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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Quantum Drude oscillators coupled with Coulomb potential as an efficient model for bonded and non-covalent interactions in atomic dimers

The quantum Drude oscillator (QDO) model has been widely used as an efficient surrogate to describe the electric response properties of matter as well as long-range interactions in molecules and materials. Most commonly, QDOs are coupled within the dipole approximation so that the Hamiltonian can be exactly diagonalized, which forms the basis for the many-body dispersion method [Phys. Rev. Lett. 108, 236402 (2012)]. The dipole coupling is efficient and allows us to study non-covalent many-body effects in systems with thousands of atoms. However, there are two limitations: (i) the need to regularize the interaction at short distances with empirical damping functions and (ii) the lack of multipolar effects in the coupling potential. In this work, we convincingly address both limitations of the dipole-coupled QDO model by presenting a numerically exact solution of the Coulomb-coupled QDO model by means of quantum Monte Carlo methods. We calculate the potential-energy surfaces of homogeneous QDO dimers, analyzing their properties as a function of the three tunable parameters: frequency, reduced mass, and charge. We study the coupled-QDO model behavior at short distances and show how to parameterize this model to enable an effective description of chemical bonds, such as the covalent bond in the H2 molecule.

Chemistry↗
DOE OSTI2024

Enhanced spin pumping in heterostructures of coupled ferrimagnetic garnets

Spin pumping has significant implications for spintronics, providing a mechanism to manipulate and transport spins for information processing. Understanding and harnessing spin currents through spin pumping is critical for the development of efficient spintronic devices. The use of a magnetic insulator with low damping enhances the signal-to-noise ratio in crucial experiments such as spin-torque ferromagnetic resonance (FMR) and spin pumping. A magnetic insulator coupled with a heavy metal or quantum material offers a more straightforward model system, especially when investigating spin-charge interconversion processes to greater accuracy. This simplicity arises from the absence of unwanted effects caused by conduction electrons unlike in ferromagnetic metals. Here, we investigate the spin pumping in coupled ferrimagnetic (FiM) Y 3 Fe 5 O 12 (YIG)/Tm 3 Fe 5 O 12 (TmIG) bilayers combined with heavy-metal (Pt) using the inverse spin Hall effect. It is observed that magnon transmission occurs at both of the FiMs FMR positions. The enhancement of spin pumping voltage (V sp ) in the FiM garnet heterostructures is observed. The plausible reason might be the interfacial exchange coupling between FiMs. The modulation of V sp is achieved by tuning the bilayer structure. Further, the spin mixing conductance for these coupled systems is found to be ≈ 10 18 m –2 . Furthermore, our findings describe a coupled FiM system for the investigation of magnon coupling providing opportunities for magnonic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Efficient simulation of open quantum systems coupled to a reservoir through multiple channels

It is challenging to simulate open quantum systems that are connected to a reservoir through multiple channels. For example, vibrations may induce fluctuations in both energy gaps and electronic couplings, which represent two independent channels of system–bath couplings. Systems of this kind are ubiquitous in the processes of excited state radiationless decay. Combined with density matrix renormalization group (DMRG) and matrix product states (MPS) methods, we develop an interaction-picture chain mapping strategy for vibrational reservoirs to simulate the dynamics of these open systems, resulting in time-dependent spatially local system–bath couplings in the chain-mapped Hamiltonian. This transformation causes the entanglement generated by the system–bath interactions to be restricted within a narrow frequency window of vibrational modes, enabling efficient DMRG/MPS dynamical simulations. We demonstrate the utility of this approach by simulating singlet fission dynamics using a generalized spin-boson Hamiltonian with both diagonal and off-diagonal system–bath couplings. This approach generalizes an earlier interaction-picture chain mapping scheme, allowing for efficient and exact simulation of systems with multi-channel system–bath couplings using matrix product states, which may further our understanding of nonlocal exciton–phonon couplings in exciton transport and the non-Condon effect in energy and electron transfer.

