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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Electron-ion recombination in composite interactions in liquid xenon

The response of liquid xenon to various types of ionizing radiation has been extensively studied theoretically and experimentally. Recent progress in direct detection dark matter experiments highlights the significance of composite events, where multiple particles interact with xenon simultaneously and generate overlapping ionization signatures. In these events, recombination of electrons and ions associated with different primary particles leads to additional suppression of the ionization signal, introducing a new source of uncertainty in dark matter searches and Migdal effect studies. We developed a model to estimate the recombination enhancement for overlapping low-energy particle interactions. This method, which has minimal dependence on xenon microphysics and is primarily driven by existing experimental data, yields predictions that are consistent with available measurements of composite interactions. Furthermore, we demonstrate that the model predictions are robust against xenon microphysics assumptions.

charge↗
DOE OSTI2018

Universal temperature dependence of the ion-neutral-neutral three-body recombination rate

A classical approach based on hyperspherical coordinates is used to derive a first-principles formulation of the the ion-neutral-neutral three-body recombination rate, A + + A + A → A 2 + + A, in terms of the mass of the atom and its polarizability. The robustness and predictive power of our approach have been checked in comparison with experimental data of rare gas three-body recombination as well as previous theoretical frameworks, which need one or two atom-dependent fitting parameters. Furthermore, our approach is general and applicable to any ion-atom-atom system.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Morphology effects on spin-dependent transport and recombination in polyfluorene thin films

Here, we have studied the role of spin-dependent processes on conductivity in polyfluorene (PFO) thin films by preforming continuous wave (cw) electrically detected magnetic resonance (EDMR) spectroscopy at temperatures between 10 K and room temperature using microwave frequencies between about 1 GHz and 20 GHz, as well as pulsed EDMR at the X band (10 GHz). Variable frequency EDMR allows us to establish the role of spin-orbit coupling in spin-dependent processes whereas pulsed EDMR allows for the observation of coherent spin motion effects. We used PFO for this study in order to allow for the investigation of the effects of microscopic morphological ordering since this material can adopt two distinct intrachain morphologies: an amorphous (glassy) phase, in which monomer units are twisted with respect to each other, and an ordered (β) phase, where all monomers lie within one plane. In thin films of organic light-emitting diodes, the appearance of a particular phase can be controlled by deposition parameters and solvent vapor annealing, and is verified by electroluminescence spectroscopy. Under bipolar charge-carrier injection conditions, we conducted multifrequency cw EDMR, electrically detected Rabi spin-beat experiments, and Hahn echo and inversion-recovery measurements. Coherent echo spectroscopy reveals electrically detected electron-spin-echo envelope modulation due to the coupling of the carrier spins to nearby nuclear spins. Our results demonstrate that, while conformational disorder can influence the observed EDMR signals, including the sign of the current changes on resonance as well as the magnitudes of local hyperfine fields and charge-carrier spin-orbit interactions, it does not qualitatively affect the nature of spin-dependent transitions in this material. In both morphologies, we observe the presence of at least two different spin-dependent recombination processes. At room temperature and 10 K, polaron-pair recombination through weakly spin-spin coupled intermediate charge-carrier pair states is dominant, while at low temperatures, additional signatures of spin-dependent charge transport through the interaction of polarons with triplet excitons are seen in the half-field resonance of a triplet spin-1 species. This additional contribution arises since triplet lifetimes are increased at lower temperatures. We tentatively conclude that spectral broadening induced by hyperfine coupling is slightly weaker in the more ordered β-phase than in the glassy phase since protons are more evenly spaced, whereas broadening effects due to spin-orbit coupling, which impacts the distribution of g-factors, appear to be somewhat more significant in the β-phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Comparative analysis of magnetic resonance in the polaron pair recombination and the triplet exciton-polaron quenching models

