Quasielastic Signal Research Articles

Protein dynamics on the picosecond timescale as affected by the environment: a quasielastic neutron scattering study

We performed an inelastic and quasielastic neutron scattering investigation of the molecular dynamics of lysozyme solvated in glycerol, at different water contents h (g water/g lysozyme). At low temperature, dry lysozyme solvated with glycerol shows a low-frequency vibrational bump located at around 4 meV. Contrary to the behaviour of strongly hydrated protein powders, this peak is well visible also at room temperature and persists in the experimental spectra at all the hydration degrees. The quasielastic contribution increases remarkably with h, thus suggesting that water molecules perform their plasticizing action by allowing the onset of confined diffusive motions of protein groups. The quasielastic signal could be decomposed in two Lorentzian components, representing motions with characteristic times of, respectively, 15 and 0.8 ps. The Lorentzian curves exhibit constant widths but variable intensities as a function of the hydration degree. The trend of the estimated quasielastic structure factors suggests that a preferential hydration effect may take place in lysozyme solvated with glycerol and water.

Coherent quasielastic neutron scattering and correlations between rotational jumps of molecules on a periodic lattice

We previously [Coddens, Phys. Rev. 63, 064105 (2001)] derived a theorem about the coherent quasielastic neutron-scattering signal from a d-dimensional lattice of N molecules that are undergoing rotational jump diffusion (around an n-fold axis), assuming that there are no correlations between the molecules. In the present paper molecular correlations are treated, but only in the sense that several molecules could reorient simultaneously as in a cog-wheel mechanism. Moreover, we do not examine the possibility that the relaxation times of these combined reorientations could depend on details of the local environment created by the neighbouring molecules. Finally also an ergodicity condition has to be fulfilled. Admitting for all these assumptions we can show that the correlations do not affect the coherent quasielastic scattering pattern in the following sense: The functions of Q that intervene in the description of the intensities remain unaltered, while the functions of ω can undergo a renormalization of the time scales. The latter changes cannot be detected as the time scales that would occur if the dynamics were independent are not available for comparison. In other words: Coherent quasielastic neutron scattering is not able to betray the existence of correlations of the restricted type that occur in our model. The assumptions that underly the model we present were made to allow a mathematically rigorous calculation of the scattering function. Other, perhaps more realistic cases may entail correlations of a type that is too difficult to be treated rigorously with our method of calculation. But our result presents an important non-trivial counterexample to show that the absence of a clue for the presence of correlations in the data is not a sufficient criterium to conclude that such correlations are indeed absent.

Magnetic fluctuations and superconductivity in YbPd 2Sn

We report muon spin relaxation and inelastic neutron measurements on the Heusler system YbPd 2Sn. Localized anisotropic and quasi-elastic Yb magnetic fluctuations are observed below T=150 K . Both μSR and neutron data indicate a slowing-down of the spin fluctuations upon lowering the temperature, similar to that observed in Kondo lattices. The temperature dependence of the quasi-elastic neutron signal is compatible with a crystal-electric field scheme having a Γ 7 CEF ground state. The muon depolarization rate exhibits an additional contribution upon decreasing the temperature below T c suggesting a close interplay between magnetic fluctuations and the superconducting state.

Time-domain interferometry using synchrotron radiation applied to diffusion in ordered alloys

Time-domain interferometry of synchrotron radiation (TDI) has recently been used as a tool for investigating diffusion in glasses. This work deals with an extension of this technique to ordered structures. In a TDI experiment performed on the B2 alloy CoGa at the APS the intensity scattered into Bragg directions showed no detectable quasielastic signal. Experimental lower limits of the elastic contribution are given. They are in accordance with the coherent scattering function derived in this paper. This result indicates that TDI can be applied to diffusion in crystalline solids, e.g. intermetallic alloys, by using diffuse scattering. Requirements and limitations of diffuse scattering experiments are discussed.

