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

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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Lattice-level observation of the elastic-to-plastic relaxation process with subnanosecond resolution in shock-compressed Ta using time-resolved in situ Laue diffraction

We report direct lattice level measurements of plastic relaxation kinetics through time-resolved, in-situ Laue diffraction of shock-compressed single-crystal [001] Ta at pressures of 27-210 GPa. For a 50 GPa shock, a range of shear strains is observed extending up to the uniaxial limit for early data points (<0.6 ns) and the average shear strain relaxes to a near steady state over ~1 ns. For 80 and 125 GPa shocks, the measured shear strains are fully relaxed already at 200 ps, consistent with rapid relaxation associated with the predicted threshold for homogeneous nucleation of dislocations occurring at shock pressure ~65 GPa. The relaxation rate and shear stresses are used to estimate the dislocation density and these quantities are compared to the Livermore Multiscale Strength model as well as various molecular dynamics simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electron scattering, charge order, and pseudogap physics in La 1.6–x Nd 0.4 Sr x CuO 4 : An angle-resolved photoemission spectroscopy study

We report an angle-resolved photoemission study of the charge stripe ordered La 1.6–x Nd 0.4 Sr x CuO 4 (Nd-LSCO) system. A comparative and quantitative line-shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side (x = 0.20), a normal-state antinodal spectral gap opens upon cooling below 80 K. In this process, spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different line shape is observed in the antinodal region of charge ordered Nd-LSCO x = 1/8. Significant low-energy spectral weight appears to be lost. As a result, these observations are discussed in terms of spectral-weight redistribution and gapping originating from charge stripe ordering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Laser angle-resolved photoemission as a probe of initial state k z dispersion, final-state band gaps, and spin texture of Dirac states in the Bi 2 Te 3 topological insulator

Here, we have obtained angle-resolved photoemission (ARPES) spectra from single crystals of the topological insulator material Bi 2 Te 3 using tunable laser spectrometer. The spectra were collected for eleven different photon energies ranging from 5.57 to 6.70 eV for incident light polarized linearly along two different in-plane directions. Parallel first-principles, fully relativistic computations of photo-intensities were carried out using the experimental geometry within the framework of the one-step model of photoemission. Good overall accord between theory and experiment is used to gain insight into how properties of the initial and final state band structures as well as those of the topological surface states and their spin-textures are reflected in the laser-ARPES spectra. In conclusion, our analysis reveals that laser-ARPES is sensitive to both the initial state k z dispersion and the presence of delicate gaps in the final state electronic spectrum.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Angle-resolved photoemission observation of Mn-pnictide hybridization and negligible band structure renormalization in BaMn 2 As 2 and BaMn 2 Sb 2

We performed an angle-resolved photoemission spectroscopy study of BaMn 2 As 2 and BaMn 2 Sb 2 , which are isostructural to the parent compound BaFe 2 As 2 of the 122 family of ferropnictide superconductors. We show the existence of a strongly k z -dependent band gap with a minimum at the Brillouin zone center, in agreement with their semiconducting properties. Despite the half filling of the electronic 3d shell, we show that the band structure in these materials is almost not renormalized from the Kohn-Sham bands of density functional theory. Finally, our photon-energy-dependent study provides evidence for Mn-pnictide hybridization, which may play a role in tuning the electronic correlations in these compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Orientation-resolved domain mapping in tetragonal SrTiO 3 using polarized Raman spectroscopy

Here, we present microscopically resolved, polarized spectroscopy of Raman scattering collected from tetragonal SrTiO 3 . The anisotropic response of first-order Raman peaks within a single tetragonal domain has been measured. From these data, we assign symmetries to the phonons seen in the first-order Raman spectrum which is normally complicated by uncontrolled domain structure. Using a translation stage, we map the local domain orientation of a 3–μm 3 crystal volume near the laser focus and compare it to wide-field polarized images. This technique can be performed with readily available instruments and is relevant to the study of a wide range of related materials, interfaces, and devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Covalent versus localized nature of 4 f electrons in ceria: Resonant angle-resolved photoemission spectroscopy and density functional theory

