Real-space, real-time approach to quantum-electrodynamical time-dependent density functional theory.

The quantum-electrodynamical time-dependent density functional theory equations are solved by time propagating the wave function on a tensor product of a Fock-space and real-space grid. Applications for molecules in cavities show the accuracy of the approach. Examples include the coupling strength and light frequency dependence of the energies, wave functions, optical absorption spectra, and Rabi splitting magnitudes in cavities, as well as a description of high harmonic generation in cavities.

Laser-Driven Petahertz Electron Ratchet Nanobubbles.

A laser-driven quantum electron ratchet nanodevice is proposed. The ratchet consists of a series of disconnected bubble-shaped nanodiode structures with a sharp tip to induce a large field enhancement. A laser pulse is used to create a plasmon oscillation in the vertical direction, and the shape of the bubble funnels the electrons toward the sharp tip leading to net electron transport in the horizontal direction. The electron current carries the fingerprint of the driving laser field. The system is modeled by using the time-dependent orbital free density functional theory with nanostructures containing thousands of atoms.

Deformed explicitly correlated Gaussians.

Deformed explicitly correlated Gaussian (DECG) basis functions are introduced, and their matrix elements are calculated. All matrix elements can be calculated analytically in a closed form, except the Coulomb one, which has to be approximated by a Gaussian expansion. The DECG basis functions can be used to solve problems with nonspherical potentials. One example of such potential is the dipole self-interaction term in the Pauli-Fierz Hamiltonian. Examples are presented showing the accuracy and necessity of deformed Gaussian basis functions to accurately solve light-matter coupled systems in cavity QED.

2D PtS nanorectangles/g-C3N4 nanosheets with a metal sulfide-support interaction effect for high-efficiency photocatalytic H2 evolution.

Cocatalyst design is a key approach to acquire high solar-energy conversion efficiency for photocatalytic hydrogen evolution. Here a new in situ vapor-phase (ISVP) growth method is developed to construct the cocatalyst of 2D PtS nanorectangles (a length of ∼7 nm, a width of ∼5 nm) on the surface of g-C3N4 nanosheets. The 2D PtS nanorectangles/g-C3N4 nanosheets (PtS/CN) show an unusual metal sulfide-support interaction (MSSI), which is evidenced by atomic resolution HAADF-STEM, synchrotron-based GIXRD, XPS and DFT calculations. The effect of MSSI contributes to the optimization of geometrical structure and energy-band structure, acceleration of charge transfer, and reduction of hydrogen adsorption free energy of PtS/CN, thus yielding excellent stability and an ultrahigh photocatalytic H2 evolution rate of 1072.6 µmol h-1 (an apparent quantum efficiency of 45.7% at 420 nm), up to 13.3 and 1532.3 times by contrast with that of Pt nanoparticles/g-C3N4 nanosheets and g-C3N4 nanosheets, respectively. This work will provide a new platform for designing high-efficiency photocatalysts for sunlight-driven hydrogen generation.

Simulation of photo-electron spectrum and electron scattering by dual time propagation.

A dual time propagation approach is introduced to describe electron scattering and ionization. The space is divided into two regions, a central region with a full time-dependent Hamiltonian and an outer region where the kinetic operator and the laser field dominate. The two regions are connected by a source term. Time-dependent density functional theory calculations of wave packet scattering on molecules and photoelectron spectrum due to circularly polarized laser are presented to illustrate the efficiency and applicability of the approach.

Catalytically potent and selective clusterzymes for modulation of neuroinflammation through single-atom substitutions.