We present a comparative theoretical study of magnetic resonance within the polaron pair recombination (PPR) and the triplet exciton-polaron quenching (TPQ) models. Both models have been invoked to interpret the photoluminescence detected magnetic resonance (PLDMR) results in π-conjugated materials and devices. We show that resonance line shapes calculated within the two models differ dramatically in several regards. First, in the PPR model, the line shape exhibits unusual behavior upon increasing the microwave power: it evolves from fully positive at weak power to fully negative at strong power. In contrast, in the TPQ model, the PLDMR is completely positive, showing a monotonic saturation. Second, the two models predict different dependencies of the resonance signal on the photoexcitation power, P L . At low P L , the resonance amplitude Δ I/I is ∝ P L within the PPR model, while it is ∝ P 2 L crossing over to P 3 L within the TPQ model. On the physical level, the differences stem from different underlying spin dynamics. Most prominently, a negative resonance within the PPR model has its origin in the microwave-induced spin-Dicke effect, leading to the resonant quenching of photoluminescence. The spin-Dicke effect results from the spin-selective recombination, leading to a highly correlated precession of the on-resonance pair partners under the strong microwave power. This effect is not relevant for TPQ mechanism, where the strong zero-field splitting renders the majority of triplets off resonance. On the technical level, the analytical evaluation of the line shapes for the two models is enabled by the fact that these shapes can be expressed via the eigenvalues of a complex Hamiltonian. This bypasses the necessity of solving the much larger complex linear system of the stochastic Liouville equations. Lastly, our findings pave the way towards a reliable discrimination between the two mechanisms via cw PLDMR.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comment on “Comparative study of ab initio nonradiative recombination rate calculations under different formalisms”

In a recent article [Phys. Rev. B 91, 205315 (2015)] Shi, Xu, and Wang presented a comparison of several formalisms to calculate nonradiative recombination rates and concluded the “one-dimensional (1D) quantum formula” that was used by Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] is insufficient to accurately describe nonradiative capture rates. Our analysis of the results of Shi, Xu, and Wang indicates that their conclusions about the 1D quantum formula are unfounded and stem from an error in their calculations. Further, our own calculations demonstrate that the 1D quantum formula approach yields reliable and accurate results for nonradiative recombination rates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Copper interstitial recombination centers in Cu 3 N

We present a comprehensive study of the earth-abundant semiconductor Cu 3 N as a potential solar energy conversion material, using density functional theory and experimental methods. Density functional theory indicates that among the dominant intrinsic point defects, copper vacancies V Cu have shallow defect levels while copper interstitials Cu i behave as deep potential wells in the conduction band which mediate Shockley-Read-Hall recombination. The existence of Cu i defects has been experimentally verified using photothermal deflection spectroscopy. A Cu 3 N/ZnS heterojunction diode with good current-voltage rectification behavior has been demonstrated experimentally, but no photocurrent is generated under illumination. Finally, the absence of photocurrent can be explained by a large concentration of Cu i recombination centers capturing electrons in p-type Cu 3 N.

14 SOLAR ENERGY↗
DOE OSTI2024

Recombination of 𝐵 𝑐 mesons in ultrarelativistic heavy-ion collisions

High-energy heavy-ion collisions have been suggested as a favorable environment for the production of 𝐵 𝑐 mesons, due to a much larger abundance of charm and bottom quarks compared to elementary reactions. Motivated by recent CMS data for 𝐵$^{+}_{𝑐}$ production in Pb-Pb(5.02 TeV) collisions at the Large Hadron Collider (LHC), we deploy a previously developed transport approach for charmonia and bottomonia to evaluate the kinetics of 𝐵 𝑐 mesons throughout the fireball formed in these reactions. The main inputs to our approach are two transport parameters: the 𝐵 𝑐 's reaction rate and equilibrium limit. Both quantities are determined by previous calculations via a combination of charm and bottom sectors. In-medium binding energies of 𝐵 𝑐 mesons are calculated from a thermodynamic 𝑇 matrix with a lattice-QCD constrained potential, and figure in their inelastic reaction rates. Temperature-dependent equilibrium limits include charm- and bottom-quark fugacities based on their initial production. We compute the centrality dependence of inclusive 𝐵 𝑐 production and transverse-momentum (𝑝 𝑇 ) spectra using two different recombination models: instantaneous coalescence and resonance recombination. The main uncertainty in the resulting nuclear modification factors, 𝑅 AA , is currently associated with the 𝐵 𝑐 cross section in elementary 𝑝⁢𝑝 collisions, caused by the uncertainty in the branching ratio for the 𝐵$^{−}_{𝑐}$ → 𝐽/𝜓⁢𝜇 − ⁢$\overline{𝜈}$ decay. Our results indicate a large enhancement of the 𝑅 AA at low 𝑝 𝑇 , with significant regeneration contributions up to 𝑝 𝑇 ≃ 20GeV. Comparisons to CMS data are carried out but firm conclusions will require a more accurate value of the branching ratio, or alternative channels to measure the 𝐵 𝑐 production in 𝑝⁢ collisions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Phenomenology of modified gravity at recombination

