490 research outputs found
Electron-phonon interaction dressed by electronic correlations near charge ordering as the origin for superconductivity in cobaltates
We consider possible routes to superconductivity in hydrated cobaltates
Na_xCoO_2.yH_2O on the basis of the t-J-V model plus phonons on the triangular
lattice. We studied the stability conditions for the homogeneous Fermi liquid
(HFL) phase against different broken symmetry phases. Besides the
sqrt(3)xsqrt(3)-CDW phase, triggered by the nearest-neighbour Coulomb
interaction V, we have found that the HFL is unstable, at very low doping,
against a bond-ordered phase due to J. We also discuss the occurrence of phase
separation at low doping and V. The interplay between the electron-phonon
interaction and correlations near the sqrt(3)xsqrt(3)-CDW leads to
superconductivity in the unconventional next-nearest neighbour f-wave (NNN-f)
channel with a dome shape for Tc around x ~ 0.35, and with values of a few
Kelvin as seen in experiments. Near the bond-ordered phase at low doping we
found tendencies to superconductivity with d-wave symmetry for finite J and
x<0.15. Contact with experiments is given along the paper.Comment: 21 pages, 6 figure
Atomic and itinerant effects at the transition metal x-ray absorption K-pre-edge exemplified in the case of VO
X-ray absorption spectroscopy is a well established tool for obtaining
information about orbital and spin degrees of freedom in transition metal- and
rare earth-compounds. For this purpose usually the dipole transitions of the L-
(2p to 3d) and M- (3d to 4f) edges are employed, whereas higher order
transitions such as quadrupolar 1s to 3d in the K-edge are rarely studied in
that respect. This is due to the fact that usually such quadrupolar transitions
are overshadowed by dipole allowed 1s to 4p transitions and, hence, are visible
only as minor features in the pre-edge region. Nonetheless, these features
carry a lot of valuable information, similar to the dipole L-edge transition,
which is not accessible in experiments under pressure due to the absorption of
the diamond anvil pressurecell. We recently performed a theoretical and
experimental analysis of such a situation for the metal insulator transition of
(V(1-x)Crx)2O3. Since the importance of the orbital degrees of freedom in this
transition is widely accepted, a thorough understanding of quadrupole
transitions of the vanadium K-pre-edge provides crucial information about the
underlying physics. Moreover, the lack of inversion symetry at the vanadium
site leads to onsite mixing of vanadium 3d- and 4p- states and related quantum
mechanical interferences between dipole and quadrupole transitions. Here we
present a theoretical analysis of experimental high resolution x-ray absorption
spectroscopy at the V pre-K edge measured in partial fluorescence yield mode
for single crystals. We carried out density functional as well as configuration
interaction calculations in order to capture effects coming from both,
itinerant and atomic limits
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
Hard x-ray spectroscopy in NaxCoO2 and superconducting NaxCoO2 - yH2O: A view on the bulk Co electronic properties
The electronic properties of Co in bulk Na0.7CoO2 and the superconducting
hydrated compound Na0.35CoO2 - y H2O have been investigated by x-ray absorption
spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) using hard
x-rays. The XAS spectra at the Co K-edge were measured in both compounds with
two different polarization directions. The changes in the XAS spectra upon
hydration and their polarization dependence are well accounted for by linear
muffin- tin orbital calculations in the local density approximation. The
underlying electronic structure indicates the strong hybridization between the
Co 3d and O 2p states in both compounds, while the electron localization is
enhanced in the hydrated compound due to the increase of the Co-Co interplanar
distance. The Co K pre-edge further highlights the splitting of the d band as a
result of the crystal field effect and demonstrates the Co valency increase
when Na0.7CoO2 is hydrated. The RIXS spectra measured at the Co K-edge show an
energy loss feature around 10 eV in both compounds in fair agreement with the
calculated dynamical structure factor. The RIXS feature is associated to a
damped plasmon excitation.Comment: 8 page
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