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 Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. 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

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

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

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, $\alpha^*$, 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 $\sigma$ bands. We find that the static, limiting value of $\alpha^*$ 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 $\pi \rightarrow \pi^*$ particle-hole continuum, and background screening from the $\sigma$-bonded electrons. We find that $\sigma$-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

Measurement of the dynamic charge response of materials using low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS)

One of the most fundamental properties of an interacting electron system is its frequency- and wave-vector-dependent density response function, $\chi({\bf q},\omega)$. The imaginary part, $\chi''({\bf q},\omega)$, defines the fundamental bosonic charge excitations of the system, exhibiting peaks wherever collective modes are present. $\chi$ quantifies the electronic compressibility of a material, its response to external fields, its ability to screen charge, and its tendency to form charge density waves. Unfortunately, there has never been a fully momentum-resolved means to measure $\chi({\bf q},\omega)$ at the meV energy scale relevant to modern elecronic materials. Here, we demonstrate a way to measure $\chi$ with quantitative momentum resolution by applying alignment techniques from x-ray and neutron scattering to surface high-resolution electron energy-loss spectroscopy (HR-EELS). This approach, which we refer to here as "M-EELS," allows direct measurement of $\chi''({\bf q},\omega)$ with meV resolution while controlling the momentum with an accuracy better than a percent of a typical Brillouin zone. We apply this technique to finite-q excitations in the optimally-doped high temperature superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi2212), which exhibits several phonons potentially relevant to dispersion anomalies observed in ARPES and STM experiments. Our study defines a path to studying the long-sought collective charge modes in quantum materials at the meV scale and with full momentum control.Comment: 26 pages, 10 sections, 7 figures, and an appendi

Electronic reconstruction at SrMnO3-LaMnO3 superlattice interfaces

We use resonant soft x-ray scattering to study electronic reconstruction at the interface between the Mott insulator LaMnO3 and the "band" insulator SrMnO3. Superlattices of these two insulators were shown previously to have both ferromagnetism and metallic tendencies [Koida et al., Phys. Rev. B 66, 144418 (2002)]. By studying a judiciously chosen superlattice reflection we show that the interface density of states exhibits a pronounced peak at the Fermi level, similar to that predicted by Okamoto et al. [Phys. Rev. B 70, 241104(R) (2004)]. The intensity of this peak correlates with the conductivity and magnetization, suggesting it is the driver of metallic behavior. Our study demonstrates a general strategy for using RSXS to probe the electronic properties of heterostructure interfaces.Comment: 4.2 pages, 4 figure