We have conducted resonant angle-resolved photoemission spectroscopy of well-defined CeO 2 (111) and c-Ce 2 O 3 (111) model surfaces, revealing distinct f contributions in the valence band of the two compounds. In conjunction with density functional theory calculations, we show that the f contribution in CeO 2 is of a covalent nature, arising from hybridization with the O2p bands. In contrast, c-Ce 2 O 3 exhibits an almost nondispersive f state at 1.3 eV, which is indicative of almost negligible c-f hybridization.

Duchoň, Tomáš↗
DOE OSTI2017

Electronic structure of R Sb ( R = Y , Ce, Gd, Dy, Ho, Tm, Lu) studied by angle-resolved photoemission spectroscopy

Here, we use high-resolution angle-resolved photoemission spectroscopy (ARPES) and electronic structure calculations to study the electronic properties of rare-earth monoantimonides RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu). The experimentally measured Fermi surface (FS) of RSb consists of at least two concentric hole pockets at the Γ point and two intersecting electron pockets at the X point. These data agree relatively well with the electronic structure calculations. Detailed photon energy dependence measurements using both synchrotron and laser ARPES systems indicate that there is at least one Fermi surface sheet with strong three-dimensionality centered at the Γ point. Due to the “lanthanide contraction”, the unit cell of different rare-earth monoantimonides shrinks when changing the rare-earth ion from CeSb to LuSb. This results in the differences in the chemical potentials in these compounds, which are demonstrated by both ARPES measurements and electronic structure calculations. Interestingly, in CeSb, the intersecting electron pockets at the X point seem to be touching the valence bands, forming a fourfold-degenerate Dirac-like feature. On the other hand, the remaining rare-earth monoantimonides show significant gaps between the upper and lower bands at the X point. Furthermore, similar to the previously reported results of LaBi, a Dirac-like structure was observed at the Γ point in YSb, CeSb, and GdSb, compounds showing relatively high magnetoresistance. This Dirac-like structure may contribute to the unusually large magnetoresistance in these compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Continuous doping of a cuprate surface: Insights from in situ angle-resolved photoemission [Continuous doping of a cuprate surface: Insights from in situ ARPES]

Here, we report a technique of a continuously doped surface of Bi 2 Sr 2 CaCu 2 O 8+x through ozone/vacuum annealing and a systematic measurement over the nearly whole superconducting dome on the same sample surface by in situ angle-resolved photoemission spectroscopy. We find that the quasiparticle weight on the antinode is proportional to the doped carrier concentration x within the entire superconducting dome, while the nodal quasiparticle weight changes more mildly. More significantly, we discover that a d-wave pairing energy gap extracted from the nodal region scales well with the onset temperature of the Nernst signal. These findings suggest that the emergence of superconducting pairing is concomitant with the onset of free vortices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Depth-resolved charge reconstruction at the LaNi O 3 / CaMn O 3 interface

Here, rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic LaNiO 3 and antiferromagnetic CaMnO 3 exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO 3 . Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the LaNiO 3 /CaMnO 3 interface. Our measurements reveal an increased concentration of Mn 3+ and Ni 2+ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the Mn 4+ -Mn 3+ ferromagnetic double exchange and Ni 2+ -O-Mn 4+ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Depth-resolved resonant inelastic x-ray scattering at a superconductor/half-metallic-ferromagnet interface through standing wave excitation

We demonstrate that combining standing wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La 1.85 Sr 0.15 CuO 4 (LSCO) and a half-metallic ferromagnet La 0.67 Sr 0.33 MnO 3 (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the RIXS excitations. Here, the magnetic excitations are found to arise from the LSCO/LSMO interfaces, and there is also a suggestion that one of the dd excitations comes from the interfaces. SW-RIXS measurements of correlated-oxide and other multilayer heterostructures should provide unique layer-resolved insights concerning their orbital and magnetic excitations, as well as a challenge for RIXS theory to specifically deal with interface effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Time-resolved study of nonlinear three-magnon processes in yttrium iron garnet films