Here, we discuss the phenomenological imprints of modifications to gravity in the early Universe with a specific focus on the time of recombination. We derive several interesting results regarding the effect that such modifications have on cosmological observables, especially on the driving and phasing of acoustic oscillations, observed in the cosmic microwave background (CMB) and baryon acoustic oscillations, as well as the weak gravitational lensing of the CMB and of galaxy shapes. This widens the pool of measurements that can be used to test gravity with present and future surveys, in particular realizing the full constraining power of the structure of the primary peaks of the CMB spectrum. We investigate whether such a phenomenology can relax tensions between cosmological measurements and find that a modification of the gravitational constant at recombination would help in reconciling measurements of the CMB with local measurements of the Hubble constant.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Direct Cation Exchange in Monolayer MoS 2 via Recombination-Enhanced Migration

In addition to their unique optical and electronic properties, two-dimensional materials provide opportunities to directly observe atomic-scale defect dynamics. Here we use scanning transmission electron microscopy to observe substitutional Re impurities in monolayer MoS 2 undergo direct exchanges with neighboring Mo atoms in the lattice. Density-functional-theory calculations find that the energy barrier for direct exchange, a process that has only been studied as a diffusion mechanism in bulk materials, is too large for either thermal activation or energy directly transferred from the electron beam. The presence of multiple sulfur vacancies next to the exchanged Re-Mo pair, as observed by electron microscopy, does not lower the energy barrier sufficiently to account for the observed atomic exchange. Instead, the calculations find that a Re dopant and surrounding sulfur vacancies introduce an ever-changing set of deep levels in the energy gap. We propose that these levels mediate an “explosive” recombination-enhanced migration via multiple electron-hole recombination events. As a proof of concept, we also show that Re-Mo direct exchange can be triggered via controlled creation of sulfur vacancies. The present experimental and theoretical findings lay a fundamental framework towards manipulating single substitutional dopants in two-dimensional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Phonon-Assisted Auger-Meitner Recombination in Silicon from First Principles

Here, we present a consistent first-principles methodology to study both direct and phonon-assisted Auger-Meitner recombination (AMR) in indirect-gap semiconductors that we apply to investigate the microscopic origin of AMR processes in silicon. Our results are in excellent agreement with experimental measurements and show that phonon-assisted contributions dominate the recombination rate in both n-type and p-type silicon, demonstrating the critical role of phonons in enabling AMR. We also decompose the overall rates into contributions from specific phonons and electronic valleys to further elucidate the microscopic origins of AMR. Our results highlight potential pathways to modify the AMR rate in silicon via strain engineering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Dissociative electron recombination and rotational cooling of the deuterated triatomic hydrogen ions H 2 ⁡D + and D 2 ⁢H +

