21 research outputs found
Dielectric constant boost in amorphous sesquioxides
High-kappa dielectrics for insulating layers are a current key ingredient of
microelectronics. X2O3 sesquioxide compounds are among the candidates. Here we
show for a typical material of this class, ScO3, that the relatively modest
dielectric constant of its crystalline phase is enhanced in the amorphous phase
by over 40% (from ~15 to ~22). This is due to the disorder-induced activation
of low frequency cation-related modes which are inactive or inefficient in the
crystal, and by the conservation of effective dynamical charges (a measure of
atomic polarizability). The analysis employs density-functional energy-force
and perturbation-theory calculations of the dielectric response of amorphous
samples generated by pair-potential molecular dynamics.Comment: 3 pages, 3 figures, submitted to AP
Conservation of dielectric constant upon amorphization in perovskite oxides
We report calculations indicating that amorphous RAO oxides, with R and A
trivalent cations, have approximately the same static dielectric constant as
their perovskite crystal phase. The effect is due to the disorder-activated
polar response of non-polar crystal modes at low frequency, which compensates a
moderate but appreciable reduction of the ionic dynamical charges. The
dielectric response was studied via density-functional perturbation theory.
Amorphous samples were generated by molecular dynamics melt-and-quench
simulations.Comment: 5 pages, 3 figure
Magnon-phonon interactions enhance the gap at the Dirac point in the spin-wave spectra of CrI 2D magnets
Recent neutron-diffraction experiments in honeycomb CrI quasi-2D
ferromagnets have evinced the existence of a gap at the Dirac point in their
spin-wave spectra. The existence of this gap has been attributed to strong
in-plane Dzyaloshinskii-Moriya or Kitaev (DM/K) interactions and suggested to
set the stage for topologically protected edge states to sustain
non-dissipative spin transport. We perform state-of-the-art simulations of the
spin-wave spectra in monolayer CrI, based on time-dependent
density-functional perturbation theory (TDDFpT) and fully accounting for
spin-orbit couplings (SOC) from which DM/K interactions ultimately stem. While
our results are in qualitative agreement with experiments, the computed TDDFpT
magnon gap at the Dirac point is found to be 0.47~meV, roughly 6 times smaller
than the most recent experimental estimates, so questioning that intralayer
anisotropies alone can explain the observed gap. Lattice-dynamical
calculations, performed within density-functional perturbation theory (DFpT),
indicate that a substantial degeneracy and a strong coupling between
vibrational and magnetic excitations exist in this system, providing a possible
additional gap-opening mechanism in the spin-wave spectra. In order to pursue
this path, we introduce an interacting magnon-phonon Hamiltonian featuring a
linear coupling between lattice and spin fluctuations, enabled by the magnetic
anisotropy induced by SOC. Upon determination of the relevant interaction
constants by DFpT and supercell calculations, this model allows us to propose
magnon-phonon interactions as an important microscopic mechanism responsible
for the enhancement of the gap in the range of ~meV around the Dirac
point of the CrI monolayer
First-principles study of the gap in the spin excitation spectrum of the CrI honeycomb ferromagnet
The nature of the gap observed at the zone border in the spin-excitation
spectrum of CrI quasi-2D single crystals is still controversial. We perform
first-principles calculations based on time-dependent density-functional
perturbation theory, which indicate that the observed gap results from a
combination of spin-orbit and inter-layer interaction effects. The former give
rise to the anisotropic spin-spin interactions that are responsible for its
very existence, while the latter determine both its displacement from the K
point of the Brillouin zone due to the in-plane lattice distortions induced by
them, and an enhancement of its magnitude, in agreement with experiments and
previous theoretical work based on a lattice model
Roadmap on Electronic Structure Codes in the Exascale Era
Electronic structure calculations have been instrumental in providing many
important insights into a range of physical and chemical properties of various
molecular and solid-state systems. Their importance to various fields,
including materials science, chemical sciences, computational chemistry and
device physics, is underscored by the large fraction of available public
supercomputing resources devoted to these calculations. As we enter the
exascale era, exciting new opportunities to increase simulation numbers, sizes,
and accuracies present themselves. In order to realize these promises, the
community of electronic structure software developers will however first have
