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Zhenglu Li

Publications and source records attributed to Zhenglu Li.

At least 19 recordsLinked to original sources

First-principles predictions of carrier mobility with record accuracy using GW perturbation theory

Accurate prediction of carrier mobility is critical for the discovery and design of next-generation electronic materials. Despite sustained progress, state-of-the-art ab initio methods remain limited by the approximate treatment of electron-phonon interactions at the density functional theory level. Here, we demonstrate that incorporating many-body GW corrections to both the electronic band structure and electron--phonon couplings when solving the ab initio Boltzmann transport equation yields a mean absolute relative error of just 11% for electron mobilities across benchmark semiconductors, including Si, GaAs, GaP, diamond, and SiC. The common practice of neglecting GW corrections to the electron--phonon interaction can lead to mobility errors exceeding 50%. The present findings highlight the importance of many-body GW self-energy effects in carrier transport simulations, and provides fundamental insights into how many-body electron--phonon interactions govern charge transport in crystalline solids.

cond-mat.mtrl-sci

Ab initio time-dependent GW approach for nonequilibrium exciton-phonon coupled dynamics across momentum space

The dynamics of optical excitations in materials generally involves intertwined electron-hole (e-h) and electron-phonon (e-ph) interactions out of equilibrium. However, a full theoretical description of such nonequilibrium dynamics requires a systematic treatment of the coherent excitonic excitations and exciton-phonon interactions across the entire crystal momentum space in real time, which remains a major challenge and out of reach for first-principles approaches. Here, we present a new ab initio time-dependent adiabatic GW methodology that incorporates full finite-momentum e-h and e-ph couplings, enabling real-time simulations of the coherently coupled exciton-phonon dynamics. The excitonic excitations are naturally described by the equation of motion of the interacting single-particle density matrix, whereas their couplings to phonons are formulated within a linear-response framework, hence the simulations can be efficiently carried out within a primitive unit cell. We demonstrate the capabilities of this new approach by investigating the direct-to-indirect exciton transitions in monolayer WSe2 in a pump-probe setup of time-resolved and angle-resolved photoemission spectroscopy. Our results reveal that the phonon-mediated ultrafast intervalley dynamics of excitons of this system is within ~0.5 ps, manifested as in-gap photoemission intensity transfer from the K-valley to the Q-valley. This work establishes a comprehensive and practical nonequilibrium Green's function framework for accurately simulating nonequilibrium and coherent excitations involving coupled excitons and phonons from first principles.

cond-mat.mtrl-sci

Photogeneration and signatures of coherent phonons in time-resolved photoemission spectroscopy: First-principles time-dependent adiabatic GW approach

Coherent lattice dynamics can be observed in pump-probe time-resolved and angle-resolved photoemission spectroscopy (TR-ARPES) as a periodic modulation of intensity and energy of photoelectrons over probe time. We present an ab initio time-dependent GW approach including electron-phonon (e-ph) couplings to simulate the photogeneration of coherent phonons and their effects on the TR-ARPES of monolayer MoS2. We demonstrate that state-resolved e-ph coupling strength can be obtained from analyzing coherent phonon modulations on TR-ARPES. Features of both the impulsive stimulated Raman scattering mechanism and the displacive excitation mechanism of coherent phonon generation are identified in our simulations. We clarify their origins and the coincident selection rule of coherent phonon generation and Raman scattering intensity. This method provides supports to analyze coherent phonon dynamics and e-ph couplings in TR-ARPES and enables quantitative engineering of band structure through coherent phonons.

cond-mat.mtrl-sci

Layer-tunable Hubbard bands probed via moir\'e excitons in MoSe$_2$/WS$_2$ heterostructures

