We study the luminescence of hexagonal boron nitride (h−BN) by means of nonequilibrium Green's functions plus finite-difference electron-phonon coupling. We derive a formula for light emission in ...solids in the limit of a weak excitation that includes perturbatively the contribution of electron-phonon coupling at the first order. This formula is applied to study luminescence in bulk h−BN. This material has attracted interest due to its strong luminescence in the ultraviolet region of the electromagnetic spectrum K. Watanabe et al., Nat. Mater. 3, 404 (2004). The origin of this intense luminescence signal has been widely discussed, but only recently a clear signature of phonon-mediated light emission started emerging from the experiments G. Cassabois et al., Nat. Photonics 10, 262 (2016). By means of our theoretical framework, we provide a clear and full explanation of light emission in h−BN.
We report diffusion quantum Monte Carlo (DMC) calculations of the quasiparticle and excitonic gaps of hydrogen-terminated oligoynes and extended polyyne. The electronic gaps are found to be very ...sensitive to the atomic structure in these systems. We have therefore optimised the geometry of polyyne by directly minimising the DMC energy with respect to the lattice constant and the Peierls-induced carbon-carbon bond-length alternation. We find the bond-length alternation of polyyne to be 0.136(2) Å and the excitonic and quasiparticle gaps to be 3.30(7) and 3.4(1) eV, respectively. The DMC zone-centre longitudinal optical phonon frequency of polyyne is 2084(5) cm
−1
, which is consistent with Raman spectroscopic measurements for large oligoynes.
The charge density of a one-dimensional sp-bonded chain composed of 26 carbon atoms terminated by H with alternating single and triple bonds.
By including phonon-assisted transitions within plane-wave density functional theory methods for calculating the x-ray absorption spectrum (XAS), we obtain the Al K-edge XAS at 300 K for two ...crystalline Al2O3 phases. The 300 K XAS reproduces the pre-edge peak for α-Al2O3, which is not visible at the static lattice level of approximation. Configurations from Monte Carlo sampling of the γ-Al2O3 phase space at the 300 K XAS correctly describe two out of the three experimental peaks. We show that the second peak arises from 1s to mixed s-p transitions and is absent in the 0 K XAS. This letter serves as an insight into the electronic origins of the characteristic peaks in the Al K-edge XAS for alumina crystals.
Fundamental physical constants provide the upper bound for the speed of sound in condensed matter phases.
Two dimensionless fundamental physical constants, the fine structure constant α and the ...proton-to-electron mass ratio
m
p
m
e
, are attributed a particular importance from the point of view of nuclear synthesis, formation of heavy elements, planets, and life-supporting structures. Here, we show that a combination of these two constants results in a new dimensionless constant that provides the upper bound for the speed of sound in condensed phases,
v
u
. We find that
v
u
c
=
α
(
m
e
2
m
p
)
1
2
, where
c
is the speed of light in vacuum. We support this result by a large set of experimental data and first-principles computations for atomic hydrogen. Our result expands the current understanding of how fundamental constants can impose new bounds on important physical properties.
We present kinetic Monte Carlo simulations of submonolayer epitaxial growth with a mobile intermediate polyatomic cluster, inspired by the observations of graphene epitaxy on Ru(0001) and lr(111). We ...suppose that deposited atoms diffuse on the surface and collide to form tetra mers, which also diffuse on the surface. Immobile islands are formed when j tetramers collide. We study the temperature dependence of all surface species as a function of j. Unlike all other growth scenarios, the adatom density at the onset of nucleation increases with temperature for large enough j. Other characteristics, such as the island-size distribution and the temperature dependence of the island density, also show qualitatively different behavior from growth based on adatom kinetics.
The optical transitions between spin-polarized bands of the quasi-two-dimensional Rashba system BiTeI are investigated using polarization-resolved resonant Raman spectroscopy. We detect chiral ...excitations between states with opposite helicity and compare spectra to calculations within a three-band model. Using the resonant Raman excitation profile, we deduce the Rashba parameters and band gaps of the higher conduction bands near the Fermi level, and compare the parameters to values obtained by ab initio density functional theory.
Searching for adequate input parameters of a Contrast Enhancement Algorithm is a fundamental task, aimed to get more suitable images, in terms of contrast, for decision making or further ...applications. Several Optimization approaches take a single image as the input of the process, so they get poor performance. In this proposal, a group of several images of the same type are taken as input of a Robust Multi-Objective Particle Swarm Optimization approach, in order to achieve more suitable input parameters for a kind of images, instead of a single image. The results are a set of input parameters for the Contrast Enhancement Algorithm, suitable for Contrast Enhancement of a group of images of the same type.
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