Ab initio optical and energy loss spectra of transition metal monopnictides TaAs, TaP, NbAs, and NbP

Transition metal monopnictides represent a new class of topological semimetals with low-energy excitations, namely, Weyl fermions. We report optical properties across a wide spectral energy range for TaAs, TaP, NbAs and NbP, calculated within density functional theory. Spectra are found to be somewhat independent of the anion and the light polarization. Their features are explained in terms of the upper $s$, $p$, $d$, and $f$ electrons. Characteristic absorption features are related to the frequency dependence of the Fresnel reflectivity. While the lower part of the energy loss spectra is dominated by plasmonic features, the high-energy structures are explained by interband transitions.Comment: Added reference 21 to "S.-Y. Xu, et al, Science 349, 613 (2015)

Dielectric tensor of monoclinic Ga2_2O3_3 single crystals in the spectral range 0.5−8.5 0.5 - 8.5\,eV

The dielectric tensor of $\beta$-Ga$_2$O$_3$ was determined by generalized spectroscopic ellipsometry in a wide spectral range from $0.5\,\mathrm{eV}$ to $8.5\,\mathrm{eV}$ as well as by calculation including quasiparticle bands and excitonic effects. The dielectric tensors obtained by both methods are in excellent agreement with each other and the observed transitions in the dielectric function are assigned to the corresponding valence bands. It is shown that the off-diagonal element of the dielectric tensor reaches values up to $|\varepsilon_{xz} | \approx 0.30$ and cannot be neglected. Even in the transparent spectral range where it is quite small ($|\varepsilon_{xz} | < 0.02$) it causes a rotation of the dielectric axes around the symmetry axis of up to $20^\circ$

Giant excitonic absorption and emission in two-dimensional group-III nitrides

Absorption and emission of pristine-like semiconducting monolayers of BN, AlN, GaN, and InN are here systematically studied by ab-initio methods. We calculate the absorption spectra for in-plane and out-of-plane light polarization including quasiparticle and excitonic effects. Chemical trends with the cation of the absorption edge and the exciton binding are discussed in terms of the band structures. Exciton binding energies and localization radii are explained within the Keldysh model for excitons in two dimensions. The strong excitonic effects are due to the interplay of low dimensionality, confinement effects, and reduced screening. We find exciton radiative lifetimes ranging from tenths of picoseconds (BN) to tenths of nanoseconds (InN) at room temperature, thus making 2D nitrides, especially InN, promising materials for light-emitting diodes and high-performance solar cells

Electronic and Optical Properties of Alkaline Earth Metal Fluoride Crystals with the Inclusion of Many-Body Effects: A Comparative Study on Rutile MgF2 and Cubic SrF2

We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF2 and cubic SrF2 . For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF2 has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF2 and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF2 , cubic SrF2 , and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems

Infrared absorbance of silicene and germanene

n/

Structural, Electronic and Vibrational Properties of B24N24 Nanocapsules: Novel Anodes for Magnesium Batteries

We report on DFT-TDDFT studies of the structural, electronic and vibrational properties of B24N24 nanocapsules and the effect of encapsulation of homonuclear diatomic halogens (Cl-2, Br-2 and I-2) and chalcogens (S-2 and Se-2) on the interaction of the B24N24 nanocapsules with the divalent magnesium cation. In particular, to foretell whether these BN nanostructures could be proper negative electrodes for magnesium-ion batteries, the structural, vibrational and electronic properties, as well as the interaction energy and the cell voltage, which is important for applications, have been computed for each system, highlighting their differences and similarities. The encapsulation of halogen and chalcogen diatomic molecules increases the cell voltage, with an effect enhanced down groups 16 and 17 of the periodic table, leading to better performing anodes and fulfilling a remarkable cell voltage of 3.61 V for the iodine-encapsulated system

Transitions in Xenes between excitonic, topological and trivial insulator phases: influence of screening, band dispersion and external electric field

Using a variational approach, the binding energies $E_b$ of the lowest bound excitons in Xenes under varying electric field are investigated. The internal exciton motion is described both by Dirac electron dispersion and in effective-mass approximation, while the screened electron-hole attraction is modeled by a Rytova-Keldysh potential with a 2D electronic polarizability $\alpha_{2{\rm D}}$. The most important parameters as spin-orbit-induced gap $E_g$, Fermi velocity $v_F$ and $\alpha_{2{\rm D}}$ are taken from ab initio density functional theory calculations. In addition, $\alpha_{2{\rm D}}$ is approximated in two different ways. The relation of $E_b$ and $E_g$ is ruled by the screening. The existence of an excitonic insulator phase with $E_b>E_g$ sensitively depends on the chosen $\alpha_{2{\rm D}}$. The values of $E_g$ and $\alpha_{2{\rm D}}$ are strongly modified by a vertical external electric bias $U$, which defines a transition from the topological into a trivial insulator at $U=E_g/2$, with the exception of plumbene. Within the Dirac approximation, but also within the effective mass description of the kinetic energy, the treatment of screening dominates the appearance or non-appearance of an excitonic insulator phase. Gating does not change the results: the prediction done at zero electric field is confirmed when a vertical electric field is applied. Finally, Many-Body perturbation theory approaches based on the Green's function method, applied to stanene, confirm the absence of an excitonic insulator phase, thus validating our results obtained by ab initio modeling of $\alpha_{2{\rm D}}$