We have measured the dissociative recombination (DR) of the deuterated triatomic hydrogen ions H 2 ⁡D + and D 2 ⁢H + as a function of storage time at the Cryogenic Storage Ring (CSR). Both molecular ions were stored for up to 1000 s inside the cryogenic vacuum of the CSR prior to the electron recombination measurements, allowing them to cool to their lowest rotational states. We implement a comprehensive model for all relevant processes to predict the internal state evolution of the ions during storage inside the CSR, employing calculated radiative transition strengths and state-selective rate coefficients for electron collisions. Our DR rate coefficient measurements with deuterated triatomic hydrogen ions in defined quantum states allow for meaningful comparisons with state-of-the-art theoretical calculations, paving the way for a better understanding of the complex DR process for polyatomic molecular ions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Simultaneous Examination of Grain-Boundary Potential, Recombination, and Photocurrent in CdTe Solar Cells Using Diverse Nanometer-Scale Imaging

We report on the simultaneous measurements of Kelvin probe force microscopy (KPFM), cathodoluminescence (CL), and electron-beam induced current (EBIC) for hundreds of grain boundaries (GBs) after different processing steps. The GB potential steadily increases from tens of mV for as-deposited samples, 80 mV after the Cl treatment, to about 120 mV after Cu diffusion (Cl+Cu). The latter is consistent with the emergence of deep Cu defects observed in CL GB spectra. At the same time, EBIC indicates that GB photocurrent is enhanced by 0% to 50% after Cu is introduced. However, this increase is very modest relative to expectations based on computational modeling and the measured GB potential. The EBIC enhancement coincides with increased GB recombination observed by CL. A plausible explanation is that GB defects introduce GB potentials and increase GB recombination; this can reconcile the CL, EBIC, and KPFM data.

14 SOLAR ENERGY↗
DOE OSTI2018

Characterizing Recombination in CdTe-Based Solar Cells by the Temperature and Excitation Dependence of Open-Circuit Voltage and Photoluminescence

Further improvement of the open-circuit voltage (V oc ) of thin-film photovoltaic devices requires that the rate of non-radiative recombination be monitored and reduced. In this study, we investigate a method that attempts to account for the excitation and temperature dependence of V oc and photoluminescence (PL), to extract detailed information about the spatial distribution of recombination and the effects of back contact offset.

14 SOLAR ENERGY↗
DOE OSTI2014

SU-E-T-625: Use and Choice of Ionization Chambers for the Commissioning of Flattened and Flattening-Filter-Free Photon Beams: Determination of Recombination Correction Factor (ks)

Purpose: To evaluate the recombination effect for some ionization chambers to be used for linacs commissioning for Flattened Filter (FF) and Flattening Filter Free (FFF) photon beams. Methods: A Varian TrueBeam linac with five photon beams was used: 6, 10 and 15 MV FF and 6 and 10 MV FFF. Measurements were performed in a water tank and in a plastic water phantom with different chambers: a mini-ion chamber (IC CC01, IBA), a plane-parallel ion chamber (IC PPC05, IBA) and two Farmer chambers (NE2581 and FPC05-IBA). Measurement conditions were Source- Surface Distance of 100 cm, two field sizes (10x10 and 40x40 cm2) and five depths (1cm, maximum buildup, 5cm, 10cm and 20cm). The ion recombination factors (kS), obtained from the Jaffe's plots (voltage interval 50-400 V), were evaluated at the recommended operating voltage of +300V. Results: Dose Per Pulse (DPP) at dmax was 0.4 mGy/pulse for FF beams, 1.0 mGy/pulse and 1.9 mGy/pulse for 6MV and 10 MV FFF beams respectively. For all measurement conditions, kS ranged between 0.996 and 0.999 for IC PPC05, 0.997 and 1.008 for IC CC01. For the FPC05 IBA Farmer IC, kS varied from 1.001 to 1.011 for FF beams, from 1.004 to 1.015 for 6 MV FFF and from 1.009 to 1.025 for 10 MV FFF. Whereas, for NE2581 IC the values ranged from 1.002 to 1.009 for all energy beams and measurement conditions. Conclusion: kS depends on the chamber volume and the DPP, which in turn depends on energy beam but is independent of dose rate. Ion chambers with small active volume can be reliably used for dosimetry of FF and FFF beams even without kS correction. On the contrary, for absolute dosimetry of FFF beams by Farmer ICs it is necessary to evaluate and apply the kS correction. Partially supported by Lega Italiana Lotta contro i Tumori (LILT)