In this work, the temporal aspects of two nonlinear spin-wave processes, i.e., three-magnon splitting and confluence, were investigated in a yttrium iron garnet film. Three-magnon splitting involves the conversion of a magnon at the microwave pumping frequency f p into two magnons, each with a frequency near f p /2, while confluence refers to the subsequent combination of two of the split magnons into one with frequency f c < f p . Time-resolved Brillouin scattering measurements confirm that these processes occur close to the driving antenna (<1.7mm away), which is expected. Here, the data indicate, however, that magnons with larger group velocities are more likely to undergo confluence, which is not predicted by existing theory. Understanding the details of the splitting and confluence processes may have important implications for the use of short-wavelength spin waves for spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Efficient prediction of time- and angle-resolved photoemission spectroscopy measurements on a nonequilibrium BCS superconductor

We study how time- and angle-resolved photoemission (tr-ARPES) reveals the dynamics of BCS-type, s -wave superconducting systems with time-varying order parameters. Approximate methods are discussed, based on previous approaches to either optical conductivity or quantum dot transport, to enable computationally efficient prediction of photoemission spectra. One use of such predictions is to enable extraction of the underlying order parameter dynamics from experimental data, which is topical given the rapidly growing use of tr-ARPES in studying unconventional superconductivity. The methods considered model the two-time lesser Green's functions with an approximated lesser self-energy that describes relaxation by coupling of the system to two types of baths. The approach primarily used here also takes into consideration the relaxation of the excited states into equilibrium by explicitly including the level-broadening of the retarded and advanced Green's functions. Here, we present equilibrium and nonequilibrium calculations of tr-ARPES spectrum from our model and discuss the signatures of different types of superconducting dynamics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Detailed band structure of twinned and detwinned BaFe 2 As 2 studied with angle-resolved photoemission spectroscopy

We study the band structure of twinned and detwinned BaFe 2 As 2 using angle-resolved photoemission spectroscopy. The combination of measurements in the ordered and normal states along four high-symmetry momentum directions Γ/Z-X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the d xz and d yz orbitals as well as folding due to magnetic order with a wave vector of (π,π,π) . We are able to assign all observed bands. In particular we suggest an assignment of the electron bands different from previous reports. The high-quality spectra allow us to achieve a comprehensive understanding of the band structure of BaFe 2 As 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Fractional Chern insulator edges and layer-resolved lattice contacts

Fractional Chern insulators (FCIs) realized in fractional quantum Hall systems subject to a periodic potential are topological phases of matter for which space group symmetries play an important role. In particular, lattice dislocations in an FCI can host non-Abelian topological defects, known as genons. Genons can increase the ground-state degeneracy of the system and are thus potentially useful for topological quantum computing. In this work, we study FCI edges and how they can be used to detect genons. We find that translation symmetry can impose a quantized momentum difference between the edge electrons of a partially filled Chern band. We propose layer-resolved lattice contacts, which utilize this momentum difference to selectively contact a particular FCI edge electron. The relative current between FCI edge electrons can then be used to detect the presence of genons in the bulk FCI. Recent experiments have demonstrated graphene is a viable platform to study FCI physics. Here, we describe how the lattice contacts proposed here could be implemented in graphene subject to an artificial lattice, thereby outlining a path forward for experimental dectection of non-Abelian topological defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Building a free-energy functional from atomically resolved imaging: Atomic-scale phenomena in La-doped BiFe O 3

