7,731 research outputs found
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 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 5 grain boundary could be rather high
given that their formation energies are as low as 0.10 eV and 0.06 eV/
respectively. In addition, our results also suggest that borophene can react
easily with H, O and N 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 TiO Anatase: Role of Interstitial Co
TiO anatase doped with Co has been recently reported to exhibit
room-temperature ferromagnetism. 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 anatase tend to cluster and
then the neighboring interstitial tetrahedral sites become energetically
favorable for Co to reside, yielding a local environment more like CoO
than CoTiO. 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 TiO rutile
Room-temperature ferromagnetism has been recently discovered in Co-doped
TiO rutile. Our density-functional theory investigations show
that the substitutional Co ions incorporated into TiO 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 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
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