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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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Mapping the unoccupied state dispersions in Ta 2 NiSe 5 with resonant inelastic x-ray scattering
The transition metal chalcogenide Ta 2 NiSe 5 undergoes a second-order phase transition at T c = 328 K involving a small lattice distortion. Below T c , a band gap at the center of its Brillouin zone increases up to about 0.35 eV. In this work, we study the electronic structure of Ta 2 NiSe 5 in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ni L 3 -edge. In addition to a weak fluorescence response, we observe a collection of intense Raman-like peaks that we attribute to electron-hole excitations. Using density functional theory calculations of its electronic band structure, we identify the main Raman-like peaks as interband transitions between valence and conduction bands. Further, by performing angle-dependent RIXS measurements, we uncover the dispersion of these electron-hole excitations that allows us to extract the low-energy boundary of the electron-hole continuum. From the dispersion of the valence band measured by angle-resolved photoemission spectroscopy, we derive the effective mass of the lowest unoccupied conduction band.
Electron- and light-induced stimulated Raman spectroscopy for nanoscale molecular mapping
We propose and theoretically analyze a new vibrational spectroscopy, termed electron- and light-induced stimulated Raman (ELISR) scattering, that combines the high spatial resolution of elec-tron microscopy with the molecular sensitivity of surface-enhanced Raman spectroscopy. WithELISR, electron-beam excitation of plasmonic nanoparticles is utilized as a spectrally-broadband butspatially-confined Stokes beam in the presence of a diffraction-limited pump laser. To characterizethis technique, we develop a numerical model and conduct full-field electromagnetic simulations toinvestigate two distinct nanoparticle geometries, nanorods and nanospheres, coated with a Raman-active material. Our results show the significant (10 6 -10 7 ) stimulated Raman enhancement that isachieved with dual electron and optical excitation of thesenanoparticle geometries. Importantly,the spatial resolution of this vibrational spectroscopy for electron microscopy is solely determinedby the nanoparticle geometry and the plasmon mode volume. Our results highlight the promiseof ELISR for simultaneous high-resolution electron microscopy with sub-diffraction-limited Ramanspectroscopy, complementing advances in superresolutionmicroscopy, correlated light and electronmicroscopy, and vibrational electron energy loss spectroscopy.
Mapping out the emergence of topological features in the highly alloyed topological Kondo insulators Sm 1-$x$ M $x$ B 6 ( M =Eu, Ce)
SmB 6 is a strongly correlated material that has been attributed as a topological insulator and a Kondo insulator. Recent studies have found the topological surface states and low temperature insulating character to be profoundly robust against magnetic and nonmagnetic impurities. Here, we use angle resolved photoemission spectroscopy to chart the evolution of topologically linked electronic structure features versus magnetic doping and temperature in Sm 1-$x$ M $x$ B 6 ( M =Eu, Ce). Topological coherence phenomena are observed out to ~30% Eu and 50% Ce concentrations that represent extreme nominal hole and electron doping, respectively. Finally, theoretical analysis reveals that a recent redesignation of the topologically inverted band symmetries provides a natural route to reconciling the persistence of topological surface state emergence even as the insulating gap is lost through decoherence.
Mapping spin interactions from conductance peak splitting in Coulomb blockade
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Mapping the three-dimensional fermiology of the triangular lattice magnet EuAg 4 Sb 2
In this paper, we report the temperature-field phase diagram as well as present a comprehensive study of the electronic structure and three-dimensional fermiology of the triangular-lattice magnet EuAg 4 Sb 2 , utilizing quantum oscillation measurements, angle-resolved photoemission spectroscopy, and first-principles calculations. The complex magnetic phase diagram of EuAg 4 Sb 2 highlights many transitions through nontrivial AFM states. Shubnikov-de Haas and de Haas-van Alphen oscillations were observed in the polarized ferromagnetic state of EuAg 4 Sb 2 , revealing three pairs of distinct spin-split frequency branches with small effective masses. A comparison of the angle-dependent oscillation data with first-principles calculations in the ferromagnetic state and angle-resolved photoemission spectra shows good agreement, identifying tubular hole pockets and hourglass-shaped hole pockets at the Brillouin zone center, as well as diamond-shaped electron pockets at the zone boundary. As the temperature increases, the frequency branches of the tiny hourglass pockets evolve into a more cylindrical shape, while the larger pockets remain unchanged. This highlights that variations in exchange splitting, driven by changes in the magnetic moment, primarily impact the small Fermi pockets without significantly altering the overall band structure. As a result, this is consistent with first-principles calculations, which show minimal changes near the Fermi level across ferromagnetic and simple antiferromagnetic states or under varying on-site Coulomb repulsion.
