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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Liquid-state polaron theory of the hydrated electron revisited

The quantum path integral/classical liquid-state theory of Chandler and co-workers, created to describe an excess electron in solvent, is re-examined for the hydrated electron. The portion that models electron-water density correlations is replaced by two equations: the range optimized random phase approximation (RO-RPA), and the Donley, Rajasekaran, and Liu (DRL) approximation to the “two-chain” equation, both shown previously to describe accurately the static structure and thermodynamics of strongly charged polyelectrolyte solutions. The static equilibrium properties of the hydrated electron are analyzed using five different electron-water pseudopotentials. The theory is then compared with data from mixed quantum/classical Monte Carlo and molecular dynamics simulations using these same pseudopotentials. It is found that the predictions of the RO-RPA and DRL-based polaron theories are similar and improve upon previous theory, with values for almost all properties analyzed in reasonable quantitative agreement with the available simulation data. Also, it is found using the Larsen, Glover, and Schwartz pseudopotential that the theories give values for the solvation free energy that are at least three times larger than that from experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Monte Carlo study of efficiency roll-off of phosphorescent organic light-emitting diodes: Evidence for dominant role of triplet-polaron quenching

We present an advanced molecular-scale organic light-emitting diode (OLED) model, integrating both electronic and excitonic processes. Using this model, we can reproduce the measured efficiency roll-off for prototypical phosphorescent OLED stacks based on the green dye tris[2-phenylpyridine]iridium (Ir(ppy){sub 3}) and the red dye octaethylporphine platinum (PtOEP) and study the cause of the roll-off as function of the current density. Both the voltage versus current density characteristics and roll-off agree well with experimental data. Surprisingly, the results of the simulations lead us to conclude that, contrary to what is often assumed, not triplet-triplet annihilation but triplet-polaron quenching is the dominant mechanism causing the roll-off under realistic operating conditions. Simulations for devices with an optimized recombination profile, achieved by carefully tuning the dye trap depth, show that it will be possible to fabricate OLEDs with a drastically reduced roll-off. It is envisaged that J{sub 90}, the current density at which the efficiency is reduced to 90%, can be increased by almost one order of magnitude as compared to the experimental state-of-the-art.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ground state energy of large polaron systems

The last unsolved problem about the many-polaron system, in the Pekar–Tomasevich approximation, is the case of bosons with the electron-electron Coulomb repulsion of strength exactly 1 (the “neutral case”). We prove that the ground state energy, for large N, goes exactly as −N{sup 7/5}, and we give upper and lower bounds on the asymptotic coefficient that agree to within a factor of 2{sup 2/5}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Intrinsic emittance properties of an Fe-doped β-Ga 2 O 3 (010) photocathode: Ultracold electron emission at 300 K and the polaron self-energy

Measurements of the spectral emission properties of an iron-doped a β-Ga 2 O 3 (010) photocathode at 300 K reveal the presence of an ultracold contribution to the total electron beam emission with a 6 meV mean transverse energy (MTE) in the 3.5–4.4 eV photon energy range (282–354 nm). This extreme sub-thermal photoemission signal is consistent with direct emission of electrons photoexcited from the Fe dopant states into the low effective mass and positive electron affinity primary conduction band, and it is superimposed on a stronger signal with a larger MTE associated with an (optical)phonon-mediated momentum-resonant Franck–Condon (FC) emission process from a thermally populated and negative electron affinity upper conduction band. For photon energies above 4.5 eV, a transition from a long to a short transport regime is forced by an absorption depth reduction to below 100 nm and both MTE signals exhibit spectral trends consistent with phonon-mediated FC emission if the polaron formation self-energy is included in the temperature of the initial thermalized photoexcited electron distribution.

Angeloni, Louis A. [University of Illinois, Chicag↗
DOE OSTI2024

Colossal magnetoresistance from spin-polarized polarons in an Ising system

Recent experiments suggest a new paradigm toward novel colossal magnetoresistance (CMR) in a family of materials EuM 2 X 2 (M = Cd, In, Zn; X = P, As), distinct from the traditional avenues involving Kondo–Ruderman–Kittel–Kasuya–Yosida crossovers, magnetic phase transitions with structural distortions, or topological phase transitions. Here, we use angle-resolved photoemission spectroscopy and density functional theory calculations to explore their origin, particularly focusing on EuCd 2 P 2 . While the low-energy spectral weight royally tracks that of the resistivity anomaly near the temperature with maximum magnetoresistance ( T MR ) as expected from transport-spectroscopy correspondence, the spectra are completely incoherent and strongly suppressed with no hint of a Landau quasiparticle. Using systematic material and temperature dependence investigation complemented by theory, we attribute this nonquasiparticle caricature to the strong presence of entangled magnetic and lattice interactions, a characteristic enabled by the p - f mixing. Given the known presence of ferromagnetic clusters, this naturally points to the origin of CMR being the scattering of spin-polarized polarons at the boundaries of ferromagnetic clusters. These results are not only illuminating to investigate the strong correlations and topology in EuCd 2 X 2 family, but, in a broader view, exemplify how multiple cooperative interactions can give rise to extraordinary behaviors in condensed matter systems.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Probing Quantum Anomalous Hall States in Twisted Bilayer WSe 2 via Attractive Polaron Spectroscopy