Emerging artificial enzymes with reprogrammed and augmented catalytic activity and substrate selectivity have long been pursued with sustained efforts. The majority of current candidates have rather poor catalytic activity compared with natural molecules. To tackle this limitation, we design artificial enzymes based on a structurally well-defined Au25 cluster, namely clusterzymes, which are endowed with intrinsic high catalytic activity and selectivity driven by single-atom substitutions with modulated bond lengths. Au24Cu1 and Au24Cd1 clusterzymes exhibit 137 and 160 times higher antioxidant capacities than natural trolox, respectively. Meanwhile, the clusterzymes demonstrate preferential enzyme-mimicking catalytic activities, with Au25, Au24Cu1 and Au24Cd1 displaying compelling selectivity in glutathione peroxidase-like (GPx-like), catalase-like (CAT-like) and superoxide dismutase-like (SOD-like) activities, respectively. Au24Cu1 decreases peroxide in injured brain via catalytic reactions, while Au24Cd1 preferentially uses superoxide and nitrogenous signal molecules as substrates, and significantly decreases inflammation factors, indicative of an important role in mitigating neuroinflammation.

Stochastic Variational Approach to Small Atoms and Molecules Coupled to Quantum Field Modes in Cavity QED.

In this work, we present a stochastic variational calculation (SVM) of energies and wave functions of few particle systems coupled to quantum fields in cavity QED. The spatial wave function and the photon spaces are optimized by a random selection process. Using correlated basis functions, the SVM approach solves the problem accurately and opens the way to the same precision that is reached the nonlight coupled quantum systems. Examples for a two-dimensional trion and confined electrons as well as for the He atom and the H_{2} molecule are presented showing that the light-matter coupling drastically changes the electronic states.

Ground and excited state exciton polarons in monolayer MoSe2.

Monolayer transition metal dichalcogenide semiconductors, with versatile experimentally accessible exciton species, offer an interesting platform for investigating the interaction between excitons and a Fermi sea of charges. Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact of charge density tuning on the A and B series of exciton Rydberg states, including A:1s, A:2s, B:1s, and B:2s. The doping dependence of the A:2s state provides an opportunity to examine such interactions with greatly reduced exciton binding energy and more spatially diffuse structures, and we found that the impact of the Fermi sea becomes much more dramatic compared to the A:1s state. Using photoluminescence upconversion, we verify that the B:2s exciton state displays similar behavior when interacting with the Fermi sea despite being well above the bare bandgap in energy. Photoluminescence and reflection spectra of the A:1s state show clear evidence that the interaction of the exciton with a Fermi sea is best described by the exciton-polaron model, rather than a trion model. Our experimental results demonstrate that overall features of charge interaction are quite generic and highly robust, offering key insights into the dressed many body states in a Fermi sea.

Simulation of a hydrogen atom in a laser field using the time-dependent variational principle.

The time-dependent variational principle is used to optimize the linear and nonlinear parameters of Gaussian basis functions to solve the time-dependent Schrödinger equation in one and three dimensions for a one-body soft Coulomb potential in a laser field. The accuracy is tested comparing the solution to finite difference grid calculations using several examples. The approach is not limited to one particle systems and the example presented for two electrons demonstrates the potential to tackle larger systems using correlated basis functions.

Time propagation of the coupled Maxwell and Kohn-Sham equations using the Riemann-Silberstein formalism.

The coupled Maxwell and time-dependent Kohn-Sham equations are solved using the Riemann-Silberstein vectors to represent the electromagnetic fields. Momentum-space time propagation of the Riemann-Silberstein vectors are proposed and test calculations are presented to show the accuracy of the approach.

Publisher Correction: Controlled dynamic screening of excitonic complexes in 2D semiconductors.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Controlled dynamic screening of excitonic complexes in 2D semiconductors.

We report a combined theoretical/experimental study of dynamic screening of excitons in media with frequency-dependent dielectric functions. We develop an analytical model showing that interparticle interactions in an exciton are screened in the range of frequencies from zero to the characteristic binding energy depending on the symmetries and transition energies of that exciton. The problem of the dynamic screening is then reduced to simply solving the Schrodinger equation with an effectively frequency-independent potential. Quantitative predictions of the model are experimentally verified using a test system: neutral, charged and defect-bound excitons in two-dimensional monolayer WS2, screened by metallic, liquid, and semiconducting environments. The screening-induced shifts of the excitonic peaks in photoluminescence spectra are in good agreement with our model.

