Research themes ================ Our group works at the interface between condensed matter theory, quantum materials research, and high-performance computing. We develop computational methods to predict the properties of materials at the atomic scale, starting from the first principles of quantum mechanics. Our activities span fundamental theory, algorithm and software development, and the application of these tools to technologically-relevant materials. We frequently collaborate with experimental groups to interpret complex measurements and to discover new materials that have not yet been realized in the laboratory. Electron-phonon physics ----------------------- The interaction between electrons and phonons :cite:`giustino2017electron`, the quanta of lattice vibrations, governs many important phenomena in materials, from electrical and thermal transport to superconductivity and ultrafast carrier dynamics. A central focus of our group is to develop theoretical and computational frameworks that describe these interactions with predictive accuracy using quantum many-body methods. We work on: - Many-body Green's function approaches to electron-phonon physics :cite:`lafuente2022unified` - Wannier-Fourier interpolation schemes for electron-phonon coupling matrix elements :cite:`giustino2007electron` - Quantum zero-point and temperature-induced renormalization of electron band structures :cite:`giustino2010electron` - Phonon-assisted optical processes :cite:`zacharias2015stochastic` - Electrical and thermal transport :cite:`ponce2020first` - Interpretation of photoemission spectroscopy and ultrafast spectroscopy :cite:`verdi2017origin` Emergent phenomena ------------------ Emergent phenomena arise when the coupled dynamics of electrons, phonons, spins, or photons produce collective behaviors that cannot be inferred from the properties of individual particles. One research direction of our group is to develop *ab initio* many-body frameworks to explain how such complex quasiparticles form, evolve, and manifest in experiments. The polaron, i.e., an electron dressed by a distortion of the crystal lattice, represents an example of emergent quasiparticle in solids. We work toward a quantitative and predictive understanding of these quasiparticles across regimes ranging from small, strongly localized Holstein-type polarons to large, delocalized Fröhlich quasiparticles, all within a unified first-principles formalism. These ideas are currently being generalized to other emergent excitations, including topological quasiparticles. Recent work in this area includes: - Development of *ab initio* methods to model polaron energetics, formation, dynamics, and mobility :cite:`sio2019polarons` - Discovery and characterization of topological polarons :cite:`lafuente2024topological` - Search for unconventional polaron species in transition-metal oxides :cite:`dai2024identification` Light-matter interactions ------------------------- We investigate how light couples to matter, and how optical absorption and emission spectra carry the signatures of this interaction. We ar eprimarily interested in regimes where electrons, holes, excitons, photons, and phonons interact to produce rich composite excitation with distinctive spectral fingerprints. This work involves the development of new electronic-structure methods, algorithms, and software for studying excitonic effects, phonon-mediated optical transitions, and exciton-phonon couplings. Recent work includes: - Method development for exciton polarons and self-trapped excitons :cite:`dai2024theory` - Calculations of optical spectra in direct, indirect, and quasi-direct materials :cite:`tiwari2024unified` - Understanding excitonic effects in emerging optoelectronic materials :cite:`dai2024excitonic` Semiconductors -------------- A large part of our work is devoted to the design and discovery of semiconductors for next-generation electronics. We work closely with experimental collaborators to identify, characterize, and design materials with improved carrier transport properties in extreme regimes, for example in ultrascaled transistors or in high-power applications. Current research directions include: - High-throughput searches of 2D atomically-thin semiconductors with high carrier mobilities :cite:`ha2024high` - Computational design of wide-bandgap semiconductors for high-power, high-frequency, and radiation-hard applications :cite:`chen2025design` - Assessment of the viabilit of *p*-type oxides for back-end-of-the-line applications in next-generation chips. :cite:`chae2025controlling` Materials discovery ------------------- We often use first-principles approaches to designing entirely new materials from scratch. Our work combines high-throughput *ab initio* screening and advanced electronic-structure methods to identify materials with targeted optical and electronic properties. Much of this research focuses on sustainable and functional semiconductors for optoelectronics, with an emphasis on materials that can be discovered and optimized through a closed theory–experiment feedback loop. Successful predictions include: - We identified lead-free alternatives to lead-based perovskites, and several compounds that we predicted have been synthesized, for example Cs₂AgInCl₆, the first single-material white-light emitter :cite:`volonakis2017cs2inagcl6` - We identified a novel nitride perovskite, CeTaN₃, and predicted that it could be a potential ferroelectric semiconductor :cite:`ha2022cetan3`; our prediction was subsequently confirmed by experiments Software, HPC, and ML/AI ----------------------------------------------------------- Much of our time is devoted to developing open-source software for advanced quantum materials simulations. We design, implement, and maintain large-scale codes for leadership-class supercomputers, with a focus on predictive electronic-structure methods, electron-phonon interactions, many-body perturbation theory, and automated high-throughput workflows. All the code generated by our group is open-source and is continuously shared with the community without any restrictions. We currently maintain several software projects (see the :doc:`software <../software/index>` page for more details): - EPW :cite:`lee2023electron` - EPWpy - SternheimerGW :cite:`schlipf2020sternheimergw` - MatCSSI .. raw:: html
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