Chemistry↗
DOE OSTI2025

Action at a distance: The remarkable coupling of CO 2 uptake to electron transfer in specialized cyanobacterial NDH-1 complexes

Cyanobacteria achieve highly efficient photosynthesis using a CO 2 -concentrating mechanism relying on specialized Type I (NDH-1) complexes. Among these, NDH-1 3 and NDH-1 4 catalyze redox-coupled hydration of CO 2 to bicarbonate, supporting carbon fixation in carboxysomes. The mechanism of coupling electron transfer to CO 2 -hydration by these variant NDH-1 complexes remains unknown. We engineered a Synechococcus PCC7942 strain that expresses exclusively the high flux/low affinity NDH-14 complex, enabling the observation of the coupling of CO 2 hydration to cyclic electron flow in isolation from the other NDH-1 isoforms normally present in cells. We found that inhibition of the CupB protein by the carbonic anhydrase inhibitor ethoxzolamide (EZ) suppressed CO 2 uptake, slowed photosystem I rereduction, and abolished proton pumping as probed by acridine orange fluorescence. These effects were absent in strains lacking Cup proteins, confirming specificity. The results demonstrate that CO 2 hydration and electron transfer through NDH-14 are tightly coupled via proton translocation across the thylakoid membrane. These findings provide direct evidence for the bidirectional interaction in bioenergetic coupling between the plastoquinone reduction and the CO 2 uptake at the distal Zn-site over a span of ~150 Å and support a proton-removal hypothesis involving the proton transfer pathways from the Zn-site of CO2 hydration to an energetically coupled proton loading site evolutionarily repurposed from the ancestral proton pumping mechanism to enable energetic CO 2 uptake.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Multiphysics Coupling Methods for Molten Salt Reactor Modeling and Simulation in VERA

New multiphysics coupling capabilities for molten salt reactor (MSR) analysis have been developed in the Virtual Environment for Reactor Applications (VERA). This development consisted of two main efforts. First, a generic species transport module was added in the CTF code, which is the thermal-hydraulics (TH) code for VERA. This module uses the velocity fields for which CTF solves during the TH calculation to transport species through the core and around the primary loop. Additionally, a gas sparging model has been added to CTF to model the movement of certain species, namely, fission products such as xenon gas, to transport between the molten salt and gas bubbles present in the salt. The second effort in this development was coupling this capability to VERA’s neutron transport code MPACT. This effort focused on coupling the detailed TH transport models in CTF to MPACT to account for feedback effects in the neutron transport calculations. Finally, the thermochemistry code Thermochimica has also been coupled to VERA. Thermochimica performs pointwise calculations for chemical potential and Gibbs free energy and determines what phases are produced by the temperature, pressure, and elemental concentrations at different locations in the primary loop. These capabilities are demonstrated using a model of the Molten Salt Reactor Experiment (MSRE). Furthermore, this reactor operated at Oak Ridge National Laboratory in the 1960s, providing sources of experimental data that were used to develop the model. Various combinations of species were modeled using VERA’s new multiphysics coupling capabilities. Species distributions and reactivity effects behaved as expected for the MSRE model, demonstrating that the coupling is behaving correctly and causing appropriate feedback. The results of these calculations show the potential for VERA to be used for a wide variety of MSR analyses.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2016

Electron phonon coupling in Ni-based binary alloys with application to displacement cascade modeling

Energy transfer between lattice atoms and electrons is an important channel of energy dissipation during displacement cascade evolution in irradiated materials. On the assumption of small atomic displacements, the intensity of this transfer is controlled by the strength of electron–phonon (el–ph) coupling. The el–ph coupling in concentrated Ni-based alloys was calculated using electronic structure results obtained within the coherent potential approximation. It was found that Ni 0.5 Fe 0.5 , Ni 0.5 Co 0.5 and Ni 0.5 Pd 0.5 are ordered ferromagnetically, whereas Ni 0.5 Cr 0.5 is nonmagnetic. Since the magnetism in these alloys has a Stoner-type origin, the magnetic ordering is accompanied by a decrease of electronic density of states at the Fermi level, which in turn reduces the el–ph coupling. Thus, the el–ph coupling values for all alloys are approximately 50% smaller in the magnetic state than for the same alloy in a nonmagnetic state. As the temperature increases, the calculated coupling initially increases. After passing the Curie temperature, the coupling decreases. The rate of decrease is controlled by the shape of the density of states above the Fermi level. Introducing a two-temperature model based on these parameters in 10 keV molecular dynamics cascade simulation increases defect production by 10–20% in the alloys under consideration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Coupled scalar fields in the late Universe: the mechanical approach and the late cosmic acceleration

In this paper, we consider the Universe at the late stage of its evolution and deep inside the cell of uniformity. At these scales, we consider the Universe to be filled with dust-like matter in the form of discretely distributed galaxies, a minimally coupled scalar field and radiation as matter sources. We investigate such a Universe in the mechanical approach. This means that the peculiar velocities of the inhomogeneities (in the form of galaxies) as well as fluctuations of other perfect fluids are non-relativistic. Such fluids are designated as coupled because they are concentrated around inhomogeneities. In the present paper we investigate the conditions under which a scalar field can become coupled, and show that, at the background level, such coupled scalar field behaves as a two component perfect fluid: a network of frustrated cosmic strings with EoS parameter w =-1/3 and a cosmological constant. The potential of this scalar field is very flat at the present time. Hence, the coupled scalar field can provide the late cosmic acceleration. The fluctuations of the energy density and pressure of this field are concentrated around the galaxies screening their gravitational potentials. Therefore, such scalar fields can be regarded as coupled to the inhomogeneities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Nuclear magnetic resonance dark-matter searches are sensitive to dark photons and the axion-photon coupling

