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We investigate the effects of spin-orbit coupling on the optical response of materials. In particular, we study the effects of the commutator between the spin-orbit coupling part of the potential and the position operator on the optical matrix elements. Using a formalism that separates a fullyrelativistic Kleinman-Bylander pseudopotential into the scalar-relativistic and spin-orbit-coupling parts, we calculate the contribution of the commutator arising from spin-orbit coupling to the squared optical matrix elements of isolated atoms, monolayer transition metal dichalcogenides, and topological insulators. In the case of isolated atoms from H ($Z = 1$) to Bi ($Z = 83$), the contribution of spin-orbit coupling to the squared matrix elements can be as large as 14 %. On the other hand, in the cases of monolayer transition metal dichalcogenides and topological insulators, we find that this contribution is less than 1 % and that it is sufficient to calculate the optical matrix elements and subsequent physical quantities without considering the commutator arising from spin-orbit coupling., Comment: 13 pages, 14 figures

Most theoretical studies of nanoscale transport in molecular junctions rely on the combination of the Landauer formalism with Kohn-Sham density functional theory (DFT) using standard local and semilocal functionals to approximate exchange and correlation effects. In many cases, the resulting conductance is overestimated with respect to experiments. Recent works have demonstrated that this discrepancy may be reduced when including many-body corrections on top of DFT. Here we study benzene-dithiol (BDT) gold junctions and analyze the effect of many-body perturbation theory (MBPT) on the calculation of the conductance with respect to different bonding geometries. We find that the many-body corrections to the conductance strongly depend on the metal-molecule coupling strength. In the BDT junction with the lowest coupling, many-body corrections reduce the overestimation on the conductance to a factor two, improving the agreement with experiments. In contrast, in the strongest coupling cases, many-body corrections on the conductance are found to be sensibly smaller and standard DFT reveals a valid approach., Comment: 9 pages, 4 figures

We present a straightforward, noniterative projection scheme that can represent the electronic ground state of a periodic system on a finite atomic-orbital-like basis, up to a predictable number of electronic states and with controllable accuracy. By cofiltering the projections of plane-wave Bloch states with high-kinetic-energy components, the richness of the finite space and thus the number of exactly-reproduced bands can be selectively increased at a negligible computational cost, an essential requirement for the design of efficient algorithms for electronic structure simulations of realistic material systems and massive high-throughput investigations. © 2013 American Physical Society.

The conductance of single molecule junctions is calculated using a Landauer approach combined with many-body perturbation theory to account for electron correlation. Contrary to intuition, a mere correction of the density-functional theory eigenvalues is found not to affect noticeably the zero-bias conductance. To improve the agreement with experiments, it is necessary to go beyond this standard procedure by also updating the wave functions. This leads to both the reduction of the molecular character and the increase of the e(g)(d(z)2) character on gold atoms around the Fermi energy.

We present a cross-section scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and ab initio density-functional theory simulations study of the cleaved nonpolar $(1\overline{1}00)$ surface ($m$-plane) of $n$-type HVPE GaN free-standing quasisubstrates. Atomically resolved empty and filled states STM topographies show that no reconstruction occurs upon cleavage, as predicted by theory. STS measurements on clean and atomically flat cleaved surfaces (defect concentration ${\ensuremath{\sigma}}_{d}\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}2}$) show that the Fermi energy is not pinned and the tunneling current flows through Ga-like electronic states lying outside the fundamental band gap. On surface areas with defect concentration ${\ensuremath{\sigma}}_{d}\ensuremath{\ge}3\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }{\text{cm}}^{\ensuremath{-}2}$, the Fermi energy is pinned inside the band gap in defect-derived surface states and tunneling through filled (empty) N-like (Ga-like) states takes place.

We report on first-principles density functional calculations of nonpolar low-index surfaces of hexagonal silicon carbide. We provide an accurate analysis of the macroscopic bulk spontaneous polarization as a function of the hexagonality of the compound, and we describe in detail the electronic and structural properties of the relaxed surfaces. We revise the methodology to achieve a detailed description of the surface polarization effects. Our results on low-index surfaces reveal a strong in-plane polar contribution, opposing the spontaneous polarization field present in hexagonal polytypes. This in-plane surface polarization component has not been considered before, although it is of significant impact in adsorption experiments, affecting functionalization and growth processes, as well as the electronic properties of confined, low-dimensional systems.

We present an in-depth investigation of the structural and electronic properties of a $p(2\ifmmode\times\else\texttimes\fi{}2)$ mercaptobenzoxazole (MBO) monolayer on the $\mathrm{Cu}(100)$ surface by means of repeated supercell density functional theory simulations. Our results show that the formation of the interface, with the molecular $\mathrm{S}$ headgroups lying at four-fold coordination sites on the metal substrate, is a strongly exothermic reaction that brings an energy gain of $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$∕molecule with respect to the free surface and gas-phase thione molecules. The electronic structure of the most stable atomic configuration is characterized by bonding and antibonding hybrid metal-molecule electron states obtained from coupling between the $p$ orbitals of the $\mathrm{S}$ atoms and the $d$ orbitals of the $\mathrm{Cu}$ atoms. A detailed assignment of the experimental photoemission peaks, which were revealed by recent measurements, can be traced on the basis of our computational findings. In addition, we are able to show that the deposition of MBO on $\mathrm{Cu}(100)$ results in chemisorption rather than physisorption and to fix the relative position of the metal Fermi level with respect to the molecular levels of the highest occupied and lowest unoccupied molecular orbitals.