Papers by Roberto Caciuffo
Magnetohydrodynamics
Hydrates are crystals consisting of small molecules enclathrated within an ice-like water cage. S... more Hydrates are crystals consisting of small molecules enclathrated within an ice-like water cage. Suppression of their growth is important in the oil industry. The presence of small quantities of specific polymers during hydrate crystallization can induce a transition from an octahedral to planar growth habit. This symmetry breaking is surprising because of the suppression of two 111 planes relative to the other six crystallographically equivalent faces. To better understand the surface effects leading to this behavior, we have studied the surface adsorption of these growth-modifing polymers onto the hydrate crytals using SANS. The total hydrate surface area, as measured by Porod scattering, increases in the presence of the growth modifier, but, no significant increase in polymer concentration on the crystal surfaces is found. Implications for possible growth mechanisms will be discussed.
Journal of Physics: Condensed Matter
In 1966 Roger Cowley (together with Gerald Dolling) reported the first neutron inelastic scatteri... more In 1966 Roger Cowley (together with Gerald Dolling) reported the first neutron inelastic scattering from the magnetic excitations from UO 2 below its antiferromagnetic ordering temperature of 30 K. They showed the strong magnon-phonon coupling in this material and that the excitations appeared to contain an additional mode that was not anticipated. Cowley never returned to UO 2 , but showed a keen interest in the developments. 40 years after this pioneering work, unambiguous evidence was found (using resonance X-ray techniques) for the ordering below T N of the electric quadrupoles involving the anisotropy of the 5f charge distribution around the uranium nuclei. A further 10 years later, now armed with a full theory for the excitation spectrum expected for phonons, magnons, and quadrupoles, we can identify the latter as the source of the "extra" mode reported first in 1966. The story is a long winding one, with the expected serendipity and dead ends, but is now (almost) completed.
Journal of Nuclear Materials
Physical Review B, 2014
We present the result of an extended experimental characterization of the hexagonal intermetallic... more We present the result of an extended experimental characterization of the hexagonal intermetallic Haucke compound NpNi5. By combining macroscopic and shell-specific techniques, we determine the 5f-shell occupation number n f close to 4 for the Np ions, together with orbital and spin components of the ordered moment in the ferromagnetic phase below TC = 16 K (µS =-1.88 µB and µL = 3.91 µB). The apparent coexistence of ordered and disordered phases observed in the Mössbauer spectra is explained in terms of slow relaxation between the components of a quasi-triplet ground state. The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment (3 Tz / Sz = +1.43) is positive and large, suggesting a localized character of the 5f electrons. The angular part of the spin-orbit coupling (ℓ • s =-5.55) is close to the value of-6.25 calculated for trivalent Np ions in intermediate coupling approximation. The results are discussed against the prediction of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction, and of a mean field model taking into account crystal field and exchange interactions.
Phys Rev B, 2011
... Rev. Lett. 103, 107202 (2009). 15P. Santini, S. Carretta, G. Amoretti, R. Caciuffo, N. Magnan... more ... Rev. Lett. 103, 107202 (2009). 15P. Santini, S. Carretta, G. Amoretti, R. Caciuffo, N. Magnani, and GH Lander, Rev. Mod. Phys. 81, 807 (2009). 16C. R. Wiebe, GM Luke, Z. Yamani, AA Menovsky, and WJ L. Buyers, Phys. Rev. B 69, 132418 (2004). 17C. R. Wiebe et al., Nat. ...
Journal of Solid State Chemistry, 2009
The crystal structure of β-BaZr(PO4)2, archetype of the high-temperature forms of BaM(PO4)2 phosp... more The crystal structure of β-BaZr(PO4)2, archetype of the high-temperature forms of BaM(PO4)2 phosphates (with M=Ti, Zr, Hf and Sn), has been solved ab initio by Rietveld analysis from synchrotron X-ray powder diffraction data. The phase transition appears as a topotactic modification of the monoclinic (S.G. C2/m) lamellar α-structure into a trigonal one (S.G. P3¯m1) through a simple mechanism involving the
Physica B, 1997
The dynamics of the cubic Laves phase ferromagnet UFe2 (TC=165 K) have recently been reported up ... more The dynamics of the cubic Laves phase ferromagnet UFe2 (TC=165 K) have recently been reported up to a maximum energy transfer of ~ 7 meV. The most surprising aspect is the strong Fe-Fe interactions as judged by the dispersion of the Fe only mode, and the absence of any observable mode involving the U 5f electrons. Using the IN8 triple-axis spectrometer at the ILL we have now extended these measurements to ~ 20 meV. The steep dispersion observed at low energy continues to ~ 10 meV. Between 12 and 17 meV the Fe mode is broad and difficult to observe, and it appears to interact with another mode, almost certainly nuclear (phonon) in origen. Above 17 meV we observe the mode again, although with a reduced dispersion.
Physica B Condensed Matter
Journal of Nuclear Materials
Journal of Physics Condensed Matter
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Papers by Roberto Caciuffo