We demonstrate that nuclear magnetic resonance based searches for dark matter (DM) have intrinsic and powerful sensitivity to dark photons and the axion-photon coupling. The reason is conceptually straightforward. An instrument such as CASPEr-Gradient begins with a large sample of nuclear spins polarized in a background magnetic field. In the presence of axion DM coupled to nucleons, the spin ensemble feels an effective magnetic field 𝐁 ∝ ∇𝑎 that tilts the spins, generating a potentially observable precession. If the magnetic field is real rather than effective, the system responds identically. A real field can be generated by a kinetically mixed dark photon within the shielded region the sample is placed or an axion coupled to photons through its interaction with the background magnetic field. We show that all three signals are detectable and distinguishable. If CASPEr-Gradient were to reach the QCD axion prediction of the axion-nucleon coupling, it would simultaneously be sensitive to kinetic mixings of 𝜀 ≃ 3 × 10 −16 and axion-photon couplings of 𝑔 𝑎⁢𝛾⁢𝛾 ≃ 2 × 10 −16 GeV −1 for 𝑚 ≃ 1 𝜇⁢eV. Our analysis demonstrates that CASPEr-Gradient has the tantalizing possibility of simultaneously measuring two separate couplings of axion dark matter, thereby providing a unique window into the axion’s UV completion.

Beadle, Carl [University of Geneva (Switzerland)] ↗
DOE OSTI2023

Strongly Coupled Plasmon and Phonon Polaritons as Seen by Photon and Electron Probes

The ability to control and modify infrared excitations in condensed matter is of both fundamental and applied interest. Here we explore a system supporting low-energy excitations, in particular, mid-infrared localized plasmon modes and phonon polaritons that are tuned to be strongly coupled. We study the coupled modes by using far-field infrared spectroscopy, state-of-the-art monochromated electron energy-loss spectroscopy, numerical simulations, and analytical modeling. Here we demonstrate that the electron probe facilitates a precise characterization of polaritons constituting the coupled system, and enables an active control over the coupling and the resulting sample response both in frequency and space. Although far-field optical spectra can be substantially different from near-field electron energy-loss spectra, we show that a direct comparison is possible via postprocessing and right positioning of the electron beam. The resulting spectra allow us to evaluate the key parameters of the coupled system, such as the coupling strength, which we demonstrate to be probe independent. Our work establishes a rigorous description of the spectral features observed in light- and localized electron-based spectroscopies, which can be extended to the analysis of analogous optical systems with applications in heat management and electromagnetic field concentration or nanofocusing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Ultrastrong coupling of vibrationally dressed organic Frenkel excitons with Bloch surface waves in a one-sided all-dielectric structure

We demonstrate a transition from weak, to strong, to ultrastrong coupling of Frenkel molecular excitons and Bloch surface wave photons at room temperature using a one-sided, all-dielectric optical structure. The all-dielectric structure comprises an organic semiconductor thin film on the surface of a distributed Bragg reflector. We investigate the evolution of multiple vibronic polariton branches and their dominant absorption peaks as a function of coupling and in-plane momentum, which is absent in previous ultrastrong coupling systems. Measurements are interpreted using both the transfer matrix method and a coupled-oscillator model without the rotating wave approximation. The dependence of Rabi splitting on the number of excitons and electrical field amplitude is also modeled showing a transition to ultrastrong coupling at film thicknesses ≥ 50 nm. Furthermore, this low-loss polaritonic structure enables us to study phenomena such as organic exciton-polariton dynamics, ultralong range polariton propagation, and high efficiency energy transport in the ultrastrong coupling regime at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Strain dependence of antiferromagnetic interface coupling in La 0.7 Sr 0.3 MnO 3 /SrRuO 3 superlattices

