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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Nano-photocurrent Mapping of Local Electronic Structure in Twisted Bilayer Graphene

In this work, we report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene (TBG) enabling access to the local electronic phenomena at length scales as short as 20 nm. We show that the photocurrent changes sign at carrier densities tracking the local superlattice density of states of TBG. We use this property to identify domains of varying local twist angle by local photothermoelectric effect. Consistent with the photocurrent study, infrared nanoimaging experiments reveal optical conductivity features dominated by twist-angle-dependent interband transitions. Our results provide a fast and robust method for mapping the electronic structure of TBG and suggest that similar methods can be broadly applied to probe electronic inhomogeneities of Moiré superlattices in other van der Waals heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Local Atomic Configuration Control of Superconductivity in the Undoped Pnictide Parent Compound BaFe 2 As 2

Emergent superconductivity is strongly correlated with the symmetry of local atomic configuration in the parent compounds of iron-based superconductors. While chemical doping or hydrostatic pressure can change the local geometry, conventional approaches do not provide a clear pathway in predictably tuning the detailed atomic arrangement due to the parent compound’s complicated structural deformation in the presence of the tetragonal-to-orthorhombic phase transition. Here, we demonstrate a systematic approach to manipulate local structural configurations in BaFe 2 As 2 epitaxial thin films by controlling two independent structural factors, orthorhombicity (in-plane anisotropy) and tetragonality (out-of-plane/in-plane balance), from lattice parameters. We tune superconductivity without doping utilizing both structural factors separately and controlling local tetrahedral coordination in the designed thin film heterostructures with substrate clamping and biaxial strain. We further show this allows quantitative control of the structural phase transition, the associated magnetism, and superconductivity in parent material BaFe 2 As 2 . Furthermore, this approach will advance the development of tunable thin film superconductors in a reduced dimension.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Modeling High Current Pulsed Discharge in AA Battery Cathodes: The Effect of Localized Charging during Rest

During high current operation, substantial heterogeneity develops within battery cathodes, particularly when their thickness is large. Heterogeneity relaxation during subsequent rest is important for understanding battery performance under pulsed conditions. Localized charge balancing phenomena within batteries at zero net current are not well understood and merit investigation. In this work, the heterogeneity within cathodes of commercial alkaline Zn–MnO 2 batteries is measured during discharge and monitored during rest using energy dispersive X-ray diffraction (EDXRD). Significant gradients in protonation form during discharge and partially relax under rest. It is demonstrated that the proton gradient relaxation is through local redox activity at zero net current, where local (de)protonation works to redistribute charge across the cathode thickness. To support this redox-based relaxation, a fundamental kinetic study on prismatic MnO 2 cathodes is conducted to determine an appropriate model to describe both discharge and charge kinetics of MnO 2 . These kinetics are incorporated into a computational model to simulate the proton gradient formation and partial relaxation under identical discharge conditions as the operando EDXRD experiments. Model and experimental data are found to be in excellent agreement, correctly predicting localized charge balancing at rest.

Batteries↗
DOE OSTI2021

Chemical Modulation of Local Transition Metal Environment Enables Reversible Oxygen Redox in Mn-Based Layered Cathodes

Oxygen redox plays a prominent role in enhancing the energy density of Mn-based layered cathodes. However, understanding the factors affecting the reversibility of oxygen redox is nontrivial due to complicated structural and chemical transformations. Here in this paper, we show that local Mn–O symmetry induced structural/chemical evolutions majorly dictate the reversibility of oxygen redox of Na x Li y Mn 1–y O 2 in Na cells. Na x Li y Mn 1–y O 2 with Jahn–Teller distorted MnO 6 octahedra undergoes severe Mn dissolution during cycling, which destabilizes the transition metal layer resulting in poor Li retention and irreversible oxygen redox. Jahn–Teller distortion of MnO 6 octahedra can be suppressed by modulating the local charge of Mn and Mn–O distance through Mg/Ti dual doping. This leads to reduced Mn dissolution and more reversible oxygen redox. Such stabilization significantly improves the electrochemical performance of Mg/Ti dual doped Na x Li y Mn 1–y O 2 . Through this work, we show that local structural stabilization through local chemical environment modification can promote reversible oxygen redox in layered cathodes.

