Terahertz chiral photonic-crystal cavities for Dirac gap engineering in graphene.

Strong coupling between matter and vacuum electromagnetic fields in a cavity can induce novel quantum phases in thermal equilibrium via symmetry breaking. Particularly intriguing is the coupling with circularly polarized cavity fields, which can break time-reversal symmetry (TRS) and lead to topological bands. This has spurred significant interest in developing chiral cavities that feature broken TRS, especially in the terahertz (THz) frequency range, where various large-oscillator-strength resonances exist.

Recent Developments in DFTB+, a Software Package for Efficient Atomistic Quantum Mechanical Simulations.

DFTB+ is a flexible, open-source software package developed by its community, designed for fast and efficient atomistic quantum mechanical simulations. It employs various methods that approximate density functional theory (DFT), such as density functional-based tight binding (DFTB) and the extended tight binding (xTB) approach allowing simulations of large systems over extended time scales with reasonable accuracy, while being significantly faster than traditional ab initio methods. In recent years, several new extensions of the DFTB method have been developed and implemented in the DFTB+ program package in order to improve the accuracy and generality of the available simulation results.

Beyond Electric-Dipole Treatment of Light-Matter Interactions in Materials: Nondipole Harmonic Generation in Bulk Si.

A beyond electric-dipole light-matter theory is needed to describe emerging x-ray and THz applications for characterization and control of quantum materials but inaccessible as nondipole lattice-aperiodic terms impede on the use of Bloch's theorem. To circumvent this, we derive a formalism that captures dominant nondipole effects in intense electromagnetic fields while conserving lattice translational symmetry. Our approach enables the first accurate nondipole first-principles microscopic simulation of nonperturbative harmonic generation in Si.

Frustrated phonon with charge density wave in vanadium Kagome metal.

The formation of a star-of-David charge density wave superstructure, resulting from the coordinated displacements of vanadium ions on a corner-sharing triangular lattice, has garnered significant attention to comprehend the influence of electron-phonon interaction within geometrically intricate lattice of Kagome metals, specifically AVSb (where A represents K, Rb, or Cs). However, understanding of the underlying mechanism behind charge density wave formation, coupled with symmetry-protected lattice vibrations, remains elusive. Here, from femtosecond time-resolved X-ray scattering experiments, we reveal that the phonon mode, associated with cesium ions' out-of-plane motion, becomes frustrated in the charge density wave phase.

Quantum electrodynamics formulation of multidimensional spectroscopy.

We present a description of multidimensional spectroscopy, where all the light pulses are treated quantum-mechanically and the signal is expressed as a quantum dynamical map of the broadband light fields. Focusing in particular on the rephasing contribution to two-dimensional spectroscopy, we demonstrate how the semiclassical description emerges naturally as a limiting case of the quantum description. Our work establishes a formalism to apply quantum information methods to the optimization of multidimensional spectroscopy and address, e.

Quantum melting of generalized electron crystal in twisted bilayer MoSe.

Electrons can form an ordered solid crystal phase ascribed to the interplay between Coulomb repulsion and kinetic energy. Tuning these energy scales can drive a phase transition from electron solid to liquid, i.e.

Momentum-resolved fingerprint of Mottness in layer-dimerized NbBr.

Crystalline solids can become band insulators due to fully filled bands, or Mott insulators due to strong electronic correlations. While Mott insulators can theoretically occur in systems with an even number of electrons per unit cell, distinguishing them from band insulators experimentally has remained a longstanding challenge. In this work, we present a unique momentum-resolved signature of a dimerized Mott-insulating phase in the experimental spectral function of NbBr: the top of the highest occupied band along the out-of-plane direction k has a momentum-space separation Δk = 2π/d, whereas that of a band insulator is less than π/d, where d is the average interlayer spacing.

GARFIELD, a toolkit for interpreting ultrafast electron diffraction data of imperfect quasi-single crystals.

The analysis of ultrafast electron diffraction (UED) data from low-symmetry single crystals of small molecules is often challenged by the difficulty of assigning unique Laue indices to the observed Bragg reflections. For a variety of technical and physical reasons, UED diffraction images are typically of lower quality when viewed from the perspective of structure determination by single-crystal x-ray or electron diffraction. Nevertheless, time series of UED images can provide valuable insight into structural dynamics, providing that an adequate interpretation of the diffraction patterns can be achieved.

Density-Functional Theory for the Dicke Hamiltonian.

A detailed analysis of density-functional theory for quantum-electrodynamical model systems is provided. In particular, the quantum Rabi model, the Dicke model, and a generalization of the latter to multiple modes are considered. We prove a Hohenberg-Kohn theorem that manifests the magnetization and displacement as internal variables, along with several representability results.

Geometrical design of 3D superconducting diodes.

The design of advanced functionality in superconducting electronics usually focuses on materials engineering, either in heterostructures or in compounds of unconventional quantum materials. Here we demonstrate a different strategy to bespoke function by controlling the 3D shape of superconductors on the micron-scale. As a demonstration, a large superconducting diode effect is engineered solely by 3D shape design of a conventional superconductor, ion-beam deposited tungsten.

Gate-Defined Single-Electron Transistors in Twisted Bilayer Graphene.

Twisted bilayer graphene (tBLG) near the magic angle is a unique platform where the combination of topology and strong correlations gives rise to exotic electronic phases. These phases are gate-tunable and related to the presence of flat electronic bands, isolated by single-particle band gaps. This enables gate-controlled charge confinements, essential for the operation of single-electron transistors (SETs), and allows one to explore the interplay of confinement, electron interactions, band renormalization, and the moiré superlattice, potentially revealing key paradigms of strong correlations.

Probing optically driven KC thin films with an ultrafast voltmeter.

