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Liang, Yufeng

Publications and source records attributed to Liang, Yufeng.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Controlled Experiments and Optimized Theory of Absorption Spectra of Li Metal and Salts

Investigation of Li metal and ionic compounds through experimental and theoretical spectroscopy has been of tremendous interest due to their prospective applications in Li-metal and Li-ion batteries. Li K-edge soft X-ray absorption spectroscopy (sXAS) provides the most direct spectroscopic characterization; unfortunately, due to the low core-level energy and the highly reactive surface, Li-K sXAS of Li metal has been extremely challenging, as evidenced by many controversial reports. Here, through controlled and ultra-high energy resolution experiments of two kinds of in situ prepared samples, we report the intrinsic Li-K sXAS of Li-metal that displays a prominent leading peak that has not been revealed before. Furthermore, theoretical simulations show that, due to the low number of valence electrons in Li, the Li-K sXAS is strongly affected by the response of the valence electrons to the core hole. We successfully reproduce the Li-K sXAS by state-of-the-art calculations with considerations of a number of relevant parameters such as temperature, energy resolution, and, especially, contributions from transitions which are forbidden in the single-particle treatment. Such a comparative experimental and theoretical investigation is further extended to a series of Li ionic compounds, which highlight the importance of considering the total and single-particle energies for obtaining an accurate alignment of the spectra. Furthermore, our work provides the first reliable Li-K sXAS of the Li metal surface with advanced theoretical calculations. The experimental and theoretical results provide a critical benchmark for studying Li chemistry in both metallic and ionic states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Taming convergence in the determinant approach for x-ray excitation spectra

A determinant formalism in combination with ab initio calculations was proposed recently and has paved a way for simulating and interpreting x-ray excitation spectra in condensed-phase systems. The method systematically takes into account many-electron effects in the Mahan-Noziéres-De Dominicis (MND) theory, including core-level excitonic effects, the Fermi-edge singularity, shakeup excitations, and wave function overlap effects such as the orthogonality catastrophe, all within a universal framework using many-electron configurations. A heuristic search algorithm was introduced to search for the configurations that are important for defining the x-ray spectral line shape, instead of enumerating them in a brute-force way. The algorithm has proven to be efficient for calculating O K edges of transition metal oxides, which converge at the second excitation order (denoted as $f^{(n)}$ with n = 2), i.e., the final-state configurations with two e-h pairs (with one hole being the core hole). However, it remains unknown how the determinant x-ray spectra converge for general cases and at which excitation order n should one stop the determinant calculation. Even with the heuristic algorithm, the number of many-electron configurations still grows exponentially with the excitation order n. In this work we prove two theorems that can indicate the order of magnitude of the contribution of the $f^{(n)}$ configurations, so that one can estimate their contribution very quickly without actually calculating their amplitudes. The two theorems are based on singular-value decomposition (SVD) analysis, a method that is widely used to quantify entanglement between two quantum many-body systems. Finally, we examine the K edges of several metallic systems with the determinant formalism up to $f^{(5)}$ to illustrate the usefulness of the theorems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electronic Structure and Performance Bottlenecks of CuFeO 2 Photocathodes

The path to realizing low-cost, stable, and earth-abundant photoelectrodes can be enabled through a detailed understanding of the optoelectronic properties of these materials by combining theory and experimental techniques. Of the limited set of oxide photocathode materials currently available, CuFeO 2 has emerged as a promising candidate warranting detailed attention. Here, highly compact thin films of rhombohedral (3R) CuFeO 2 were prepared via reactive co-sputtering. Despite its 1.43 eV indirect band gap, a cathodic photocurrent of 0.85 mA/cm 2 was obtained at 0.4 V versus reversible hydrogen electrode in the presence of a sacrificial electron acceptor. This unexpected performance was related to inefficient bulk charge separation because of the ultrafast (<1 ps) self-trapping of photogenerated free carriers. The electronic structure of 3R-CuFeO 2 was elucidated through a combination of optical and X-ray spectroscopic techniques and further complemented by first-principles computational methods including a many-body approach for computing the O K-edge X-ray absorption spectrum. Through resonant inelastic X-ray scattering spectroscopy, the visible absorption edges of CuFeO 2 were found to correspond to Cu → Fe metal-to-metal charge transfer, which exhibits a high propensity toward self-trapping. Findings of the present work enable us to understand the performance bottlenecks of CuFeO 2 photocathodes and suggest feasible strategies for improving material limitations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Understanding the effect of thionation on naphthalene diimide using first-principles predictions of near-edge x-ray absorption fine structure spectra

The near edge X-ray absorption fine structure (NEXAFS) spectra of naphthalene diimide molecules with increasing degrees of thionation show distinct and systematic changes in the C 1s → π* manifold. However, interpretation of such spectra is difficult using experimental data alone, due to the limitation in experimental NEXAFS resolution. In this work, we have calculated the NEXAFS spectra of naphthalene diimide molecules with increasing degrees of thionation using the density functional theory-based eXcited electron and Core Hole approach. Furthermore, we find that the systematic broadening and intensity reduction in the peaks observed in the π* manifold with increasing thionation are the result of distinct changes in the chemical environment of the outer carbon atoms that are bonded directly to either oxygen or sulfur. Specifically, the C 1s → lowest unoccupied molecular orbital (LUMO) transition energy dramatically decreases with thionation, as the valence electron density of these carbon atoms is increased when highly electronegative oxygen atoms are replaced by less-oxidizing sulfur atoms. It is also shown that significant core level shifts present in naphthalene diimide-based molecule result in a mixing of the LUMO and LUMO + 1 character in the C 1s → π* manifold, meaning that experimentally observed peaks cannot be uniquely associated with the transitions of LUMO, LUMO + 1, etc.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Quantum many-body effects in x-ray spectra efficiently computed using a basic graph algorithm

The mounting interest in using x-ray spectroscopy for refined materials characterization calls for an accurate electronic-structure theory to interpret the x-ray near-edge fine structure. Here, we propose an efficient and unified framework to describe all the many-electron processes in a Fermi liquid after a sudden perturbation (such as a core hole). This problem has been visited by the Mahan-Noziéres-De Dominicis (MND) theory, but it is intractable to implement various Feynman diagrams within first-principles calculations. In our report, we adopt a nondiagrammatic approach and treat all the many-electron processes in the MND theory on an equal footing. Starting from a recently introduced determinant formalism [Phys. Rev. Lett. 118, 096402 (2017)], we exploit the linear dependence of determinants describing different final states involved in the spectral calculations. An elementary graph algorithm, breadth-first search, can be used to quickly identify the important determinants for shaping the spectrum, which avoids the need to evaluate a great number of vanishingly small terms. This search algorithm is performed over the tree-structure of the many-body expansion, which mimics a path-finding process. We demonstrate that the determinantal approach is computationally inexpensive even for obtaining x-ray spectra of extended systems. Using Kohn-Sham orbitals from two self-consistent fields (ground and core-excited state) as input for constructing the determinants, the calculated x-ray spectra for a number of transition metal oxides are in good agreement with experiments. Many-electron aspects beyond the Bethe-Salpeter equation, as captured by this approach, are also discussed, such as shakeup excitations and many-body wave function overlap considered in Anderson's orthogonality catastrophe.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