Coherent quasielastic neutron scattering: A theorem about total neutron scattering functions for rotational jump diffusion of molecules on a lattice

We derive a theorem about the coherent quasielastic neutron-scattering signal from a lattice of N molecules that are undergoing rotational jump diffusion. If no correlations between the molecular jumps exist then the coherent quasielastic scattering signal is just N times that of a single molecule. The elastic contribution is just the diffraction diagram of the system. This implies, among others, that between the Bragg peaks there is no coherent elastic contribution, i.e., between the Bragg peaks the coherent signal is purely quasielastic. These results for coherent scattering are in marked contrast to those in the case of incoherent scattering where one also can find elastic scattering outside the Bragg peaks.

Magnetic excitations in heavy-fermionCePd2Si2

We have studied the magnetic interactions of single-crystalline ${\mathrm{CePd}}_{2}{\mathrm{Si}}_{2}$ by measurements of the electrical resistivity, specific heat, thermal expansion, magnetic susceptibility, and elastic and inelastic neutron scattering. In the paramagnetic phase the system is characterized by Kondo-type spin fluctuations which give rise to a strong quasielastic signal in the inelastic neutron scattering cross section. Below the antiferromagnetic ordering temperature ${(T}_{N}=8.5$ K), the quasielastic signal coexists with spin-wave excitations. The spin waves are strongly dispersive, with a minimum energy of 0.83 meV at the magnetic zone center $\mathbf{k}=(1/2,1/2,0).$ The dispersion can be described in terms of a two-sublattice model with a reduced magnetic exchange interaction and a strong damping of the spin waves due to Kondo screening. The volume dependence of the magnetic interactions has been analyzed using the Gr\"uneisen parameters in order to study the magnetic interactions close to the quantum critical point, which is reached at an applied pressure of about 30 kbar.

Time-Temperature Superposition of Structural Relaxation in a Viscous Metallic Liquid

Bulk metallic glass-forming ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{30}{\mathrm{P}}_{20}$ has been investigated in its equilibrium liquid by quasielastic neutron scattering. The quasielastic signal exhibits a structural relaxation as known from nonmetallic viscous liquids. Even well above the melting point, the structural relaxation is nonexponential and obeys a universal time-temperature superposition. From the mean relaxation times average diffusivities have been determined, resulting in values on a ${10}^{\ensuremath{-}10}{\mathrm{m}}^{2}{\mathrm{s}}^{\ensuremath{-}1}$ scale, 3 orders of magnitude slower than in simple metallic liquids.

Brillouin and Umklapp scattering in polybutadiene: comparison of neutron and x-ray scattering.

We report a comparison of high resolution inelastic x-ray Brillouin scattering to coherent inelastic neutron scattering for amorphous deuterated polybutadiene, done for one temperature in the glass phase and another one in the melt. The x-ray scattering proves to be by far the better technique for such a polymer within its present resolution bounds. The neutron scattering allows one to extend these measurements to a much better resolution, showing an additional quasielastic signal in the melt. The results suggest x-ray measurements at higher momentum transfer, to see whether they are complementary to neutrons.

Use of the 3He filter in search of the in-chain spin correlations in YBa 2Cu 3O 6+ x

We report on a first neutron quasielastic scattering experiment using the 3He spin filter as analyser. In order to detect a weak quasielastic signal from the spin fluctuations in the Cu–O chains we had to integrate over an energy range of about 10 meV while performing polarisation analysis of the scattered neutron beam to discriminate the magnetic signal from the general background. The 3He filter technique, permitting to decouple polarisation analysis from the neutron energy analysis, proved ideally suited in such a case. The experimental results obtained indicate the presence of a weak magnetic scattering whose profile in the momentum space as well as the cross-section magnitude of about 36(10) mb/sr are compatible with theoretical predictions for an average hole concentration of about 50% in the chains.

