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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 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Coupling-Assisted Renormalization of Excitons and Vibrations in Compressed MoSe 2 –WSe 2 Heterostructure

Vertical heterostructures (HSs) constructed with two-dimensional (2D) materials is expected to generate fascinating properties due to interlayer coupling between neighboring layers. However, interlayer coupling can be easily obscured by cross-contamination during transfer processes, rendering their experimental demonstration challenging. Here, we explore the coupling-assisted renormalization of excitons and vibrations in a mechanically fabricated MoSe 2 –WSe 2 HS through high-pressure photoluminescence, Raman spectra, and density functional theory calculations. Accompanied by the interlayer coupling enhancement, the excitonic and vibrational renormalizations involving dimensionality and composition variations were achieved. A cycle of 2D–3D–2D excitonic evolution was disclosed and pressure-induced emergence of X – exciton of MoSe 2 in HS was found reflecting the band structure transition in the MoSe 2 –WSe 2 HS. The Raman spectra reveals that the coupled A 2 " vibrations of WSe 2 and MoSe 2 in HS was stiffened and out-of-plane A 1 ' vibrations of WSe 2 and MoSe 2 in HS got coherent upon pressure modulation. Here, this coupling-assisted renormalization in MoSe 2 –WSe 2 HS can be extended to other 2D layered HSs, which indicates the possibility to design a flexible HS with controlled excitonic and vibrational system for light-emitting diodes, excitonic, and photovoltaic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Turn-On Conductivity with Proton-Coupled Electron Transport in Metal–Organic Frameworks

Proton-coupled electron transfer (PCET) has been studied for decades in the context of molecular reactivity, but its impact on long-range electron transport is barely understood. When defined broadly as ion-coupled charge transport (ICCT), relevant systems include lithium-ion battery electrodes, electrochromic coatings, and myriad electrocatalysts. Despite ample evidence that ion-electron coupling enhances or diminishes the performance of these devices, little is known about the experimental signatures of ICCT and the microscopic factors that govern its mechanism. Here, we expect that ion-electron coupling becomes especially relevant in high surface area materials, such as the layered electrodes of intercalation batteries, due to the close proximity of itinerant electrons and electrolyte. Here, we report an electrochemical investigation into a family of metal-organic frameworks (MOFs) that serves as a well-defined platform for understanding the effect of ICCT on both elec- tronic and ionic conductivity. Through photochemical doping of e - –H + pairs and introduction of solvent guest molecules, the Ti-containing MOFs convert from electronic-only insulators conductors (σ e ≈ 10 -12 S cm -1 ) to mixed ion-electron semiconduc- tors (σ e ≈ 10 -7 S cm -1 , σ ion ≈ 10 -5 S cm -1 ). Direct current and alternating current techniques support the existence of proton- electron coupling and, critically, that improved ionic conductivity enhances electronic conductivity. Taken together, these results provide direct evidence that PCET enables long-range charge transport and generalized electrochemical tools and synthetic methods for studying ion-electron coupling in materials broadly.

Charge transport↗
DOE OSTI2018

Structural Coupling Throughout the Active Site Hydrogen Bond Networks of Ketosteroid Isomerase and Photoactive Yellow Protein

Hydrogen bonds are fundamental to biological systems and are regularly found in networks implicated in folding, molecular recognition, catalysis, and allostery. Given their ubiquity, we asked the fundamental questions of whether, and to what extent, hydrogen bonds within networks are structurally coupled. To address these questions, we turned to three protein systems, two variants of ketosteroid isomerase and one of photoactive yellow protein. We perturbed their hydrogen bond networks via a combination of site-directed mutagenesis and unnatural amino acid substitution, and we used 1 H NMR and high-resolution X-ray crystallography to determine the effects of these perturbations on the lengths of the two oxyanion hole hydrogen bonds that are donated to negatively charged transition state analogs. Perturbations that lengthened or shortened one of the oxyanion hole hydrogen bonds had the opposite effect on the other. The oxyanion hole hydrogen bonds were also affected by distal hydrogen bonds in the network, with smaller perturbations for more remote hydrogen bonds. Across 19 measurements in three systems, the length change in one oxyanion hole hydrogen bond was propagated to the other, by a factor of –0.30 ± 0.03. This common effect suggests that hydrogen bond coupling is minimally influenced by the remaining protein scaffold. The observed coupling is reproduced by molecular mechanics and quantum mechanics/molecular mechanics (QM/MM) calculations for changes to a proximal oxyanion hole hydrogen bond. However, effects from distal hydrogen bonds are reproduced only by QM/MM, suggesting the importance of polarization in hydrogen bond coupling. These results deepen our understanding of hydrogen bonds and their networks, providing strong evidence for long-range coupling and for the extent of this coupling. In conclusion, we provide a broadly predictive quantitative relationship that can be applied to and can be further tested in new systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Robust lasing modes in coupled colloidal quantum dot microdisk pairs using a non-Hermitian exceptional point

