Elastic properties of filled-Skutterudite compounds probed by Mössbauer nuclei

We have carried out Mössbauer spectroscopy and nuclear resonant inelastic scattering to elucidate the lattice dynamics in filled-Skutterudite compounds, especially phosphides. The second-order Doppler shift obeys the Debye model in RFe₄P₁₂. Nuclear quadrupole interaction reveals an unusual temperature dependence in these compounds. An anomaly is observed in ⁵⁷Fe nuclear resonant inelastic scattering of these compounds. The energy where the anomaly observed in SmFe₄P₁₂ agrees with the phonon excitation energy observed by ¹⁴⁹Sm nuclear resonant inelastic scattering. We have also performed the ⁹⁹Ru Mössbauer measurements of SmRu₄P₁₂.     

Nuclear Resonant Inelastic and Forward Scattering of Synchrotron Radiation by 40K

We achieved excitation of the first excited state of ⁴⁰K and confirmed both energy and lifetime. Furthermore, we observed nuclear resonant inelastic scattering by ⁴⁰K in a powdered KCl sample at room temperature using a high-resolution monochromator. The time spectrum of the nuclear resonant forward scattering was measured at 50 K. Our observations of nuclear resonant inelastic and forward scattering by ⁴⁰K make electronic and dynamic studies for potassium practical. The measurements of nuclear resonant scattering for the radioactive ⁴⁰K nuclide will enable and lead to further studies of other radioactive nuclides. This revised version was published online in August 2006 with corrections to the Cover Date.     

The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×10¹³ to 4.1× 10¹⁸ cm^?3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl⁰ acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ T ^q (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 10¹⁸ cm^?3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |A _hf (Al)/2π| ～ 2.5×10⁶s^?1.     

Slow dynamics of supercooled liquid revealed by Rayleigh scattering of Mössbauer radiation method in time domain

Characteristic relaxation times τ of the microscopic correlation in o-terphenyl and glycerol were obtained by a time-domain interferometry method using nuclear resonant scattering of synchrotron radiation. From the temperature dependence of τ, a new decoupling temperature 278 K, which is below the conventional decoupling temperature 290 K, was obtained for o-terphenyl. Furthermore, we found a liquid-like relation between τ and q (momentum transfer) as $\tau \propto q^{-2}$ at 285 K in o-terphenyl and 260 and 265 K in glycerol. However, an anomalous q dependence of the slow β relaxation time, $\tau \propto q^{-2.9}$ and q ^?3.8, was observed in deeply supercooled (265 K) and glass (240 K) states, respectively, in o-terphenyl, indicating specially restricted dynamics of the slow β process.     

Exciton-phonon interaction in mixed silver halides: Resonant Raman scattering vs photoluminescence

An investigation of resonant Raman scattering in mixed crystals of AgBr:Cl at 1.8 K shows that the zero-phonon and LO phonon-assisted exciton luminescence excited in the free indirect exciton absorption, exhibits an anomalous dependence on the exciton photon energy E_L. Close to the exciton gap, the bands show a Raman-like behaviour with their peaks at constant energetic distance from E_L. As E_L is tuned further into the absorption, the bands gradually develop into normal photoluminescence. The effect is explained by taking into account exciton relaxation via scattering by long-wavelength acoustic phonons, a process which is strongly energy dependent. In addition, resonant Raman scattering observed for excitation in the zero-phonon absorption suggests study for the first time of the mode behaviour of certain off-zone center phonons in this system.     

An in situ M?ssbauer study using synchrotron radiation

We have been developing a system for in situ M?ssbauer studies using synchrotron radiation (SR) to elucidate the mechanism of hydrogenation processes. In the system, samples reacts in a pressure-temperature chamber and SR-based M?ssbauer spectra using variable-frequency nuclear monochromator and energy spectra of inelastic nuclear resonant scattering (NRS) of SR are measured. As a feasibility study, the temperature dependence of the M?ssbauer and inelastic NRS spectra of ⁵⁷Fe in c-GdFe₂H₃ under vacuum were measured. In both spectra, clear differences were observed between 373?K and 573?K. These differences can be interpreted by the change of microscopic environment around Fe at the dehydrogenation. Thus, it is confirmed that this system works well enough to perform the in-situ M?ssbauer study on the dehydrogenation of c-GdFe₂H₃.     