25 ENERGY STORAGE↗
DOE OSTI2024

Unveiling Spatial and Temporal Dynamics of Plasmon-Enhanced Localized Fields in Metallic Nanoframes through Ultrafast Electron Microscopy

Plasmonic nanomaterials, particularly noble metal nanoframes (NFs), are important for applications such as catalysis, biosensing, and energy harvesting due to their ability to enhance localized electric fields and atomic efficiency via localized surface plasmon resonance (LSPR). Yet the fundamental structure-function relationships and plasmonic dynamics of the NFS are difficult to study experimentally and thus far rely predominately on computational methodologies, limiting their utilization. This study leverages the capabilities of ultrafast electron microscopy (UEM), specifically photon-induced near-field electron microscopy (PINEM), to probe the light-matter interactions within plasmonic NF structures. Here, the effects of shape, size, and plasmonic coupling of Pt@Au core-shell NFs on spatial and temporal characteristics of plasmon-enhanced localized electric fields are explored. Importantly, time-resolved PINEM analysis reveals that the plasmonic fields around hexagonal NF prisms exhibit a spatially dependent excitation and decay rate, indicating a nuanced interplay between the spatial geometry of the NF and the temporal evolution of the localized electric field. These results and observations uncover nanophotonic energy transfer dynamics in NFs and highlight their potential for applications in biosensing and photocatalysis.

Plasmonics↗
DOE OSTI2019

How Substitutional Point Defects in Two-Dimensional WS 2 Induce Charge Localization, Spin–Orbit Splitting, and Strain

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step toward impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS 2 ) grown by chemical vapor deposition using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (Cr W ) and molybdenum (Mo W ) at a tungsten site, oxygen at sulfur sites in both top and bottom layers (O S top/bottom), and two negatively charged defects (CD type I and CD type II). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. Cr W forms three deep unoccupied defect states, two of which arise from spin-orbit splitting. The formation of such localized trap states for Cr W differs from the Mo W case and can be explained by their different d shell energetics and local strain, which we directly measured. Utilizing a tight-binding model the electronic spectra of the isolectronic substitutions O S and Cr W are mimicked in the limit of a zero hopping term and infinite on-site energy at a S and W site, respectively. The abundant CDs are negatively charged, which leads to a significant band bending around the defect and a local increase of the contact potential difference. In addition, CD-rich domains larger than 100 nm are observed, causing a work function increase of 1.1 V. While most defects are electronically isolated, we also observed hybrid states formed between CrW dimers. Finally, the important role of charge localization, spin-orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS 2 as reported here will guide future efforts of targeted defect engineering and doping of TMDs.

2D materials↗
DOE OSTI2025

Elucidation of Local Ordering and Atomic-Scale Structure in Polymer-Derived SiOC

Silicon oxycarbide (SiOC) is a versatile ceramic material with tunable microstructure and compositions that can be modulated through precursor chemistry and processing conditions. Though there are several noteworthy uses of SiOC across a range of application spaces, the difficulties in elucidating the short- to medium-range order within these materials have limited the maturation of strategies to precisely control SiC x O 4–x compositions for user-tailored applications. In this contribution, we implement a range of synchrotron scattering and spectroscopy methods coupled with stochastic modeling techniques to elucidate changes in local chemistry and structure associated with the pyrolysis of a commercially available SiOC polymer precursor. Stochastic modeling approaches provide valuable insights into decoupling local Si–O and Si–C environments while confirming predominate heterogeneous phases in materials. Using pyrolysis temperatures between 250 to 800 °C results in a heterogeneous material predominately composed of SiOC and amorphous SiO 2 domains. At 1100 °C, redistribution of Si–C pairs in the SiOC network and Si–O from the SiO 2 domains create a more ordered SiOC phase with local cubic SiC-like ordering. In addition, residual carbon leads to a detectable carbon phases at 1100 °C that persist at higher temperatures. These efforts address the difficulties of obtaining atomic-scale insights into the local structure and nanoscale heterogeneities in SiOC, providing pathways toward establishing structure–property relationships for future materials development.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides

High-entropy oxides (HEOs) have attracted great interest in diverse fields because of their inherent opportunities to tailor and combine materials functionalities. The control of local order/disorder in the class is by extension a grand challenge toward realizing their vast potential. In this paper, we report the first examples of pyrochlore HEOs with five M-site cations, for Nd 2 M 2 O 7 , in which the local structure has been investigated by neutron diffraction and pair distribution function (PDF) analysis. The average structure of the pyrochlores is found to be orthorhombic Imma, in agreement with radius-ratio rules governing the structural archetype. The computed PDFs from density functional theory relaxed special quasirandom structure models are compared with real space PDFs in this work to evaluate M-site order/disorder. Reverse Monte Carlo combined with ab initio molecular dynamics and Metropolis Monte Carlo simulations demonstrates that Nd 2 (Ta 0.2 Sc 0.2 Sn 0.2 Hf 0.2 Zr 0.2 ) 2 O 7 is synthesized with its M-site local to nanoscale order highly randomized/disordered, while Nd 2 (Ti 0.2 Nb 0.2 Sn 0.2 Hf 0.2 Zr 0.2 ) 2 O 7+x exhibits a strong distortion of the TiO 6 octahedron and small degree of Ti chemical short-range order (SRO) on the subnanometer scale. Calculations suggest that this may be intrinsic, energetically favored SRO rather than due to sample processing. These results offer an important demonstration that the engineered variation of participating ions in HEOs, even among those with very similar radii, provides richly diverse opportunities to control local order/disorder motifs—and therefore materials properties for future designs. This work also hints at the exquisite level of detail that may be needed in computational and experimental data analysis to guide structure–property tuning in the emerging HEO materials class.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Elucidating the Roles of Local and Nonlocal Rate Enhancement Mechanisms in Plasmonic Catalysis

