45 research outputs found
Confinement effects in ultra-thin ZnO polymorph films: electronic and optical properties
Relying on generalized-gradient and hybrid first-principles simulations, this
work provides a complete characterization of the electronic properties of ZnO
ultra-thin films, cut along the Body-Centered-Tetragonal(010), Cubane(100),
h-BN(0001), Zinc-Blende(110), Wurtzite(100) and (0001) orientations.
The characteristics of the local densities of states are analyzed in terms of
the reduction of the Madelung potential on under-coordinated atoms and surface
states/resonances appearing at the top of the VB and bottom of the CB. The gap
width in the films is found to be larger than in the corresponding bulks, which
is assigned to quantum confinement effects. The components of the high
frequency dielectric constant are determined and the absorption spectra of the
films are computed. They display specific features just above the absorption
threshold due to transitions from or to the surface resonances. This study
provides a first understanding of finite size effects on the electronic
properties of ZnO thin films and a benchmark which is expected to foster
experimental characterization of ultra-thin films via spectroscopic techniques
Exciton interference in hexagonal boron nitride
In this letter we report a thorough analysis of the exciton dispersion in
bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation
at finite , and we compare our results with
recent high-accuracy electron energy loss data. Simulations reproduce the
measured dispersion and the variation of the peak intensity. We focus on the
evolution of the intensity, and we demonstrate that the excitonic peak is
formed by the superposition of two groups of transitions that we call and
from the k-points involved in the transitions. These two groups
contribute to the peak intensity with opposite signs, each damping the
contributions of the other. The variations in number and amplitude of these
transitions determine the changes in intensity of the peak. Our results
contribute to the understanding of electronic excitations in this systems along
the direction, which is the relevant direction for spectroscopic
measurements. They also unveil the non-trivial relation between valley physics
and excitonic dispersion in h--BN, opening the possibility to tune excitonic
effects by playing with the interference between transitions. Furthermore, this
study introduces analysis tools and a methodology that are completely general.
They suggest a way to regroup independent-particle transitions which could
permit a deeper understanding of excitonic properties in any system
Mapping of the energetically lowest exciton in bulk -HfS
By combining electron energy-loss spectroscopy and state-of-the-art
computational methods, we were able to provide an extensive picture of the
excitonic processes in -HfS. The results differ significantly from the
properties of the more scrutinized group VI semiconducting transition metal
dichalcogenides such as MoS and WSe. The measurements revealed a
parabolic exciton dispersion for finite momentum parallel to the
K direction which allowed the determination of the effective exciton
mass. The dispersion decreases monotonically for momentum exchanges parallel to
the M high symmetry line. To gain further insight into the excitation
mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The
results matched the experimental loss spectra closely, thereby confirming the
excitonic nature of the observed transitions, and produced the
momentumdependent binding energies. The simulations also demonstrated that the
excitonic transitions for || M occur exactly along that
particular high symmetry line. For || K on the other hand,
the excitations traverse the Brillouin zone crossing various high symmetry
lines. A particular interesting aspect of our findings was that the calculation
of the electron probability density revealed that the exciton assumes a
six-pointed star-like shape along the real space crystal planes indicating a
mixed Frenkel-Wannier character.Comment: 12 pages, 10 figure
Structural classification of boron nitride twisted bilayers and ab initio investigation of their stacking-dependent electronic structure
Since the discovery of superconductive twisted bilayer graphene which
initiated the field of twistronics, moir\'e systems have not ceased to exhibit
fascinating properties. We demonstrate that in boron nitride twisted bilayers,
for a given moir\'e periodicity, there are five different stackings which
preserve the monolayer hexagonal symmetry (i.e. the invariance upon rotations
of 120) and not only two as always discussed in literature. We
introduce some definitions and a nomenclature that identify unambiguously the
twist angle and the stacking sequence of any hexagonal bilayer with order-3
rotation symmetry. Moreover, we employ density functional theory to study the
evolution of the band structure as a function of the twist angle for each of