Evanescently coupled pairs of microdisk lasers have emerged as a useful platform for studying the non-Hermitian physics of exceptional points. It remains an open question how scalable and versatile such phenomena can be when carried over to other designs. Here we have studied the effect of gain/loss modulation in an evanescently coupled pair of microdisk optical resonators fabricated from solution-processed colloidal quantum dots. The emission spectra of these structures are sensitive to small imperfections, which cause frequency-splitting of the whispering gallery modes. Despite this inherent disorder, we found that when spatially modulating the optical pump to vary the gain differential between the coupled microdisks, the coupling drives the split parasitic intra-cavity modes into coalescence at an exceptional point of the resulting three-mode system. This unusual behavior is rationalized via a Hamiltonian that incorporates the intra-cavity coupling as well as the anisotropic inter-cavity coupling between modes in the microdisk pair.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Analysis of vibronic coupling in a 4f molecular magnet with FIRMS

Vibronic coupling, the interaction between molecular vibrations and electronic states, is a fundamental effect that profoundly affects chemical processes. In the case of molecular magnetic materials, vibronic, or spin-phonon, coupling leads to magnetic relaxation, which equates to loss of magnetic memory and loss of phase coherence in molecular magnets and qubits, respectively. The study of vibronic coupling is challenging, and most experimental evidence is indirect. Here we employ far-infrared magnetospectroscopy to directly probe vibronic transitions in [Yb(trensal)] (where H 3 trensal = 2,2,2-tris(salicylideneimino)trimethylamine). We find intense signals near electronic states, which we show arise due to an “envelope effect” in the vibronic coupling Hamiltonian, which we calculate fully ab initio to simulate the spectra. We subsequently show that vibronic coupling is strongest for vibrational modes that simultaneously distort the first coordination sphere and break the C 3 symmetry of the molecule. With this knowledge, vibrational modes could be identified and engineered to shift their energy towards or away from particular electronic states to alter their impact. Hence, these findings provide new insights towards developing general guidelines for the control of vibronic coupling in molecules.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Quantum dynamical effects of vibrational strong coupling in chemical reactivity

Abstract Recent experiments suggest that ground state chemical reactivity can be modified when placing molecular systems inside infrared cavities where molecular vibrations are strongly coupled to electromagnetic radiation. This phenomenon lacks a firm theoretical explanation. Here, we employ an exact quantum dynamics approach to investigate a model of cavity-modified chemical reactions in the condensed phase. The model contains the coupling of the reaction coordinate to a generic solvent, cavity coupling to either the reaction coordinate or a non-reactive mode, and the coupling of the cavity to lossy modes. Thus, many of the most important features needed for realistic modeling of the cavity modification of chemical reactions are included. We find that when a molecule is coupled to an optical cavity it is essential to treat the problem quantum mechanically to obtain a quantitative account of alterations to reactivity. We find sizable and sharp changes in the rate constant that are associated with quantum mechanical state splittings and resonances. The features that emerge from our simulations are closer to those observed in experiments than are previous calculations, even for realistically small values of coupling and cavity loss. This work highlights the importance of a fully quantum treatment of vibrational polariton chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Quantum reservoir computing implementation on coherently coupled quantum oscillators

Quantum reservoir computing is a promising approach for quantum neural networks, capable of solving hard learning tasks on both classical and quantum input data. However, current approaches with qubits suffer from limited connectivity. We propose an implementation for quantum reservoir that obtains a large number of densely connected neurons by using parametrically coupled quantum oscillators instead of physically coupled qubits. We analyze a specific hardware implementation based on superconducting circuits: with just two coupled quantum oscillators, we create a quantum reservoir comprising up to 81 neurons. We obtain state-of-the-art accuracy of 99% on benchmark tasks that otherwise require at least 24 classical oscillators to be solved. Our results give the coupling and dissipation requirements in the system and show how they affect the performance of the quantum reservoir. Beyond quantum reservoir computing, the use of parametrically coupled bosonic modes holds promise for realizing large quantum neural network architectures, with billions of neurons implemented with only 10 coupled quantum oscillators.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Super-resolved time–frequency measurements of coupled phonon dynamics in a 2D quantum material

