudinal and transverse acoustic modes are found in the vicinity of quasiperiodically distributed special points in k space, the \ensuremath{\Gamma} points of the reciprocal quasilattice. At higher energy, a hierarchy of stationary modes is predicted around other high-symmetry points corresponding to quasi-Brillouin-zone boundaries. For the propagating low-energy modes, the predictions are in very good agreement with inelastic neutron-scattering experiments on single crystals of i-Al-Cu-Li. However, certain discrepancies subsist between the predicted vibrational density of states (DOS) and the experiments with powdered samples. Quite generally, the calculated DOS shows more structure (with again only minor differences between the Penrose- and canonical-cell models) than the experimental result. This is probably to be attributed to the influence of phason disorder in the real quasicrystals.

Structural fluctuations and the order-disorder phase transition in calcite

Molecular-dynamics calculations have been carried out on the ordered (low-temperature) and orientationally disordered (high-temperature) phases of calcite. The potential model used is shown to reproduce the main features of the experimentally observed order-disorder transition. The significance of the results for the interpretation of recent inelastic neutron-scattering experiments is discussed with particular reference to a possible competition between different ordering processes. Comparison is made with previously reported simulations of the corresponding phase transition in sodium nitrate and differences in behaviour between the two systems are noted.

Magnetic excitations in the dipole-coupled singlet-singlet system HoF3

Well defined magnetic excitations in holmium trifluoride are observed in inelastic neutron-scattering experiments, both in the paramagnetic phase at 1.6 K and in the ferrimagnetic phase at 90 mK. The dispersion relations at the two temperatures have been determined along the high-symmetry directions, and the field dependence of the excitations has been studied at 1.6 K. These measurements and the previous studies of the magnetic properties of HoF3 are analysed within the mean-field/random-phase approximation (RPA). The two lowest electronic states of Ho3+ ions are singlets, which are well separated from the remaining levels. The dominating coupling between the ions is the classical dipole interaction, which forces the system to order at Tc=0.53 K. The two-ion coupling is below the threshold value for inducing the ordering of the electronic system; it only occurs because the magnetic susceptibility is enhanced by the hyperfine interaction between the electronic and nuclear moments on the Ho ions. The classical dipole coupling is calculated directly from first principles, whereby the response function is nearly fixed by the macroscopic properties of the system. The calculated response is found to agree accurately with the observations in the paramagnetic phase, whereas some discrepancies occur in the ordered phase. These may indicate that correlation effects beyond the RPA are important or that two-ion (magnetoelastic) quadrupole couplings are present.

Molecular-dynamics simulation of crystalline trans-polyacetylene.

Constant-pressure molecular-dynamics simulations of rigid, infinitely long polyacetylene chains were performed for various temperatures and pressures. The experimentally observed anisotropic lattice thermal expansion and compressibility were qualitatively well reproduced by the simulations. The model lattice exhibits considerable jump diffusion along chain axes already at room temperature, with the onset of liquidlike diffusion above 400 K accompanied by orientational disorder of chain projections in the ab plane. The polarized vibrational density of states for E\ensuremath{\le}25 meV is in good agreement with G(E) spectra derived from inelastic incoherent neutron-scattering experiments. By analyzing different contributions to chain motions, peaks in G(E) at 3.5, \ensuremath{\sim}8, and \ensuremath{\sim}20 meV can be assigned to rigid-chain vibrations along the chain axis, vibrations in the ab plane, and chain librations, respectively.

Distributions of excitation energies of protons in hydrogen-bonded crystalline KH2PO4: Monte Carlo calculations

Monte Carlo calculations are performed on a model that was proposed in our previous letter to explain a large isotope effect on the transition temperature (T,) from a ferroelectric phase to a paraelectric phase in KH2PO4 (KDP). On the basis of the distribution of the electric dipole moments obtained from these calculations, the distribution of the excitation energies for protons is calculated; it is found that their features are in agreement with the excitation spectra in KDP, observed by inelastic neutron-scattering experiments.

Electron-phonon interactions in oxide superconductors

We present recent results on the lattice dynamics of oxide superconductors as determined by inelastic neutron-scattering experiments. Included are data on the high- T c cuprate superconductors, for example YBa 2Cu 3O 7, as well as Nb-doped SrTiO 3, which is a low- T c system ( T c∼1 K). Particular emphasis is laid on direct indications of electron-phonon coupling such as phonon softening by renormalization or changes in phonon frequencies when passing through T c .

Crossover from propagating spin waves to localized Ising-cluster excitations in diluted Heisenberg antiferromagnets.

We performed inelastic neutron-scattering experiments on three-dimensional diluted Heisenberg antiferromagnets, ${\mathrm{RbMn}}_{\mathit{c}}$${\mathrm{Mg}}_{1\mathrm{\ensuremath{-}}\mathit{c}}$${\mathrm{F}}_{3}$ with c=0.74 and 0.63. Magnetic excitations were investigated throughout the entire Brillouin zone. As the wave-vector approaches the zone-boundary (ZB) values, the observed spectra show a crossover at around q=0.25${\mathit{q}}_{\mathrm{ZB}}$ from propagating sharp spin waves to six well-resolved localized excitations. In the crossover region, the spectra show a characteristic line shape with two pronounced peaks and additional small peaks. These results are in contrast with recent measurements for a three-dimensional diluted antiferromagnet, ${\mathrm{Mn}}_{0.5}$${\mathrm{Zn}}_{0.5}$${\mathrm{F}}_{2}$, with Ising anisotropy, which were concerned with the finding of fractons. The origin of these characteristic line shapes is discussed.

Magnetic excitations in TmH observed using inelastic neutron scattering

We have performed inelastic neutron-scattering experiments to study the magnetic excitations in α-TmH 0.06 and compared these with “pure” thulium metal. Our results are similar to earlier measurements on the metal but there are additional features caused by changes in the crystal field due to hydrogen ordering, most notably a large dispersionless feature at 15 meV. We also see magnetic excitations coupled to the hydrogen vibration states.

Selected aspects of the lattice dynamics in high-Tc oxide superconductors

We present selected results of inelastic neutron-scattering experiments on single-crystalline La2CuO4. For this compound we find very unusual features in the nuclear scattering law S(K, ω) such as strong asymmetries in S(K, ω) for K = τ ±q and in particular an unexpected splitting of the breathing mode. For YBa2Cu3O7−δ, very recent measurements seem to exhibit similar extra modes. A possible explanation for these pronounced anharmonicities would be hybridization of lattice modes with electronic excitations thus indicating very strong electron-lattice coupling.