1,338 research outputs found
Ytterbium divalency and lattice disorder in near-zero thermal expansion YbGaGe
While near-zero thermal expansion (NZTE) in YbGaGe is sensitive to
stoichiometry and defect concentration, the NZTE mechanism remains elusive. We
present x-ray absorption spectra that show unequivocally that Yb is nearly
divalent in YbGaGe and the valence does not change with temperature or with
nominally 1% B or 5% C impurities, ruling out a valence-fluctuation mechanism.
Moreover, substantial changes occur in the local structure around Yb with B and
C inclusion. Together with inelastic neutron scattering measurements, these
data indicate a strong tendency for the lattice to disorder, providing a
possible explanation for NZTE in YbGaGe.Comment: 4 pages, 4 figure, supplementary inf
Understanding the complex phase diagram of uranium: the role of electron-phonon coupling
We report an experimental determination of the dispersion of the soft phonon
mode along [1,0,0] in uranium as a function of pressure. The energies of these
phonons increase rapidly, with conventional behavior found by 20 GPa, as
predicted by recent theory. New calculations demonstrate the strong pressure
(and momentum) dependence of the electron-phonon coupling, whereas the
Fermi-surface nesting is surprisingly independent of pressure. This allows a
full understanding of the complex phase diagram of uranium, and the interplay
between the charge-density wave and superconductivity
Elastic and thermodynamic properties of the shape-memory alloy AuZn
The current work reports on the elastic shear moduli, internal friction, and
the specific heat of the B2 cubic ordered alloy AuZn as a function of
temperature. Measurements were made on single-crystal and polycrystalline
samples using Resonant Ultrasound Spectroscopy (RUS), semi-adiabatic
calorimetry and stress-strain measurements. Our results confirm that this alloy
exhibits the shape-memory effect and a phase transition at 64.75 K that appears
to be continuous (second-order) from the specific heat data. It is argued that
the combination of equiatomic composition and a low transformation temperature
constrain the chemical potential and its derivatives to exhibit behavior that
lies at the borderline between that of a first-order (discontinuous) and a
continuous phase transition. The acoustic dissipation does not peak at the
transtion temperature as expected, but shows a maximum well into the
low-temperature phase. The Debye temeprature value of 219 K, obtained from the
low-temperature specific heat data is in favorable agreement with that
determined from the acoustic data (207 K) above the transition.Comment: 25 pages, 6 figures, submitted to Phys. Rev.
Experimental electronic heat capacities of and Plutonium; heavy-fermion physics in an element
We have measured the heat capacities of PuAl and
Pu over the temperature range 2-303 K. The availability of data below
10 K plus an estimate of the phonon contribution to the heat capacity based on
recent neutron-scattering experiments on the same sample enable us to make a
reliable deduction of the electronic contribution to the heat capacity of
PuAl; we find
mJKmol as . This is a factor larger than that
of any element, and large enough for PuAl to be
classed as a heavy-fermion system. By contrast,
mJKmol in Pu. Two distinct anomalies are seen in the
electronic contribution to the heat capacity of
PuAl, one or both of which may be associated with
the formation of the martensitic phase. We suggest that the large
-value of PuAl may be caused by proximity to
a quantum-critical point.Comment: 4 pages, 4 figure
In vivo staging of frontotemporal lobar degeneration TDP-43 type C pathology
Background:
TDP-43 type C is one of the pathological forms of frontotemporal lobar degeneration (FTLD) and mainly associated clinically with the semantic variant of primary progressive aphasia (svPPA). We aimed to define in vivo the sequential pattern of neuroanatomical involvement in a cohort of patients with FTLD-TDP type C pathology.
Methods:
We extracted the volumes of a set of cortical and subcortical regions from MRI scans of 19 patients with post mortem confirmed TDP-43 type C pathology (all with left hemisphere-predominant atrophy at baseline). In the initial development phase, we used w-scores computed from 81 cognitively normal controls to define a set of sequential stages of neuroanatomical involvement within the FTLD-TDP type C cohort where a w-score of < − 1.65 was considered abnormal. In a subsequent validation phase, we used 31 follow-up scans from 14 of the 19 patients in the same cohort to confirm the staging model.
Results:
Four sequential stages were identified in the initial development phase. Stage 1 was defined by atrophy in the left amygdala, medial temporal cortex, temporal pole, lateral temporal cortex and right medial temporal cortex; Stage 2 by atrophy in the left supratemporal cortex; Stage 3 by atrophy in the right anterior insula; and Stage 4 by atrophy in the right accumbens. In the validation phase, calculation of w-scores in the longitudinal scans confirmed the staging system, with all patients either staying in the same stage or progressing to a later stage at follow-up.
Conclusion:
In vivo imaging is able to detect distinct stages of neuroanatomical involvement in FTLD-TDP type C pathology. Using an imaging-derived staging system allows a more refined stratification of patients with svPPA during life
Coulomb-U and magnetic moment collapse in -Pu
The around-the-mean-field version of the LDA+U method is applied to
investigate electron correlation effects in -Pu. It yields a
non-magnetic ground state of Pu, and provides a good agreement with
experimental equilibrium volume, bulk modulus and explains important features
of the photoelectron spectra
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