Abstract Methods to probe and understand the dynamic response of materials following impulsive excitation are important for many fields, from materials and energy sciences to chemical and neuroscience. To design more efficient nano, energy, and quantum devices, new methods are needed to uncover the dominant excitations and reaction pathways. In this work, we implement a newly-developed superlet transform—a super-resolution time-frequency analytical method—to analyze and extract phonon dynamics in a laser-excited two-dimensional (2D) quantum material. This quasi-2D system, 1 T -TaSe 2 , supports both equilibrium and metastable light-induced charge density wave (CDW) phases mediated by strongly coupled phonons. We compare the effectiveness of the superlet transform to standard time-frequency techniques. We find that the superlet transform is superior in both time and frequency resolution, and use it to observe and validate novel physics. In particular, we show fluence-dependent changes in the coupled dynamics of three phonon modes that are similar in frequency, including the CDW amplitude mode, that clearly demonstrate a change in the dominant charge-phonon couplings. More interestingly, the frequencies of the three phonon modes, including the strongly-coupled CDW amplitude mode, remain time- and fluence-independent, which is unusual compared to previously investigated materials. Our study opens a new avenue for capturing the coherent evolution and couplings of strongly-coupled materials and quantum systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Exploration of the bio-analogous asymmetric C–C coupling mechanism in tandem CO 2 electroreduction

C–C coupling is a critical step of CO 2 fixation in constructing the carbon skeleton of value-added multicarbon products. The Wood–Ljungdahl pathway is an efficient natural process through which microbes transform CO 2 into methyl and carbonyl groups and subsequently couple them together. This asymmetric coupling mechanism remains largely unexplored in inorganic CO 2 electroreduction. Here we experimentally validate the asymmetric coupling pathway through isotope-labelled co-reduction experiments on a Cu surface where 13 CH 3 I and 12 CO are co-fed externally as the methyl and the carbonyl source, respectively. Isotope-labelled multicarbon oxygenates were detected, which confirms an electrocatalytic asymmetric coupling on the Cu surface. We further employed tandem Cu–Ag nanoparticle systems in which *CH x and *CO intermediates can be generated to achieve asymmetric C–C coupling for a practical CO 2 electroreduction. We found that the production of multicarbon oxygenates is correlated with the generation rate of two intermediate indicators, CH 4 and CO. By aligning their rates, the oxygenates generation rate can be maximized.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Understanding plasmon coupling in nanoparticle dimers using molecular orbitals and configuration interaction

Here, we perform a theoretical investigation of the electronic structure and optical properties of atomic nanowire and nanorod dimers using DFT and TDDFT. In both systems at separation distances larger than 0.75 nm, optical spectra show a single feature that resembles the bonding dipole plasmon (BDP) mode. A configuration interaction (CI) analysis shows that the BDP mode arises from constructive coupling of transitions, whereas the destructive coupling does not produce significant oscillator strength for such separation distances. At shorter separation distances, both constructive and destructive coupling produce oscillator strength due to wave-function overlap, which results in multiple features in the calculated spectra. Our analysis shows that a charge-transfer plasmon (CTP) mode arises from destructive coupling of transitions, whereas the BDP results from constructive coupling of the same transitions at shorter separation distances. Furthermore, the coupling elements between these transitions are shown to depend heavily on the amount of exact Hartree–Fock exchange (HFX) in the functional, which affects the splitting of CTP and BDP modes. With 50% HFX or more, the CTP and BDP modes mainly merge into a single feature in the spectra. These findings suggest that the effects of exact exchange must be assessed during the prediction of CTP modes in plasmonic systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Two-bond 13 C– 13 C spin-coupling constants in saccharides: dependencies on exocyclic hydroxyl group conformation

