ALBERT

Notes: A Raman heterodyne detection of magnetic resonance has been performed for probing the local structure of a sodium phosphate glass doped with trivalent europium. High resolution rf modulation spectra between 0.5 and 10 MHz were observed without the laser field in resonance with electronic transitions of the glass. The intensity of the observed Raman heterodyne signal depends on the external static magnetic field and sample temperature as well as the intensity of the laser and rf fields. The ability of monitoring the rf resonance spectra with micrometer spatial resolution may offer a potentially important means to probe the variation of local structure in disordered solid state materials. © 1996 American Institute of Physics.

Notes: We report the measured phonon density of states of a bulk GaN powder by time-of-flight neutron spectroscopy. The observed one-phonon excitation spectrum consists of two broad bands centered at about 23 and 39 meV corresponding to the acoustic and the first group of optical phonons; two sharp bands of upper optic modes at about 75 and 86 meV; and a gap of 45–65 meV. The phonon dispersion curves, lattice specific heat, and Debye temperature are calculated from fitting the data with a rigid-ion model. © 1998 American Institute of Physics.

Notes: The effects on the crystal lattice of a NdFe12−xMox (x≅1.7) during controlled nitrogenation over the 25–600 °C temperature range were studied by neutron powder diffraction. Prior to nitrogenation the sample contained a major phase of NdFe10.3Mo1.7 and a minor phase (∼12 vol %) of bcc-Fe. The sample inside the furnace was connected to a closed volume of ultrapure nitrogen gas while neutron data were collected over regular time intervals during sequential heating. Substantial nitrogen absorption occurred between 500 and 600 °C. During the nitrogenation process the NdFe12−xMoxNy lattice expanded while the bcc-Fe lattice contracted. An increasing decomposition of the compound into bcc-Fe at 600 °C was observed. The average size of the NdFe12−xMoxNy crystalline grains decreased starting at ∼300 °C, reaching a minimum at ∼500 °C and then increased markedly at higher temperatures. The development of lattice strains, on the other hand, showed an opposite trend, i.e., a maximum at 500 °C. © 1998 American Institute of Physics.

Notes: The complete sets of elastic constants of 4H and 6H silicon carbide single crystals were determined by Brillouin scattering. The elastic constants of 6H SiC are C11=501±4, C33=553±4, C44=163±4, C12=111±5, and C13=52±9 GPa; the corresponding ones of 4H SiC are the same within experimental uncertainties. The compressibility, 4.5×10−3 GPa, is about 3–5 times smaller than those reported for polycrystalline SiC materials. © 1997 American Institute of Physics.

Notes: Polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced by an externally applied magnetic field in a single crystal of CeCu6, a heavy-fermion compound that remains paramagnetic and nonsuperconducting down to ∼10 mK. The measurements were performed at 4.2 K and 92 mK in an applied magnetic field of 50 kOe. We find that the induced magnetization is predominately of 4f electronic character and exhibits Pauli-like behavior in the 4.2 K–92 mK temperature region.

Notes: Early experiments at the pulsed neutron source at Argonne National Laboratory (ANL) showed that it was possible to observe crystal-field (CF) levels at energy transfers above 150 meV in UO2. More recent investigations at the UK pulsed source at Rutherford Laboratory have improved the resolution and seen a series of well-resolved peaks. Based on these measurements the CF parameters are V4=−130 meV and V6=+30 meV. Since these parameters should be independent of the precise An ion, there is interest in looking at the tetravalent oxides UO2, NpO2, and PuO2. We have now performed experiments at ANL on NpO2 at temperatures between 15 and 200 K. We find no sharp peak in the excitation spectrum up to energy transfers of about 250 meV. This result is contrary to the expected strong transition at about 53 meV using the above CF parameters in a complete intermediate coupling calculation. The observed phonon spectrum of NpO2, in comparison with that of ThO2, shows larger widths in all of the three vibrational bands above ∼30 meV, which may indicate strong interaction between CF levels with optic phonons in NpO2.

