41 research outputs found
Fingerprints for spin-selection rules in the interaction dynamics of O2 at Al(111)
We performed mixed quantum-classical molecular dynamics simulations based on
first-principles potential-energy surfaces to demonstrate that the scattering
of a beam of singlet O2 molecules at Al(111) will enable an unambiguous
assessment of the role of spin-selection rules for the adsorption dynamics. At
thermal energies we predict a sticking probability that is substantially less
than unity, with the repelled molecules exhibiting characteristic kinetic,
vibrational and rotational signatures arising from the non-adiabatic spin
transition.Comment: 4 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Signatures of nonadiabatic O2 dissociation at Al(111): First-principles fewest-switches study
Recently, spin selection rules have been invoked to explain the discrepancy
between measured and calculated adsorption probabilities of molecular oxygen
reacting with Al(111). In this work, we inspect the impact of nonadiabatic spin
transitions on the dynamics of this system from first principles. For this
purpose the motion on two distinct potential-energy surfaces associated to
different spin configurations and possible transitions between them are
inspected by means of the Fewest Switches algorithm. Within this framework we
especially focus on the influence of such spin transitions on observables
accessible to molecular beam experiments. On this basis we suggest experimental
setups that can validate the occurrence of such transitions and discuss their
feasibility.Comment: 13 pages, 7 figure
Ab initio Green-Kubo simulations of heat transport in solids: Method and implementation
Ab initio Green-Kubo (aiGK) simulations of heat transport in solids allow for
assessing lattice thermal conductivity in anharmonic or complex materials from
first principles. In this work, we present a detailed account of their
practical application and evaluation with an emphasis on noise reduction and
finite-size corrections in semiconductors and insulators. To account for such
corrections, we propose strategies in which all necessary numerical parameters
are chosen based on the dynamical properties displayed during molecular
dynamics simulations in order to minimize manual intervention. This paves the
way for applying the aiGK method in semi-automated and high-throughput
frameworks. The proposed strategies are presented and demonstrated for
computing the lattice thermal conductivity at room temperature in the mildly
anharmonic periclase MgO, and for the strongly anharmonic marshite CuI.Comment: 13 pages, 9 figure
Formation of Vacancies in Si- and Ge-based Clathrates: Role of Electron Localization and Symmetry Breaking
The formation of framework vacancies in Si- and Ge-based type-I clathrates is
studied as function of filling the cages with K and Ba atoms using
density-functional theory. Our analysis reveals the relevance of structural
disorder, geometric relaxation, electronic saturation, as well as vibrational
and configurational entropy. In the Si clathrates we find that vacancies are
unstable, but very differently, in Ge clathrates up to three vacancies per unit
cell can be stabilized. This contrasting behavior is largely driven by the
different energy gain on populating the electronic vacancy states, which
originates from the different degree of localization of the valence orbitals of
Si and Ge. This also actuates a qualitatively different atomic relaxation of
the framework.Comment: 5 pages, 6 figures, submitted to a journa
Spontaneous polarization in NaNbO film on NdGaO and DyScO substrates
Pure NaNbO is an antiferroelectric material at room temperature that
irreversibly transforms to a ferroelectric polar state when subjected to an
external electrical field or lattice strain. Experimentally, it has been
observed that NaNbO films grown on NdGaO exhibit an electrical
polarization along the [001] direction, whereas films on
DyScO substrates exhibit a polarization along the [011]
direction. These effects have been attributed to the realization of different
lattice symmetries in the films due to the incorporation of lattice strain
imposed by the use of oxide substrates with different lattice parameters.
However, the underlying atomistic mechanisms of the resulting phase symmetry in
the films are hardly clear, given that NaNbO features a diverse and
complex phase diagram. In turn, these also impede a straightforward tailoring
and optimization of the resulting macroscopic properties on different
substrates. To clarify this issue, we perform all-electron first-principles
calculations for several potential NaNbO polymorphs under stress and
strain. The computed properties, including the ferroelectric polarization,
reveal that an orthorhombic phase is realized on NdGaO
substrates since this is the only phase with an out-of-plane polarization under
a compressive strain. Conversely, the monoclinic phase is consistent for
the samples grown on DyScO substrate, since this phase exhibits a
spontaneous in-plane polarization along [011] under tensile
strain.Comment: 9 pages, 5 figures, and supplementary material
Anharmonicity in Thermal Insulators: An Analysis from First Principles
The anharmonicity of atomic motion limits the thermal conductivity in
crystalline solids. However, a microscopic understanding of the mechanisms
active in strong thermal insulators is lacking. In this letter, we classify 465
experimentally known materials with respect to their anharmonicity and perform
fully anharmonic ab initio Green-Kubo calculations for 58 of them, finding 28
thermal insulators with W/mK including 6 with ultralow W/mK. Our analysis reveals that the underlying strong anharmonic
dynamics is driven by the exploration of meta-stable intrinsic defect
geometries. This is at variance with the frequently applied perturbative
approach, in which the dynamics is assumed to evolve around a single stable
geometry.Comment: 5 pages, 4 figure
Anharmonicity Measure for Materials
Theoretical frameworks used to qualitatively and quantitatively describe
nuclear dynamics in solids are often based on the harmonic approximation.