Moir\'e superlattices in transition metal dichalcogenide heterostructures provide a highly tunable platform for engineering strongly interacting states at the nanoscale. However, quantitatively determining and in-situ tuning of the underlying Hubbard parameters remains experimentally challenging. Here, we report electric-field-driven reordering of layer-specific Hubbard bands by performing optical spectroscopy on a dual-gated, 60{\deg}-aligned MoSe$_2$/WS$_2$ heterobilayer. Using two spatially distinct moir\'e excitons as local optical probes and tracking them as a function of carrier filling and vertical electric field, we quantitatively extract the layer-dependent on-site Coulomb repulsions, U$_M$~60 meV in MoSe$_2$ and U$_W$~30 meV in WS$_2$. Furthermore, we stabilize generalized Wigner crystal and stripe phases by electrostatically tuning the system to a type-II band alignment, shifting the ground state into the WS$_2$ layer where reduced on-site repulsion allows inter-site Coulomb interactions to dominate. Our results establish vertical electric fields as a deterministic tuning knob for layer-selective Hubbard physics, enabling device-level control of complex many-body phases.

cond-mat.mes-hall

Many-Body Correlation Effects in Fr\"ohlich Electron-Phonon Coupling

In compound semiconductors and insulators, the polar electron-phonon coupling diverges at long range, known as the Fr\"ohlich interaction. Modern first-principles electron-phonon calculations treat the Fr\"ohlich interaction in a semiclassical electrostatic formalism based on density-functional perturbation theory. Here, using many-body $GW$ perturbation theory, we reveal important electron correlation effects in the Fr\"ohlich-type electron-phonon coupling, which are missed by the prevailing approaches and rooted in the fundamentally distinct behavior between quasiparticle self-energy and semi-local exchange-correlation functionals in the long-range limit. Going beyond the electrostatic treatment, we derive and implement the $GW$ self-energy contribution to the long-range polar electron-phonon coupling, and demonstrate its critical role and nontrivial behaviors in properties such as electron linewidth and polaron formation in several prototype semiconductors. Remarkably, calculations on photoemission kink in electron-doped TiO$_2$ achieve excellent agreement with experiment. Our work establishes the many-body generalization of the Fr\"ohlich interaction that is essential for accurate electron-phonon calculations at the full $GW$ level combined with Wannier interpolation techniques.

cond-mat.mtrl-sci

Advancing Quantum Many-Body GW Calculations on Exascale Supercomputing Platforms

Advanced ab initio materials simulations face growing challenges as increasing systems and phenomena complexity requires higher accuracy, driving up computational demands. Quantum many-body GW methods are state-of-the-art for treating electronic excited states and couplings but often hindered due to the costly numerical complexity. Here, we present innovative implementations of advanced GW methods within the BerkeleyGW package, enabling large-scale simulations on Frontier and Aurora exascale platforms. Our approach demonstrates exceptional versatility for complex heterogeneous systems with up to 17,574 atoms, along with achieving true performance portability across GPU architectures. We demonstrate excellent strong and weak scaling to thousands of nodes, reaching double-precision core-kernel performance of 1.069 ExaFLOP/s on Frontier (9,408 nodes) and 707.52 PetaFLOP/s on Aurora (9,600 nodes), corresponding to 59.45% and 48.79% of peak, respectively. Our work demonstrates a breakthrough in utilizing exascale computing for quantum materials simulations, delivering unprecedented predictive capabilities for rational designs of future quantum technologies.

cond-mat.mtrl-sci

Moir\'e excitons in generalized Wigner crystals

Moir\'e superlattices of transition-metal dichalcogenide bilayers host strong Coulomb interactions residing in narrow electron bands, leading to correlated insulating states at fractional carrier doping densities, known as generalized Wigner crystals. In excited states, the formation of moir\'e excitons is expected to be fundamentally shaped by the Wigner-crystal ground states, manifesting an intricate interplay between electronic and excitonic correlations. However, the microscopic description of these Wigner crystalline excitons (WCEs) remains elusive, largely subject to speculations, and is further needed for the understanding of exotic excitonic phases (e.g., exciton insulators and exciton density waves) and their unique properties (e.g., anomalous exciton diffusion). Here, using first-principles many-body GW-Bethe-Salpeter-equation calculations, we directly reveal the internal structures of WCEs in angle-aligned MoSe2/MoS2 moir\'e heterostructure at hole fillings of 1/3 and 2/3. Our results unveil the propagation of correlation effects from the ground state to excited states, shaping the real-space characteristics of WCEs. The strong two-particle excitonic correlations dominate over the kinetic energy of free electron-hole pairs, in analog to the strong single-particle correlations of flat bands. We propose that such unusual excited-state correlation effects of WCEs can be experimentally probed by photocurrent tunneling microscopy. Our work provides a microscopic understanding of strongly correlated WCEs, suggesting them as a highly tunable mixed boson-fermion platform to study many-body interactions and phenomena.

