599 research outputs found
Accurate Ab-initio Predictions of III-V Direct-Indirect Band Gap Crossovers
We report the compositional dependence of the electronic band structure for a
range of III-V alloys. Density functional theory with the PBE functional is
insufficient to mimic the electronic gap energies at different symmetry points
of the Brillouin zone. The HSE hybrid functional with screened exchange
accurately reproduces the experimental band gaps and, more importantly, the
alloy concentration of the direct-indirect gap crossovers for the III-V alloys
studied here: AlGaAs, InAlAs, AlInP, InGaP, and GaAsP.Comment: 3 pages, 2 figure
Negative dynamic conductance from photon-assisted tunneling in superconducting junctions
We show that a superconductor-insulator-superconductor (SIS) junction may exhibit regions of negative dynamic conductance if it is irradiated by a time-varying signal source which deviates from the conventionally treated constant ac voltage limit. This phenomenon reflects the strong dependence of the junction absorption cross section upon dc bias voltage. Analytic estimates for the magnitude of the negative conductance and its impact upon the frequency down conversion process are obtained in the constant ac current limit
Magnetic states and optical properties of single-layer carbon-doped hexagonal boron nitride
We show that carbon-doped hexagonal boron nitride (h-BN) has extraordinary
properties with many possible applications. We demonstrate that the
substitution-induced impurity states, associated with carbon atoms, and their
interactions dictate the electronic structure and properties of C-doped h-BN.
Furthermore, we show that stacking of localized impurity states in small C
clusters embedded in h-BN forms a set of discrete energy levels in the wide gap
of h-BN. The electronic structures of these C clusters have a plethora of
applications in optics, magneto-optics, and opto-electronics
Relative stability of extended interstitial defects in silicon: First-principles calculations
Interstitials stored in {311} or {111} habit planes form rows of interstitial chains elongated in ⟨011⟩ direction. Exploiting the large aspect ratio to treat chains as infinite, first-principles calculations of large computation supercells reveal a unique formation energy trend for each defect, which is closely correlated with its distinct shape. The most energetically favorable structure changes from {311} rodlike defects to Frank loops as the number of interstitials in the defect increases. These results are consistent with transmission electron microscopy studies
Effects of morphology on phonons of nanoscopic silver grains
The morphology of nanoscopic Ag grains significantly affects the phonons.
Atomistic simulations show that realistic nanograin models display complex
vibrational properties. (1) Single-crystalline grains. Nearly-pure torsional
and radial phonons appear at low frequencies. For low-energy, faceted models,
the breathing mode and acoustic gap (lowest frequency) are about 10% lower than
predicted by elasticity theory (ET) for a continuum sphere of the same volume.
The sharp edges and the atomic lattice split the ET-acoustic-gap quintet into a
doublet and triplet. The surface protrusions associated with nearly spherical,
high-energy models produce a smaller acoustic gap and a higher vibrational
density of states (DOS) at frequencies \nu<2 THz. (2) Twined icosahedra. In
contrast to the single-crystal case, the inherent strain produce a larger
acoustic gap, while the core atoms yield a DOS tail extending beyond the
highest frequency of single-crystalline grains. (3) Mark's decahedra, in
contrast to (1) and (2), do not have a breathing mode; although twined and
strained, do not exhibit a high-frequency tail in the DOS. (4) Irregular
nanograins. Grain boundaries and surface disorder yield non-degenerate phonon
frequencies, and significantly smaller acoustic gap. Only these nanograins
exhibit a low-frequency \nu^2 DOS in the interval 1-2 THz.Comment: Version published in Phys. Rev.
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