Inelastic neutron scattering study of the superionic solid solutions Ba1−xLaxF2+x

Inelastic neutron scattering spectra of Ba1−xLaxF2+x single crystals have been measured at temperatures from 450 to 1000°C. A noticeable result at x=0.16 consists in a low-energy inelastic mode (E=0.8 THz) which is observed at a specific wavevector only and takes the form of a quasielastic signal for other wavevectors. This is explained by the assumption that the mode comprises mobile ions and defect clusters; the characteristic wavevector corresponds to the short-distance correlation length between the clusters. For x=0.31, this mode seems overdamped, which we explain by enhanced interactions between the clusters.

Low energy excitations in CeNiSn

We have studied the magnetic excitation spectrum of CeNiSn at low energies both on a polycrystalline sample using time-of-flight technique and on a single crystal with a triple axis spectrometer. The energy gap in the excitation spectrum is clearly observed in the polycrystalline sample reconciling the earlier discrepancies between the two kinds of measurements. The experimental results are consistent with the occurrence of a quasielastic signal within the gap without any significant wave vector dependence and characterized by an energy scale Γ≈0.2 meV.

Neutron study of the spin dynamics in superconducting La 2−xSr xCuO 4

In addition to the incommensurate magnetic fluctuations already well-established in La 2−xSr xCuO 4 , we report inelastic neutron scattering experiments that provide clear evidence for a new set of low energy excitations. This novel feature of the dynamical structure factor consists of a two-dimensional quasi-elastic antiferromagnetic signal that sets in at low temperature well below the superconducting critical temperature.

Dynamics of a globular protein as studied by neutron scattering and solid-state NMR

Effect of hydration on the dynamics of parvalbumin, a 11.5 kDa Ca 2+/Mg 2+-binding globular protein has been studied, at room temperature, by incoherent quasi-elastic neutron scattering and 13C solid-state NMR. Samples were protein powders hydrated at different hydration levels. The increase of the quasi-elastic signal observed in neutron scattering upon hydration is interpreted as an increase of the local mobility of charged side-chain protons (Asp, Glu, Lys) and is in agreement with a parallel study of parvalbumin by solid-state natural abundance 13C NMR under cross-polarization and magic angle spinning (CP MAS) conditions.

Dynamics of Guanosine Self-Assembled Aggregates in the Hexagonal Columnar Phase by Quasielastic Neutron Scattering

Abstract Guanosine, one component of the nucleic acids, presents a simple model system to study the self-assembling process and the properties of columnar liquid-crystalline phases. In a recent x-ray study we investigated the structure of the aggregates at medium concentrations and explored the phase-diagram as a function of salt concentration in the solution. A microscopic model has been presented describing the size and distribution of the rod-shaped aggregates consisting of stacked guanosine tetramers. Here we present a first study of the dynamics of guanosine in the two-dimensional hexagonal phase. Inelastic neutron scattering gives further evidence for the “persistent-flexible-hard-rod” model; displacements in the hexagonal plane are found to be extremely high. To a detection limit of 0.04 meV we find no evidence for propagating phonon modes. The quasielastic signal measured is attributed to relaxational motions that is extremely overdamped motions of the guanosine rods.

Study of Na 3Fe 2(PO 4) 3 and Na 3Cr 2(PO 4) 3 in their paramagnetic phases by neutron scattering

The paramagnetic scattering of the title compounds (noted as NaFe and NaCr, respectively) has been studied by neutron scattering on the time of flight instrument IN6 at ILL. For NaFe the strong quasielasticity observed at 60,300 and 573 K is mainly Lorentzian for small Q s and Gaussian for large Q s in conformity with de Gennes' theory. An average exchange coupling is estimated to be about 0.36 meV. For NaCr a quasi-elastic signal coexists with an inelastic feature centered between 1.5 and 1.8 mev over the whole investigated temperature range (12 K up to 600 K). The quasi-elastic scattering, as for NaFe, can be interpreted in terms of de Gennes' theory and the corresponding average exchange coupling of 0.083 meV was found to be much smaller than that of NaFe; the interpretation of the inelastic component is discussed in a forthcoming article. [Barj et al., J. Phys. Chem. Solids 53 (1992)].

