5,701 research outputs found
Large scale GW calculations
We present GW calculations of molecules, ordered and disordered solids and
interfaces, which employ an efficient contour deformation technique for
frequency integration, and do not require the explicit evaluation of virtual
electronic states, nor the inversion of dielectric matrices. We also present a
parallel implementation of the algorithm which takes advantage of separable
expressions of both the single particle Green's function and the screened
Coulomb interaction. The method can be used starting from density functional
theory calculations performed with semi-local or hybrid functionals. We applied
the newly developed technique to GW calculations of systems of unprecedented
size, including water/semiconductor interfaces with thousands of electrons
Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies
Spin defects in wide-band gap semiconductors are promising systems for the
realization of quantum bits, or qubits, in solid-state environments. To date,
defect qubits have only been realized in materials with strong covalent bonds.
Here, we introduce a strain-driven scheme to rationally design defect spins in
functional ionic crystals, which may operate as potential qubits. In
particular, using a combination of state-of-the-art ab-initio calculations
based on hybrid density functional and many-body perturbation theory, we
predicted that the negatively charged nitrogen vacancy center in piezoelectric
aluminum nitride exhibits spin-triplet ground states under realistic uni- and
bi-axial strain conditions; such states may be harnessed for the realization of
qubits. The strain-driven strategy adopted here can be readily extended to a
wide range of point defects in other wide-band gap semiconductors, paving the
way to controlling the spin properties of defects in ionic systems for
potential spintronic technologies.Comment: In press. 32 pages, 4 figures, 3 tables, Scientific Reports 201
Role of surface states in the Casimir force between semiconducting films
We present results of first principle calculations of the Casimir force
between Si films of nanometric size, which show that it depends significantly
upon the configuration of the surface atoms, and give evidence of the
importance of surface states.Comment: to be published on J.Phys.
Carrier multiplication between interacting nanocrystals for fostering silicon-based photovoltaics
Being a source of clean and renewable energy, the possibility to convert
solar radiation in electric current with high efficiency is one of the most
important topics of modern scientific research. Currently the exploitation of
interaction between nanocrystals seems to be a promising route to foster the
establishment of third generation photovoltaics. Here we adopt a fully
ab-initio scheme to estimate the role of nanoparticle interplay on the carrier
multiplication dynamics of interacting silicon nanocrystals. Energy and charge
transfer-based carrier multiplication events are studied as a function of
nanocrystal separation showing benefits induced by the wavefunction sharing
regime. We prove the relevance of these recombinative mechanisms for
photovoltaic applications in the case of silicon nanocrystals arranged in dense
arrays, quantifying at an atomistic scale which conditions maximize the
outcome.Comment: Supplementary materials are freely available onlin
Nonempirical Range-separated Hybrid Functionals for Solids and Molecules
Dielectric-dependent hybrid (DDH) functionals were recently shown to yield
accurate energy gaps and dielectric constants for a wide variety of solids, at
a computational cost considerably less than that of GW calculations. The
fraction of exact exchange included in the definition of DDH functionals
depends (self-consistently) on the dielectric constant of the material. Here we
introduce a range-separated (RS) version of DDH functionals where short and
long-range components are matched using system dependent, non-empirical
parameters. We show that RS DDHs yield accurate electronic properties of
inorganic and organic solids, including energy gaps and absolute ionization
potentials. Furthermore we show that these functionals may be generalized to
finite systems.Comment: In press. 13 pages, 7 figures, 8 tables, Physical Review B 201
A Finite-field Approach for Calculations Beyond the Random Phase Approximation
We describe a finite-field approach to compute density response functions,
which allows for efficient and calculations beyond
the random phase approximation. The method is easily applicable to density
functional calculations performed with hybrid functionals. We present results
for the electronic properties of molecules and solids and we discuss a general
scheme to overcome slow convergence of quasiparticle energies obtained from
calculations, as a function of the basis set used to represent
the dielectric matrix
Designing defect-based qubit candidates in wide-gap binary semiconductors for solid-state quantum technologies
The development of novel quantum bits is key to extend the scope of
solid-state quantum information science and technology. Using first-principles
calculations, we propose that large metal ion - vacancy complexes are promising
qubit candidates in two binary crystals: 4H-SiC and w-AlN. In particular, we
found that the formation of neutral Hf- and Zr-vacancy complexes is
energetically favorable in both solids; these defects have spin-triplet ground
states, with electronic structures similar to those of the diamond NV center
and the SiC di-vacancy. Interestingly, they exhibit different spin-strain
coupling characteristics, and the nature of heavy metal ions may allow for easy
defect implantation in desired lattice locations and ensure stability against
defect diffusion. In order to support future experimental identification of the
proposed defects, we report predictions of their optical zero-phonon line,
zero-field splitting and hyperfine parameters. The defect design concept
identified here may be generalized to other binary semiconductors to facilitate
the exploration of new solid-state qubits.Comment: 23 pages, 5 figures, 6 tables, Supplementary Information is added at
the en
Fundamental Principles for Calculating Charged Defect Ionization Energies in Ultrathin Two-Dimensional Materials
Defects in 2D materials are becoming prominent candidates for quantum
emitters and scalable optoelectronic applications. However, several physical
properties that characterize their behavior, such as charged defect ionization
energies, are difficult to simulate with conventional first-principles methods,
mainly because of the weak and anisotropic dielectric screening caused by the
reduced dimensionality. We establish fundamental principles for accurate and
efficient calculations of charged defect ionization energies and electronic
structure in ultrathin 2D materials. We propose to use the vacuum level as the
reference for defect charge transition levels (CTLs) because it gives robust
results insensitive to the level of theory, unlike commonly used band edge
positions. Furthermore, we determine the fraction of Fock exchange in hybrid
functionals for accurate band gaps and band edge positions of 2D materials by
enforcing the generalized Koopmans' condition of localized defect states. We
found the obtained fractions of Fock exchange vary significantly from 0.2 for
bulk -BN to 0.4 for monolayer -BN, whose band gaps are also in good
agreement with experimental results and calculated GW results. The combination
of these methods allows for reliable and efficient prediction of defect
ionization energies (difference between CTLs and band edge positions). We
motivate and generalize these findings with several examples including
different defects in monolayer to few-layer hexagonal boron nitride (-BN),
monolayer MoS and graphane. Finally, we show that increasing the number of
layers of -BN systematically lowers defect ionization energies, mainly
through CTLs shifting towards vacuum, with conduction band minima kept almost
unchanged
A finite field approach to solving the Bethe Salpeter equation
We present a method to compute optical spectra and exciton binding energies
of molecules and solids based on the solution of the Bethe-Salpeter equation
(BSE) and the calculation of the screened Coulomb interaction in finite field.
The method does not require the explicit evaluation of dielectric matrices nor
of virtual electronic states, and can be easily applied without resorting to
the random phase approximation. In addition it utilizes localized orbitals
obtained from Bloch states using bisection techniques, thus greatly reducing
the complexity of the calculation and enabling the efficient use of hybrid
functionals to obtain single particle wavefunctions. We report exciton binding
energies of several molecules and absorption spectra of condensed systems of
unprecedented size, including water and ice samples with hundreds of atoms
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