60 research outputs found
Crystallographic refinement of collective excitations using standing wave inelastic x-ray scattering
We propose a method for realizing true, real-space imaging of charge dynamics
in a periodic system, with angstrom spatial resolution and attosecond time
resolution. In this method, inelastic x-ray scattering (IXS) is carried out
with a coherent, standing wave source, which provides the off-diagonal elements
of the generalized dynamic structure factor, S(q_1,q_2,\omega), allowing
complete reconstruction of the inhomogeneous response function of the system,
\chi(x_1,x_2,t). The quantity \chi has the physical meaning of a propagator for
charge, so allows one to observe - in real time - the disturbance in the
electron density created by a point source placed at a specified location, x_1
(on an atom vs. between atoms, for example). This method may be thought of as a
generalization of x-ray crystallography that allows refinement of the excited
states of a periodic system, rather than just its ground state.Comment: 28 pages, 8 figures, submitted to Chemical Physic
Temperature-resolution anomalies in the reconstruction of time dynamics from energy-loss experiments
Inelastic scattering techniques provide a powerful approach to studying
electron and nuclear dynamics, via reconstruction of a propagator that
quantifies the time evolution of a system. There is now growing interest in
applying such methods to very low energy excitations, such as lattice
vibrations, but in this limit the cross section is no longer proportional to a
propagator. Significant deviations occur due to the finite temperature Bose
statistics of the excitations. Here we consider this issue in the context of
high-resolution electron energy loss experiments on the copper-oxide
superconductor BiSrCaCuO. We find that simple division of a
Bose factor yields an accurate propagator on energy scales greater than the
resolution width. However, at low energy scales, the effects of resolution and
finite temperature conspire to create anomalies in the dynamics at long times.
We compare two practical ways for dealing with such anomalies, and discuss the
range of validity of the technique in light of this comparison.Comment: 19 pages, 2 figures, submitted to Journal of Physics
Microscopic theory of resonant soft x-ray scattering in systems with charge order
We present a microscopic theory of resonant soft x-ray scattering (RSXS) that
accounts for the delocalized character of valence electrons. Unlike past
approaches defined in terms of form factors for atoms or clusters, we develop a
functional determinant method that allows us to treat realistic band
structures. This method builds upon earlier theoretical work in mesoscopic
physics and accounts for both excitonic effects as well as the orthogonality
catastrophe arising from interaction between the core hole and the valence band
electrons. Comparing to RSXS measurements from stripe-ordered LBCO, we show
that the two-peak structure observed near the O K edge can be understood as
arising from dynamic nesting within the canonical cuprate band structure. Our
results provide evidence for reasonably well-defined, high-energy
quasiparticlesComment: 7 pages, 2 figure
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LEEM investigations of clean surfaces driven by energetic ion beams
The original purpose of this award was to use low‐energy electron microscopy (LEEM) to explore the dynamics of surfaces of clean single crystal surfaces when driven by a beam of energetic ions. The goal was to understand the nanoscience of hyperthermal growth, surface erosion by sublimation and irradiation, operation of surface sinks in irradiated materials, diffusion on driven surfaces, and the creation of structural patterns. This project was based on a novel LEEM system constructed by C. P. Flynn, which provided real‐time imaging of surface dynamics by scattering low energy electrons. With the passing of Prof. Flynn in late 2011, this project was completed under a slightly different scope by constructing a low‐energy, inelastic electron scattering (�EELS�) instrument. Consistent with Flynn�s original objectives for his LEEM system, this device probes the dynamics of crystal surfaces. However the measurements are not carried out in real time, but instead are done in the frequency domain, through the energy lost from the probe electrons. The purpose of this device is to study the collective bosonic excitations in a variety of materials, including high temperature superconductors, topological insulators, carbon allotropes including (but not limited to) graphene, etc. The ultimate goal here is to identify the bosons that mediate interactions in these and other materials, with hopes of shedding light on the origin of many exotic phenomena including high temperature superconductivity. We completed the construction of a low‐energy EELS system that operates