147 research outputs found
Interrelation between the pseudogap and the incoherent quasi-particle features of high-Tc superconductors
Using a scenario of a hybridized mixture of localized bipolarons and
conduction electrons, we demonstrate for the latter the simultaneous appearance
of a pseudogap and of strong incoherent contributions to their quasi-particle
spectrum which arise from phonon shake-off effects. This can be traced back to
temporarily fluctuating local lattice deformations, giving rise to a
double-peak structure in the pair distribution function, which should be a key
feature in testing the origin of these incoherent contributions, recently seen
in angle-resolved photoemission spectroscopy (ARPES).Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let
Multiconfigurational nature of 5f orbitals in uranium and plutonium intermetallics
Uranium and plutonium's 5f electrons are tenuously poised between strongly
bonding with ligand spd-states and residing close to the nucleus. The unusual
properties of these elements and their compounds (eg. the six different
allotropes of elemental plutonium) are widely believed to depend on the related
attributes of f-orbital occupancy and delocalization, for which a quantitative
measure is lacking. By employing resonant x-ray emission spectroscopy (RXES)
and x-ray absorption near-edge structure (XANES) spectroscopy and making
comparisons to specific heat measurements, we demonstrate the presence of
multiconfigurational f-orbital states in the actinide elements U and Pu, and in
a wide range of uranium and plutonium intermetallic compounds. These results
provide a robust experimental basis for a new framework for understanding the
strongly-correlated behavior of actinide materials.Comment: 30 pages, concatenated article and supporting information, 10 figure
Competing Ground States of the New Class of Halogen-Bridged Metal Complexes
Based on a symmetry argument, we study the ground-state properties of
halogen-bridged binuclear metal chain complexes. We systematically derive
commensurate density-wave solutions from a relevant two-band Peierls-Hubbard
model and numerically draw the the ground-state phase diagram as a function of
electron-electron correlations, electron-phonon interactions, and doping
concentration within the Hartree-Fock approximation. The competition between
two types of charge-density-wave states, which has recently been reported
experimentally, is indeed demonstrated.Comment: 4 pages, 5 figures embedded, to appear in J. Phys. Soc. Jp
Possible charge inhomogeneities in the CuO2 planes of YBa2Cu3O6+x (x=0.25, 0.45, 0.65, 0.94) from pulsed neutron diffraction
The atomic pair distribution functions (PDF) of four powder samples of
YBa2Cu3O6+x (x=0.25, 0.45, 0.65, 0.94) at 15 K have been measured by means of
pulsed neutron diffraction. The PDF is modelled using a full-profile fitting
approach to yield structural parameters. In contrast to earlier XAFS work we
find no evidence of a split apical oxygen site. However, a slightly improved
fit over the average crystallographic model results when the planar Cu(2) site
is split along the z-direction. This is interpreted in terms of charge
inhomogeneities in the CuO2 planes.Comment: 8 pages, 3 figure
Probing Electronic Correlations in Actinide Materials Using Multipolar Transitions
We report nonresonant inelastic x-ray scattering from the semi-core 5d levels
of several actinide compounds. Dipole-forbidden, high-multipole features form a
rich bound-state spectrum dependent on valence electron configuration and
spin-orbit and Coulomb interactions. Cross-material comparisons, together with
the anomalously high Coulomb screening required for agreement between atomic
multiplet theory and experiment, demonstrate sensitivity to the neighboring
electronic environment, such as is needed to address long-standing questions of
electronic localization and bonding in 5f compounds.Comment: LA-UR 09-0782
Studies of Vibrational Properties in Ga Stabilized d-Pu by Extended X-ray Absorption Fine Structure
Temperature dependent extended x-ray absorption fine structure (EXAFS)
spectra were measured for a 3.3 at% Ga stabilized Pu alloy over the range T= 20
- 300 K at both the Ga K-edge and the Pu L_III-edge. The temperature dependence
of the pair-distance distribution widths, \sigma(T) was accurately modeled
using a correlated-Debye model for the lattice vibrational properties,
suggesting Debye-like behavior in this material. We obtain pair- specific
correlated-Debye temperatures, \Theta_cD, of 110.7 +/- 1.7 K and 202.6 +/- 3.7
K, for the Pu-Pu and Ga-Pu pairs, respectively. These results represent the
first unambiguous determination of Ga-specific vibrational properties in PuGa
alloys, and indicate the Ga-Pu bonds are significantly stronger than the Pu-Pu
bonds. This effect has important implications for lattice stabilization
mechanisms in these alloys.Comment: 7 pages, 4 figures, Phys. Rev. B in pres
Soliton excitations in halogen-bridged mixed-valence binuclear metal complexes
Motivated by recent stimulative observations in halogen (X)-bridged binuclear
transition-metal (M) complexes, which are referred to as MMX chains, we study
solitons in a one-dimensional three-quarter-filled charge-density-wave system
with both intrasite and intersite electron-lattice couplings. Two distinct
ground states of MMX chains are reproduced and the soliton excitations on them
are compared. In the weak-coupling region, all the solitons are degenerate to
each other and are uniquely scaled by the band gap, whereas in the
strong-coupling region, they behave differently deviating from the scenario in
the continuum limit. The soliton masses are calculated and compared with those
for conventional mononuclear MX chains.Comment: 9 pages, 10 figures embedded, to be published in J. Phys. Soc. Jpn.
71, No. 1 (2002
Correlated local distortions of the TlO layers in TlBaCuO: An x-ray absorption study
We have used the XAFS (x-ray-absorption fine structure) technique to
investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic
Tl-2201 with a superconducting transition temperature T=60 K. Our results
clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as
given by the diffraction measurements, while the Tl and O(3) atoms are more
disordered than suggested by the average crystal structure. The Tl-Tl distance
at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at
3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks
are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33
\AA, significantly shorter than the distance calculated with both the Tl and
O(3) atoms at their ideal sites ( 0 or ). A model based
on these results shows that the Tl atom is displaced along the
directions from its ideal site by about 0.11 \AA; the displacements of
neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$
site by about 0.53 \AA{ } roughly along the directions. A comparison of
the Tl L-edge XAFS spectra from three samples, with T=60 K, 76 K,
and 89 K, shows that the O environment around the Tl atom is sensitive to T
while the Tl local displacement is insensitive to T and the structural
symmetry. These conclusions are compared with other experimental results and
the implications for charge transfer and superconductivity are discussed. This
paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.
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