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Density functional theory (DFT) and thermal DFT (thDFT) calculations were used to evaluate the energy band structure, bandgap, and the total energy of various graphene quantum dots (GQDs). The DFT calculations were performed using local density approximation for the exchange-correlation functional and norm-conserving pseudopotentials. We consider the triangular and hexagonal GQDs with zigzag and armchair edges and 1-3 nm dimensions with many hundred atoms. The simulation results show that all of these GQDs are direct bandgap semiconductors with a flat band structure, and they are suitable for electronics and optoelectronics applications. Analysis of GQDs in which the A and B sublattice symmetries were broken showed degenerate zero-energy shells. Using the thDFT calculations carried out at temperatures up to 1400 K, we evaluated the temperature dependence of the GQDs bandgaps and total energies via entropy-term and electron kinetic energy. The obtained results indicate that the ground-state DFT calculations are valid for determining the electronic properties of GQDs up to room temperature. Moreover, we tune semi-empirical parameters of the tight-binding model by the DFT results in small GQDs to reduce the computational cost of electronic structure calculations for large GQDs, which contained up to thousands of atoms.

We present a rigorous and quantum-consistent description of dispersive non-Hermitian optical bilayers in the framework of the canonical quantization scheme. Then we investigate the propagation of a normally incident squeezed coherent state of light through such media, particularly at a frequency for which the bilayers become parity-time (PT) symmetric. Furthermore, to check the realization of PT-symmetry in quantum optics, we reveal how dispersion and loss/gain-induced noises and thermal effects in such bilayers can affect quantum features of the incident light, such as squeezing and sub-Poissonian statistics. The numerical results show thermally-induced noise at room temperature has an insignificant effect on the propagation properties in these non-Hermitian bilayers. Moreover, tuning the bilayers loss/gain strength, we show that the transmitted squeezed coherent states through the structure can retain to some extent their nonclassical characteristics, specifically for the frequencies far from the emission frequency of the gain layer. Furthermore, we demonstrate, only below a critical value of gain, quantum optical effective medium theory can correctly predict the propagation of quantized waves in non-Hermitian and PT-symmetric bilayers., Comment: 8 figures

Unique properties of lead chalcogenides have enabled multiple exciton generation (MEG) in their nanocrystals that can be beneficial in enhancing the efficiency of the third generation solar cells. Although the intrinsic electric field plays an imperative role in a solar cell, its effect on the multiple exciton generation (MEG) has been overlooked, so far. Using EOM-CCSD as a many-body approach, we show that any electric field can affect the absorptivity spectra of the lead chalcogenide nanocrystals (Pb4Te4, Pb4Se4, and Pb4S4). The same electric field, however, has insignificant effects on the MEG quantum probabilities and the thresholds in these nanocrystals. Furthermore, simulations show that Pb4Te4, among the aforementioned nanocrystals, has the lowest MEG threshold and the strongest absorptivity peak that is located in the multi-excitation window, irrespective of the field strength, making it the most suitable candidate for MEG applications. Simulations also demonstrate that an electric field affects the MEG characteristics in the Pb4Te4 nanocrystal, in general, less than it perturbs MEG characteristics in Pb4Se4 and Pb4S4 nanocrystals. Our results can have a great impact in designing optoelectronic devices whose performance can be significantly influenced by MEG., Comment: Twenty pages, Six figures, Forty seven References