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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Observation of anisotropic Dirac cones in the topological material Ti 2 Te 2 P

Anisotropic bulk Dirac (or Weyl) cones in three-dimensional systems have recently gained intense research interest as they are examples of materials with tilted Dirac (or Weyl) cones indicating the violation of Lorentz invariance. In contrast, the studies on anisotropic surface Dirac cones in topological materials which contribute to anisotropic carrier mobility have been limited. By employing angle-resolved photoemission spectroscopy and first-principles calculations, we reveal the anisotropic surface Dirac dispersion in a tetradymite material Ti 2 Te 2 P on the (001) plane of the Brillouin zone. We observe quasielliptical Fermi pockets at the M ¯ point of the Brillouin zone forming the anisotropic surface Dirac cones. Our calculations of the Z 2 indices confirm that the system is topologically nontrivial with multiple topological phases in the same material. In addition, the observed nodal-line-like feature formed by bulk bands makes this system topologically rich.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Observation of nonlinear planar Hall effect in magnetic-insulator–topological-insulator heterostructures

Interfacing topological insulators (TIs) with magnetic insulators (MIs) have been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of the magnetic proximity effect (MPE) imposed to TIs. Here, in this work, we report the observation of the nonlinear planar Hall effect (NPHE) in Bi 2 ⁢Se 3 films grown on MI thulium and yttrium-iron-garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi 2 ⁢Se 3 grown on nonmagnetic gadolinium-gallium-garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi 2 ⁢Se 3 depends linearly on the external magnetic field, while that in YIG/Bi 2 ⁢Se 3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate that the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Observations of twinning microstructure in iron ramp-compressed through the α – ε phase transition

Single crystal iron was ramp-compressed along the [100] axis at 4 GPa/ns through the α–ϵ phase transformation at 13 GPa. Unlike shock experiments, in these ramp compression experiments the bcc lattice is observed to be nearly isotropically relaxed before the phase transition. There was a mixed α/ϵ phase region starting at ~13 GPa which was observed for 0.75 ns until the peak stress of 18 GPa was reached. In situ x-ray diffraction measurements show the formation of new hcp orientations not reported in shock or quasistatic compression experiments. The new hcp orientations appear to be caused by {10¯11} <10¯1 ¯2> → {¯1011} <¯101¯2> sequential twins that occur during the phase transition. This twinning mechanism relieves the shear strain caused by the bcc-hcp phase transition from the isotropically relaxed bcc phase while subject to uniaxial compression. Furthermore, these results show that in iron the induced microstructure through a phase transition and the phase transition mechanism depend on the loading history.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Observation of Coulomb blockade and Coulomb staircases in superconducting Pr 0.8 Sr 0.2 NiO 2 films

Motivated by the discovery of superconductivity in the infinite-layer nickelate family, we report an experimental endeavor to clean the surface of nickelate superconductor Pr 0.8 Sr 0.2 NiO 2 films by Ar + ion sputtering and subsequent annealing, and we study their electronic structures by cryogenic scanning tunneling microscopy and spectroscopy. The annealed surfaces are characterized by nano-sized clusters and Coulomb staircases with periodicity inversely proportional to the projected area of the nanoclusters, consistent with a double-barrier tunneling junction model. Moreover, the dynamical Coulomb blockade effects are observed and result in well-defined energy gaps around the Fermi level, which correlate closely with the specific configuration of the junctions. These Coulomb blockade-related phenomena provide an alternative plausible cause of the observed gap structure that should be considered in the spectroscopic understanding of nickelate superconductors with the nano-clustered surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Flow of the normal component of He-II about bluff objects observed with He$^*_2$ excimers

Flow of the normal fluid component of He II about a cylinder and flat plate in a channel with a square cross-section was observed by tracking clouds of He$^*_2$ excimers. Flow was produced using a lithographically patterned heater. The direction and speed of flow projected onto a plane parallel to the axis of the channel and perpendicular to the heater exhibited significant change with time even after the heater was turned off. Flow was recorded in movies. Excimers moved at least as fast as could be recorded by the camera. Velocity vector field maps of flow suggest formation of structures downstream of the cylinder object that are consistent with parts of eddies. Here, in this paper, we establish a foundation to observe normal component flow of a quantum fluid over centimeters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Experimental observation of spin glass state in the highly disordered quaternary Heusler alloy FeRuMnGa