cond-mat.mtrl-sci

Correlation between Exciton Dynamics and Spin Structure in van der Waals Antiferromagnet NiPS3

The emerging magnetic van der Waals (vdW) materials provide a platform for exploring novel physics regarding magnetism in low dimensions and developing ultrathin spintronic applications. Here, we investigate the ultrafast dynamics of excitons in a vdW NiPS3 crystal. The temporal evolution of the transient reflection spectra indicates that the spin-correlated exciton is formed through photocarrier localization, the rate of which is independent of the magnetic degrees of freedom. However, the recombination rate of these excitons is connected with the long-range magnetic order, and this connection probably arise from a spin-flip rooted in the underlying antiferromagnetic background during the recombination. Our findings uncover intertwined coupling between carrier, lattice and spin degrees of freedom in NiPS3, which may pave the path toward ultrafast optical manipulation of spin-related quantum states in vdW antiferromagnets.

cond-mat.mtrl-sci

Non-excitonic mechanism for electronic and structural phase transitions in Ta2Ni(Se,S)5

We present a first-principles study based on density functional theory (DFT) on the electronic and structural properties of Ta2NiSe5, a layered transition metal chalcogenide that has been considered as a possible candidate for an excitonic insulator. Our systematic DFT results however provide a non-excitonic mechanism for the experimentally observed electronic and structural phase transitions in Ta2NiSe5, in particular explaining why sulfur substitution of selenium reduces the distortion angle in the low-temperature phase and potassium dosing closes the gap in the electronic structure. Moreover, the calculations show that these two effects couple to each other. Further, our first-principles calculations predict several changes in both the crystal structure and electronic structure under the effects of uniform charge dosing and uniaxial strain, which could be tested experimentally.

cond-mat.mtrl-sci

Correlated insulating states in slow Dirac fermions on a honeycomb moir{\'e} superlattice

Strong Coulomb repulsion is predicted to open a many-body charge gap at the Dirac point of graphene, transforming the semimetal into a Mott insulator. However, this correlated insulating phase has remained inaccessible in pristine graphene, where a large Fermi velocity dominates the interaction effects. To overcome this limitation, we realize a honeycomb moir{\'e} superlattice in a twisted MoSe$_2$ homobilayer, where a graphene-like band structure forms with a Fermi velocity reduced by nearly two orders of magnitude. These slow moir{\'e} bands are folded from the valence band maximum at the $\Gamma$ valley of the extended Brillouin zone with negligible spin-orbital coupling, and can therefore simulate massless Dirac fermions in the strongly correlated regime with full SU(2) symmetry. By correlating Rydberg exciton sensing with moir{\'e} trions of different spatial characters, we detect a Mott gap at the Dirac point that persists up to 110 K. We further identify correlated insulating states at $\nu=-1$ with a weak ferromagnetic coupling as well as at several fractional fillings. Our results highlight the potential of studying a wide range of quantum many-body phenomena in twisted two-dimensional materials.

cond-mat.mes-hall

Observation of collective charge excitations in a cuprate superconductor

Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolution resonant inelastic x-ray scattering (RIXS) study of charge excitations in the stripe-ordered cuprate La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. The RIXS spectra consist of both charge and phonon excitations around the charge ordering wave vector. By modeling the momentum-dependent phonon intensity, the charge-excitation spectral weight is extracted for a wide range of energy. As such, we reveal the highly dispersive nature of the charge excitations, with an energy scale comparable to the spin excitations. Since charge order and superconductivity in cuprates are possibly driven by the same electronic correlations, determining the interaction strength underlying charge order is essential to establishing a comprehensive microscopic model of high-temperature superconductivity.