Plasmonic metal nanoparticles (e.g., Ag, Au, and Cu) constitute a class of materials that interact with light via the excitation of localized surface plasmon resonance (LSPR). Numerous studies have reported substantial enhancements in the rates of chemical reactions on illuminated plasmonic nanoparticle catalysts compared to corresponding systems in the absence of illumination. There are two mechanisms that have been proposed to explain the LSPR-induced chemical reactivity. One mechanism assumes a local plasmon-induced hot charge-carrier-mediated activation of the reactants, while the other assumes an LSPR-induced equilibrium heating of the catalyst, which leads to energy transfer to and chemical reaction of the adsorbed reactants. Here, in this contribution, we developed a setup amenable to accurate in situ catalyst temperature and kinetic reaction rate measurements. We employed this setup to study the LSPR-induced rate enhancement in a case study of the CO oxidation reaction on plasmonic, monometallic Ag nanoparticle catalysts supported on α-Al 2 O 3 . We explored various Ag loadings and clustering levels. Our data show that the equilibrium heating of the catalyst cannot fully explain the illumination-induced plasmonic rate enhancements. This is the case even for high loading and clustering of Ag nanoparticles, where the equilibrium heating significantly increases. Based on the analysis, we propose that local effects, related to the plasmon-induced activation of adsorbates (reactants) via electronic excitation of the reactant or photothermal heating of the reactants that is highly localized to the individual nanoparticles, play a critical role in driving LSPR-induced chemical reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electron localization of anions probed by nitrile vibrations

Localization and delocalization of electrons is a key concept in chemistry, and is one of the important factors determining the efficiency of electron transport through organic conjugated molecules, which have potential to act as “molecular wires”. This, in turn, substantially influences the efficiencies of organic solar cells and other molecular electronic devices. It is also necessary to understand the electronic energy landscape and the dynamics of electrons through molecular chain that govern their transport capabilities in one-dimensional conjugated chains so that we can better define the design principles of conjugated molecules for their applications. We show that nitrile ν(C≡N) vibrations respond to the degree of electron localization in nitrile-substituted organic anions by utilizing time-resolved infrared (TRIR) detection combined with pulse radiolysis. Measurements of a series of aryl nitrile anions allow us to construct a semi-empirical calibration curve between the changes in the ν(C≡N) IR shifts and the changes in the electronic charges from the neutral to the anion states in the nitriles; more electron localization in the nitrile anion results in larger IR shifts. Furthermore, the IR linewidth in anions can report a structural change accompanying changes in the electronic density distribution. Probing the shift of the nitrile ν(C≡N) IR vibrational bands enables us to determine how the electron is localized in anions of nitrile-functionalized oligofluorenes, considered as organic mixed-valence compounds. We estimate the diabatic electron transfer distance, electronic coupling strengths, and energy barriers in these organic mixed-valence compounds. The analysis reveals a dynamic picture, showing that the electron is moving back and forth within the oligomers with a small activation energy of ≤ k B T, likely controlled by the movement of dihedral angles between monomer units. Thus, implications for the electron transport capability in "molecular wires" are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Characterizing Oxygen Local Environments in Paramagnetic Battery Materials via 17 O NMR and DFT Calculations

Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of 17 O NMR in the same materials has not yet been reported. In the work reported here, we present a combined 17 O NMR and hybrid density functional theory study of the local O environments in Li 2 MnO 3 , a model compound for layered Li-ion batteries. After a simple 17 O enrichment procedure, we observed five resonances with large 17 O shifts ascribed to the Fermi contact interaction with directly bonded Mn 4+ ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the 17 O shifts of environments similar to the 4i and 8j sites in pristine Li 2 MnO 3 , respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li 2 MnO 3 structure. From the ratio of the intensities of the different 17 O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying 17 O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides.

