Single NV centers in diamond coupled by hyperfine interaction (hfi) to neighboring 13 C nuclear spins are now widely used in emerging quantum technologies as elements of quantum memory adjusted to a nitrogen-vacancy (NV) center electron spin qubit. For nuclear spins with low flip-flop rate, single shot readout was demonstrated under ambient conditions. Here we report on a systematic search for such stable NV− 13 C systems using density functional theory to simulate the hfi and spatial characteristics of all possible NV− 13 C complexes in the H-terminated cluster C 510 [NV] -H 252 hosting the NV center. Along with the expected stable 'NV-axial− 13 C' systems wherein the 13 C nuclear spin is located on the NV axis, we found for the first time new families of positions for the 13 C nuclear spin exhibiting negligible hfi-induced flipping rates due to near-symmetric local spin density distribution. Spatially, these positions are located in the diamond bilayer passing through the vacancy of the NV center and being perpendicular to the NV axis. Analysis of available publications showed that, apparently, some of the predicted non-axial near-stable NV− 13 C systems have already been observed experimentally. A special experiment performed on one of these systems confirmed the prediction made.

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In this paper we study the effect of absorption peak correlation in finite length carbon nanotubes and graphene nanoribbons. It is shown, in the orthogonal π-orbital tight-binding model with the nearest neighbor approximation, that if the ribbon width is a half of the tube circumference the effect takes place for all achiral ribbons (zigzag, armchair and bearded), and corresponding tubes, starting from lengths of about 30 nm. This correlation should be useful in designing nanoribbon-based optoelectronics devices fully integrated into a single layer of graphene.

Effective positronium ͓Psϭ(e ϩ e Ϫ )͔ quadrupole interaction in ␣-quartz has been observed using the Zeeman effect on Ps. In order to reduce sample dependence and to achieve a good resolution, the two-dimensional angular correlation of annihilation radiation technique has been used. The quadrupole coupling constant d in ␣-quartz has been determined to be (3.0Ϯ0.9)ϫ10 Ϫ5 eV.

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