1,678 research outputs found
Investigation of the magnetic structure and crystal field states of pyrochlore antiferromagnet Nd2Zr2O7
We present synchrotron x-ray diffraction, neutron powder diffraction and
time-of-flight inelastic neutron scattering measurements on the rare earth
pyrochlore oxide Nd2Zr2O7 to study the ordered state magnetic structure and
cystal field states. The structural characterization by high-resolution
synchrotron x-ray diffraction confirms that the pyrochlore structure has no
detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals
long range all-in/all-out antiferromagnetic order below T_N ~ 0.4 K with
propagation vector k = (0 0 0) and an ordered moment of 1.26(2) \mu_B/Nd at 0.1
K. The ordered moment is much smaller than the estimated moment of 2.65
\mu_B/Nd for the local Ising ground state of Nd3+ (J=9/2) suggesting that
the ordering is partially suppressed by quantum fluctuations. The strong Ising
anisotropy is further confirmed by the inelastic neutron scattering data which
reveals a well-isolated dipolar-octupolar type Kramers doublet ground state.
The crystal field level scheme and ground state wavefunction have been
determined.Comment: 12 pages, 15 figures, 2 table
Electron-phonon coupling in the conventional superconductor YNiBC at high phonon energies studied by time-of-flight neutron spectroscopy
We report an inelastic neutron scattering investigation of phonons with
energies up to 159 meV in the conventional superconductor YNiBC. Using
the SWEEP mode, a newly developed time-of-flight technique involving the
continuous rotation of a single crystal specimen, allowed us to measure a four
dimensional volume in (Q,E) space and, thus, determine the dispersion surface
and linewidths of the (~ 102 meV) and (~ 159 meV) type phonon
modes for the whole Brillouin zone. Despite of having linewidths of , modes do not strongly contribute to the total electron-phonon
coupling constant . However, experimental linewidths show a remarkable
agreement with ab-initio calculations over the complete phonon energy range
demonstrating the accuracy of such calculations in a rare comparison to a
comprehensive experimental data set.Comment: accepted for publication in PR
Order within disorder: the atomic structure of ion-beam sputtered amorphous tantala (a-Ta2O5)
Amorphous tantala (a-Ta2O5) is a technologically important material often used in high-performance coatings. Understanding this material at the atomic level provides a way to further improve performance. This work details extended X-ray absorption fine structure measurements of a-Ta2O5 coatings, where high-quality experimental data and theoretical fits have allowed a detailed interpretation of the nearest-neighbor distributions. It was found that the tantalum atom is surrounded by four shells of atoms in sequence; oxygen, tantalum, oxygen, and tantalum. A discussion is also included on how these models can be interpreted within the context of published crystalline Ta 2O5 and other a-T2O5 studies
Methane and Nitrogen Abundances On Pluto and Eris
We present spectra of Eris from the MMT 6.5 meter telescope and Red Channel
Spectrograph (5700-9800 angstroms; 5 angstroms per pix) on Mt. Hopkins, AZ, and
of Pluto from the Steward Observatory 2.3 meter telescope and Boller and
Chivens spectrograph (7100-9400 angstroms; 2 angstroms per pix) on Kitt Peak,
AZ. In addition, we present laboratory transmission spectra of methane-nitrogen
and methane-argon ice mixtures. By anchoring our analysis in methane and
nitrogen solubilities in one another as expressed in the phase diagram of
Prokhvatilov and Yantsevich (1983), and comparing methane bands in our Eris and
Pluto spectra and methane bands in our laboratory spectra of methane and
nitrogen ice mixtures, we find Eris' bulk methane and nitrogen abundances are
about 10% and about 90%, and Pluto's bulk methane and nitrogen abundances are
about 3% and about 97%. Such abundances for Pluto are consistent with values
reported in the literature. It appears that the bulk volatile composition of
Eris is similar to the bulk volatile composition of Pluto. Both objects appear
to be dominated by nitrogen ice. Our analysis also suggests, unlike previous
work reported in the literature, that the methane and nitrogen stoichiometry is
constant with depth into the surface of Eris. Finally, we point out that our
Eris spectrum is also consistent with a laboratory ice mixture consisting of
40% methane and 60% argon. Although we cannot rule out an argon rich surface,
it seems more likely that nitrogen is the dominant species on Eris because the
nitrogen ice 2.15 micron band is seen in spectra of Pluto and Triton.Comment: The manuscript has 44 pages, 15 figures, and four tables. It will
appear in the Astrophysical Journa
Spin Excitations in BaFe1.84Co0.16As2 Superconductor Observed by Inelastic Neutron Scattering
Superconductivity appears to compete against the spin-density-wave in Fe
pnictides. However, optimally cobalt doped samples show a quasi-two-dimensional
spin excitation centered at the (0.5, 0.5, L) wavevector, "the spin resonance
peak", that is strongly tied to the onset of superconductivity. By inelastic
neutron scattering on single crystals we show the similarities and differences
of the spin excitations in BaFe1.84Co0.16As2, with respect to the spin
excitations in the high-temperature superconducting cuprates. As in the
cuprates the resonance occurs as an enhancement to a part of the spin
excitation spectrum which extends to higher energy transfer and higher
temperature. However, unlike in the cuprates, the resonance peak in this
compound is asymmetric in energy.Comment: 12 pages, 6 figures; PACS # 74.70.-b, 74.20.Mn, 78.70.Nx, 74.25.Ha;
corrected discussion of figures in tex
- …