Exponential integrators in time-dependent density-functional calculations.

The integrating factor and exponential time differencing methods are implemented and tested for solving the time-dependent Kohn-Sham equations. Popular time propagation methods used in physics, as well as other robust numerical approaches, are compared to these exponential integrator methods in order to judge the relative merit of the computational schemes. We determine an improvement in accuracy of multiple orders of magnitude when describing dynamics driven primarily by a nonlinear potential. For cases of dynamics driven by a time-dependent external potential, the accuracy of the exponential integrator methods are less enhanced but still match or outperform the best of the conventional methods tested.

Accurate time propagation method for the coupled Maxwell and Kohn-Sham equations.

An accurate method for time propagation of the coupled Maxwell and time-dependent Kohn-Sham (TDKS) equation is presented. The new approach uses a simultaneous fourth-order Runge-Kutta-based propagation of the vector potential and the Kohn-Sham orbitals. The approach is compared to the conventional fourth-order Taylor propagation and predictor-corrector methods. The calculations show several computational and numerical advantages, including higher computational performance, greater stability, better accuracy, and faster convergence.

Accuracy and computational efficiency of real-time subspace propagation schemes for the time-dependent density functional theory.

Time-dependent Density Functional Theory (TDDFT) has become successful for its balance of economy and accuracy. However, the application of TDDFT to large systems or long time scales remains computationally prohibitively expensive. In this paper, we investigate the numerical stability and accuracy of two subspace propagation methods to solve the time-dependent Kohn-Sham equations with finite and periodic boundary conditions. The bases considered are the Lánczos basis and the adiabatic eigenbasis. The results are compared to a benchmark fourth-order Taylor expansion of the time propagator. Our results show that it is possible to use larger time steps with the subspace methods, leading to computational speedups by a factor of 2-3 over Taylor propagation. Accuracy is found to be maintained for certain energy regimes and small time scales.

Matrix elements of explicitly correlated Gaussian basis functions with arbitrary angular momentum.

A new algorithm for calculating the Hamiltonian matrix elements with all-electron explicitly correlated Gaussian functions for quantum-mechanical calculations of atoms with arbitrary angular momentum is presented. The calculations are checked on several excited states of three and four electron systems. The presented formalism can be used as unified framework for high accuracy calculations of properties of small atoms and molecules.

Structural, electronic and vibrational properties of few-layer 2H- and 1T-TaSe2.

Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. Few-layer tantalum diselenides (TaSe2) are typical metallic TMDs exhibiting rich CDW phase transitions. However, a description of the structural, electronic and vibrational properties for different crystal phases and stacking configurations, essential for interpretation of experiments, is lacking. We present first- principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries. Evolution of the Fermi surfaces as well as the phonon dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material. Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.

Excited Biexcitons in Transition Metal Dichalcogenides.

The Stochastic Variational Method (SVM) is used to show that the effective mass model correctly estimates the binding energies of excitons and trions but fails to predict the experimental binding energy of the biexciton. Using high-accuracy variational calculations, it is demonstrated that the biexciton binding energy in transition metal dichalcogenides is smaller than the trion binding energy, contradicting experimental findings. It is also shown that the biexciton has bound excited states and that the binding energy of the L = 0 excited state is in very good agreement with experimental data. This excited state corresponds to a hole attached to a negative trion and may be a possible resolution of the discrepancy between theory and experiment.

Real-space, real-time calculation of dynamic hyperpolarizabilities.

The finite-difference method to calculate hyperpolarizabilities is generalized for dynamical case. The calculation of the dynamical hyperpolarizabilities from non-perturbative, explicitly time-dependent single particle states obtained in the framework of the time-dependent density functional theory, is implemented in real space and real time. The optical response functions up to the third order are extracted in frequency domain. The present approach is free of deficiencies associated with atom centered basis sets and allows treatment of large molecules. The calculated results are in good agreement with experiments and with other theoretical calculations for various test cases.