A simple (5 + 6 + 7)-sp(3) carbon (denoted as F-carbon) with eight atoms per unit cell predicted by a newly developed ab initio particle-swarm optimization methodology on crystal structure prediction is proposed. F-carbon can be seen as the reconstruction of AA-stacked or 3R-graphite, and is energetically more stable than 2H-graphite beyond 13.9 GPa. Band structure and hardness calculations indicate that F-carbon is a transparent superhard carbon with a gap of 4.55 eV at 15 GPa and a hardness of 93.9 GPa at zero pressure. Compared with the previously proposed Bct-, M- and W-carbons, the simulative x-ray diffraction pattern of F-carbon also well matches the superhard intermediate phase of the experimentally cold-compressed graphite. The possible transition route and energy barrier were observed using the variable cell nudged elastic band method. Our simulations show that the cold compression of graphite can produce some reversible metastable carbons (e.g. M- and F-carbons) with energy barriers close to diamond or lonsdaleite.

A supersaturation of nitrogen atoms is found in the surface layer of microstructured silicon after femtosecond (fs) laser irradiation in NF3. The average nitrogen concentration in the uppermost 50 nm is about 0.5 ± 0.2 at. %, several orders of magnitude higher than the solid solubility of nitrogen atoms in silicon. The nitrogen-hyperdoped silicon shows high crystallinity in the doped layer, which is due to the repairing effect of nitrogen on defects in silicon lattices. Nitrogen atoms and vacancies can be combined into thermal stable complexes after fs laser irradiation, which makes the nitrogen-hyperdoped silicon exhibit good thermal stability of optical properties.

We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.

The physical mechanisms behind the reduction in the bulk modulus of a high-pressure cubic TiO 2 phase are revealed by first-principles calculations. An unusual and abrupt change occurs in the dependence of energy on pressure at 43 GPa, indicating a pressure-induced phase transition from columbite TiO 2 to a modified fluorite TiO 2 with a Pca21 symmetry. Oxygen atom displacement in Pca21 TiO 2 unexpectedly reduces the bulk modulus by 34% relative to fluorite TiO 2 . This discovering provides a direct evidence for understanding the compressive properties of such groups of homologous materials.Titanium dioxide ͑TiO 2 ͒ has rich phase diagrams, namely, the rutile ͑P42/ mnm͒, anatase ͑I41/ amd͒, brookite ͑Pbca͒, columbite ͑Pbcn͒, baddeleyite ͑P21/ c͒, and cotunnite ͑Pnma͒ phases. 1-6 Due to its versatile physical and chemical properties, TiO 2 is extensively used in many industrial applications, such as high efficiency solar cells, photocatalysis, dynamic random access memory modules, and superhard materials. 7-12 The rutile and anatase phases of TiO 2 are abundant in nature. 13,14 Since the phase sequence of TiO 2 is very similar to that of other bulk materials, such as ZrO 2 and HfO 2 , it is highly expected to transform into its cubic polymorphs under pressure. 15 Modified cubic fluorite-structured RuO 2 , SnO 2 , and PbO 2 that possess a Pa3 symmetry, have been successfully synthesized. 16 In particular, RuO 2 is considered to be a potential ultrahard material because of its measured Knoop hardness ͑ϳ20 GPa͒ and bulk modulus ͑399 GPa͒, which is only 10% less than that of sintered diamonds. 17 Moreover, synthesized cotunnite TiO 2 has an extremely high bulk modulus of 431 GPa and is considered as the hardest oxide to date. 1 After the synthesis of cotunnite TiO 2 , scientists expected to synthesize cubic TiO 2 because it showed potential for use as a solar cell or ultrahard material. Ultimately, the highly anticipated cubic TiO 2 was successfully synthesized by heating anatase TiO 2 between 1900 and 2100 K in diamond-anvil cells under a pressure of 48 GPa. 18 Some ambiguities, however, remained both in experiment and in theory. For instance, the theoretical bulk modulus calculated for cubic TiO 2 in the pyrite and fluorite phases was significantly larger than that obtained during the experiments. Kim et al. 13 showed that pyrite TiO 2 is unstable because of the presence of imaginary frequencies in the phonon spectra throughout the entire pressure range, whereas fluorite TiO 2 is stable because of the absence of these imaginary frequencies under pressure. Swamy and Muddle 19 reported that pyrite TiO 2 has theoretical properties closer to the experimental values because it has a relatively lower bulk modulus. In terms of mechanical properties, however, Liang et al. 20 found a minor difference between the fluorite and pyrite phases. At the present stage, there is no theory of the cubic phase of TiO 2 , and that although there is some disagreement between existing calculations on candidate phases fluorite and pyrite, ...

We present a new approach to optical coherence elastography (OCE), which probes the local elastic properties of tissue by using optical coherence tomography to measure the effect of an applied stimulus in the audio frequency range. We describe the approach, based on analysis of the Bessel frequency spectrum of the interferometric signal detected from scatterers undergoing periodic motion in response to an applied stimulus. We present quantitative results of sub-micron excitation at 820 Hz in a layered phantom and the first such measurements in human skin in vivo.