An adaptive prefix-assignment technique for symmetry reduction

This paper presents a technique for symmetry reduction that adaptively assigns a prefix of variables in a system of constraints so that the generated prefix-assignments are pairwise nonisomorphic under the action of the symmetry group of the system. The technique is based on McKay's canonical extension framework [J.~Algorithms 26 (1998), no.~2, 306--324]. Among key features of the technique are (i) adaptability---the prefix sequence can be user-prescribed and truncated for compatibility with the group of symmetries; (ii) parallelizability---prefix-assignments can be processed in parallel independently of each other; (iii) versatility---the method is applicable whenever the group of symmetries can be concisely represented as the automorphism group of a vertex-colored graph; and (iv) implementability---the method can be implemented relying on a canonical labeling map for vertex-colored graphs as the only nontrivial subroutine. To demonstrate the practical applicability of our technique, we have prepared an experimental open-source implementation of the technique and carry out a set of experiments that demonstrate ability to reduce symmetry on hard instances. Furthermore, we demonstrate that the implementation effectively parallelizes to compute clusters with multiple nodes via a message-passing interface.Comment: Updated manuscript submitted for revie

Effect of correlation and dielectric confinement on 1S1/2(e)nS3/2(h)Excitons in CdTe/CdSe and CdSe/CdTe Type-II quantum dots

Controlled reduction of graphene oxide is an alternative and promising method to tune the electronic and optically active energy gap of this two-dimensional material in the energy range of the visible light spectrum. By means of ab initio calculations, based on hybrid density functional theory, that combine the Hartree–Fock method with the generalized gradient approximation (GGA), we investigated the electronic, optical, and radiative recombination properties of partially reduced graphene oxide, modelled as small islands of pristine graphene formed in an infinite sheet of graphene oxide. We predict that tuning of optically active gaps, in the wide range from ∼6.5 eV to ∼0.25 eV, followed by the electron radiative transition times in the range from ns to μs, can be effected by controlling the level of oxidization

First-Principles Modeling of Core/Shell Quantum Dot Sensitized Solar Cells

We report on the density functional theory (DFT) modeling of core/shell quantum dot (QD) sensitized solar cells (QDSSCs), a device architecture that holds great potential in photovoltaics but has not been fully exploited so far. To understand the working mechanisms of this kind of solar cells, we have investigated ZnSe- and ZnSe/CdS-sensitized TiO2 models. Both the core-only and the core/shell QDs are predicted to strongly adsorb on the oxide surface, driven by the electrostatic interaction between the metal atoms on the QD surface and the O atoms exposed by the oxide substrate. Accordingly, the QD conduction states are strongly mixed with the TiO2 acceptor states, giving rise to bridge states that should funnel the interfacial electron transfer. Accordingly, quite fast electron injection processes are predicted, with computed rates of 135 and 163 fs. The back-electron transfer is much slower for ZnSe/CdS, due to the weak coupling between the newly injected charge and the holes trapped in the sensitizer core. Therefore, the core/shell QDs deliver much better efficiencies. Moreover, the interfacial dipole established between the TiO2-injected electrons and the holes confined in the QD are found to shift the conduction band edge of the oxide, which further improves the performance of the device in terms of the open circuit voltage (VOC). We believe that this work sets the ground for future computational works in the field, which could in turn guide the fabrication of new device architectures with improved efficiencies