arXiv subjects

Lu Qiao

Publications and source records attributed to Lu Qiao.

4 recordsLinked to original sources

arXiv2026

Disentangling Electronic and Lattice Contributions to Transient Absorption in Metal Halide Perovskites: A First-Principles Study of CH3NH3PbBr3

Soft lattices combined with strong electron-phonon coupling in metal halide perovskites result in a complex interplay between electronic and lattice degrees of freedom. This interplay complicates the interpretation of time-resolved excitation spectra like pump-probe spectra. Here, we develop a first-principles approach that combines a nonequilibrium extension of the Bethe-Salpeter equation with \textit{ab initio} molecular dynamics to resolve the origin of transient absorption. This approach can quantitatively disentangle electronic and thermal lattice contributions across femtosecond-to-picosecond timescales. Exemplified with \ce{CH3NH3PbBr3}, we find that on the femtosecond scale, both X-ray and optical transient absorption spectra are dominated by electronic contributions: Photoinduced Coulomb screening weakens the effective electron-hole interaction and blueshifts the excitonic resonances, whereas Pauli blocking is negligible. On the picosecond scale, thermal lattice contributions become essential, with distinct mechanisms dominating different spectral regions: Lattice vibrations lead to spectral redistribution in the X-ray transient absorption spectrum, whereas lattice expansion blueshifts the optical transient absorption spectrum.

cond-mat.mtrl-sci↗

arXiv2026

An exciting approach to theoretical spectroscopy

Theoretical spectroscopy, and more generally, electronic-structure theory, are powerful concepts for describing the complex many-body interactions in materials. They comprise a variety of methods that can capture all aspects, from ground-state properties to lattice excitations to different types of light-matter interaction, including time-resolved variants. Modern electronic-structure codes implement either a few or several of these methods. Among them, exciting is an all-electron full-potential package that has a very rich portfolio of all levels of theory, with a particular focus on excitations. It implements the linearized augmented planewave plus local orbital (LAPW+LO) basis, which is known as the gold standard for solving the Kohn-Sham equations of density-functional theory (DFT). Based on this, it also offers benchmark-quality results for a wide range of excited-state methods. In this review, we provide a comprehensive overview of the features implemented in exciting in recent years, accompanied by short summaries on the state of the art of the underlying methodologies. They comprise DFT and time-dependent DFT (TDDFT), density-functional perturbation theory (DFPT) for phonons and electron-phonon coupling, and many-body perturbation theory in terms of the $GW$ approach and the Bethe-Salpeter equation (BSE). Moreover, exciting can handle resonant inelastic x-ray scattering (RIXS), pump-probe spectroscopy as well as exciton-phonon coupling (EXPC). Finally, we cover workflows and a view on data and machine learning (ML). All aspects are demonstrated with examples for scientifically relevant materials.

cond-mat.mtrl-sci↗

arXiv2025

First-principles approach to ultrafast pump-probe spectroscopy in solids

Pump-probe spectroscopy is a powerful tool to study ultrafast exciton dynamics, revealing the underlying complex interactions on the electronic scale. Despite significant advances in experimental techniques, developing a comprehensive and rigorous theoretical framework for modeling and interpreting the transient response in photoexcited materials remains a challenge. Here, we present a first-principles approach to simulating pump-probe spectroscopy and disentangling the electronic and thermal contributions underlying exciton dynamics. We showcase our method to three materials, representative for different classes of solids: the transition-metal dichalcogenides WSe$_2$, the halide perovskite CsPbBr$_3$, and the transition-metal oxide TiO$_2$, showing remarkable agreement with experimental counterparts. We find that (i) photoinduced Coulomb screening is the primary electronic effect, responsible for a blue shift of exciton resonances, while (ii) Pauli blocking plays a minor role, and (iii) thermal lattice expansion leads to a red shift of the spectra. We further demonstrate how key parameters such as excitation density, pump photon energy, and pump polarization modulate the transient absorption spectra, offering direct control over the exciton-resonance energy. Our approach establishes a quantitative and predictive framework for interpreting pump-probe experiments, providing actionable insights for the design of energy-selective optoelectronic devices through exciton engineering.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast dynamic Coulomb screening of X-ray core excitons in photoexcited semiconductors

Ultrafast X-ray spectroscopy has been revolutionized in recent years due to the advent of fourth-generation X-ray facilities. In solid-state materials, core excitons determine the energy and line shape of absorption features in core-level spectroscopies such as X-ray absorption spectroscopy. The screening of core excitons is an inherent many-body process that can reveal insight into charge-transfer excitations and electronic correlations. Under non-equilibrium conditions such as after photoexcitation, however, core-exciton screening is still not fully understood. Here we demonstrate the dynamic Coulomb screening of core excitons induced by photoexcited carriers by employing X-ray transient absorption (XTA) spectroscopy with picosecond time resolution. Our interpretation is supported by state-of-the-art ab initio theory, combining constrained and real-time time-dependent density functional theory with many-body perturbation theory. Using ZnO as an archetypal wide band-gap semiconductor, we show that the Coulomb screening by photoexcited carriers at the Zn K-edge leads to a decrease in the core-exciton binding energy, which depends nonlinearly on both the excitation density and the distribution of photoexcited carriers in reciprocal space. The effect of Coulomb screening dominates over Pauli blocking in the XTA spectra. We show that dynamic core-exciton screening is also observed at other X-ray absorption edges and theoretically predict the effect of core-exciton screening on the femtosecond time scale for the case of ZnO, a major step towards hard X-ray excitonics. The results have implications for the interpretation of ultrafast X-ray spectra in general and their use in tracking charge carrier dynamics in complex materials on atomic length scales.

cond-mat.mtrl-sci↗