82 research outputs found
Local Moment Instability of Os in Honeycomb Li2.15Os0.85O3.
Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li2.15(3)Os0.85(3)O3 (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μB, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)-Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state
Ab Initio Approach to Second-order Resonant Raman Scattering Including Exciton-Phonon Interaction
Raman spectra obtained by the inelastic scattering of light by crystalline
solids contain contributions from first-order vibrational processes (e.g. the
emission or absorption of one phonon, a quantum of vibration) as well as
higher-order processes with at least two phonons being involved. At second
order, coupling with the entire phonon spectrum induces a response that may
strongly depend on the excitation energy, and reflects complex processes more
difficult to interpret. In particular, excitons (i.e. bound electron-hole
pairs) may enhance the absorption and emission of light, and couple strongly
with phonons in resonance conditions. We design and implement a
first-principles methodology to compute second-order Raman scattering,
incorporating dielectric responses and phonon eigenstates obtained from
density-functional theory and many-body theory. We demonstrate our approach for
the case of silicon, relating frequency-dependent relative Raman intensities,
that are in excellent agreement with experiment, to different vibrations and
regions of the Brillouin zone. We show that exciton-phonon coupling, computed
from first principles, indeed strongly affect the spectrum in resonance
conditions. The ability to analyze second-order Raman spectra thus provides
direct insight into this interaction.Comment: 10 pages, 8 figure
Generating and grading 34 Optimised Norm-Conserving Vanderbilt Pseudopotentials for Actinides and Super Heavy Elements in the PseudoDojo
In the last decades, material discovery has been a very active research field
driven by the necessity of new materials for different applications. This has
also included materials incorporating heavy elements, beyond the stable
isotopes of lead. Most of actinides exhibit unique properties that make them
useful in various applications. Further, new heavy elements, taking the name of
super-heavy elements, have been synthesized, filling previously empty space of
Mendeleev periodic table. Their chemical bonding behaviour, of academic
interest at present, would also benefit of state-of-the-art modelling
approaches. In particular, in order to perform first-principles calculations
with planewave basis sets, one needs corresponding pseudopotentials. In this
work, we present a series of fully-relativistic optimised norm-conserving
Vanderbilt pseudopotentials (ONCVPs) for thirty-four actinides and super-heavy
elements. The scalar relativistic version of these ONCVPs is tested by
comparing equations of states for crystals, obtained with \textsc{abinit} 9.6,
with those obtained by all-electron zeroth-order regular approximation (ZORA)
calculations performed with the Amsterdam Modelling Suite BAND code.
-Gauge and -Gauge indicators are used to validate these
pseudopotentials. This work is a contribution to the PseudoDojo project, in
which pseudopotentials for the whole periodic table are developed and
systematically tested. The fully-relativistic pseudopotential files (i.e.
including spin-orbit coupling) are available on the PseudoDojo web-interface
pseudo-dojo.org under the name NC FR (ONCVPSP) v4.x. Pseudopotentials are made
available in psp8 and UPF2 formats, both convenient for \textsc{abinit}, the
latter being also suitable for Quantum ESPRESSO
Convergence and pitfalls of density functional perturbation theory phonons calculations from a high-throughput perspective
The diffusion of large databases collecting different kind of material
properties from high-throughput density functional theory calculations has
opened new paths in the study of materials science thanks to data mining and
machine learning techniques. Phonon calculations have already been employed
successfully to predict materials properties and interpret experimental data,
e.g. phase stability, ferroelectricity and Raman spectra, so their availability
for a large set of materials will further increase the analytical and
predictive power at hand. Moving to a larger scale with density functional
perturbation calculations, however, requires the presence of a robust framework
to handle this challenging task. In light of this, we automatized the phonon
calculation and applied the result to the analysis of the convergence trends
for several materials. This allowed to identify and tackle some common problems
emerging in this kind of simulations and to lay out the basis to obtain
reliable phonon band structures from high-throughput calculations, as well as
optimizing the approach to standard phonon simulations
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