with an electron kinetic energy of 7 - 10 eV. With this instrument now running, we hope to identify, among other things, the bosons that mediate pairing in high temperature superconductors. Using this instrument, we have already made our first discovery. Studying freshly cleaved single crystals of Bi{sub 2}Se{sub 3}, which is a topological insulator, we have observed a surface excitation at an energy loss of ~ 90 meV. This excitation disperses quadratically, exhibits a critical momentum of q{sub c} = 0.11 �{sup ‐1}, and may be identified as the surface collective mode of the helical Dirac liquid. To make a stronger connection between the behavior of this excitation and the known surface physics of Bi{sub 2}Se{sub 3}, we are carrying out a doping‐dependent study, as a function of Se vacancy content, of this excitation. From this study we will be able to quantify the strength of interactions in the spin‐polarized surface states in a manner analogous to our past work on graphene
Dynamics of confined water reconstructed from inelastic x-ray scattering measurements of bulk response functions
Nanoconfined water and surface-structured water impacts a broad range of fields. For water confined between hydrophilic surfaces, measurements and simulations have shown conflicting results ranging from “liquidlike” to “solidlike” behavior, from bulklike water viscosity to viscosity orders of magnitude higher. Here, we investigate how a homogeneous fluid behaves under nanoconfinement using its bulk response function: The Green's function of water extracted from a library of S(q,ω) inelastic x-ray scattering data is used to make femtosecond movies of nanoconfined water. Between two confining surfaces, the structure undergoes drastic changes as a function of surface separation. For surface separations of ≈9 Å, although the surface-associated hydration layers are highly deformed, they are separated by a layer of bulklike water. For separations of ≈6 Å, the two surface-associated hydration layers are forced to reconstruct into a single layer that modulates between localized “frozen’ and delocalized “melted” structures due to interference of density fields. These results potentially reconcile recent conflicting experiments. Importantly, we find a different delocalized wetting regime for nanoconfined water between surfaces with high spatial frequency charge densities, where water is organized into delocalized hydration layers instead of localized hydration shells, and are strongly resistant to `freezing' down to molecular distances (<6 Å)
A reexamination of the effective fine structure constant of graphene, as measured in graphite
We present a refined and improved study of the influence of screening on the
effective fine structure constant of graphene, , as measured in
graphite using inelastic x-ray scattering. This follow-up to our previous study
[J. P. Reed, et al., Science 330, 805 (2010)] was carried out with two times
better energy resolution, five times better momentum resolution, and improved
experimental setup with lower background. We compare our results to RPA
calculations and evaluate the relative importance of interlayer hopping,
excitonic corrections, and screening from high energy excitations involving the
bands. We find that the static, limiting value of falls in
the range 0.25 to 0.35, which is higher than our previous result of 0.14, but
still below the value expected from RPA. We show the reduced value is not a
consequence of interlayer hopping effects, which were ignored in our previous
analysis, but of a combination of excitonic effects in the particle-hole continuum, and background screening from the
-bonded electrons. We find that -band screening is extremely
strong at distances of the order of a few nm, and should be highly effective at
screening out short-distance, Hubbard-like interactions in graphene, as well as
other carbon allotropes.Comment: 23 pages, 5 figure
First-principles method of propagation of tightly bound excitons: exciton band structure of LiF and verification with inelastic x-ray scattering
We propose a simple first-principles method to describe propagation of
tightly bound excitons. By viewing the exciton as a composite object (an
effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic
kernel to encapsulate the exciton propagation and decay for all binding energy.
Applied to prototypical LiF, our approach produces three exciton bands, which
we verified quantitatively via inelastic x-ray scattering. The proposed
real-space picture is computationally inexpensive and thus enables study of the
full exciton dynamics, even in the presence of surfaces and impurity
scattering. It also provides intuitive understanding to facilitate practical
exciton engineering in semiconductors, strongly correlated oxides, and their
nanostructures.Comment: 5 pages, 4 figures. Accepted by PR
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