Nearsightedness of Electronic Matter and the Size of Viruses

I conjecture that the nearsightedness of component electronic matter largely determines the size of a virus. These two length scales, one from physics and one from biochemistry, are in fact the same dimension which connects our quantum and everyday worlds. Learning how viruses interact with microscopic molecules and macroscopic biological cells might help us understand the quantum-to-classical transition in general cases of multiscale phenomena.Comment: 6 pages, 1 figur

One-for-multiple substitution in solid solutions

It is generally assumed that one solute atom will occupy only one lattice site in a substitutional solid solution. We here report an interesting discovery by first-principles calculations that a large solute atom can replace multiple matrix atoms in the elemental crystal of beryllium. Examination on Groups IIIB, IVB, VB, VIB, and VA elements shows that Cr will substitute for one, V and Mo for three, Sc, Y, Ti, Zr, Hf, W, Nb, Ta, As, Sb, and Bi for four, and La for five Be atoms. Dissolution of Zr, Hf, Sc, and Y is exothermic, suggesting a good solubility. At low concentration, the configurational entropy resulted from one-for-multiple substitution is larger than in the one-for-one substitution case. We find that Sc, Y, Zr, and Hf all have tendency to aggregate in Be, but Sc is the weakest among them and thus can be expected to improve the superplasticity of Be.Comment: 12 pages, 2 figure

Lattice Defects and the Mechanical Anisotropy of Borophene

Using density functional theory combined with a semi-empirical van der Waals dispersion correction, we have investigated the stability of lattice defects including boron vacancy, substitutional and interstitial X (X=H, C, B, N, O) and $\Sigma$5 tilt grain boundaries in borophene and their influence on the anisotropic mechanical properties of this two-dimensional system. The pristine borophene has significant in-plane Young's moduli and Poisson's ratio anisotropy due to its strong and highly coordinated B-B bonds. The concentration of B vacancy and $\Sigma$5 grain boundary could be rather high given that their formation energies are as low as 0.10 eV and 0.06 eV/$\AA$ respectively. In addition, our results also suggest that borophene can react easily with H$_2$, O$_2$ and N$_2$ when exposed to these molecules. We find that the mechanical properties of borophene are remarkably degraded by these defects. The anisotropy in Poisson's ratio, however, can be tuned by some of them. Furthermore, the adsorbed H or substitutional C may induce remarkably negative Poisson's ratio in borophene, and the substitutional C or N can significantly increase the Poisson's ratio by contrast.Comment: 9 pages, 8 figure

Room-Temperature Ferromagnetism in Co-Doped TiO2_2 Anatase: Role of Interstitial Co

TiO$_2$ anatase doped with Co has been recently reported to exhibit room-temperature ferromagnetism. $Ab$ $initio$ study on substitutional Co doping, however, yielded much larger magnetic moment for Co than experiment. Our calculations based on density-functional theory show that the substitutional Co ions incorporated into TiO$_2$ anatase tend to cluster and then the neighboring interstitial tetrahedral sites become energetically favorable for Co to reside, yielding a local environment more like Co$_3$O$_4$ than CoTiO$_3$. The interstitial Co destroys the spin-polarization of the surrounding substitutional Co but enhances the stability of the ferromagnetism significantly. In the absence of carriers, this room-temperature ferromagnetism can only be accounted for by superexchange interaction.Comment: 5 pages, 3 figures; submitted to PRL on December 26,200

Structural, electronic, and magnetic properties of a ferromegnetic semiconductor: Co-doped TiO2_2 rutile

Room-temperature ferromagnetism has been recently discovered in Co-doped TiO$_2$ rutile. Our $ab$ $initio$ density-functional theory investigations show that the substitutional Co ions incorporated into TiO$_2$ rutile tend to cluster and then the neighboring interstitial sites become energetically favorable for Co to reside. This suggests that a Co-doped rutile containing only substitutional Co is not an appropriate reference bulk system in derterming the local environment of Co in polycrystalline (Ti,Co)O$_2$ rutile. We also find that the interstitial Co is in the low spin state and destroys the spin-polarization of the surrounding substitutional Co, hence reduces the average magnetic moment of impurity atoms.Comment: 4 pages, 3 figures; submitted to PRB on February 17, 200