However, this approximation is known to become inaccurate or to break down
completely in many modern functional materials. Interestingly, there is no
reliable measure to quantify anharmonicity so far. Thus, a systematic
classification of materials in terms of anharmonicity and a benchmark of
methodologies that may be appropriate for different strengths of anharmonicity
is currently impossible. In this work, we derive and discuss a statistical
measure that reliably classifies compounds across temperature regimes and
material classes by their "degree of anharmonicity". This enables us to
distinguish "harmonic" materials, for which anharmonic effects constitute a
small perturbation on top of the harmonic approximation, from strongly
"anharmonic" materials, for which anharmonic effects become significant or even
dominant and the treatment of anharmonicity in terms of perturbation theory is
more than questionable. We show that the analysis of this measure in real and
reciprocal space is able to shed light on the underlying microscopic
mechanisms, even at conditions close to, e.g., phase transitions or defect
formation. Eventually, we demonstrate that the developed approach is
computationally efficient and enables rapid high-throughput searches by
scanning over a set of several hundred binary solids. The results show that
strong anharmonic effects beyond the perturbative limit are not only active in
complex materials or close to phase transitions, but already at moderate
temperatures in simple binary compounds.Comment: 17 figures, 2 tables, and 12 page
Evaluating the Role of Anharmonic Vibrations in Zeolite β Materials
The characterization of zeolitic materials is often facilitated by spectroscopic analysis of vibrations, which informs about the bonding character of the substrate and any adsorbents. Computational simulations aid the interpretation of the spectra but often ignore anharmonic effects that can affect the spectral characteristics significantly. Here, the impact of anharmonicity is demonstrated with a combination of dynamical and static simulations applied to the structures formed during the synthesis of Sn-BEA via solid-state incorporation (SSI): the initial siliceous BEA (Si-β), aluminosilicate BEA (H-β), dealuminated BEA (deAl-β), and Sn-BEA (Sn-β). Heteroatom and defect-containing BEA are shown to have strong anharmonic vibrational contributions, with atomic and elemental resolution highlighting particularly the prevalence for H atoms (H-β, deAl-β) as well as localization to heteroatoms at defect sites. We simulate the vibrational spectra of BEA accounting for anharmonic contributions and observe an improved agreement with experimental data compared to harmonic methods, particularly at wavenumbers below 1500 cm–1. The results demonstrate the importance of incorporating anharmonic effects in simulations of vibrational spectra, with consequences toward future characterization and application of zeolitic materials
On the Uncertainty Estimates of Equivariant-Neural-Network-Ensembles Interatomic Potentials
Machine-learning (ML) interatomic potentials (IPs) trained on
first-principles datasets are becoming increasingly popular since they promise
to treat larger system sizes and longer time scales, compared to the {\em ab
initio} techniques producing the training data. Estimating the accuracy of
MLIPs and reliably detecting when predictions become inaccurate is key for
enabling their unfailing usage. In this paper, we explore this aspect for a
specific class of MLIPs, the equivariant-neural-network (ENN) IPs using the
ensemble technique for quantifying their prediction uncertainties. We
critically examine the robustness of uncertainties when the ENN ensemble IP
(ENNE-IP) is applied to the realistic and physically relevant scenario of
predicting local-minima structures in the configurational space. The ENNE-IP is
trained on data for liquid silicon, created by density-functional theory (DFT)
with the generalized gradient approximation (GGA) for the exchange-correlation
functional. Then, the ensemble-derived uncertainties are compared with the
actual errors (comparing the results of the ENNE-IP with those of the
underlying DFT-GGA theory) for various test sets, including liquid silicon at
different temperatures and out-of-training-domain data such as solid phases
with and without point defects as well as surfaces. Our study reveals that the
predicted uncertainties are generally overconfident and hold little
quantitative predictive power for the actual errors