Moiré superlattices in semiconductors exhibit a rich variety of interaction-induced topological states, including quantum anomalous Hall (QAH) effects. A recent study hinted that twisted WSe 2 homobilayer (tWSe 2 ) could host a QAH state but lacked direct evidence of ferromagnetism, a key hallmark of this phase. Here, we report the first direct evidence of QAH states in tWSe 2 with spontaneous ferromagnetism. Specifically, we employ polarization-resolved attractive polaron spectroscopy on a dual-gated, 2° tWSe 2 and observe direct signatures of spontaneous time-reversal symmetry breaking at hole filling 𝜈 = 1. Together with a Chern number (𝐶) measurement via Streda formula analysis, we identify this magnetized state as a topological state, characterized by 𝐶 = 1. Furthermore, we demonstrate that these topological and magnetic properties are tunable via a finite displacement field, between a QAH ferromagnetic state and an antiferromagnetic state. Our findings position tWSe 2 as a highly versatile, stable, and optically addressable platform for investigating topological order and strong correlations in two-dimensional landscapes.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Polaronic Quasiparticles in the Valence-Transition Compound TmSe 1−𝑥⁢ Te 𝑥

Exotic quasiparticle states have been proposed in mixed-valent compounds exhibiting valence transitions. However, clear spectroscopic evidence identifying these states has remained elusive. Here, using synchrotron-based hard x-ray and extreme ultraviolet photoemission spectroscopy, we have probed the Tm 3⁢𝑑 and 4⁢𝑓 emissions in TmSe 1−𝑥 ⁢Te 𝑥 , where a Te concentration-dependent semimetal–insulator transition occurs alongside the valence transition. Our photoemission results, which are characteristic of the bulk, track this combined transition across the critical concentration (𝑥 𝑐 = 0.29). Notably, our results reveal a noninteger valence for the insulating phase and a novel quasiparticle excitation in the semimetallic phase: a Holstein polaron that extends beyond the standard periodic Anderson model.

Min, Chul H. [Christian-Albrechts-Univ. zu Kiel (G↗
DOE OSTI2021

Dynamic properties of a polaron coupled to dispersive optical phonons

We study static and dynamic properties of an electron coupled to dispersive quantum optical phonons in the framework of the Holstein model defined on a one-dimensional lattice. Calculations are performed using the Lanczos algorithm based on a highly efficient construction of the variational Hilbert space. Even small phonon dispersion has a profound effect on the low-energy optical response. While the upward phonon dispersion broadens the optical spectra due to single-phonon excitations, the downward dispersion has the opposite effect. With increasing dispersion, a multiphonon excitation (MPE) state becomes the lowest excited state of the system at zero momentum and determines the low-frequency response of the optical conductivity where the threshold for optical absorption moves below the single-phonon frequency. Multiphonon states form a well-defined bandlike feature just above the polaron band as clearly seen in the electron spectral function. Low-energy MPEs should be observable in systems with strong optical phonon dispersion in optical as well as angle-resolved photoemission experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Polaronic transport and thermoelectricity in Mn 3 Si 2 Te 6 single crystals

Here, we carried out a comprehensive study of the structural, electrical transport, thermal, and thermodynamic properties in ferrimagnetic Mn 3 Si 2 Te 6 single crystals. Mn and Te K -edge x-ray absorption spectroscopy and synchrotron powder x-ray diffraction were measured to provide information on the local atomic environment and the average crystal structure. The dc and ac magnetic susceptibility measurements indicate a second-order paramagnetic to ferrimagnetic transition at T c ~ 74 K, which is further confirmed by the specific heat measurement. Mn 3 Si 2 Te 6 exhibits semiconducting behavior along with a large negative magnetoresistance of -87% at T c and a relatively high value of thermopower up to ~10 mV/K at 5 K. Besides the rapidly increasing resistivity ρ(T) and thermopower S(T) below 20 K, the large discrepancy between the activation energy for resistivity E ρ and thermopower E S above 20 K indicates the polaronic transport mechanism. Furthermore, the thermal conductivity κ(T) of Mn 3 Si 2 Te 6 is notably rather low, comparable to Cr 2 Si 2 Te 6 , and is strongly suppressed in the magnetic field across T c , indicating the presence of strong spin-lattice coupling, also similar with Cr 2 Si 2 Te 6 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Polaronic transport and thermoelectricity in Fe 1 – x Co x Sb 2 S 4 ( x = 0 , 0.1, and 0.2)

