Topological crystalline insulator states in Pb(1-x)Sn(x)Se.

Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In this study we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.

Optical electron spin pumping in n-doped quantum wells.

A theoretical model for optical spin pumping of electrons in a quantum well with low intrinsic electron density is presented. A system of electrons under continuous-wave illumination by circularly polarized light tuned to the electron-trion resonance is considered. The simultaneous off-resonant creation of excitons is also taken into account. The spin flip of trions and their radiative decay as the basic processes which allow the electronic spin pumping, as well as other processes, such as the formation of trions from excitons and electrons, are accounted for in the appropriate kinetic equations. The results obtained for CdTe and GaAs quantum wells indicate that significant electron spin polarization can be achieved in a time range of a few nanoseconds.

Nucleation of single-walled carbon nanotubes.

The nucleation pathway for single-wall carbon nanotubes on a metal surface is demonstrated by a series of total energy calculations using density functional theory. Incorporation of pentagons at an early stage of nucleation is energetically favorable as they reduce the number of dangling bonds and facilitate curvature of the structure and bonding to the metal. In the presence of the metal surface, nucleation of a closed cap or a capped single-wall carbon nanotube is overwhelmingly favored compared to any structure with dangling bonds or to a fullerene.

Origin of the residual acceptor ground-state splitting in silicon.

The residual ground-state splitting of acceptors in high-quality silicon has been studied intensely by different experimental techniques for several decades. Recently, photoluminescence studies of isotopically pure silicon revealed the ground-state splitting to result from the random distribution of isotopes in natural silicon. Here we present a new model that explains these surprising experimental results, and discuss the implications for acceptor ground-state splittings observed in other isotopically mixed semiconductors, as well as for the acceptor ground state in semiconductor alloys.

Core-hole effects on energy-loss near-edge structure.

We present first-principles electron energy-loss near-edge structure calculations that incorporate electron-hole interactions and are in excellent agreement with experimental data obtained with X-ray absorption spectroscopy (XAS) and electron energy-loss spectroscopy (EELS). The superior energy resolution in XAS spectra and the new calculations make a compelling case that core-hole effects dominate core-excitation edges of the materials investigated: Si, SiO2, MgO, and SiC. These materials differ widely in the dielectric constant leading to the conclusion that core-hole effects dominate all core-electron excitation spectra in semiconductors and insulators. The implications of the importance of core-holes for simulations of core-electron excitation spectra at interfaces will be discussed.

Bonding arrangements at the Si-SiO2 and SiC-SiO2 interfaces and a possible origin of their contrasting properties

We report ab initio calculations designed to explore the relative energetics of different interface bonding structures. We find that, for Si (001), abrupt (no suboxide layer) interfaces generally have lower energy because of the surface geometry and the softness of the Si-O-Si angle. However, two energetically degenerate phases are possible at the nominal interface layer, so that a mix of the two is the likely source of the observed suboxide and dangling bonds. In principle, these effects may be avoidable by low-temperature deposition. In contrast, the topology and geometry of SiC surfaces is not suitable for abrupt interfaces.

Atomic arrangement of iodine atoms inside single-walled carbon nanotubes

We report atomic resolution Z-contrast scanning transmission electron microscopy images that reveal the incorporation of I atoms in the form of helical chains inside single-walled carbon nanotubes. Density functional calculations and topological considerations provide a consistent interpretation of the experimental data. Charge transfer between the nanotube walls and the I chains is associated with the intercalation.

Excitonic effects in core-excitation spectra of semiconductors

Core-electron excitation spectra are used widely for structural and chemical analysis of materials, but interpretation of the near-edge structure remains unsettled, especially for semiconductors. For the important Si L(2,3) edge, there are two mutually inconsistent interpretations, in terms of effective-mass excitons and in terms of Bloch conduction-band final states. We report ab initio calculations and show that neither interpretation is valid and that the near-edge structure is in fact dominated by short-range electron-hole interactions even though the only bound excitons are effective-mass-like.