Seven doubly 13 C-labeled isotopomers of methyl β-D-glucopyranoside, methyl β-D-xylopyranoside, methyl β-D-galactopyranoside, methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside and methyl β-D-galactopyranosyl-(1→4)-β-D-xylopyranoside were prepared, crystallized, and studied by single-crystal X-ray crystallography and solid-state 13 C NMR spectroscopy to determine experimentally the dependence of 2 J C1,C3 values in aldopyranosyl rings on the C1–C2–O2–H torsion angle, θ 2 , involving the C2 carbon of the C1–C2–C3 coupling pathway. Using X-ray crystal structures to determine θ 2 in crystalline samples and by selecting compounds that exhibit a relatively wide range of θ 2 values in the crystalline state, 2 J C1,C3 values measured in crystalline samples were plotted against θ 2 and the resulting plot compared to that obtained from density functional theory (DFT) calculations. For θ 2 values ranging from ~90° to ~240°, very good agreement was observed between the experimental and theoretical plots, providing strong validation of DFT-calculated spin-coupling dependencies on exocyclic C–O bond conformation involving the central carbon of geminal C–C–C coupling pathways. we report that these findings provide new experimental evidence supporting the use of 2 J CCC values as non-conventional spin-coupling constraints in MA'AT conformational modeling of saccharides in solution, and the use of NMR spin-couplings not involving coupled hydroxyl hydrogens as indirect probes of C–O bond conformation. Solvomorphism was observed in crystalline βGal-(1→4)-βGlcOCH 3 wherein the previously-reported methanol solvate form was found to spontaneously convert to a monohydrate upon air-drying, leading to small but discernible conformational changes in, and a new crystalline form of, this disaccharide.

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

Optically induced strong intermodal coupling in mechanical resonators at room temperature

Strong parametric mode coupling in mechanical resonators is demonstrated at room temperature by using the photothermal effect in thin membrane structures. Thanks to the large stress modulation by laser irradiation, the coupling rate of the mechanical modes, defined as half of the mode splitting, reaches 2.94 kHz, which is an order of magnitude larger than electrically induced mode coupling. This large coupling rate exceeds the damping rates of the mechanical resonators and results in the strong coupling regime, which is a signature of coherent mode interaction. Room-temperature coherent mode coupling will enable us to manipulate mechanical motion at practical operation temperatures and provides a wide variety of applications of integrated mechanical systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Recoupling the singlet- and triplet-pairing channels in single-reference coupled cluster theory

It is well known that single-reference coupled cluster theory truncated to low orders of excitations gives the right answer for the right reason when systems are dominated by dynamical or weak correlation. Static or strong correlation is more problematic, causing often catastrophic breakdown of restricted coupled cluster. This failure can be remedied, e.g., by allowing symmetry breaking in the reference or taking a multi-reference approach, but poses an interesting theoretical problem, especially since many groups have found that simplifying the T 2 operator or the doubles amplitude equations gives better results. In singlet-paired coupled cluster, eliminating the triplet-pairing channel recovers reasonable qualitative behavior for strong correlation at the cost of a decreased description of dynamical correlation in weakly correlated situations. This behavior seems to hold for both closed- and open-shell systems. In this work, we explore the coupling of the singlet- and triplet-pairing channels of T 2 and attempt to recouple them in order to recover dynamical correlation without reintroducing catastrophic failure due to strong correlation. In the weakly correlated regime, these pairing channels are only weakly coupled, and a simple recoupling gives good results. Furthermore, as strong correlation dominates, the coupling strength between the singlet- and triplet-pairing channels increases, making it difficult to perturbatively recouple the singlet- and triplet-pairing channels in this regime.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Stability of entrainment of a continuum of coupled oscillators

Complex natural and engineered systems are ubiquitous, and their behavior is challenging to characterize and control. Here, we examine the design of the entrainment process for an uncountably infinite collection of coupled phase oscillators that are all subject to the same periodic driving signal. In the absence of coupling, an appropriately designed input can result in each oscillator attaining the frequency of the driving signal, with a phase offset determined by its natural frequency. We also consider a special case of interacting oscillators in which the coupling tends to destabilize the phase configuration to which the driving signal would send the collection in the absence of coupling. In this setting, we derive stability results that characterize the trade-off between the effects of driving and coupling, and compare these results to the well-known Kuramoto model of a collection of free-running coupled oscillators.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Rank reduced coupled cluster theory. I. Ground state energies and wavefunctions