Optically enhanced superconductivity in KC is supported by transient optical spectra, by pressure responses, and by ultrafast nonlinear transport measurements. However, the underlying physics and in fact the similarity or dissimilarity to most properties of equilibrium superconductivity are not clear. In this paper, we study the ultrafast voltage response of optically driven KC thin films.

Local-Density Correlation Functional from the Force-Balance Equation.

The force-balance equation of time-dependent density-functional theory presents a promising route toward obtaining approximate functionals; however, so far, no practical correlation functionals have been derived this way. In this work, starting from a correlated wave function proposed originally by Colle and Salvetti [Theoret. Chim.

Largely Enhanced Oxygen Evolution Performance in a Ferromagnetic Ruddlesden-Popper Phase Cobaltate.

The oxygen evolution reaction (OER), as a key half-reaction in water splitting, plays a critical role in various energy conversion and storage systems. Novel high-performance catalysts have always been desirable. In this work, we have fabricated insulating perovskite LaCoO and its corresponding Ruddlesden-Popper (RP) phase LaCoO films.

On-demand heralded MIR single-photon source using a cascaded quantum system.

We propose a mechanism for generating single photons in the mid-infrared (MIR) using a solid-state or molecular quantum emitter. The scheme uses cavity quantum electrodynamics (QED) effects to selectively enhance a Frank-Condon transition, deterministically preparing a single Fock state of a polar phonon mode. By coupling the phonon mode to an antenna, the resulting excitation is then radiated to the far field as a single photon with a frequency matching the phonon mode.

Probing water-electrified electrode interfaces: Insights from Au and Pd.

The water/electrode interface under an applied bias potential is a challenging out-of-equilibrium phenomenon, which is difficult to accurately model at the atomic scale. In this study, we employ a combined approach of density functional theory and non-equilibrium Green's function methods to analyze the influence of an external bias on the properties of water adsorbed on Au(111) and Pd(111) metallic electrodes. Our results demonstrate that while both Au and Pd-electrodes induce qualitatively similar structural responses in adsorbed water molecules, the quantitative differences are substantial, driven by the distinct nature of water-metal bonding.

Frozen non-equilibrium dynamics of exciton Mott insulators in moiré superlattices.

Moiré superlattices, such as those formed from transition metal dichalcogenide heterostructures, have emerged as an exciting platform for exploring quantum many-body physics. They have the potential to serve as solid-state analogues to ultracold gases for quantum simulations. A key open question is the coherence and dynamics of the quantum phases arising from photoexcited moiré excitons, particularly amid dissipation.

Correlated states controlled by a tunable van Hove singularity in moiré WSe bilayers.

Twisted transition metal dichalcogenide (TMD) bilayers have enabled the discovery of superconductivity, ferromagnetism, correlated insulators, and a series of new topological phases of matter. However, the connection between these electronic phases of matter and the underlying band structure singularities has remained largely unexplored. Here, combining magnetic circular dichroism and exciton sensing measurements, we investigate the influence of a van Hove singularity (vHS) on the correlated phases in bilayer WSe with twist angle between 2 and 3 degrees.

Learning the Electrostatic Response of the Electron Density through a Symmetry-Adapted Vector Field Model.

A current challenge in atomistic machine learning is that of efficiently predicting the response of the electron density under electric fields. We address this challenge with symmetry-adapted kernel functions that are specifically derived to account for the rotational symmetry of a three-dimensional vector field. We demonstrate the equivariance of the method on a set of rotated water molecules and show its high efficiency with respect to number of training configurations and features for liquid water and naphthalene crystals.

Nonadiabatic Dynamics with the Mapping Approach to Surface Hopping (MASH).

The mapping approach to surface hopping (MASH) combines the rigor of quasiclassical mapping approaches with the pragmatism of surface hopping to obtain a practical trajectory-based method for simulating nonadiabatic dynamics in molecular systems. In this review, we outline the derivation of MASH, prove a number of important properties that ensure its reliability, and illustrate its accuracy for computing nonadiabatic rate constants as well as ultrafast photochemical dynamics.

Quantum-Electrodynamical Density-Functional Theory Exemplified by the Quantum Rabi Model.

The key features of density-functional theory (DFT) within a minimalistic implementation of quantum electrodynamics are demonstrated, thus allowing to study elementary properties of quantum-electrodynamical density-functional theory (QEDFT). We primarily employ the quantum Rabi model that describes a two-level system coupled to a single photon mode and also discuss the Dicke model, where multiple two-level systems couple to the same photon mode. In these settings, the density variables of the system are the polarization and the displacement of the photon field.

Super-Resolution Imaging of Nanoscale Inhomogeneities in hBN-Covered and Encapsulated Few-Layer Graphene.

Encapsulating few-layer graphene (FLG) in hexagonal boron nitride (hBN) can cause nanoscale inhomogeneities in the FLG, including changes in stacking domains and topographic defects. Due to the diffraction limit, characterizing these inhomogeneities is challenging. Recently, the visualization of stacking domains in encapsulated four-layer graphene (4LG) has been demonstrated with phonon polariton (PhP)-assisted near-field imaging.

Correlation-driven attosecond photoemission delay in the plasmonic excitation of C fullerene.

Extreme light confinement in plasmonic nanosystems enables novel applications in photonics, sensor technology, energy harvesting, biology, and quantum information processing. Fullerenes represent an extreme case for nanoplasmonics: They are subnanometer carbon-based molecules showing high-energy and ultrabroad plasmon resonances; however, the fundamental mechanisms driving the plasmonic response and the corresponding collective electron dynamics are still elusive. Here, we uncover the dominant role of electron correlations in the dynamics of the giant plasmon resonance (GPR) of the subnanometer system C by using attosecond photoemission chronoscopy.