Unraveling the dislocation core structure at a van der Waals gap in bismuth telluride.

Tetradymite-structured chalcogenides such as bismuth telluride (Bi2Te3) are of significant interest for thermoelectric energy conversion and as topological insulators. Dislocations play a critical role during synthesis and processing of such materials and can strongly affect their functional properties. The dislocations between quintuple layers present special interest since their core structure is controlled by the van der Waals interactions between the layers. In this work, using atomic-resolution electron microscopy, we resolve the basal dislocation core structure in Bi2Te3, quantifying the disregistry of the atomic planes across the core. We show that, despite the existence of a stable stacking fault in the basal plane gamma surface, the dislocation core spreading is mainly due to the weak bonding between the layers, which leads to a small energy penalty for layer sliding parallel to the van der Waals gap. Calculations within a semidiscrete variational Peierls-Nabarro model informed by first-principles calculations support our experimental findings.

Vacancy dissociation in body-centered cubic screw dislocation cores.

The interaction between screw dislocations and vacancies in body-centered cubic metals is investigated using molecular dynamics simulations. For thirteen different classical interatomic potentials, materials properties relating to vacancies, dislocations, and the interaction between the two are evaluated. The potentials include six for iron, two for molybdenum, and five for tantalum, and they are a mix of embedded atom method (EAM), modified embedded atom method (MEAM), and angular dependent potential (ADP) styles. A previously unknown behavior was identified during the interaction simulations. Out of the thirteen potentials investigated, ten predict a vacancy on the dislocation core to no longer remain as a discrete point defect, but rather to dissociate along the dislocation line. The structure of the dissociation is dependent on the potential and is characterized here. As this vacancy dissociation alters the core structure of the dislocation, it may prove to be a new mechanism for dislocation pinning and pipe diffusion.

Choroidal malignant melanoma in siblings.

Choroidal malignant melanoma was diagnosed on pathologic examination of enucleated globes from a 54-year-old man and his 58-year-old sister. The brother had been monitored at four-month intervals for four years because of a choroidal lesion, which abruptly increased in size, in his left eye. Three years after an examination with normal findings, the sister developed a retinal detachment with a pigmented dome in her right eye. Both patients are doing well five years (Patient 1) and two years (Patient 2) after enucleation. Screening examinations detected two cases of benign melanoma (nevi) in 18 close relatives but this incidence did not exceed chance.

Conjunctival biopsy in sarcoidosis.

Excisional biopsy, under slit lamp visualization, makes conjunctival biopsies a viable alternative to bronchoscopy as the initial diagnostic step. The authors report their experience on excisional biopsies of the palpebral conjunctiva in 16 patients suspected of having sarcoidosis.