cond-mat.supr-con

IsoME: Streamlining High-Precision Eliashberg Calculations

This paper introduces the Julia package IsoME, an easy-to-use yet accurate and robust computational tool designed to calculate superconducting properties. Multiple levels of approximation are supported, ranging from the basic McMillan-Allen-Dynes formula and its machine learning-enhanced variant to Eliashberg theory including static Coulomb interactions derived from $GW$ calculations, offering a fully ab initio approach to determine superconducting properties, such as the critical superconducting temperature ($T_\text{c}$) and the superconducting gap function ($\Delta$). We validate IsoME by benchmarking it against various materials, demonstrating its versatility and performance across different theoretical levels. The findings indicate that the previously held assumption that Eliashberg theory overestimates $T_\text{c}$ is no longer valid when $\mu^*$ is appropriately adjusted to account for the finite Matsubara frequency cutoff. Furthermore, we conclude that the constant density of states (DOS) approximation remains accurate in most cases. By unifying multiple approximation schemes within a single framework, IsoME combines first-principles precision with computational efficiency, enabling seamless integration into high-throughput workflows through its $T_\text{c}$ search mode. This makes IsoME a powerful and reliable tool for advancing superconductivity research.

cond-mat.supr-con

Theory of ab initio downfolding with arbitrary range electron-phonon coupling

Ab initio downfolding describes the electronic structure of materials within a low-energy subspace, often around the Fermi level. Typically starting from mean-field calculations, this framework allows for the calculation of one- and two-electron interactions, and the parametrization of a many-body Hamiltonian representing the active space of interest. The subsequent solution of such Hamiltonians can provide insights into the physics of strongly-correlated materials. While phonons can substantially screen electron-electron interactions, electron-phonon coupling has been commonly ignored within ab initio downfolding, and when considered this is done only for short-range interactions. Here we propose a theory of ab initio downfolding that accounts for all mechanisms of electron-phonon coupling on equal footing, regardless of the range of the interactions. Our practical computational implementation is readily compatible with current downfolding approaches. We apply our approach to polar materials MgO and GeTe, and we reveal the importance of both short-range and long-range electron-phonon coupling in determining the magnitude of electron-electron interactions. Our results show that in the static limit, phonons reduce the on-site repulsion between electrons by 40% for MgO, and by 79% for GeTe. Our framework also predicts that overall attractive nearest-neighbor interactions arise between electrons in GeTe, consistent with superconductivity in this material.

cond-mat.mtrl-sci

Phonon-mediated electron attraction in SrTiO$_3$ via the generalized Fr\"ohlich and deformation potential mechanisms

Superconductivity in doped SrTiO$_3$ was discovered in 1964, the first superconducting transition observed in a doped semiconductor. However, the mechanism of electron pairing in SrTiO$_3$ remains a subject of debate. By developing a theoretical framework to incorporate dynamical lattice screening in the electronic Coulomb interactions of semiconductors and insulators, we demonstrate analytically that linear long-range coupling of electrons to multiple longitudinal optical phonons, described by a generalized Fr\"ohlich mechanism, can result in superconductivity in SrTiO$_3$. Moreover, by combining our theory with first-principles calculations, we reveal an additional attractive interaction between electrons in SrTiO$_3$ due to the deformation potential mechanism, arising from the mixed ionic-covalent character of the Ti-O bond. Our results may have implications for the emergence of phonon-mediated electron attraction and superconductivity in a broad range of materials.

cond-mat.supr-con

Excitonic effects on infrared vibrational and Raman spectroscopy from first principles

