GPAW: An open Python package for electronic structure calculations.

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for the implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE), providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation, variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support for graphics processing unit (GPU) acceleration has been achieved with minor modifications to the GPAW code thanks to the CuPy library. We end the review with an outlook, describing some future plans for GPAW.

Electronic Raman scattering in the 2D antiferromagnet NiPS3.

Correlated-electron systems have long been an important platform for various interesting phenomena and fundamental questions in condensed matter physics. As a pivotal process in these systems, d-d transitions have been suggested as a key factor toward realizing optical spin control in two-dimensional (2D) magnets. However, it remains unclear how d-d excitations behave in quasi-2D systems with strong electronic correlation and spin-charge coupling. Here, we present a systematic electronic Raman spectroscopy investigation on d-d transitions in a 2D antiferromagnet­NiPS3, from bulk to atomically thin samples. Two electronic Raman modes originating from the scattering of incident photons with d electrons in Ni2+ ions are observed at ~1.0 eV. This electronic process persists down to trilayer flakes and exhibits insensitivity to the spin ordering of NiPS3. Our study demonstrates the utility of electronic Raman scattering in investigating the unique electronic structure and its coupling to magnetism in correlated 2D magnets.

Microscopic Theory of Plasmons in Substrate-Supported Borophene.

We compute the dielectric properties of freestanding and metal-supported borophene from first-principles time-dependent density functional theory. We find that both the low- and high-energy plasmons of borophene are fully quenched by the presence of a metallic substrate at borophene-metal distances smaller than ≃9 Å. Based on these findings, we derive an electrodynamic model of the interacting, momentum-dependent polarizability for a two-dimensional metal on a model metallic substrate, which quantitatively captures the evolution of the dielectric properties of borophene as a function of metal-borophene distance. Applying this model to a series of metallic substrates, we show that maximizing the plasmon energy detuning between borophene and substrate is the key material descriptor for plasmonic performance.

Compact broadband low-loss taper for coupling to a silicon nitride photonic wire.

We demonstrate an ultra-compact waveguide taper on a silicon nitride platform. The proposed taper provides a coupling efficiency of 95% at a length of 19.5 µm in comparison to the standard linear taper of length 50 µm, which connects a 10 µm wide waveguide to a 1 µm wide photonic wire. The taper has a spectral response >75% spanning over 800 nm and resilience to fabrication variations; ±200 nm change in taper and end waveguide width varies transmission by <5%. We experimentally demonstrate taper insertion loss of <0.1 dB/transition for a taper as short as 19.5 µm, and reduce the footprint of the photonic device by 50.8% compared to the standard adiabatic taper. To the best of our knowledge, the proposed taper is the shortest waveguide taper ever reported in silicon nitride.

Spectroscopic Study of the Reversible Chemical Reduction and Reoxidation of Substitutional Cr Ions in Sr2TiO4.

The solid-state synthesis and controllable speciation of Cr dopants in the layered perovskite Sr2TiO4 is reported. We employed a chemical reduction procedure with NaBH4 at relatively mild temperatures (<450 °C) to impart sensitive control over the relative concentration of Cr3+ dopants, the charge-state of oxygen-vacancy defects, and presence of Ti3+ defects in highly reduced Cr-doped Sr2TiO4. The electron paramagnetic resonance (EPR) spectra of the reduced powder samples reveal a 12-fold increase in the Cr3+ concentration within the axially compressed Ti4+-site of the Sr2TiO4 host. The increase in Cr3+ content is achieved through the reduction of higher-valence Cr ions that are either EPR silent or diamagnetic. The spin-Hamiltonian parameters for Cr3+ substituted at the B-site of Sr2TiO4 were refined to D = -201 × 10-4 cm-1, g⊥ = 1.980, and g∥ = 1.978. In addition, the Cr3+ ion exhibits a temperature-dependent axial component to the zero-field splitting of the 4A2 ground term that is accounted for by ligand field theory and an isotropic contraction of the Sr2TiO4 lattice with decreasing temperature. The observed changes to the electronic structure upon reduction are quantitatively reversible upon reoxidation of the sample under aerobic annealing at the same temperature and duration as the reduction conditions. This temperature dependence of the Cr3+ content in the Cr-doped Sr2TiO4 powders is discussed and contrasted to our recent study on Cr-doped SrTiO3.