Here, we report a study of Co-doped berthierite Fe 1–x Co x Sb 2 S 4 (x=0, 0.1, and 0.2). The alloy series of Fe 1–x Co x Sb 2 S 4 crystallize in an orthorhombic structure with the Pnma space group, similar to FeSb 2 , and show semiconducting behavior. The large discrepancy between activation energy for conductivity, E ρ (146 ~270meV), and thermopower, E S (47 ~108 meV), indicates the polaronic transport mechanism. Bulk magnetization and heat-capacity measurements of pure FeSb 2 S 4 (x=0) exhibit a broad antiferromagnetic transition (T N = 46K) followed by an additional weak transition (T* = 50K). Transition temperatures (T N and T*) slightly decrease with increasing Co content x. This is also reflected in the thermal conductivity measurement, indicating strong spin-lattice coupling. Fe 1–x Co x Sb 2 S 4 shows relatively high value of thermopower (up to ~624μVK –1 at 300 K) and thermal conductivity much lower when compared to FeSb 2 , a feature desired for potential applications based on FeSb 2 materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Role of Magnon-Magnon Scattering in Magnon Polaron Spin Seebeck Effect

The spin Seebeck effect (SSE) signal of magnon polarons in bulk-Y 3 Fe 5 O 12 (YIG)/Pt heterostructures is found to drastically change as a function of temperature. It appears as a dip in the total SSE signal at low temperatures, but as the temperature increases, the dip gradually decreases before turning to a peak. We attribute the observed dip-to-peak transition to the rapid rise of the four-magnon scattering rate. Our analysis provides important insights into the microscopic origin of the hybridized excitations and the overall temperature dependence of the SSE anomalies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Universal van der Waals Force between Heavy Polarons in Superfluids

Here, we investigate the long-range behavior of the induced Casimir interaction between two spinless heavy impurities, or polarons, in superfluid cold atomic gases. With the help of effective field theory (EFT) of a Galilean invariant superfluid, we show that the induced impurity-impurity potential at long distance universally shows a relativistic van der Waals-like attraction (~1/r 7 ) resulting from the exchange of two superfluid phonons. We also clarify finite temperature effects from the same two-phonon exchange process. The temperature $\mathrm{T}$ introduces the additional length scale c s /$\mathrm{T}$ with the speed of sound c s . Leading corrections at finite temperature scale as $\mathrm{T}$ 6 /r for distances r$\ll$c s /$\mathrm{T}$ smaller than the thermal length. For larger distances the potential shows a nonrelativistic van der Waals behavior (~$\mathrm{T}$/r 6 ) instead of the relativistic one. Our EFT formulation applies not only to weakly coupled Bose or Fermi superfluids but also to those composed of strongly correlated unitary fermions with a weakly coupled impurity. The sound velocity controls the magnitude of the van der Waals potential, which we evaluate for the fermionic superfluid in the BCS-BEC crossover.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Evidence of Polaron Formation in Halide Perovskites via Carrier Effective Mass Measurements

Carrier effective mass is a central parameter in solid-state physics. It is a measure of the strength of the coupling between a carrier and excitations arising from its surrounding medium, and features prominently in transport and optical calculations. Experimental techniques employed to determine it are steady-state ones, and so are unable to detect any change in the effective mass after a strong perturbation to the system, e.g., strong optical excitation. By combining time-resolved terahertz spectroscopy and transient absorption spectroscopy, on a mixed-cation mixed-halide perovskite thin film, we observe a large and long-lived photoinduced enhancement of the carrier effective mass, and from it deduce a twofold increase of the carrier-phonon coupling constant, giving evidence of polaron formation. Our work demonstrates a new approach to track the strength and ultrafast lifetimes of photoinduced carrier-boson interactions down to picosecond timescales that can be applied to a wide range of solid-state systems.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The Generalized Green’s function Cluster Expansion: A Python package for simulating polarons

We present an efficient implementation of the Generalized Green’s function Cluster Expansion (GGCE), which is a new method for computing the ground-state properties and dynamics of polarons (single electrons coupled to lattice vibrations) in model electron-phonon systems. The GGCE works at arbitrary temperature and is well suited for a variety of electron-phonon couplings, including, but not limited to, site and bond Holstein and Peierls (Su-SchriefferHeeger) couplings, and couplings to multiple phonon modes with different energy scales and coupling strengths. Quick calculations can be performed efficiently on a laptop using solvers from NumPy and SciPy, or in parallel at scale using the PETSc sparse linear solver engine.

36 MATERIALS SCIENCE↗
NASA NTRS1967

Polarons in Anisotropic Energy Bands

Calculation of polaron properties in anisotropic energy bands, and results for electron on spheroidal energy surface interacting with optical phonons

Kahn, A. H.↗
NASA NTRS1968

Polarons in anisotropic energy bands.

Polaron coupling approximation applied to electron-longitudinal optical phonon interaction on spheroidal energy band surface of strontium titanate

Kahn, A. H.↗