Neutron scattering study of the dynamics of iron spins in iron doped Bi 2CaSr 2Cu 2O 8 superconducting compounds

We have studied the dynamics of the iron spins in a 6.5% iron doped Bi 2CaSr 2Cu 2O 8 superconducting compound with T c =46.5 K by time of flight (TOF) neutron scattering experiments. The quasielastic and elastic intensities and the width of the quasielastic signal strongly vary with temperature. At low temperature, a slowing down of the iron spin dynamics is observed, starting around 60 K, i.e. above T c. At higher temperature, an unusual temperature dependence of the quasielastic intensity is observed in the doped Bi compound as previously described in Fe doped YBa 2Cu 3O 7.

High-energy-neutron spectroscopy of crystal-field excitations in NpO2.

Neutron spectroscopy has been applied to study the crystal-field potential in ${\mathrm{NpO}}_{2}$. The inelastic-scattering cross section shows a broad magnetic peak that is split into two components and is centered at about 55 meV. A quasielastic magnetic signal extending up to about 30 meV is also observed at temperatures below the phase transition at 25 K. The origin of the inelastic peak is attributed to excitations between the ${\mathrm{\ensuremath{\Gamma}}}_{8}^{(2)}$ and ${\mathrm{\ensuremath{\Gamma}}}_{8}^{(1)}$ crystal-field quartets. A mechanism that could be responsible for the observed splitting is proposed.

Quasielastic gamma -ray scattering from pentadecane.

A recently developed M\ossbauer instrument has been used to study the liquid dynamics of pentadecane at various temperatures and scattering angles. Widths of the quasielastic signal extracted from the data ranged between 16 GHz (at 12 \ifmmode^\circ\else\textdegree\fi{}C) and 48 GHz (at 74 \ifmmode^\circ\else\textdegree\fi{}C) at a scattering vector of 1.15 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$. Similar measurements at five different scattering angles (at 12 \ifmmode^\circ\else\textdegree\fi{}C) were also taken to investigate the Q dependence between 0.82 and 1.56 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$. The results obtained are in good agreement with classical theory for coherent scattering from liquids.

Study of the dynamics of a single crystal of Na +β-Al 2O 3 by neutron scattering

The present study has been undertaken on a single crystal of Na +β-Al 2O 3. The quasi elastic signal in the −2, +2 meV range, for Q parallel to the sample conduction plane has been much improved compared with the powder data. Two incident wavelengths, 5.1 and 4.1 Å, have been used and data were collected between 400 and 25°C. As found previously, at 300°C, only a small fraction of Na + ions are mobile on a picosecond time scale. The data are interpreted within a model in which about 18% of ions, corresponding to doubly occupied cells, undergo “localized” motions restricted to a given cell with a residence time of about 3×10 −11s at 300°C and about 63% of ions are able to undergo long-range motions on a time scale of about 1.5×10 −10s. The conductivity mechanism is discussed in terms of a caterpillar mechanism combined with an itinerant polaron model. Inelastic data have also been collected between 30 and 300 cm −1. Some dispersion curves for low frequency phonons are proposed.

Neutron scattering in Ag + and Na + β-aluminas

Na + and Ag + β-alumina for various stoichiometries and for various hydration states have been studied by neutron scattering using incident energy of 0.8 meV with a resolution of 20 μeV in the 200–325° C temperature range. From the weak quasielastic signal (QES) observed for the non- stoichiometric (NS) Ag + β-alumina a translational diffusion constant D t has been estimated at about 10 -5 cm 2 s -1 at 200°C. No such QES has been observed for stoichiometric Ag + and Na + compounds. The dehydration of Na + β-alumina has been evidenced by the disappearance of the broad QES above 300°C; correlatively the low frequency inelastic spectrum of Na + of β-alumina is changed.