We have investigated the magnetic response of La 0.7 Sr 0.3 MnO 3 /SrRuO 3 superlattices to biaxial in-plane strain applied in situ. Superlattices grown on piezoelectric substrates of 0.72PbMg 1/3 Nb 2/3 O 3 -0.28PbTiO 3 (001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu H-0(AF) = 1.8 T is found to change by mu(0)Delta H-AF/Delta epsilon similar to -520 mT %(-1) under reversible biaxial strain Delta epsilon at 80 K in a [La 0.7 Sr 0.3 MnO 3 (22 angstrom)/SrRuO 3 (55 angstrom)] 15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers, which are under as-grown tensile strain, leading to a larger net coupling of SrRuO 3 layers at the interface. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface for the strength of the apparent antiferromagnetic coupling. We discuss our results in the framework of available models.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Ab initio study of cross-interface electron-phonon couplings in FeSe thin films on SrTiO 3 and BaTiO 3

We study the electron-phonon coupling strength near the interface of monolayer and bilayer FeSe thin films on SrTiO 3 , BaTiO 3 , and oxygen-vacant SrTiO 3 substrates, using ab initio methods. The calculated total electron-phonon coupling strength λ = 0.2 – 0.3 cannot account for the high T c ~ 70 K observed in these systems through the conventional phonon-mediated pairing mechanism. In all of these systems, however, we find that the coupling constant of a polar oxygen branch peaks at q = 0 with negligible coupling elsewhere, while the energy of this mode coincides with the offset energy of the replica bands measured recently by angle-resolved photoemission spectroscopy experiments. However, the integrated coupling strength for this mode from our current calculations is still too small to produce the observed high T c , even through the more efficient pairing mechanism provided by the forward scattering. Also, we arrive at the same qualitative conclusion when considering a checkerboard antiferromagnetic configuration in the Fe layer. In light of the experimental observations of the replica band feature and the relatively high T c of FeSe monolayers on polar substrates, our results point towards a cooperative role for the electron-phonon interaction, where the cross-interface interaction acts in conjunction with a purely electronic interaction. Finally, we discuss a few scenarios where the coupling strength obtained here may be enhanced.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Detecting Weak Coupling of Quantum Systems with a Nonequilibrium Fano Resonance

A critical aspect of quantum mechanics is the nonlocal nature of the wavefunction, a characteristic that may yield unexpected coupling of nominally-isolated systems. The capacity to detect this coupling can be vital in many situations, especially those in which its strength is weak. In this work we address this problem in the context of mesoscopic physics, by implementing an electron-wave realization of a Fano interferometer using pairs of coupled quantum point contacts (QPCs). Within this scheme, the discrete level required for a Fano resonance is provided by pinching off one of the QPCs, thereby inducing the formation of a quasi-bound state at the center of its self-consistent potential barrier. Using this system, we demonstrate a form of $\textit{nonequilibrium}$ Fano resonance (NEFR), in which nonlinear electrical biasing of the interferometer gives rise to pronounced distortions of its Fano resonance. Our experimental results are captured well by a quantitative theoretical model, which considers a system in which a standard two-path Fano interferometer is coupled to an additional, $\textit{intruder}$, continuum. According to this theory, the observed distortions in the Fano resonance arise $\textit{only}$ in the presence of coupling to the intruder, indicating that the NEFR provides a sensitive means to infer the presence of weak coupling between mesoscopic systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Piezoelectric polar nanoregions and relaxation-coupled resonances in relaxor ferroelectrics

It is a generally accepted fact that the unique dielectric properties of relaxor ferroelectrics are related to the formation of polar nanoregion (PNRs). Less well recognized is the corollary that, because they are polar and therefore lack inversion symmetry, PNRs are also piezoelectric at the nanoscale and can therefore behave as nanoresonators. Using the particular relaxor ferroelectric K 1–x Li x TaO 3 (KLT), we show that, when electrically excited into oscillation, these piezoelectric nanoresonators can drive macroscopic electromechanical resonances. Unexpectedly, however, pairs of coupled resonances corresponding to a particular type of oscillation are observed, with one of the resonance exhibiting a characteristic Fano-like line shape. The complex resonance spectra can be described equally well by two alternative but complementary models both involving two resonances coupled through a relaxation: a purely classical one based on two coupled damped harmonic oscillators and a semiclassical based on two discrete excitations coupled to each other through a continuum. Together, they provide complementary perspectives on the underlying physics of the system. Both reproduce the rapid evolution of the resonance spectrum across three wide temperature ranges, including a phase transition range. In the high-temperature range, the coupling between modes is due to the collective π relaxation of the lithium ions within PNRs and, in the phase transition range, to “heterophase relaxation” of the surrounding lattice between its high-temperature cubic and low-temperature tetragonal phases, both coherent effects. Here, the coupling is suppressed in the intermediate range of the collective π/2 relaxation of the lithium ions. Incidentally, the measured dielectric spectra are shown to bear a surprising but justifiable resemblance to the optical spectra of certain atomic vapors exhibiting electromagnetically induced transparency.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Demonstration of resonant inelastic x-ray scattering as a probe of exciton-phonon coupling