the five stacking sequences of boron nitride bilayers. We show that the gap is
indirect at any angle and in any stacking, and identify features that are
conserved within the same stacking sequence irrespective of the angle of twist.Comment: 16 pages (6.5 main text); 15 figures (5 in main); 5 tables (3 in
main). Appendixes concatenated to main tex
Evening out the spin and charge parity to increase T in unconventional superconductor Sr_{2}RuO_{4}
Unconventional superconductivity in SrRuO has been intensively
studied for decades. The origin and nature of the pairing continues to be
widely debated, in particular, the possibility of a triplet origin of Cooper
pairs. However, complexity of SrRuO with multiple low-energy
scales, involving subtle interplay among spin, charge and orbital degrees of
freedom, calls for advanced theoretical approaches which treat on equal footing
all electronic effects. Here we develop a novel approach, a detailed \emph{ab
initio} theory, coupling quasiparticle self-consistent \emph{GW} approximation
with dynamical mean field theory (DMFT), including both local and non-local
correlations. We report that the superconducting instability has multiple
triplet and singlet components. In the unstrained case the triplet eigenvalues
are larger than the singlets. Under uniaxial strain, the triplet eigenvalues
drop rapidly and the singlet components increase. This is concomitant with our
observation of spin and charge fluctuations shifting closer to wave-vectors
favoring singlet pairing in the Brillouin zone. We identify a complex mechanism
where charge fluctuations and spin fluctuations co-operate in the even-parity
channel under strain leading to increment in , thus proposing a novel
mechanism for pushing the frontier of in unconventional `triplet'
superconductors.Comment: 30 pages, 9 figure, 2 table
Gap engineering and wave function symmetry in C and BN armchair nanoribbons
Many are the ways of engineering the band gap of nanoribbons including
application of stress, electric field and functionalization of the edges. In
this article, we investigate separately the effects of these methods on
armchair graphene and boron nitride nanoribbons. By means of density functional
theory calculations, we show that, despite their similar structure, the two
materials respond in opposite ways to these stimuli. By treating them as
perturbations of a heteroatomic ladder model based on the tight-binding
formalism, we connect the two behaviours to the different symmetries of the top
valence and bottom conduction wave functions. These results indicate that
opposite and complementary strategies are preferable to engineer the gapwidth
of armchair graphene and boron nitride nanoribbons
Quantum well confinement and competitive radiative pathways in the luminescence of black phosphorus layers
Black phosphorus (BP) stands out from other 2D materials by the wide
amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from
Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness.
This singularity made the optical and excitonic properties of BP difficult to
map. Specifically, the literature lacks in presenting experimental and
theoretical data on the optical properties of BP on an extended thickness
range. Here we report the study of an ensemble of photoluminescence spectra
from 79 passivated BP flakes recorded at 4 K with thicknesses ranging from 4 nm
to 700 nm, obtained by mechanical exfoliation. We observe that the exfoliation
steps induce additional defects states that compete the radiative recombination
from bound excitons observed in the crystal. We also show that the evolution of
the photoluminescence energy versus thickness follows a quantum well
confinement model appreciable from a thickness predicted and probed at 25 nm.
The BP slabs placed in different 2D heterostructures show that the emission
energy is not significantly modulated by the dielectric environment.
Introduction Confinement effectsComment: 11 pages, 3 figures - Main text 12 pages, 5 figures - Supporting
informatio
Multiple satellites in materials with complex plasmon spectra: From graphite to graphene
International audienceThe photoemission spectrum of graphite is still debated. To help resolve this issue, we present photoemission measurements at high photon energy and analyze the results using a Green's function approach that takes into account the full complexity of the loss spectrum. Our measured data show multiple satellite replicas. We demonstrate that these satellites are of intrinsic origin, enhanced by extrinsic losses. The dominating satellite is due to the π+σ plasmon of graphite, whereas the π plasmon creates a tail on the high-binding energy side of the quasiparticle peak. The interplay between the two plasmons leads to energy shifts, broadening, and additional peaks in the satellite spectrum. We also predict the spectral changes in the transition from graphite towards graphene