881 research outputs found
Ab initio data-analytics study of carbon-dioxide activation on semiconductor oxide surfaces
The excessive emissions of carbon dioxide (CO2) into the atmosphere threaten to shift the CO2 cycle planet-wide and induce unpredictable climate changes. Using artificial intelligence (AI) trained on high-throughput first principles based data for a broad family of oxides, we develop a strategy for a rational design of catalytic materials for converting CO2 to fuels and other useful chemicals. We demonstrate that an electron transfer to the π-antibonding orbital of the adsorbed molecule and the associated bending of the initially linear molecule, previously proposed as the indicator of activation, are insufficient to account for the good catalytic performance of experimentally characterized oxide surfaces. Instead, our AI model identifies the common feature of these surfaces in the binding of a molecular O atom to a surface cation, which results in a strong elongation and therefore weakening of one molecular C-O bond. This finding suggests using the C-O bond elongation as an indicator of CO2 activation. Based on these findings, we propose a set of new promising oxide-based catalysts for CO2 conversion, and a recipe to find more
Recent advances in the SISSO method and their implementation in the SISSO++ code
Accurate and explainable artificial-intelligence (AI) models are promising
tools for the acceleration of the discovery of new materials, ore new
applications for existing materials. Recently, symbolic regression has become
an increasingly popular tool for explainable AI because it yields models that
are relatively simple analytical descriptions of target properties. Due to its
deterministic nature, the sure-independence screening and sparsifying operator
(SISSO) method is a particularly promising approach for this application. Here
we describe the new advancements of the SISSO algorithm, as implemented into
SISSO++, a C++ code with Python bindings. We introduce a new representation of
the mathematical expressions found by SISSO. This is a first step towards
introducing ``grammar'' rules into the feature creation step. Importantly, by
introducing a controlled non-linear optimization to the feature creation step
we expand the range of possible descriptors found by the methodology. Finally,
we introduce refinements to the solver algorithms for both regression and
classification, that drastically increase the reliability and efficiency of
SISSO. For all of these improvements to the basic SISSO algorithm, we not only
illustrate their potential impact, but also fully detail how they operate both
mathematically and computationally.Comment: 10 pages, 7 figures, 4 table
Evidence of orbital reconstruction at interfaces in La0.67Sr0.33MnO3 films
Electronic properties of transition metal oxides at interfaces are influenced
by strain, electric polarization and oxygen diffusion. Linear dichroism (LD)
x-ray absorption, diffraction, transport and magnetization on thin
La0.7Sr0.3MnO3 films, allow identification of a peculiar universal interface
effect. We report the LD signature of preferential 3d-eg(3z2-r2) occupation at
the interface, suppressing the double exchange mechanism. This surface orbital
reconstruction is opposite of that favored by residual strain and independent
of dipolar fields, chemical nature of the substrate and capping.Comment: 13 pages, 5 figure
Multiple double-exchange mechanism by Mn-doping in manganite compounds
Double-exchange mechanisms in REAEMnO manganites (where
RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies
on the strong exchange interaction between two Mn and Mn ions
through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions
has ever been considered "silent" with respect to the DE conducting mechanisms.
Here we show that a new path for DE-mechanism is indeed possible by partially
replacing the RE-AE elements by Mn-ions, in La-deficient
LaMnO thin films. X-ray absorption spectroscopy demonstrated
the relevant presence of Mn ions, which is unambiguously proved to be
substituted at La-site by Resonant Inelastic X-ray Scattering. Mn is
proved to be directly correlated to the enhanced magneto-transport properties
because of an additional hopping mechanism trough interfiling Mn-ions,
theoretically confirmed by calculations within the effective single band model.
The very idea to use Mn both as a doping element and an ions
electronically involved in the conduction mechanism, has never been foreseen,
revealing a new phenomena in transport properties of manganites. More
important, such a strategy might be also pursed in other strongly correlated
materials.Comment: 6 pages, 5 figure
Dynamical charge density fluctuations pervading the phase diagram of a Cu-based high-Tc superconductor
Charge density waves are a common occurrence in all families of high critical
temperature superconducting cuprates. Although consistently observed in the
underdoped region of the phase diagram and at relatively low temperatures, it
is still unclear to what extent they influence the unusual properties of these
systems. Using resonant x-ray scattering we carefully determined the
temperature dependence of charge density modulations in
(Y,Nd)BaCuO for three doping levels. We discovered
short-range dynamical charge density fluctuations besides the previously known
quasi-critical charge density waves. They persist up to well above the
pseudogap temperature T*, are characterized by energies of few meV and pervade
a large area of the phase diagram, so that they can play a key role in shaping
the peculiar normal-state properties of cuprates.Comment: 34 pages, 4 figures, 11 supplementary figure
Evidence for core-hole-mediated inelastic x-ray scattering from metallic FeTe
We present a detailed analysis of resonant inelastic scattering (RIXS) from
FeTe with unprecedented energy resolution. In contrast to the sharp
peaks typically seen in insulating systems at the transition metal edge,
we observe spectra which show different characteristic features. For low energy
transfer, we experimentally observe theoretically predicted many-body effects
of resonant Raman scattering from a non-interacting gas of fermions.
