Ewald summation on a helix : a route to self-consistent charge density-functional based tight-binding objective molecular dynamics

We explore the generalization to the helical case of the classical Ewald method, the harbinger of all modern self-consistent treatments of waves in crystals, including ab initio electronic structure methods. Ewald-like formulas that do not rely on a unit cell with translational symmetry prove to be numerically tractable and able to provide the crucial component needed for coupling objective molecular dynamics with the self-consistent charge density-functional based tight-binding treatment of the inter-atomic interactions. The robustness of the method in addressing complex hetero-nuclear nano- and bio-systems is demonstrated with illustrative simulations on a helical boron nitride nanotube, a screw dislocated zinc oxide nanowire, and an ideal DNA molecule

Enhancing ultraviolet spontaneous emission with a designed quantum vacuum

We determine how to alter the properties of the quantum vacuum at ultraviolet wavelengths to simultaneously enhance the spontaneous transition rates and the far field detection rate of quantum emitters. We find the response of several complex nanostructures in the 200 − 400 nm range, where many organic molecules have fluorescent responses, using an analytic decomposition of the electromagnetic response in terms of continuous spectra of plane waves and discrete sets of modes. Coupling a nanorod with an aluminum substrate gives decay rates up to 2.7 × 103 times larger than the decay rate in vacuum and enhancements of 824 for the far field emission into the entire upper semi-space and of 2.04 × 103 for emission within a cone with a 60º semi-angle. This effect is due to both an enhancement of the field at the emitter’s position and a reshaping of the radiation patterns near mode resonances and cannot be obtained by replacing the aluminum substrate with a second nanoparticle or with a fused silica substrate. These large decay rates and far field enhancement factors will be very useful in the detection of fluorescence signals, as these resonances can be shifted by changing the dimensions of th nanorod. Moreover, these nanostructures have potential for nano-lasing because the Q factors of these resonances can reach 107.9, higher than the Q factors observed in nano-lasers

Evaluation of E. M. fields and energy transport in metallic nanoparticles with near field excitation

We compare two ways of calculating the optical response of metallic nanoparticles illuminated by near field dipole sources. We develop tests to determine the accuracy of the calculations of internal and scattered fields of metallic nanoparticles at the boundary of the particles and in the far field. We verify the correct transport of energy by checking that the evaluation of the energy flux agrees at the surface of the particles and in the far field. A new test is introduced to check that the surface fields fulfill Maxwell's equations allowing evaluation of the validity of the internal field. Calculations of the scattering cross section show a faster rate of convergence for the principal mode theory. We show that for metallic particles the internal field is the most significant source of error

Plasmon modes in single gold nanodiscs

Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc

Directional-dependent thickness and bending rigidity of phosphorene

The strong mechanical anisotropy of phosphorene combined with the atomic-scale thickness challenges the commonly employed elastic continuum idealizations. Using objective boundary conditions and a density functional-based potential, we directly uncover the flexibility of individual α, β and γ phosphorene allotrope layers along an arbitrary bending direction. A correlation analysis with the in-plane elasticity finds that although a monolayer thickness cannot be defined in the classical continuum sense, an unusual orthotropic plate with a directional-dependent thickness can unambiguously describe the out-of-plane deformation of α and γ allotropes. Such decoupling of the in-plane and out-of-plane nanomechanics might be generic for two-dimensional materials beyond graphene

Optical control of scattering, absorption and lineshape in nanoparticles

We find exact conditions for the enhancement or suppression of internal and/or scattered fields in any smooth particle and the determination of their spatial distribution or angular momentum through the combination of simple fields. The incident fields can be generated by a single monochromatic or broad band light source, or by several sources, which may also be impurities embedded in the nanoparticle. We can design the lineshape of a particle introducing very narrow features in its spectral response

Collapsed carbon nanotubes : from nano to mesoscale via density functional theory-based tight-binding objective molecular modeling

Due to the inherent spatial and temporal limitations of atomistic modeling and the lack of efficient mesoscopic models, mesoscale simulation methods for guiding the development of super strong lightweight material systems comprising collapsed carbon nanotubes (CNTs) are currently missing. Here we establish a path for deriving ultra-coarse-grained mesoscopic distinct element method (mDEM) models directly from the quantum mechanical representation of a collapsed CNT. Atomistic calculations based on density functional-based tight-binding (DFTB) extended with Lennard-Jones interactions allow for the identification of the cross-section and elastic constants of an elastic beam idealization of a collapsed CNT. Application of the DFTB quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and angular symmetries exhibited by twisted and bent CNTs. The multiscale modeling chain established here is suitable for deriving ultra-coarse-grained mesoscopic models for a variety of microscopic filaments presenting complex interatomic bondings

Atomic Simulation Interface (ASI) : application programming interface for electronic structure codes

The Atomic Simulation Interface (ASI) is a native C-style API for density functional theory (DFT) codes. ASI provides an efficient way to import and export large arrays that describe electronic structure (e.g. Hamiltonian, overlap, and density matrices) from DFT codes that are typically monolithic. The ASI API is designed to be implemented and used with minimal performance penalty, avoiding, where possible, unnecessary data copying. It provides direct access to the internal data structures of a code, and reuses existing data distribution over MPI nodes. The ASI API also defines a set of functions that support classical, AIMD (ab initio molecular dynamics), and hybrid QM/MM simulations: exporting potential energy, forces, atomic charges, and electrostatic potential at user defined points, as well as importing nuclear coordinates and arbitrary external electrostatic potentials. The ASI API is implemented in the DFTB+ (Hourahine et al., 2020) and FHI-aims (Blum et al., 2009) codes. A Python wrapper for easy access to ASI functions is also freely available (asi4py). We hope that the ASI API will be widely adopted and used for development of universal and interoperable DFT codes without sacrificing efficiency for portability

Theoretical modelling of rare Earth dopants in GaN

We review theoretical investigations into the structure and electrical activity of rare earth (RE) dopants in III-V semiconductors especially GaN. Substitutional rare earth dopants in GaN are found to be electrically inactive and require another defect to enable them to act as strong exciton traps. In contrast AlN is distinctive as it possesses a deep donor level. The electronic structure of complexes of the RE with other defects is discussed along with implications for efficient room temperature luminescence