Platinum liquid-vapor phase boundary mapped by fluid flyer experiments
We report a direct measurement of the temperature and density of a metal along its liquid-vapor coexistence (L-V) curve. By shocking platinum to a high-pressure liquid, we imparted sufficient heat for subsequent isentropic release to place it in a state on the boundary between the liquid and vapor phases. Released material in the liquid phase acted as a high velocity flyer pinned to the L-V curve. We measured velocity and radiant emission of the flyer as well as the interface motion and transiting shock states induced in a downstream window material by its impact. We used these measurements to calculate temperature and density of the L-V curve state which we compare to density functional theory predictions.
Nuclear mass measurements map the structure of atomic nuclei and accreting neutron stars
We introduce here mass excesses (ME) of neutron-rich isotopes of Ar through Fe, obtained via time of flight B ρ mass spectrometry at the National Superconducting Cyclotron Laboratory. Our new results have significantly reduced systematic uncertainties relative to a prior analysis, enabling the first determination of ME for Ti 58 , 59 , V 62 , Cr 65 , Mn 67 , 68 , and Fe 69 , 70 . Our results show the N = 34 subshell weaken at Sc and vanish at Ti, along with the absence of an N = 40 subshell at Mn. This further leads to a cooler accreted neutron star crust, highlighting the connection between the structure of nuclei and neutron stars.
Mapping of fragmented νf 5/2 →πf 7/2 transitions in the 73 Co→ 73 Ni decay
Excited states in 73 , 75 Ni were investigated through the β decay of 73 , 75 Co in an experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The experimental results extended the level scheme of Ni 73 to 3.2-MeV excitation energy and provided the experimental information on excited states in Ni 75 . The β -delayed neutron branching ratio for Co 73 was obtained. Furthermore, the experimental results are discussed in comparison with shell-model calculations.
Mapping the N = 40 island of inversion: Precision mass measurements of neutron-rich Fe isotopes
Nuclear properties across the chart of nuclides are key to improving and validating our understanding of the strong interaction in nuclear physics. Here we present high-precision mass measurements of neutron-rich Fe isotopes performed at the TITAN facility. The multiple-reflection time-of-flight mass spectrometer (MR-ToF-MS), achieving a resolving power greater than 600000 for the first time, enabled the measurement of 63–70 Fe, including first-time high-precision direct measurements (δm/m ≈ 10 -7 ) of 68–70 Fe, as well as the discovery of a long-lived isomeric state in 69 Fe. These measurements are accompanied by both mean-field and ab initio calculations using the most recent realizations which enable theoretical assignment of the spin-parities of the 69 Fe ground and isomeric states. Together with mean-field calculations of quadrupole deformation parameters for the Fe isotope chain, these results benchmark a maximum of deformation in the N = 40 island of inversion in Fe and shed light on trends in level densities indicated in the newly refined mass surface.
Mapping out the thermodynamic stability of a QCD equation of state with a critical point using active learning
The Beam Energy Scan Theory (BEST) collaboration's equation of state (EoS) incorporates a three-dimensional Ising model critical point into the quantum chromodynamics (QCD) equation of state from lattice simulations. Furthermore, it contains four free parameters related to the size and location of the critical region in the QCD phase diagram. Certain combinations of the free parameters lead to acausal or unstable realizations of the EoS that should not be considered. In this work, we use an active learning framework to rule out pathological EoS efficiently. We find that checking stability and causality for a small portion of the parameters' range is sufficient to construct algorithms that perform with > 96 % accuracy across the entire parameter space. Though in this work we focus on a specific case, our approach can be generalized to any EoS containing a parameter space-class correspondence.
Mapping machine-learned physics into a human-readable space
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Searching for exotic production of Higgs boson + X to map out new physics
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New probe of the high-redshift Universe: Nulling CMB lensing with interloper-free line intensity mapping pair lensing
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Probing early structure and model-independent neutrino mass with high-redshift CMB lensing mass maps
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