The realization of the spin-glass (SG) state in Heusler alloys is very rare despite the presence of inherent structural and elemental disorder in those compounds. Although a few half- and full-Heusler alloys are known to exhibit the SG state, there is hardly any manifestation of the same in cases of quaternary Heusler compounds. Here we report the observation of a SG state in a highly disordered equiatomic quaternary Heusler compound: FeRuMnGa where the SG state is in between the canonical SG and the cluster glass. Different intricate features of the SG state including nonequilibrium magnetic dynamics at low temperatures in the compound are unveiled through our comprehensive magnetic, heat capacity, and neutron-diffraction studies. Further the structural disorder in the sample is neither conventional A2 - nor B2 -type whereas those two types are commonly observed for Heusler compounds. The presence of disorder also plays a significant role in electron transport properties of the alloy, which is reflected in its exhibition of semimetallic behavior and anomalous Hall effect at low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Observation of cupratelike nonlinear terahertz responses in superconducting infinite-layer nickelates via two-dimensional coherent spectroscopy

The 𝑑-wave pairing symmetry in newly discovered infinite-layer nickelates remains highly debated compared to the widely accepted 𝑑-wave superconductivity in cuprates, despite recent intense efforts using equilibrium optical and electrical transport measurements. A complementary approach involves characterizing THz coherent nonlinear responses, which is currently lacking but needed, as it would offer new insights into the superconducting gap structure by characterizing Higgs modes and quasiparticle excitations. Here, in this study, we report THz 2D coherent spectroscopy (THz-2DCS) responses from the superconducting state in an infinite-layer nickelate. The 2D spectra clearly demonstrate the absence of hallmark signatures of 𝑠-wave pairing symmetry, such as resonant 2⁢Δ peaks at superconducting gaps. Instead, we observe gapless 2D spectra in the nickelate, centered around the THz driving frequency, with no noticeable shift as the temperature increases. These findings closely resemble the nodal superconductivity behavior expected in cuprates. Together with our simulations, these findings suggest that 2⁢Δ signatures in the 2D spectra of 𝑑-wave gap structures can be obscured by significant nodal quasiparticle excitations at arbitrarily low energies. This contrasts with the long-lived Higgs modes, observed as distinct 2⁢Δ peaks in a controlled THz-2DCS experiment on niobium, a representative example of dirty-limit s-wave superconductors. Our results have implications for understanding the unconventional superconductivity in nickelates and highlight the need to explore their coherent nonlinear dynamics.

Pairing mechanisms↗
DOE OSTI2025

Magnetic excitations in Nd 𝑛+1⁢ Ni 𝑛⁢ O 3⁢𝑛+1 Ruddlesden-Popper nickelates observed via resonant inelastic x-ray scattering

Magnetic interactions are thought to play a key role in the properties of many unconventional superconductors, including cuprates, iron pnictides, and square-planar nickelates. Superconductivity was also recently observed in the bilayer and trilayer Ruddlesden-Popper nickelates, the electronic structure of which is expected to differ from that of cuprates and square-planar nickelates. Here we study how electronic structure and magnetic interactions evolve with the number of layers, 𝑛, in thin film Ruddlesden-Popper nickelates Nd 𝑛+1 ⁢Ni 𝑛 ⁢O 3⁢𝑛+1 with 𝑛 = 1,3, and 5 using resonant inelastic x-ray scattering (RIXS). The RIXS spectra are consistent with a high-spin |3⁢𝑑 8 ⁢ $\underline{𝐿}$⟩ electronic configuration, resembling that of La 2−𝑥 ⁢Sr 𝑥 ⁢NiO 4 and the parent perovskite, NdNiO 3 . The magnetic excitations soften to lower energy in the structurally self-doped, higher-𝑛 films. In conclusion, our observations confirm that structural tuning is an effective route for altering electronic properties, such as magnetic superexchange, in this prominent family of materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Traces of nonequilibrium effects, initial condition, bulk dynamics, and elementary collisions in the charm observables