We develop a first-principles approach to compute infrared (IR) vibrational absorption and Raman scattering spectra with excitonic effects included. Our method is based on a perturbative expansion of electron-phonon and electron-light couplings in the time-dependent adiabatic GW (TD-aGW) theory. We show that excitonic effects in the IR absorption spectrum can be included by replacing the free electron-hole propagators in the perturbative expression for independent particles with their interacting counterparts, which are readily available from standard GW-Bethe-Salpeter equation calculations. For Raman spectrum, our derived expression agrees with the single and double resonance terms from a diagrammatic approach. We show significant excitonic enhancement in both the IR and resonance Raman scattering intensity for monolayer MoS2, WS2, and WSe2. Moreover, the exciton-phonon coupling strength and exciton energy landscape can be accessed by analyzing resonance Raman spectrum of these materials.

cond-mat.mtrl-sci

Diverse Manifestations of Electron-Phonon Coupling in a Kagome Superconductor

Recent angle-resolved photoemission spectroscopy (ARPES) experiments on a kagome metal CsV$_3$Sb$_5$ revealed distinct multimodal dispersion kinks and nodeless superconducting gaps across multiple electron bands. The prominent photoemission kinks suggest a definitive coupling between electrons and certain collective modes, yet the precise nature of this interaction and its connection to superconductivity remain to be established. Here, employing the state-of-the-art \textit{ab initio} many-body perturbation theory computation, we present direct evidence that electron-phonon ($e$-ph) coupling induces the multimodal photoemission kinks in CsV$_3$Sb$_5$, and profoundly, drives the nodeless $s$-wave superconductivity, showcasing the diverse manifestations of the $e$-ph coupling. Our calculations well capture the experimentally measured kinks and their fine structures, and reveal that vibrations from different atomic species dictate the multimodal behavior. Results from anisotropic $GW$-Eliashberg equations predict a phonon-mediated superconductivity with nodeless $s$-wave gaps, in excellent agreement with various ARPES and scanning tunneling spectroscopy measurements. Despite of the universal origin from the $e$-ph coupling, the contributions of several characteristic phonon vibrations vary in different phenomena, highlighting a versatile role of $e$-ph coupling in shaping the low-energy excitations of kagome metals.

cond-mat.supr-con

Two-gap superconductivity and decisive role of rare-earth $d$ electrons in infinite-layer nickelates

We present a theoretical prediction of a phonon-mediated two-gap superconductivity in infinite-layer nickelates Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ by performing $\textit{ab initio}$ $GW$ and $GW$ perturbation theory calculations. Electron $GW$ self-energy effects significantly alter the characters of the two-band Fermi surface and enhance the electron-phonon coupling, compared with results based on density functional theory. Solutions of the fully $\textbf{k}$-dependent anisotropic Eliashberg equations yield two dominant $s$-wave superconducting gaps - a large gap on a band of rare-earth Nd $d$ and interstitial orbital characters and a small gap on a band of transition-metal Ni $d$ character. Increasing hole doping induces a non-rigid-band response in the electronic structure, leading to a rapid drop of the superconducting $T_c$ in the overdoped regime in agreement with experiments.

cond-mat.supr-con

Phonon screening of excitons in atomically thin semiconductors

Atomically thin semiconductors, encompassing both 2D materials and quantum wells, exhibit a pronounced enhancement of excitonic effects due to geometric confinement. Consequently, these materials have become foundational platforms for the exploration and utilization of excitons. Recent ab initio studies have demonstrated that phonons can substantially screen electron-hole interactions in bulk semiconductors and strongly modify the properties of excitons. While excitonic properties of atomically thin semiconductors have been the subject of extensive theoretical investigations, the role of phonon screening on excitons in atomically thin structures remains unexplored. In this work, we demonstrate via ab initio GW-Bethe-Salpeter equation calculations that phonon screening can have a significant impact on optical excitations in atomically thin semiconductors. We further show that the degree of phonon screening can be tuned by structural engineering. We focus on atomically thin GaN quantum wells embedded in AlN and identify specific phonons in the surrounding material, AlN, that dramatically alter the lowest-lying exciton in monolayer GaN via screening. Our studies provide new intuition beyond standard models into the interplay among structural properties, phonon characteristics, and exciton properties in atomically thin semiconductors, and have implications for future experiments.

cond-mat.mtrl-sci