Furthermore, we find that limitations to this many-body electron-only theory
are realized at high Raman shift, where an exponential lineshape reveals an
energy scale not present in these considerations. This regime, identified as
emission, requires considerations of lattice degrees of freedom to understand
the lineshape. We argue that both observations are intrinsic general features
of many-body physics of metals.Comment: 4 pages, 4 figure
Analysis of Topological Transitions in Two-dimensional Materials by Compressed Sensing
Quantum spin-Hall insulators (QSHIs), i.e., two-dimensional topological insulators (TIs) with a symmetry-protected band inversion, have attracted considerable scientific interest in recent years. In this work, we have computed the topological Z2 invariant for 220 functionalized honeycomb lattices that are isoelectronic to functionalized graphene. Besides confirming the TI character of well-known materials such as functionalized stanene, our study identifies 45 yet unreported QSHIs. We applied a compressed-sensing approach to identify a physically meaningful descriptor for the Z2 invariant that only depends on the properties of the material's constituent atoms. This enables us to draw a map of materials, in which metals, trivial insulators, and QSHI form distinct regions. This analysis yields fundamental insights in the mechanisms driving topological transitions. The transferability of the identified model is explicitly demonstrated for an additional set of honeycomb lattices with different functionalizations that are not part of the original set of 220 graphene-type materials used to identify the descriptor. In this class, we predict 74 more novel QSHIs that have not been reported in literature yet
Dispersive charge density wave excitations and temperature dependent commensuration in Bi2Sr2CaCu2O8+{\delta}
Experimental evidence on high-Tc cuprates reveals ubiquitous charge density
wave (CDW) modulations, which coexist with superconductivity. Although the CDW
had been predicted by theory, important questions remain about the extent to
which the CDW influences lattice and charge degrees of freedom and its
characteristics as functions of doping and temperature. These questions are
intimately connected to the origin of the CDW and its relation to the
mysterious cuprate pseudogap. Here, we use ultrahigh resolution resonant
inelastic x-ray scattering (RIXS) to reveal new CDW character in underdoped
Bi2Sr2CaCu2O8+{\delta} (Bi2212). At low temperature, we observe dispersive
excitations from an incommensurate CDW that induces anomalously enhanced phonon
intensity, unseen using other techniques. Near the pseudogap temperature T*,
the CDW persists, but the associated excitations significantly weaken and the
CDW wavevector shifts, becoming nearly commensurate with a periodicity of four
lattice constants. The dispersive CDW excitations, phonon anomaly, and
temperature dependent commensuration provide a comprehensive momentum space
picture of complex CDW behavior and point to a closer relationship with the
pseudogap state
Dispersion, damping, and intensity of spin excitations in the single-layer (Bi,Pb)(Sr,La)CuO cuprate superconductor family
Using Cu- edge resonant inelastic x-ray scattering (RIXS) we measured
the dispersion and damping of spin excitations (magnons and paramagnons) in the
high- superconductor (Bi,Pb)(Sr,La)CuO
(Bi2201), for a large doping range across the phase diagram (). Selected measurements with full polarization analysis
unambiguously demonstrate the spin-flip character of these excitations, even in
the overdoped sample. We find that the undamped frequencies increase slightly
with doping for all accessible momenta, while the damping grows rapidly, faster
in the (0,0)(0.5,0.5) nodal direction than in the
(0,0)(0.5,0) antinodal direction. We compare the experimental
results to numerically exact determinant quantum Monte Carlo (DQMC)
calculations that provide the spin dynamical structure factor
of the three-band Hubbard model. The theory reproduces
well the momentum and doping dependence of the dispersions and spectral weights
of magnetic excitations. These results provide compelling evidence that
paramagnons, although increasingly damped, persist across the superconducting
dome of the cuprate phase diagram; this implies that long range
antiferromagnetic correlations are quickly washed away, while short range
magnetic interactions are little affected by doping.Comment: 11 pages, 9 figure
High-energy spin and charge excitations in electron-doped copper oxide superconductors
The evolution of electronic (spin and charge) excitations upon carrier doping
is an extremely important issue in superconducting layered cuprates and the
knowledge of its asymmetry between electron- and hole-dopings is still
fragmentary. Here we combine x-ray and neutron inelastic scattering
measurements to track the doping dependence of both spin and charge excitations
in electron-doped materials. Copper L3 resonant inelastic x-ray scattering
spectra show that magnetic excitations shift to higher energy upon doping.
Their dispersion becomes steeper near the magnetic zone center and deeply mix
with charge excitations, indicating that electrons acquire a highly itinerant
character in the doped metallic state. Moreover, above the magnetic
excitations, an additional dispersing feature is observed near the
{\Gamma}-point, and we ascribe it to particle-hole charge excitations. These
properties are in stark contrast with the more localized spin-excitations
(paramagnons) recently observed in hole-doped compounds even at high
doping-levels.Comment: 20 page
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