Development of Mössbauer diffractometer by using nuclear resonant scattering at SPring-8 BL11XU

A Mössbauer diffractometer has been developed by using ⁵⁷Fe nuclear resonant scattering apparatus at SPring-8 BL11XU in order to obtain a crystal-site-selective Mössbauer spectrum. A ??-2?? goniometer was newly installed between the nuclear monochromator and a detector. From a single crystal Fe₃ O ₄ mounted on the goniometer, the 111, 222, and 220 reflected γ-rays were used to collect the diffraction spectra at room temperature. The intensity ratio of the two subspectra, corresponding to A- and B-site Fe ions, changes notably according to the reflection index. The diffraction spectrum is composed of a major absorption spectrum and a minor emission spectrum. The former is given by the γ-ray due to the electron scattering and nuclear absorption, whereas the latter is given by the γ-ray due to the nuclear resonant scattering. Interference effects between these two γ-rays are also seen as line broadenings, asymmetric line shapes, and slope of the base lines. These features can be successfully expressed by a Fano function. We consider that the emission spectrum due to the nuclear resonant scattering represents crystal-site-selective Mössbauer spectrum.     

Phenomenology of nuclear shadowing in deep-inelastic scattering

We investigate shadowing effects in deep-inelastic scattering from nuclei at small valuesx < 0.1 of the Bjorken variable. Unifying aspects of generalized vector meson dominance and color transparency we first develop a model for deep-inelastic scattering from free nucleons at smallx. In application to nuclear targets we find that the coherent interaction of quark-antiquark fluctuations with nucleons in a nucleus leads to the observed shadowing atx < 0.1. We compare our results with most of the recent data for a large variety of nuclei and examine in particular the Q² dependence of the shadowing effect. While the coherent interaction of low mass vector mesons causes a major part of the shadowing observed in the Q² range of current experiments, the coherent scattering of continuum quark-antiquark pairs is also important and guarantees a very weak overall Q² dependence of the effect. We also discuss shadowing in deuterium and its implications for the quark flavor structure of nucleons. Finally we comment on shadowing effects in high-energy photon-nucleus reactions with real photons.     

The B(E2) for the first 2 state in Zr

A model-independent measurement of the width of the first 2⁺ state (2.186 MeV) of ⁹⁰Zr has been carried out using the resonant scattering of bremsstrahlung and its detection with a Ge(Li) detector. The observed resonance fluorescence yield corresponds to a reduced E2 transition probability B(E2↑) = 608 ± 35 e² · fm⁴. The difference between this value and recent inelastic electron scattering results exceeds the range usually attributed to the model dependence of the latter.     

Neutron diffraction study on the hyperfine-induced nuclear spin order in the antiferromagnet PrSn3

At several $\text{(h+}\text{1}\text{2}\text{0 0)}$ reciprocal lattice positions of PrSn₃, we observed Bragg scattering of neutrons which is purely due to the nuclear spin polarization of ¹⁴¹Pr, and could measure the temperature dependence of the polarization between 10 mK and 4.2 K. From the perfectly polarized state of the nuclear spins, the difference between the spin-dependent nuclear scattering lengths b⁺ and b^- has been determined to be b⁺ - b^- = -0.110 ± 0.006 × 10^-12 cm, which is significantly larger in magnitude than the previously reported value. An additional electronic polarization which is linearly proportional to the nuclear spin polarization has also been observed.     

Target excitations and the optical potential for protons scattering from nuclei

We calculate the contributions to the optical potential for 30 MeV protons due to inelastic excitations of the target nucleus. The scattering due to this non-local potential is calculated exactly and some of the results subjected to conventional optical model analysis. When only one excited state is included, a resonant dependence on the excitation energy is observed. Even with ten excited states, the position of a single one can strongly influence the scattering. It is possible to account for about $\text{3}\text{4}$ of the observed absorption in ⁴⁰Ca and ²⁰⁸Pb, but only by postulating unobserved states which exhaust the remainder of the experimental sum rules at somewhat unreasonably low energies. It was not possible to find simple local potentials which gave the same scattering because of the strong L-dependence of the absorption. The constructed potentials concentrate the absorption at too small radii. It is suggested that rearrangement (pick-up) processes contribute a substantial amount of absorption at larger radii, while compound formation will give rise to a volume term in the imaginary potential.     

Importance of longitudinal contributions to backward inelastic electron scattering

Coulomb contributions of multipolarity 2 are calculated for backward (θ = 180°) inelastic electron scattering by our improved DWBA code. General features are studied such as nuclear charge (Z) dependence, energy transfer dependence, momentum transfer dependence as well as the angle dependence near θ = 180°, with the help of a simple model for the transition charge density. We compare these features with PWBA and other simple approximations. We show, as a specific example, that the C2 contribution is comparable to the M1 and E2 multipole form factors for inelastic scattering to the rotational excited states of ¹⁸¹Ta.     