We propose a compression of the opposite-spin coupled cluster doubles amplitudes of the form τ ab ij ≡ U V ia T VW U W jb , where U V ia are the n V -highest magnitude eigenvectors of the MP2 or MP3 doubles amplitudes. Together with a corresponding parameterization of the opposite-spin coupled cluster Lagrange multipliers of the form λ ij ab ≡ U V ia L VW U W jb , this yields a fully self-consistent parameterization of reduced-rank coupled cluster equations in terms of the Lagrangian L 0 ( T VW ,L VW ). Making this Lagrangian stationary with respect to the L VW parameters yields a perfectly determined set of equations for the T VW equations and coupled cluster energy. These equations can be solved using a Lyapunov equation for the first-order amplitude updates. We test this “rank-reduced coupled cluster” method for coupled cluster singles and doubles in medium sized molecules and find that substantial compression of the $\hat{T} $ 2 amplitudes is possible with acceptable accuracy.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Enabling complete multichannel nonadiabatic dynamics: A global representation of the two-channel coupled, 1,2 1 A and 1 3 A states of NH 3 using neural networks

Global coupled three-state two-channel potential energy and property/interaction (dipole and spin–orbit coupling) surfaces for the dissociation of NH 3 (Ã) into NH + H 2 and NH 2 + H are reported. The permutational invariant polynomial-neural network approach is used to simultaneously fit and diabatize the electronic Hamiltonian by fitting the energies, energy gradients, and derivative couplings of the two coupled lowest-lying singlet states as well as fitting the energy and energy gradients of the lowest-lying triplet state. The key issue in fitting property matrix elements in the diabatic basis is that the diabatic surfaces must be smooth, that is, the diabatization must remove spikes in the original adiabatic property surfaces attributable to the switch of electronic wavefunctions at the conical intersection seam. Here, we employ the fit potential energy matrix to transform properties in the adiabatic representation to a quasi-diabatic representation and remove the discontinuity near the conical intersection seam. The property matrix elements can then be fit with smooth neural network functions. The coupled potential energy surfaces along with the dipole and spin–orbit coupling surfaces will enable more accurate and complete treatment of optical transitions, as well as nonadiabatic internal conversion and intersystem crossing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Multimode two-dimensional vibronic spectroscopy. II. Simulating and extracting vibronic coupling parameters from polarization-selective spectra

We report experimental demonstrations of polarization-selection two-dimensional Vibrational-Electronic (2D VE) and 2D Electronic-Vibrational (2D EV) spectroscopies aim to map the magnitudes and spatial orientations of coupled electronic and vibrational coordinates in complex systems. The realization of that goal depends on our ability to connect spectroscopic observables with molecular structural parameters. In this paper, we use a model Hamiltonian consisting of two anharmonically coupled vibrational modes in electronic ground and excited states with linear and bilinear vibronic coupling terms to simulate polarization-selective 2D EV and 2D VE spectra. We discuss the relationships between the linear vibronic coupling and two-dimensional Huang–Rhys parameters and between the bilinear vibronic coupling term and Duschinsky mixing. We develop a description of the vibronic transition dipoles and explore how the Hamiltonian parameters and non-Condon effects impact their amplitudes and orientations. Using simulated polarization-selective 2D EV and 2D VE spectra, we show how 2D peak positions, amplitudes, and anisotropy can be used to measure parameters of the vibronic Hamiltonian and non-Condon effects. This paper, along with the first in the series, provides the reader with a detailed description of reading, simulating, and analyzing multimode, polarization-selective 2D EV and 2D VE spectra with an emphasis on extracting vibronic coupling parameters from complex spectra.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Activation of atomic transport via vibrational coupling-induced force fluctuations

Vibrational coupling, although well-explored in many fields, has seldom been investigated in the context of mass transport. In this Letter, we examine the impact of vibrational coupling on atomic transport using simple molecular dynamics simulations. Our study shows that the atomic transport can be activated when the natural frequency of the atomic slit is close to the natural frequency of the atom being transported. We uncover the presence of fluctuating forces induced by vibrational coupling with higher amplitudes observed when the coupling is strong. We show that the transport activation mechanism is due to the high force fluctuations that arise during strong vibrational coupling allowing the atom to temporarily surpass the transport barrier of the slit. Our findings will serve as a foundation for the continued examination of vibrational coupling in the realm of mass transport.

Physics↗