DAY 3

X-ray absorption spectroscopy is a premier element-specific experimental technique for materials characterization. Specifically, X-ray absorption near edge structure (XANES) encodes rich local structural and chemical information around absorber sites, making it a powerful tool for probing physical and chemical processes at large synchrotron facilities. However, the correlation between XANES spectral features and the underlying local structural motifs or electronic descriptors is obscure. Recent progress in materials discovery using smart automation and in situ / operando experiments underscores emerging challenges and opportunities of spectral analysis in real time. This challenge cannot be tackled by empirical fingerprints, which have practical limitations on data availability and diversity, or first-principles simulations due to the high computational cost. Here I discuss the recent progress of the data-driven spectral analysis pipeline by combining first-principles theory, data analytics tools and software development to decipher the structure-spectrum relationship. I highlight the utility of key modules of this pipeline, including 1) Benchmark modules that quantify the effects of key approximations and implementations in different computational methods or codes; 2) Workflow modules that aim to lower the barrier for non-expert practitioners by providing appropriate calculation input parameters based on systematic benchmarks; 3) Database modules that provide FAIR spectral datasets; 4) Machine learning modules that accelerate spectral simulation and extract physical descriptors from spectra.

In this talk, I will introduce various analytical approaches used in the literature to compute the dielectric function. These methods are based on the homogeneous electron gas/liquid. Our ultimate goal is to describe properly the plasmon peak in core level spectroscopies and model its variations as a function of the outgoing angle. For this, we need a good description of the dielectric function both in frequency AND momentum, and especially along the plasmon dispersion. I will show that most, if not all, the methods developed in the literature can be seen as particular cases of the memory function scheme, which will be explicated. In order to assess the validity of this approach for our goal, I will then compare our results based on the homogeneous electron gas with ab initio calculations (i.e. taking fully into account the band structure). This comparison will be made for a metal with a simple band structure, Al and for a semiconductor GaN.

I will present some case studies where the tight interlinking between theory and experiment has been crucial for the investigation of electronic excitations in materials ranging from simple metals to transition metal oxides [1-4].

What is calculated and what is measured are often different: I will highlight the importance of bridging the gap between theory and experiment to uncover new physics.

Finally, I will show how the idea that electronic correlation can be explained in terms of coupling of excitations promotes fruitful connections between different spectroscopies.

This work has been done in collaboration with several members and collaborators of the Palaiseau Theoretical spectroscopy group.

We will describe methodological developments to describe electronic excitations and noise in materials for interest in quantum technologies, in particular quantum sensing and communications. We will touch upon recent progress on spin-flip time-dependent DFT [1] and Bethe Salpeter (BSE) based methodologies [2] for optical excitations, quantum embedding approaches [3] and techniques to describe noise in closed and open quantum systems [4].

Dielectric response of materials is a classical phenomenon with quantum mechanical underpinnings. Atomic shell structure gives rise to x-ray absorption near edge spectra (XANES or XAS), which can be computed near edges using many approaches (e.g., self-consistent or time-dependent density functional theory or by the Bethe-Salpeter equation). Results along this line will be presented. Most simply, an absorption event can be considered as the joint product of a photoelectron and core hole that interact. Beyond this, however, there are signatures of other effects within XAS spectra and their by-products. These include features with resonant Auger and resonant inelastic x-ray scattering, as well as signatures because of Debye-Waller factors. We shall survey various aspects of XAS in solid C60, perovskites, simple metals and semiconductors, as well as resonant Auger and related spectra in MoS2 and noble metals.

There has been considerable recent interest in studies of electronic structure and spectra of materials in non-equilibrium and extreme conditions. These include, for example, the warm dense matter (WDM) regime with temperatures of order the Fermi temperature TF. Such studies are of particular interest at next-generation high intensity and pulsed XFEL light sources, where these conditions can be simulated. Complimentary theoretical simulations are essential to interpret the experimental results. Although several approaches have been introduced for calculations of WDM in equilibrium [1], they are usually not directly applicable to transient sources or to spectra. Thus, the development of theoretical methods for treating such exotic conditions remains challenging. In this presentation, we discuss several developments to this end. We first discuss a finite-temperature (FT) cumulant Green’s function approach which yields both thermodynamic properties and FT exchange-correlation potentials vxc(T,n), consistent with the PIMC fits [3]. Next, we address extensions of our real-space Green’s
function x-ray absorption spectroscopy (XAS) codes to finite temperature, up to the WDM regime [4]. Finally, we discuss recent theoretical simulations of non-equilibrium, transient, high intensity XAS from XFEL sources. This theory accounts for the transient, ultrafast rearrangement of electronic states and occupations with increasing beam intensity, that explain the observed cross-over from reverse saturable absorption (RSA) to saturable absorption (SA) [5], where the material becomes transparent.
*Supported by the Theory Institute for Materials and Energy Spectroscopies (TIMES) at SLAC, FWP 100291, funded by US DOE Contract DE-AC02-76SF0051, with computer support from the National Energy Research Scientific Computer (NERSC).

Intense and ultrashort pulses of X-ray Free Electron Lasers (XFELs) have revolutionized our ways to probe matter at atomic length and time scales. The intensities achievable at these light sources surpass those of storage-ring based x-ray sources (synchrotron x-ray sources) by up to 10-12 orders of magnitude, making it possible to study nonlinear x-ray matter interaction for the first time. Phenomena studied in the early phases of XFEL based research have been multiple sequential ionization of atomic and molecular gases, collective excitation and superfluoresecence of gas, liquid and solid samples and the creation and study of warm-dense matter. With recent capabilities to produce sub-fs pulses, stimulated electronic x-ray Raman scattering becomes experimentally tractable, opening the pathway to a plethora of nonlinear x-ray spectroscopies. Another intriguing class of nonlinear analysis techniques combining x-ray diffraction and spectroscopy are parametric wave-mixing processes. The underlying material properties addressable with these novel techniques are nonlinear, nonlocal electronic correlation functions, such as current-density current-density or density current-density correlators. In this presentation I will address the current experimental and theoretical status of nonlinear x-ray spectroscopy and will discuss the related theoretical challenges and future needs.