Heavy quarks produced in relativistic heavy-ion collisions are known to be sensitive probes of the hot and dense QCD matter they traverse. In this paper we study how their dynamics is affected by the nature of the bulk evolution of the QCD matter, the initial condition of the system, and the treatment of elementary interactions between heavy quarks and the surrounding medium. For the same initial condition and the same quark-gluon plasma (QGP) expansion scenario we discuss the consequences of the assumption of a local equilibrium by comparing the consequences for the nuclear modification factor $R_{AA}$ and the elliptic flows of charm quarks, scrutinizing the different components of the final distribution of charm quarks. For this purpose we employ the parton-hadron-string dynamics (PHSD) model, which is an off-shell microscopic transport approach, as well as the linearized-Boltzmann (LB) scheme obtained by coarse graining the PHSD bulk and assuming local equilibrium for the interactions of the charm quarks with the bulk. The $R_{AA}$ of charm quarks stemming from the later LB approach is also compared to a genuine fluid dynamics evolution initiated by the coarse grained PHSD, which allows us to further assess the consequences of reducing the full $\textit{n}$-body dynamics. We then proceed to a systematic comparison of PHSD (in its LB approximation) with MC@HQ, another transport model for heavy flavors which also relies on the LB approach. In particular, we investigate the consequences for the nuclear modification factor of charm quarks if we vary separately the initial heavy quark distribution function in matter, the expansion dynamics of the QGP, and the elementary interactions of heavy quarks of these models. We find that the results for both models vary significantly depending on the details of the calculation. However, both models achieve very similar predictions for key heavy quark observables for certain combinations of initial condition, bulk evolution, and interactions. In this work, we conclude that this ambiguity limits our ability to determine the different properties of the system based on the current set of observables.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Bayesian inference of the dense-matter equation of state encapsulating a first-order hadron-quark phase transition from observables of canonical neutron stars

The remarkable progress in recent multimessenger observations of both isolated neutron stars (NSs) and their mergers has provided some of the much needed data to improve our understanding about the equation of state (EOS) of dense neutron-rich matter. Various EOSs with or without some kinds of phase transitions from hadronic to quark matter (QM) have been widely used in many forward modelings of NS properties. Direct comparisons of these predictions with observational data sometimes also using χ 2 minimizations have provided very useful constraints on the model EOSs. Furthermore, it is normally difficult to perform uncertain quantifications and analyze correlations of the EOS model parameters involved in forward modelings especially when the available data are still very limited.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Quantum computation of nuclear observables involving linear combinations of unitary operators

Here we present the quantum computation of nuclear observables where the operators of interest are first decomposed in terms of the linear combination of unitaries. Then we utilize the Hadamard test and the linear combination of unitaries based methods to compute the expectation values. We apply these methods to calculate the electric quadrupole moment of the deuteron. The results are compared for the Jordan-Wigner transformation and Gray code encoding. We discuss the versatility of our approach that can be utilized in general to calculate several observables on a quantum computer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Properties of first-order hadron-quark phase transition from inverting neutron star observables

By inverting the observational data of several neutron star observables in the three-dimensional parameter space of the constant speed of sound (CSS) model while fixing all hadronic equation-of-state parameters at their currently known most probable values, we constrain the three parameters of the CSS model and their correlations. Using two lower radius limits of $R$ $2.01$ = $11.41$ km and $R$ $2.01$ = $12.2$ km for PSR J0740+6620 obtained from two independent analyses using different approaches by the Neutron Star Interior Composition Explorer (NICER) Collaboration, the speed of sound squared $c$ $^2_{QM}$ in quark matter is found to have a lower limit of 0.35 or 0.43 in units of $c$ $2$ , above its conformal limit of $c$ $^2_{QM}$ < $1/3$. Furthermore, an approximately linear correlation between the first-order hadron-quark transition density $ρ$ $t$ and its strength $Δε$ is found. Moreover, the presence of twin stars is deemed improbable by the present work.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Empirical radius formulas for canonical neutron stars from bidirectionally selecting features of equations of state in extended Bayesian analyses of observational data