Notes: We have performed inelastic neutron scattering study of the cerium γ↔α valence transition (T0(approximately-equal-to)150 K) in polycrystalline Ce0.74Th0.26 at the Intense Pulsed Neutron Source (IPNS) of Argonne National Laboratory. An incident neutron energy of 300 meV was used to measure the excitation energy spectra of Ce0.74Th0.26 at 100, 140, 155, and 200 K by chopper spectrometers. By comparing the neutron total scattering of Ce0.74Th0.26 with that of La0.74Th0.26, an isostructural nonmagnetic alloy measured under idential experimental conditions, the magnetic scattering function of Ce0.74Th0.26 averaged over all q in the Brillouin zone, Savemag (Q,E), is derived. We find that the magnetic response of Ce0.74Th0.26 at all temperatures is predominately due to moments of 4f character. The obtained magnetic scattering function in this temperature region consists of a broad quasielastic peak, which is well fitted by a spin relaxational spectral function, namely, a Lorentzian centered at 0 energy. The peak shifts to higher energies and broadens as the temperature is lowered. As the temperature decreases across the transition temperature, the magnetic intensity drops sharply, accompanied by an abrupt broadening of the linewidth corresponding to a spin fluctuation energy much higher than the thermal energy. Γ, the Lorentzian HWHM's, were found to be 16, 25, 63, and 110 meV at T=200, 155, 140, and 100 K, respectively. Within experimental precision, we find no evidence of additional inelastic peaks due to crystal-field excitations. These results are in good agreement with those from an earlier neutron experiment1 using thermal-energy neutrons in which the measured spectra were limited to about 70 meV. The static single-site susceptibility obtained by a Kramers–Kronig analysis agrees well with the bulk susceptibility.1 The 4f occupation per Ce atom, deduced by summing theneutron data from −100 to 230 meV, is about 0.6 and 0.4 for the γ and α phase, respectively. These values are considerably smaller than those obtained from photoemission2 and other measurements.1 This indicates that the neutron experiment did not extend to high enough energies to account for all the intensity. In the high-temperature γ phase the relaxational model represents a reasonable approximation to the spin dynamics. By integrating the Lorentzian scattering function obtained from the fits of the neutron data over an energy interval of −0.1 to 2 eV, we obtained an f occupancy close to unity in the γ phase. For the α phase there is currently no analytical expression of the magnetic scattering function from first-principle calculations. Model calculations3,4 of the ground state (T=0) for a single f impurity in the metal, on the other hand, predicts a magnetic response function having a thresholdlike rise at a finite energy, followed by a long tail extending to high energies. In order to be able to compare with the theory, we have also undertaken the measurements of the magnetic scattering function of Ce0.74Th0.26 at 10 K with an incident neutron energy of 1.2 eV. We find that Savemag shows an inelastic peak at about 138 meV and a tail at higher energies. The line shape of the measured spectrum agrees qualitatively with the single-impurity theory3,5,6 and the result of a recent polarized neutron study7 of α-Ce. By summing the measured magnetic intenstiy up to 500 meV, we estimated a 4f occupancy of 0.76 per Ce atom. The static susceptibility at 10 K and the electronic specific heat coefficient obtained from the neutron data and the theory3,8 agree well with the values obtained from bulk measurements.1,9,10 The above results are also in fair agreement with the electronic spectroscopy data.2 Since the neutron measurements were made using polycrystalline samples, we are unable to detect, if any, coherence effects6,11 due to interaction of the f electrons in the lattice. A detailed report on the inelastic neutron scattering investigations is presented elsewhere.12

Notes: We give a comparison for inelastic neutron scattering experiments performed at the high flux reactor in Grenoble and at the spallation source in Argonne. These experiments were performed at the IN4 (I. L. L. Grenoble) and at the HRMECS (IPNS Argonne) with nearly identical conditions using E0(approximately-equal-to)50 meV as energy of incident neutrons. We conclude that the neutron counting rate for a given scattering angle is less by a factor of about 30 at HRMECS than at IN4. This is due to a rather small neutron flux at HRMECS. Thus the background to signal ratio is worse by a factor of 2 at HRMECS compared to IN4.

Notes: In this paper we present and discuss experimental results on molecular mobility in propylene glycol and its three oligomers confined to the ∼100 A(ring) pores of a controlled porous glass. The objective is to elucidate the finite size effects on the dynamics of hydrogen-bonded liquids of different molecular weights but identical chemical composition. The methods of dielectric and neutron spectroscopy have been employed to investigate both the low- and high-frequency features as a function of temperature. We find that all fluids in pores separate into two distinct liquid phases. (i) molecules physisorbed at the surface which exhibit a dramatic frustration of their mobility related to a substantial positive shift of the glass transition temperature Tg by up to ΔTg≈+47 K; and (ii) relatively "free'' molecules in the inner pore space subject to only moderate retardation of the α and normal mode relaxation and substantial broadening of the distribution of relaxation times. The shift in Tg for the α process with ΔTg≈+5 K is maximal for the monomer liquid and gradually diminishes with increasing molecular weight or decreasing intermolecular hydrogen bonding. The inelastic neutron spectrum of confined propylene glycol shows the boson peak as expected in bulk strong and intermediate glass formers in the vicinity of Tg. This effect can be attributed to the finite-size induced crossover from long wave vibrations characteristic of a continuous medium to localized vibrations in a confined geometry. © 1995 American Institute of Physics.

Notes: With the advance of pulsed spallation neutron sources, neutron scattering investigation of elementary excitations in magnetic materials can now be extended to energies up to several hundreds of meV. We have measured, using chopper spectrometers and time-of-flight techniques, the magnetic response functions of a series of d and f transition metals and compounds over a wide range of energy and momentum transfer. In PrO2, UO2, BaPrO3, and CeB6 we observed crystal-field transitions between the magnetic ground state and the excited levels in the energy range from 40 to 260 meV. In materials exhibiting spin-fluctuation or mixed-valent character such as Ce0.74Th0.26, on the other hand, no sharp crystal-field lines but a broadened quasielastic magnetic peak was observed. The line width of the quasielastic component is thought to be connected to the spin-fluctuation energy of the 4f electrons. The significance of the neutron scattering results in relation to the ground state level structure of the magnetic ions and the spin dynamics of the f electrons is discussed. Recently, in a study of the spin-wave excitations in itinerant magnetic systems, we have extended the spin-wave measurements in ferromagnetic iron up to about 160 meV. Neutron scattering data at high energy transfers are of particular interest because they provide direct comparison with recent theories of itinerant magnetism.