46 research outputs found
Orbital Order and Spontaneous Orthorhombicity in Iron Pnictides
A growing list of experiments show orthorhombic electronic anisotropy in the
iron pnictides, in some cases at temperatures well above the spin density wave
transition. These experiments include neutron scattering, resistivity and
magnetoresistance measurements, and a variety of spectroscopies. We explore the
idea that these anisotropies stem from a common underlying cause: orbital order
manifest in an unequal occupation of and orbitals, arising
from the coupled spin-orbital degrees of freedom. We emphasize the distinction
between the total orbital occupation (the integrated density of states), where
the order parameter may be small, and the orbital polarization near the Fermi
level which can be more pronounced. We also discuss light-polarization studies
of angle-resolved photoemission, and demonstrate how x-ray absorption linear
dichroism may be used as a method to detect an orbital order parameter.Comment: Orig.: 4+ pages; Rev.: 4+ pages with updated content and reference
A Comparative Study of the Valence Electronic Excitations of N_2 by Inelastic X-ray and Electron Scattering
Bound state, valence electronic excitation spectra of N_2 are probed by
nonresonant inelastic x-ray and electron scattering. Within the usual
theoretical treatments, dynamical structure factors derived from the two probes
should be identical. However, we find strong disagreements outside the dipole
scattering limit, even at high probe energies. This suggests an unexpectedly
important contribution from intra-molecular multiple scattering of the probe
electron from core electrons or the nucleus. These effects should grow
progressively stronger as the atomic number of the target species increases.Comment: Submitted to Physical Review Letters April 27, 2010. 12 pages
including 2 figure pages
Uncovering selective excitations using the resonant profile of indirect inelastic x-ray scattering in correlated materials: Observing two-magnon scattering and relation to the dynamical structure factor
Resonant inelastic x-ray scattering (RIXS) is a spectroscopic technique which
has been widely used to study various elementary excitations in correlated and
other condensed matter systems. For strongly correlated materials, besides
boosting the overall signal the dependence of the resonant profile on incident
photon energy is still not fully understood. Previous endeavors in connecting
indirect RIXS, such as Cu K-edge for example where scattering takes place only
via the core-hole created as an intermediate state, with the charge dynamical
structure factor S(q,\omega) neglected complicated dependence on the
intermediate state configuration. To resolve this issue, we performed an exact
diagonalization study of the RIXS cross-section using the single-band Hubbard
model by fully addressing the intermediate state contribution. Our results are
relevant to indirect RIXS in correlated materials, such as high Tc cuprates. We
demonstrate that RIXS spectra can be reduced to S(q,\omega) when there is no
screening channel for the core-hole potential in the intermediate state. We
also show that two-magnon excitations are highlighted at the resonant photon
energy when the core-hole potential in the corresponding intermediate state is
poorly screened. Our results demonstrate that different elementary excitations
can be emphasized at different intermediate states, such that selecting the
exact incident energy is critical when trying to capture a particular
elementary excitation.Comment: 11 pages, 3 figure
Resonant Enhancement of Charge Density Wave Diffraction in the Rare-Earth Tritellurides
We performed resonant soft X-ray diffraction on known charge density wave
(CDW) compounds, rare earth tri-tellurides. Near the (3d - 4f) absorption
edge of rare earth ions, an intense diffraction peak is detected at a
wavevector identical to that of CDW state hosted on Te planes, indicating a
CDW-induced modulation on the rare earth ions. Surprisingly, the temperature
dependence of the diffraction peak intensity demonstrates an exponential
increase at low temperatures, vastly different than that of the CDW order
parameter. Assuming 4f multiplet splitting due to the CDW states,we present a
model to calculate X-ray absorption spectrum and resonant profile of the
diffraction peak, agreeing well with experimental observations. Our results
demonstrate a situation where the temperature dependence of resonant X-ray
diffraction peak intensity is not directly related to the intrinsic behavior of
the order parameter associated with the electronic order, but is dominated by
the thermal occupancy of the valence states.Comment: 7 pages, 5 figure
High pressure evolution of FeO electronic structure revealed by X-ray absorption
We report the first high pressure measurement of the Fe K-edge in hematite
(FeO) by X-ray absorption spectroscopy in partial fluorescence yield
geometry. The pressure-induced evolution of the electronic structure as
FeO transforms from a high-spin insulator to a low-spin metal is
reflected in the x-ray absorption pre-edge. The crystal field splitting energy
was found to increase monotonically with pressure up to 48 GPa, above which a
series of phase transitions occur. Atomic multiplet, cluster diagonalization,
and density-functional calculations were performed to simulate the pre-edge
absorption spectra, showing good qualitative agreement with the measurements.
The mechanism for the pressure-induced phase transitions of FeO is
discussed and it is shown that ligand hybridization significantly reduces the
critical high-spin/low-spin pressure.Comment: 5 pages, 4 figures and 1 tabl