Significant advancement in Bayesian inference of nuclear equation of state (EOS) from gravitational wave and x-ray observations of neutron stars (NSs) has been made by the nuclear astrophysics community especially since GW170817. By extending the traditional Bayesian analysis which normally ends at presenting the marginalized posterior probability distribution functions (PDFs) of individual EOS parameters and their correlations (or sometimes only the Pearson correlation coefficients which are only reliably useful when the variables are linearly correlated while they are actually often not), we search for a data-driven and robust empirical formula for the radius 𝑅 1.4 of canonical NSs in terms of the characteristic EOS parameters (features). We also identify the single most important but currently poorly known EOS parameter for determining the 𝑅 1.4 . Using three regression-model-building methodologies: bidirectional stepwise feature selection, least absolute shrinkage selection operator (LASSO) regression, and neural network regression on a large set of posterior EOSs and the corresponding 𝑅 1.4 values inferred from earlier comprehensive Bayesian analyses of NS observational data, we systematically and rigorously develop the most probable 𝑅 1.4 formulas with varying statistical accuracy and technical complexity. Here, the most important EOS parameters for determining 𝑅 1.4 are found consistently in each of the feature selection processes to be (in order of decreasing importance): curvature 𝐾 sym , slope 𝐿, skewness 𝐽 sym of nuclear symmetry energy, skewness 𝐽 0 , incompressibility 𝐾 0 of symmetric nuclear matter, and the magnitude 𝐸 sym ⁡(𝜌 0 ) of symmetry energy at the saturation density 𝜌 0 of nuclear matter.

Bayesian methods↗
DOE OSTI2024

Observation of collective modes of excitations in 59 Co, 59 Ni, and 61 Co and the influence of the $g_{9/2}$ orbital

High-spin states in 59 Co (Z = 27), 59 Ni (Z = 28), and 61 Co have been populated by the fusion evaporation reactions, 48 Ti( 14 C, p2n ) 59 Co, 48 Ti( 14 C, 3 n ) 59 Ni, and 50 Ti( 14 C, p2n ) 61 Co. The 9 MV tandem accelerator at the John D. Fox Laboratory, Florida State University (FSU) was used to accelerate the 14 C beam and the deexciting γ rays were detected by the FSU detector array consisting of six high-purity germanium (HPGe) clover detectors and three single crystals. Directional correlation of the $γ$ rays deexciting oriented states (DCO ratios) and polarization asymmetry measurements helped to establish spin and parities of the excited states whenever possible. The level scheme of 59 Co has been expanded with the inclusion of positive-parity states up to 31/2 + at around 11 MeV. The 59 Ni positive-parity states known from previous study were verified with modifications to some of the spins and parities. On the other hand, the negative-parity states were extended to 31/2 at an excitation energy of 12 MeV. No new transition was observed for 61 Co, but one of the major bands has been reassigned as consisting of positive-parity states by reason of this study. Importantly, excitations observed within the f 7/2 , p 3/2 , f 5/2 , and p 1/2 orbitals, and also across the N = 40 subshell closure into the g 9/2 orbital was established by comparison with large-scale shell-model calculations for the three nuclei studied.

59 ≤ A ≤ 89↗
DOE OSTI2020

Optically targeted search for gravitational waves emitted by core-collapse supernovae during the first and second observing runs of advanced LIGO and advanced Virgo