Molecular bond formation in Na*+N2 energy transfer: Crossed beam study of atomic alignment and orientation

We report the first full analysis of collisionally induced atomic alignment and orientation for a molecular collision process. In an experiment with crossed supersonic beams of N₀ and laser excited Na(3² P _3/2) we have studied the dependence of angular and energy resolved differential quenching cross sections as a function of the linear and circular polarization of the exciting laser light. The anisotropies observed in the linear polarization data range up to 2∶1 when corrected for electron and nuclear spin relaxation. The maximum effect is found at small scattering angles and intermediate energy transfer where the cross section is also largest. The atomic alignment angle most favourable for quenching relates to the scattering angle and can be understood in a model picture in such a way that the (NaN₂)^* molecular system is formed at internuclear distances as low asR=10a ₀. The circular asymmetry is small but with significant structure and is attributed to interaction on different potential surfaces atR>10a ₀. Full analysis of the four measurable parameter is given in terms of the density matrix in a frame withz-axis perpendicular to collision plane which allows a clear understanding of the properties of atomic reflection symmetry and coherence of the scattering process.     

Wave vector conservation in the first order resonant Raman scattering

The dependence of the shape and the intensity of the first order resonant Raman line 1LO on the direction and magnitude of phonon wave vector has been investigated in CdS crystals. Comparison of RRS in different orientations shows that in the most pure samples the phonon wave vector is determined, to a great extent, by the momentum conservation law. In Ni doped crystals one can observe violation of momentum conservation, resulting in a sharp increase of 1 LO intensity. The shape of the 1 LO line in such sample does not depend on the experimental orientation. For the first time the dependence of the Raman intensity on the scattering angle has been observed by the comparison of forward and backward scattering spectra. Observation of this dependence shows that the free excitons are the dominating intermediate states in the resonant Raman scattering in A₂B₆ compounds.     

Temperature dependence of the rotational life times in solid H2

We have studied the temperature dependence of the nuclear transverse and longitudinal relaxation times in solid H₂ for ortho concentrations 10^-3 < X < 10^-2 between 0.4 K and the triple point, 13.9 K. We find a striking temperature dependence in T₂ over the whole temperature range. This new effect is ascribed to a coupling between molecular rotation and lattice vibrations, which is brought into evidence by the narrow width of the spectral density of the rotational fluctuations at low X.     

Totalreflexion der rückstoßfreien 8,4 keV γ-Strahlung des Tm169

The total reflection of the recoilless 8.4 keVγ-radiation of Tm¹⁶⁹ was observed at various glancing angles between 3 and 7 milliradians on surfaces of Tm metal and Tm₂O₃ produced by vacuum evaporation. The Mössbauer velocity spectra of the reflected intensity show a typical dispersion pattern due to interference between electronic and nuclear resonant scattering, which can be understood quantitatively, thus permitting the determination of the nuclear scattering amplitude. From this the total conversion coefficient of the 8.4 keV transition was derived to be α=220±50.     

Resonant Raman scattering at the indirect exciton in silver bromide

By excitation in the indirect exciton absorption of AgBr at low temperatures selectively enhanced two-phonon Raman scattering is observed. Using different excitation wavelengths the resonance enhancement is found to be associated with the Г-L exciton as intermediate state for the resonant scattering process. The resonant phonons involved are pairs of LA and TA phonons with opposite wave vectors near L. Measurements in the temperature range 2 K ? T ? 40 K show a decrease of the scattering intensity with increasing temperature. The origin of this temperature dependence is due to lifetime broadening of the scattering state. Several features of the indirect exciton absorption of AgBr are discussed.     

The8Be scattering system in the framework of a microscopic theory

We present conclusions for the⁸Be scattering system from a multi-channel calculation in the framework of a microscopic nuclear cluster model. The energy region fromα-α threshold up to 30 MeV center of mass is investigated; results of an eigenphase analysis forJ ^π-values from 0⁺ to 4⁺ and 0^? to 4^? are displayed. For comparison and completion we have performed quasibound state calculations taking into account possible combinations of participating structures. The results throw light upon a complicated sequence of resonant states and moreover predict the occurrence of some additional levels in the energy spectrum.     

Single‐molecule surface‐enhanced Raman scattering of fullerene C60

We achieved single‐molecule surface‐enhanced Raman scattering (SM‐SERS) spectra from ultralow concentrations (10⁻¹⁵ M) of fullerene C₆₀ on uniformly assembled Au nanoparticles. It was found that resonant excitation at 785 nm is a powerful tool to probe SM‐SERS in this system. The appearance of additional bands and splitting of some vibrational modes were observed because of the symmetry reduction of the adsorbed molecule and a relaxation in the surface selection rules. Time‐evolved spectral fluctuation and ‘hot spot’ dependence in the SM‐SERS spectra were demonstrated to result from the single‐molecule Raman behavior of the spherical C₆₀ on Au nanoparticles. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparison of elastic and inelastic scattering of phonons by a three-level spin system

Expressions for the phonon relaxation time due to elastic and inelastic scattering by small concentrations of paramagnetic ions having a simple three-level Zeeman-type spectrum are obtained. Both resonant and nonresonant processes can occur and the frequency dependence of each is examined for a model applicable to Fe²⁺ ions in MgO. Comparison with a Green function theory of coupled spin-phonon modes is made.