Scanning transmission electron microscopy (STEM) has enabled mapping of atomic structures of solids with sub-picometer precision, providing insight to the physics of ferroic phenomena and chemical expansion. Yet, only a subset of information is available, due to projective nature of imaging in the beam direction. Correspondingly, the analysis often relies on the postulated form of macroscopic Landau-Ginzburg energy for the ferroic long-range order parameter, and some predefined relationship between experimentally determined atomic coordinates and the order-parameter field. Here, we propose an approach for exploring the structure of ferroics using reduced order-parameter models constructed based on experimental data only. We develop a four-sublattice model (FSM) for the analytical description of A -cation displacement in (anti)ferroelectric-antiferrodistortive perovskites of ABO 3 type. The model describes the displacements of cation A in four neighboring unit cells and determines the conditions of different structural phases’ appearance and stability in ABO 3 . We show that FSM explains the coexistence of rhombohedral, orthorhombic, and spatially modulated phases, observed by atomic-resolution STEM in La-doped BiFeO 3 . Using this method, we atomically resolve and theoretically model the sublattice asymmetry inherent to the case of the A -site La/Bi cation sublattice in La x Bi 1- x FeO 3 polymorphs. This approach allows the exploration of ferroic behaviors from experimental data only, without additional assumptions on the nature of the order parameter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Resolving the ultracollinear paradox with effective field theory

Naive intuition about scale decoupling breaks down in the presence of fermion masses. Kinematic enhancements can greatly extend the range where one needs to keep a finite mass in calculations to obtain a correct result at even the O ( 1 ) level. Treating a light fermion as massive though, leads to a known but somewhat obscure paradox, a seeming leading-order sensitivity to an arbitrarily small mass. We show how a proper formulation in effective field theory not only resolves the physical conundrum but also answers the very practical question of when fermion masses are required. This has important implications for the development of shower Monte Carlo event generators above the weak scale. Published by the American Physical Society 2024

Astronomy & Astrophysics↗
DOE OSTI2025

Enhancing charge ratio sensitivity to hadronization effects via jet selections on resolved SoftDrop splitting

The study of quantum chromodynamics (QCD) at ultrarelativistic energies can be performed in a controlled environment through lepton-hadron deep inelastic scatterings. In such collisions, the high-energy partonic emissions that follow from the ejected hard partons are accurately described by perturbative QCD. However, the lower energy scales at which quarks and gluons experience color confinement, i.e., hadronization mechanism, fall outside the validity regions for perturbative calculations, requiring phenomenological models tuned to data to describe it. As such, hadronization physics cannot be currently derived from first principles alone. Monte Carlo event generators are useful tools to describe these processes as they simulate both the perturbative and the nonperturbative interactions, with model-dependent energy scales that control parton dynamics. This work employs jets—experimental reconstructions of final-state particles likely to have a common partonic origin—to inspect this transition further. Although originally proposed to circumvent hadronization effects, we show that jets can be utilized as probes of nonperturbative phenomena via their substructure. The charge correlation ratio was recently shown to be sensitive to hadronization effects. Our work further improves this sensitivity to nonperturbative scales by introducing a new selection based on the relative placement of the within the clustering tree, defined as the unclustering that resolves the jet’s leading charged particles. Published by the American Physical Society 2025

Apolinário, Liliana (ORCID:0000000335009681)↗
DOE OSTI2021

Time-resolved absorption spectroscopic characterization of ultrafast laser-produced plasmas under varying background pressures

Time-resolved tunable laser absorption spectroscopy is used to characterize the physical properties of ultrafast laser-produced plasmas. The plasmas were produced from an Inconel target, with = 0.4 wt. % Al, using ~ 35 fs, ~800 nm laser pulses at varying Ar background pressures from 1-100 Torr. The absorption spectrum of atomic Al is measured with high spectral and temporal resolution when the probe laser is stepped across the selected Al transition at 394.4 nm. Spectral fitting is used to infer linewidths, kinetic temperature, Al column density, and pressure broadening coefficient. The late time physical properties of plasmas are compared for various pressure levels. Our studies highlight that a significant lower state population exists even at early times of ultrafast laser-produced plasma evolution and lower state population persistence decreases with increasing ambient pressure. We also show that the fundamental optical properties, such as pressure-broadening, can be measured using ultrafast laser-produced plasmas combined with laser absorption spectroscopy.

laser absorption spectroscopy, laser produced plas↗