Resonant inelastic x-ray scattering (RIXS) is a promising technique for obtaining electron-phonon coupling constants. However, the ability to extract these coupling constants throughout the Brillouin zone for crystalline materials remains limited. To address this need, we developed a Green's function formalism to capture electron-phonon contributions to core-level spectroscopies without explicitly solving the full vibronic problem. Our approach is based on the cumulant expansion of the Green's function combined with many-body theory calculated vibrational coupling constants. The methodology is applied to x-ray photoemission spectroscopy, x-ray absorption spectroscopy (XAS), and RIXS. In the case of the XAS and RIXS, we use a two-particle exciton Green's function, which accounts implicitly for particle-hole interference effects that have previously proved difficult. To demonstrate the methodology and gain a deeper understanding of the experimental technique, we apply our formalism to small molecules, for which unambiguous experimental data exist. This comparison reveals that the vibronic coupling constant probed by RIXS is in fact related to exciton-phonon coupling rather than electron-phonon coupling, challenging the conventional interpretation of the experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Coupling between collective modes in the deformed 98 Zr nucleus: Insights from consistent HFB + QRPA calculations with the Gogny interaction

The zirconium isotopes exhibit structural properties that present multiple challenges to nuclear theory. Investigations of the coupling present within isoscalar modes and within isovector modes are scarce but important for advancing our understanding of the microscopic picture of nuclei. To explore some of these underlying coupling features, and to test the predictive power of a state-of-the-art nuclear structure approach, we provide a detailed analysis of the properties of 90,96,98 Zr . This region includes a benchmarking case and offers insights into nuclear deformation phenomena. Here, to investigate the coupling between collective modes in deformed nuclei, we focused our analysis on the ground and excited-state properties of these isotopes, employing a consistent approach with the axially symmetric deformed Hartree-Fock-Bogoliubov (HFB) and the quasiparticle random phase approximation (QRPA) framework, both using the Gogny D1M force. This approach effectively describes both low-lying and giant-resonance states. We devoted special attention to the deformed 98 Zr nucleus, where we confirm the existence of coupling between monopole and quadrupole excitations through the 𝐾 𝜋 = 0 + QRPA components and demonstrate an analogous dipole-octupole coupling through the 𝐾 𝜋 = 0 − and 𝐾 𝜋 = 1 − components. Intrinsic transition densities and associated radial projections illustrate the coupling. Our work complements and extends earlier studies carried out using density-functional-based methods and notably, we included the complete Coulomb interaction also in the pairing fields, i.e., we treat terms exactly that are approximated in typical calculations that use the Gogny D1 and D2 interaction families.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Spectroscopy of particle couplings with gravitational waves

We discuss the possibility to measure particle couplings with stochastic gravitational wave backgrounds (SGWBs). Under certain circumstances a sequence of peaks of different amplitude and frequency — a stairway — emerges in a SGWB spectrum, with each peak probing a different coupling. The detection of such signature opens the possibility to reconstruct couplings (spectroscopy) of particle species involved in high energy phenomena generating SGWBs. Stairwaylike signatures may arise in causally produced backgrounds in the early Universe, e.g., from preheating or first order phase transitions. As a proof of principle we study a preheating scenario with an inflaton $\phi$ coupled to multiple daughter fields {$χ_j$} with different coupling strengths. As a clear stairway signature is imprinted in the SGWB spectrum, we reconstruct the relevant couplings with various detectors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Top Yukawa coupling measurement at the muon collider

We have presented a detailed study about the prospects for the measurement of the top Yukawa coupling in the vector boson fusion production of a top quark pair at high energy muon colliders. By employing the effective W approximation and the high energy limit for the helicity amplitudes of the subprocess W + W − → t t ¯ , we have derived the energy scaling of the statistical signal significance in the presence of the anomalous couplings by focusing on the interference term only. The sensitivity on the top Yukawa coupling decreases as the bin energy increases. For the anomalous triple gauge boson couplings and the gauge-boson-fermion couplings with E 2 energy growing behavior, the signal significance has mild increase at the beginning and starts to decrease for s ^ t t ¯ ∼ 0.2 s μ + μ − . The 95% CL on the anomalous top Yukawa coupling is projected to be 5.6% (1.7%) at a 10 (30) TeV muon collider, which is comparable to the sensitivity of 2% at the 100 TeV collider. Published by the American Physical Society 2024

Astronomy & Astrophysics↗