25 ENERGY STORAGE↗
DOE OSTI2014

Intrinsic Nanoscience of δ Pu-Ga Alloys: Local Structure and Speciation, Collective Behavior, Nanoscale Heterogeneity, and Aging Mechanisms

Because diffraction measurements are sensitive only to the long range average arrangement of the atoms in the coherent portion of a crystal, complementary local structure measurements are required for a complete understanding of the structure of a complex material. This is particularly an issue in solid solutions where even random distributions of a solute will result in nanometer-scale fluctuations in the local composition. The structure will be further complicated if collective and cooperative phenomena organize the solute distribution via longer range interactions between non-bonded solute sites. If the solute affects the phase stability then the question is raised of whether the atoms in domains with local compositions outside the limits of the bulk phase will rearrange into the structure stable for that composition and temperature or if the resulting stress would prevent such a local phase transition. If the former, then phase separated, heterogeneous structures at or below the diffraction limit will form. This nanometerscale competition between the phase transition and the epitaxial mismatch – exacerbated by the added strain if the transition involves a volume change – raises the potential for the formation of novel structures that do not occur in bulk material, e.g., fcc Fe. This coupling over multiple scales between inhomogeneity ordering, elastic forces, phase competition, and texture in the form of coexisting structures is a hallmark of martensites, a class of complex materials that includes δ-stabilized PuGa and that often exhibit correlated atomic and electronic properties. The enigmatic and extreme nature of Pu is consistent with its exhibiting unusual structural behavior of this type, including nanoscale heterogeneity in δ-stabilized PuGa and its enhanced homogeneity on aging that has been suggested based on earlier X-ray Absorption Fine Structure (XAFS) spectroscopy and x-ray pair distribution function (pdf) measurements. Measurements on a defined set of laboratory-prepared materials now corroborate and better describe this heterogeneity while additional aged samples demonstrate the role of heterogeneity in aging processes in Pu.

Conradson, Steven D.↗
DOE OSTI2025

Space‐Time Causal Discovery in Earth System Science: A Local Stencil Learning Approach

Causal discovery tools enable scientists to infer meaningful relationships from observational data, spurring advances in fields as diverse as biology, economics, and climate science. Despite these successes, the application of causal discovery to space-time systems remains immensely challenging due to the high-dimensional nature of the data. For example, in climate sciences, modern observational temperature records over the past few decades regularly measure thousands of locations around the globe. To address these challenges, we introduce Causal Space-Time Stencil Learning (CaStLe), a novel meta-algorithm for discovering causal structures in complex space-time systems. CaStLe leverages regularities in local space-time dependencies to learn governing global dynamics. This local perspective eliminates spurious confounding and drastically reduces sample complexity, making space-time causal discovery practical and effective. For causal discovery, CaStLe flexibly accepts any appropriately adapted time series causal discovery algorithm to recover local causal structures. These advances enable causal discovery of geophysical phenomena that were previously unapproachable, including non-periodic, transient phenomena such as volcanic eruption plumes. Regularities in local space-time dependencies are transformed into informative spatial replicates, which actually improve CaStLe's performance when applied to ever-larger spatial grids. We successfully apply CaStLe to discover the atmospheric dynamics governing the climate response to the 1991 Mount Pinatubo volcanic eruption. We provide validation experiments to demonstrate the effectiveness of CaStLe over existing causal-discovery frameworks on a range of geophysics-inspired benchmarks while identifying the method's limitations and domains where its assumptions may not hold.

Nichol, J. Jake [Univ. of New Mexico, Albuquerque,↗
DOE OSTI2017

Direct observation of localized surface plasmon field enhancement by Kelvin probe force microscopy

A surface plasmon (SP) is a fundamental excitation state that exists in metal nanostructures. Over the past several years, the performance of optoelectronic devices has been improved greatly via the SP enhancement effect. In our previous work, the responsivity of GaN ultraviolet detectors was increased by over 30 times when using Ag nanoparticles. However, the physics of the SP enhancement effect has not been established definitely because of the lack of experimental evidence. To reveal the physical origin of this enhancement, Kelvin probe force microscopy (KPFM) was used to observe the SP-induced surface potential reduction in the vicinity of Ag nanoparticles on a GaN epilayer. Under ultraviolet illumination, the localized field enhancement induced by the SP forces the photogenerated electrons to drift close to the Ag nanoparticles, leading to a reduction of the surface potential around the Ag nanoparticles on the GaN epilayer. For an isolated Ag nanoparticle with a diameter of ~200 nm, the distribution of the SP localized field is located within 60 nm of the boundary of the Ag nanoparticle. For a dimer of Ag nanoparticles, the localized field enhancement between the nanoparticles was the strongest. The results presented here provide direct experimental proof of the localized field enhancement. In conclusion, these results not only explain the high performance of GaN detectors observed with the use of Ag nanoparticles but also reveal the physical mechanism of SP enhancement in optoelectronic devices, which will help us further understand and improve the performance of SP-based optoelectronic devices in the future.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ultrafast charge localization in a stripe-phase nickelate