Migration of helium-pair in metals

Understanding helium accumulation in plasma-facing or structural materials in a fusion reactor starts from uncovering the details of the migration of single and paired He interstitials. We have carried out a first-principles density functional theory investigation into the migration of both a single interstitial He atom and an interstitial He-pair in bcc (Fe, Mo and W) and fcc (Cu, Pd and Pt) metals. By identifying the most stable configurations of an interstitial He-pair in each metal and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our first-principles calculations reveal that the migration trajectories and barriers are determined predominantly by the relatively stable He-pair configurations which depend mainly on the stability of a single He in different interstices. Contrary to atomistic studies reported in literature, the migration barrier in bcc Fe, Mo, and W is 0.07, 0.07, and 0.08 eV respectively, always slightly higher than for a single interstitial He (0.06 eV for all three). Configurations of a He-pair in fcc metals are much more complicated, due to the stability closeness of different interstitial sites for a single He atom. In both Cu and Pd, the migration of a He-pair proceeds by moving one He at a time from one tetrahedral site to neighboring octahedral site; whereas in Pt the two He move simultaneously because the bridge interstitial site presents an extremely low barrier. The migration barrier for a He-pair is 0.05, 0.15, and 0.04 eV for Cu, Pd, and Pt, slightly lower than (in Cu), or similar to (in Pd and Pt) a single He, which is 0.08, 0.15, and 0.03 eV, respectively. The associative motions of a He-pair are ensured by the strong He-He interactions in metals which are chemical bonding-like and can be described very well with Morse potentials.Comment: 34 pages, 12 figure

Native point defects in few-layer phosphorene

Using hybrid density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the structural and electronic properties of vacancies and self-interstitials in defective few-layer phosphorene. We find that both a vacancy and a self-interstitial defect are more stable in the outer layer than in the inner layer. The formation energy and transition energy of both a vacancy and a self-interstitial P defect decrease with increasing film thickness, mainly due to the upward shift of the host valence band maximum in reference to the vacuum level. Consequently, both vacancies and self-interstitials could act as shallow acceptors, and this well explains the experimentally observed p-type conductivity in few-layer phosphorene. On the other hand, since these native point defects have moderate formation energies and are stable in negatively charged states, they could also serve as electron compensating centers in n-type few-layer phosphorene.Comment: 10 pages, 12 figure

Non-constant crack tip opening angle and negligible crack tunneling of brittle fracture in Al: A first-principles prediction

Numerous measurements showed that the crack tip opening angle (CTOA) is nearly constant upon stable ductile fracture in Al alloys which widely used in modern transportation industry. The atomic structure of the very tip of a crack front has remained unknown, however. We have carried out a first-principles density functional theory study to reveal the precise alignment of atoms near the crack tip in single-crystalline Al. The calculations demonstrate that the CTOA increases with the opening displacement, thus the observed constant CTOA in millimeter scale is an entirely plastic effect during ductile crack. Besides, we find no significant crack tunneling (crack-front blunting), which can be accounted for from the very small relaxation of the Al free surface. The atomic structure thus obtained provides a solid basis for larger scale simulations using for example finite element method.Comment: 12 pages, 4 figure

A simple scaling law between the total energy of a free atom and its atomic number

A simple, approximate relation is found between the total energy of a free atom and its atomic number: E ~= -Z^{2.411}. The existence of this index is inherent in the Coulomb and many-body nature of the electron-electron interaction in the atomic system and cannot be fabricated from the existing fundamental physical constants.Comment: 5 pages, 3 figure

Atomic size effect in impurity indued grain boundary embrittlement

Bismuth segregated to the grain boundary in Cu is known to promote brittle fracture of this material. Recently, Schweinfest et al. reported first-principles quantum mechanical calculations on the electronic and structural properties of a Cu grain boundary with and without segregated Bi and argue that the grain boundary weakening induced by Bi is a simple atomic size effect. But their conclusion is invalid for both Bi and Pb because it fails to distinguish the chemical and mechanical (atomic size) contributions, as obtained with our recently developed first-principles based phenomenological theory.Comment: 4 page