to tackle a number of challenges pertaining to the efficient use of new
architectures that will rely heavily on massive parallelism and hardware
accelerators. This roadmap provides a broad overview of the state-of-the-art in
electronic structure calculations and of the various new directions being
pursued by the community. It covers 14 electronic structure codes, presenting
their current status, their development priorities over the next five years,
and their plans towards tackling the challenges and leveraging the
opportunities presented by the advent of exascale computing.Comment: Submitted as a roadmap article to Modelling and Simulation in
Materials Science and Engineering; Address any correspondence to Vikram
Gavini ([email protected]) and Danny Perez ([email protected]
Methylammonium fragmentation in amines as source of localized trap levels and the healing role of Cl in hybrid lead-iodide perovskites
The resilience to deep traps and localized defect formation is one of the important aspects that qualify a material as a suited photoabsorber in solar cell devices. Here we investigate by ab initio calculations the fundamental physics and chemistry of a number of possible localized defects in hybrid methylammonium lead-iodide perovskites. Our analysis encompasses a number of possible molecular fragments deriving from the dissociation of methylammonium. In particular, we found that in stoichiometric conditions both ammonia and methylamine molecules present lone-pair localized levels well within the perovskite band gap, while the radical cation CH2NH3+ observed by EPR after irradiation injects partially-occupied levels into the band gap but only in p-type conditions. These defects are thus potentially capable of significantly altering absorption and recombination properties. Amazingly, we found that additional interstitial Cl is capable of removing these localized states from the band gap. These results are consistent with the observed improvement of photoabsorption properties due to the Cl inclusion in the solution processing
Entropy-Suppressed Ferroelectricity in Hybrid Lead-Iodide Perovskites
The actual nature of the electric polarization in hybrid lead-iodide perovskites is unveiled on the basis of ab initio and model results. A finite, albeit small electric polarization of few \u3bcC/cm2 is found to be pervasive in this system, due to the polar-uncompensated alignment of methylammonium dimers, at least for temperature lower than the activation energy of dimer rotations; however, the presence of a large number of structural local minima corresponding to differently oriented polarization directions counteracts the stabilization of an ordered ferroelectric phase at the macroscale. According to our estimate, only for temperatures lower than 40-50 K a clear ferroelectric behavior is displayed. At higher temperature the polarization is progressively suppressed and the ferroelectric ordering hindered by the large configurational entropy, giving rise to a superparaelectric-like behavior at the macroscale
Thermally activated point defect diffusion in methylammonium lead trihalide: anisotropic and ultrahigh mobility of iodine
We study the diffusion of point defects in crystalline methylammonium lead halide (MAPI) at finite temperatures by using all-atoms molecular dynamics. We find that, for what concerns intrinsic defects, iodine diffusion is by far the dominant mechanism of ionic transport in MAPI, with diffusivities as high as 7.4 × 10-7 and 4.3 × 10-6 cm2 s-1 at 300 K and single activation energies of 0.24 and 0.10 eV, for interstitials and vacancies, respectively. The comparison with common covalent and oxide crystals reveals the ultrahigh mobility of defects in MAPI. Though at room temperature the vacancies are about 1 order of magnitude more diffusive, the anisotropic interstitial dynamics increases more rapidly with temperature, and it can be dominant at high temperatures. Present results are fully consistent with the involvement of iodide ions in hysteresis and have implications for improvement of the material quality by better control of defect diffusion
First Principles Investigation on the Modifications of the 4H-SiC Band Structure Due to the (4,4) and (3,5) Stacking Faults
Using first principle calculations, we investigated the energetic and electronic properties of two stacking faults that have been recently identified experimentally in as-grown 4H-SiC homo epitaxial films. We found that both defects generate two separate split-off bands localized below the bottom of the conduction band. The energy of the deepest intra gap state associated with each defect is in excellent agreement with photoluminescence measurements. Furthermore, we calculated formation energies of 0.3 and 2.4 mJ/m(2) for the (4,4) and (3,5) defects, respectively, much smaller than the energy of any other stacking fault; this result justifies their dominance in as-grown epilayers. (C) 2011 The Japan Society of Applied Physic