Self-organized electronically-ordered phases are a recurring feature in correlated materials, resulting in e.g. fluctuating charge stripes whose role in high-Tc superconductivity is under debate. However, the relevant cause-effect relations between real-space charge correlations and low-energy excitations remain hidden in time-averaged studies. Here, we reveal ultrafast charge localization and lattice vibrational coupling as dynamical precursors of stripe formation in the model compound La1.75Sr0.25NiO4, using ultrafast and equilibrium mid-infrared spectroscopy. The opening of a pseudogap at a crossover temperature T* far above long-range stripe formation establishes the onset of electronic localization which is accompanied by an enhanced Fano asymmetry of Ni-O stretch vibrations. Ultrafast excitation triggers a sub-picosecond dynamics exposing the synchronous modulation of electron-phonon coupling and charge localization. These results illuminate the role of localization in forming the pseudogap in nickelates, opening a path to understanding this mysterious phase in a broad class of complex oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Correspondence: Phantom phonon localization in relaxors

Manley et al. report the observation of an unexpected, weak, phonon mode located in energy between that of the transverse acoustic (TA) and soft transverse optic (TO) modes in the relaxor ferroelectric Pb[(Mg 1/3 Nb 2/3 ) 1-x Ti x ]O 3 (PMN-xPT) with Ti content x = 0.30. Referred to as the local mode (LM), the energy of this extra mode varies little with wave vector in the cubic (paraelectric) phase, and this was interpreted as evidence of phonon localization. Any excitation that is localized in space must be extended in reciprocal space Q. To confirm this property, we performed a standard test on a PMN-xPT single crystal of nominally identical composition. This was done by comparing the rates at which the Bragg, TA phonon, and LM neutron scattering cross-sections decrease as the cubic crystallographic [100] axis is tilted out of the horizontal scattering plane by rotating about the orthogonal [010] axis. We find that the LM cross section varies with tilt in a manner identical to that of the Bragg peak. A local mode with short-range spatial correlations that decay exponentially will exhibit a broad Lorentzian peak as a function of tilt. Instead, we observe a narrow Gaussian peak. We conclude that the LM is not a real mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Inducing Kondo screening of vacancy magnetic moments in graphene with gating and local curvature

In normal metals the magnetic moment of impurity-spins disappears below a characteristic Kondo temperature which marks the formation of a cloud of conduction-band electrons that screen the local-moment. In contrast, moments embedded in insulators remain unscreened at all temperatures. What then is the fate of magnetic-moments in intermediate, pseudogap systems, such as graphene? Theory predicts that coupling to the conduction-band electrons will drive a quantum phase transition between a local-moment phase and a Kondo-screened phase. However, attempts to experimentally confirm this prediction and its intriguing consequences, such as electrostatically tunable magnetic-moments, have been elusive. Here we report the observation of Kondo-screening and the quantum phase-transition between screened and unscreened phases of vacancy magnetic moments in graphene. Using scanning tunneling spectroscopy and numerical renormalization-group calculations we show that this transition enables to control the screening of local moments by tuning the gate voltage and the local curvature of the graphene membrane.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Intrinsic anharmonic localization in thermoelectric PbSe

Lead chalcogenides have exceptional thermoelectric properties and intriguing anharmonic lattice dynamics underlying their low thermal conductivities. An ideal material for thermoelectric efficiency is the phonon glass–electron crystal, which drives research on strategies to scatter or localize phonons while minimally disrupting electronic-transport. Anharmonicity can potentially do both, even in perfect crystals, and simulations suggest that PbSe is anharmonic enough to support intrinsic localized modes that halt transport. Here, we experimentally observe high-temperature localization in PbSe using neutron scattering but find that localization is not limited to isolated modes – zero group velocity develops for a significant section of the transverse optic phonon on heating above a transition in the anharmonic dynamics. Arrest of the optic phonon propagation coincides with unusual sharpening of the longitudinal acoustic mode due to a loss of phase space for scattering. Our study shows how nonlinear physics beyond conventional anharmonic perturbations can fundamentally alter vibrational transport properties.

36 MATERIALS SCIENCE↗