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2020

Emergence of SARS-CoV-2 through recombination and strong purifying selection

COVID-19 has become a global pandemic caused by the novel coronavirus SARS-CoV-2. Understanding the origins of SARS-CoV-2 is critical for deterring future zoonosis, discovering new drugs, and developing a vaccine. We show evidence of strong purifying selection around the receptor binding motif (RBM) in the spike and other genes among bat, pangolin, and human coronaviruses, suggesting similar evolutionary constraints in different host species. We also demonstrate that SARS-CoV-2’s entire RBM was introduced through recombination with coronaviruses from pangolins, possibly a critical step in the evolution of SARS-CoV-2’s ability to infect humans. Similar purifying selection in different host species, together with frequent recombination among coronaviruses, suggests a common evolutionary mechanism that could lead to new emerging human coronaviruses.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Simulation of ultraviolet- and soft X-ray-pulse generation as a result of cooperative recombination of excitons in diamond nanocrystals embedded in a polymer film

Using numerical simulation, it is shown that the recombination of free excitons photoexcited in diamond nanocrystals embedded in a polymer film can occur in the cooperative mode. It is found that this mode can be implemented despite the fact that diamond is an 'indirect' semiconductor. It is shown that the power of the generated radiation at the pulse peak during the cooperative recombination of free excitons can exceed that of the incoherent spontaneous emission of the same initial number of free excitons by more than an order of magnitude. Finally, it is shown that the process under consideration can be used to generate picosecond pulses of ultraviolet and soft X-ray electromagnetic field at a wavelength of 235 nm.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Evaluation of the recombinant proteins RlpB and VacJ as a vaccine for protection against Glaesserella parasuis in pigs

Abstract Background Glaesserella parasuis , the causative agent of Glӓsser’s disease, is widespread in swine globally resulting in significant economic losses to the swine industry. Prevention of Glӓsser’s disease in pigs has been plagued with an inability to design broadly protective vaccines, as many bacterin based platforms generate serovar or strain specific immunity. Subunit vaccines are of interest to provide protective immunity to multiple strains of G. parasuis . Selected proteins for subunit vaccination should be widespread, highly conserved, and surface exposed. Results Two candidate proteins for subunit vaccination (RlpB and VacJ) against G. parasuis were identified using random mutagenesis and an in vitro organ culture system. Pigs were vaccinated with recombinant RlpB and VacJ, outer membrane proteins with important contributions to cellular function and viability. Though high antibody titers to the recombinant proteins and increased interferon-γ producing cells were found in subunit vaccinated animals, the pigs were not protected from developing systemic disease. Conclusions It appears there may be insufficient RlpB and VacJ exposed on the bacterial surface for antibody to bind, preventing high RlpB and VacJ specific antibody titers from protecting animals from G. parasuis . Additionally, this work confirms the importance of utilizing the natural host species when assessing the efficacy of vaccine candidates.

Hau, Samantha J.↗
DOE OSTI2025

Overcoming energy limitation in post-compression of picosecond pulses by free beam propagation in air and temporal pulse splitting and recombining

Cryogenically cooled diode-pumped Yb:YAG has been demonstrated to efficiently generate pulses of >1 J energy and several picoseconds duration at a kilohertz repetition rate. Pulse broadening and post-compression techniques capable of operating at these high pulse energies will expand the use of these lasers in applications requiring shorter pulses. Towards this goal, we demonstrate the post-compression of 5.5 ps pulses with energy in the 0.1 J range by spectral broadening of self-collimated large area beams in air. In proof-of-principle experiments, pulses of 65 mJ energy from a cryogenically cooled Yb:YAG laser were compressed to 1.1 ps after 65 m propagation in a compact setup in which the beam was folded using flat mirrors. Further energy scaling was demonstrated by combining this spectral broadening technique with temporal pulse splitting and recombining. 120 mJ pulses were split into two pulses and coherently recombined with 85% efficiency after 50 m of propagation in air to yield 1.4 ps pulses.

High Energy Laser↗