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Register a new userFirst-principles study of excitons in the optical spectra of silver chloride
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Silver chloride is a material that has been investigated and used for many decades. Of particular interest are its optical properties, but only few fundamental theoretical studies exist. We present first-principles results for the optical properties of AgCl, obtained using time-dependent density functional theory and many-body perturbation theory. We show that optical properties exhibit strong excitonic effects, which are correctly captured only by solving the Bethe-Salpeter equation starting from quasiparticle self-consistent GW results. Numerical simulations are made feasible by using a model screening for the electron-hole interaction in a way that avoids the calculation of the static dielectric constant. A thorough analysis permits us to discuss localization in bright and dark excitons of silver chloride.
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Recent studies on excitons in two-dimensional materials have been widely conducted for their potential usages for novel electronic and optical devices. Especially, sophisticated manipulation techniques of quantum degrees of freedom of excitons are demanded. In this paper we propose a technique of forming an optical dipole trap for excitons in graphane, a two-dimensional wide gap semiconductor, based on first principles calculations. We develop a first principles method to evaluate the exciton transition dipole matrix and combine it with the density functional theory and GW+BSE calculations. We reveal that in graphane the huge exciton binding energy and the large dipole moments of Wannier-like excitons enable us to induce the dipole trap of the order of meV depth and $mu$m width. This work opens a new way to control light-exciton interacting systems based on a newly developed numerically robust ab initio calculations.
Cu$_2$ZnSnS$_4$ is an earth-abundant photovoltaic absorber material predicted to provide a sustainable solution for commercial solar applications. One of the main limitations restricting its commercialization is the issue of cation disorder. Raman spectroscopy has been a sought after technique to characterize disorder in CZTS while a clear consensus between theoretical and experimental results is yet to be achieved. In the present study, via the virtual crystal approximation, we take into account the progressive nature of Cu-Zn disorder in CZTS: we obtain the phonon frequencies at zone-center within the density functional perturbation theory formalism, and further compute the Raman spectra for the disordered phases, achieving a consensus between theory and experiment. These calculations confirm the presence of complete disorder in Cu-Zn 2$a$, 2$c$ and 2$d$ Wyckoff sites. They also show that the Raman intensities of two prominent $A$ phonon modes characterized by motion of S atoms, also known to be experimentally significant, play a key role in understanding the nature of disorder in CZTS.
The optical and electronic properties of Mg-Ti hydrides are studied using first-principles density functional theory. Dielectric functions are calculated for MgxTi(1-x)H2 with compositions x = 0.5, 0.75, and 0.875. The structure is that of fluorite TiH2 where both Mg and Ti atoms reside at the Ti positions of the lattice. In order to assess the effect of randomness in the Mg and Ti occupations we consider both highly ordered structures, modeled with simple unit cells of minimal size, and models of random alloys. These are simulated by super cells containing up to 64 formula units (Z = 64). All compositions and structural models turn out metallic, hence the dielectric functions contain interband and intraband free electron contributions. The former are calculated in the independent particle random phase approximation. The latter are modeled based upon the intraband plasma frequencies, which are also calculated from first-principles. Only for the models of the random alloys we obtain a black state, i.e. low reflection and transmission in the energy range from 1 to 6 eV.
The linear polarizability absorption spectra of the double-walled carbon nanotubes (DWNTs) have been calculated by using the tight-binding (TB) model and sum-over-state (SOS) method, supplemented by the first principles CASTEP calculations. It is found that the chiral symmetries of both outer and inner tubes in the DWNTs can always be identified distinctly by the characteristic peaks in the absorption spectra of the DWNTs, no matter what kind of the outer tube is, offering a powerful experimental tool to measure precisely the chiral angle of the inner tube of a DWNT.
Recent experiments reported giant magnetoresistance at room temperature in LaOMnAs. Here a density functional theory calculation is performed to investigate magnetic properties of LaOMnAs. The ground state is found to be the G-type antiferromagnetic order within the $ab$ plane but coupled ferromagnetically between planes, in agreement with recent neutron investigations. The electronic band structures suggest an insulating state which is driven by the particular G-type magnetic order, while a metallic state accompanies the ferromagnetic order. This relation between magnetism and conductance may be helpful to qualitatively understand the giant magnetoresistance effects.
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