In this work, we present the results from a search for gravitational-wave transients associated with core-collapse supernovae observed within a source distance of approximately 20 Mpc during the first and second observing runs of Advanced LIGO and Advanced Virgo. No significant gravitational-wave candidate was detected. We report the detection efficiencies as a function of the distance for waveforms derived from multidimensional numerical simulations and phenomenological extreme emission models. Furthermore, the sources with neutrino-driven explosions are detectable at the distances approaching 5 kpc, and for magnetorotationally driven explosions the distances are up to 54 kpc. However, waveforms for extreme emission models are detectable up to 28 Mpc. For the first time, the gravitational-wave data enabled us to exclude part of the parameter spaces of two extreme emission models with confidence up to 83%, limited by coincident data coverage. Besides, using ad hoc harmonic signals windowed with Gaussian envelopes, we constrained the gravitational-wave energy emitted during core collapse at the levels of 4.27×10 -4 M ⊙ c 2 and 1.28×10 -1 M ⊙ c 2 for emissions at 235 and 1304 Hz, respectively. These constraints are 2 orders of magnitude more stringent than previously derived in the corresponding analysis using initial LIGO, initial Virgo, and GEO 600 data.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

NuSTAR tests of sterile-neutrino dark matter: New Galactic bulge observations and combined impact

In this work, we analyze two dedicated NuSTAR observations with exposure ~190 ks located ~10° from the Galactic plane, one above and the other below, to search for x-ray lines from the radiative decay of sterile-neutrino dark matter. These fields were chosen to minimize astrophysical x-ray backgrounds while remaining near the densest region of the dark matter halo. We find no evidence of anomalous x-ray lines in the energy range 5–20 keV, corresponding to sterile neutrino masses 10–40 keV. Interpreted in the context of sterile neutrinos produced via neutrino mixing, these observations provide the leading constraints in the mass range 10–12 keV, improving upon previous constraints in this range by a factor ~2. We also compare our results to Monte Carlo simulations, showing that the fluctuations in our derived limit are not dominated by systematic effects. An updated model of the instrumental background, which is currently under development, will improve NuSTAR’s sensitivity to anomalous x-ray lines, particularly for energies 3–5 keV.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Observable signatures of enhanced axion emission from protoneutron stars

Here we perform general relativistic one-dimensional supernova (SN) simulations to identify observable signatures of enhanced axion emission from the pion-induced reaction π - + p →n + a inside a newly born protoneutron star (PNS). We focus on the early evolution after the onset of the supernova explosion to predict the temporal and spectral features of the neutrino and axion emission during the first 10 s. Pions are included as explicit new degrees of freedom in hot and dense matter. Their thermal population and their role in axion production are both determined consistently to include effects due to their interactions with nucleons. For a wide range of ambient conditions encountered inside a PNS, we find that the pion-induced axion production dominates over nucleon-nucleon bremsstrahlung processes. By consistently including the role of pions on the dense matter equation of state and on the energy loss, our simulations predict robust discernible features of neutrino and axion emission from a galactic supernova that can be observed in terrestrial detectors. For axion couplings that are compatible with current bounds, we find a significant suppression with time of the neutrino luminosity during the first 10 s. This suggests that current bounds derived from the neutrino signal from SN 1987A can be improved and that future galactic supernovae may provide significantly more stringent constraints.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Study of the decay D s + → π + π + π - η and observation of the W -annihilation decay D s + → a 0 ( 980 ) + ρ 0

The decay $D^+_s$ → $π^+π^+π^-η$ is observed for the first time, using e + e - collision data corresponding to an integrated luminosity of 6.32 fb -1 collected by the BESIII detector at center-of-mass energies between 4.178 and 4.226 GeV. The absolute branching fraction for this decay is measured to be $\mathscr{B}$($D^+_s$ → $π^+π^+π^-η$) = (3.12 ± 0.13 stat ± 0.90 syst )%. The first amplitude analysis of this decay reveals the substructures in $D^+_s$ → $π^+π^+π^-η$ nd determines the relative fractions and the phases among these substructures. The dominant intermediate process is $D^+_s$ → $a_1$(1260) + $η,a_1$(1260) + → ρ(770) 0 $π^+$ with a branching fraction of (1.73 ± 0.14 stat ±0.08 syst )%. We also observe the W-annihilation process $D^+_s$ → $a_0$(980) + ρ(770) 0 , $a_0$(980) + → $π^+η$ with a branching fraction of (0.21 ± 0.08 stat ± 0.05 syst )%, which is larger than the branching fractions of other measured pure W-annihilation decays by 1 order of magnitude.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