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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 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Quenching rattling modes in skutterudites with pressure

In this paper, a high-pressure study of the lattice dynamics in the filled skutterudite Eu 0.84 Fe 4 Sb 12 was carried out by means of x-ray powder diffraction and nuclear inelastic scattering. The anharmonicity of particular phonon modes was characterized by mode and element specific Grüneisen parameters. The large anharmonicity of the rattling optical mode that is hybridized with the acoustical phonons at ambient pressure is reduced at high pressure as the phonon modes decouple. Finally, this result suggests that anharmonic coupling between acoustic and optical phonon modes plays a central role in the reduced thermal conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Spin dynamics near a putative antiferromagnetic quantum critical point in Cu-substituted BaFe 2 As 2 and its relation to high-temperature superconductivity

We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting Ba(Fe 0.957 Cu 0.043 ) 2 As 2 , a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound BaFe 2 As 2 and superconducting Ba(Fe 1–x Ni x ) 2 As 2 compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe 0.957 Cu 0.043 ) 2 As 2 , which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. In addition, we show that the spin-spin correlation length ξ(T) increases rapidly as the temperature is lowered and find ω/T scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings

A scaled quantum computer with donor spins in silicon would benefit from a viable semiconductor framework and a strong inherent decoupling of the qubits from the noisy environment. Coupling neighboring spins via the natural exchange interaction according to current designs requires gate control structures with extremely small length scales. In this work, we present a silicon architecture where bismuth donors with long coherence times are coupled to electrons that can shuttle between adjacent quantum dots, thus relaxing the pitch requirements and allowing space between donors for classical control devices. An adiabatic SWAP operation within each donor/dot pair solves the scalability issues intrinsic to exchange-based two-qubit gates, as it does not rely on subnanometer precision in donor placement and is robust against noise in the control fields. In conclusion, we use this SWAP together with well established global microwave Rabi pulses and parallel electron shuttling to construct a surface code that needs minimal, feasible local control.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Tunable inverse topological heterostructure utilizing ( B i 1 - x I n x ) 2 S e 3 and multichannel weak-antilocalization effect

In typical topological insulator (TI) systems the TI is bordered by a non-TI insulator, and the surrounding conventional insulators, including vacuum, are not generally treated as part of the TI system. Here, we implement a material system where the roles are reversed, and the topological surface states form around the non-TI (instead of the TI) layers. This is realized by growing a layer of the tunable non-TI (Bi 1-x In x ) 2 Se 3 in between two layers of the TI Bi 2 Se 3 using the atomically precise molecular beam epitaxy technique. On this tunable inverse topological platform, we systematically vary the thickness and the composition of the (Bi 1-x In x ) 2 Se 3 layer and show that this tunes the coupling between the TI layers from strongly coupled metallic to weakly coupled, and finally to a fully decoupled insulating regime. This system can be used to probe the fundamental nature of coupling in TI materials and provides a tunable insulating layer for TI devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Manipulation of pure spin current in ferromagnetic metals independent of magnetization

Upon the injection of a pure spin current, a ferromagnet, similar to a nonmagnetic metal, also exhibits inverse spin Hall effect (ISHE). In our work, we show in Co/Cu/YIG, where the thin Cu layer allows transmission of spin current from YIG into Co but decouples the two ferromagnets, that the interaction between ISHE and ferromagnetic ordering in Co can be unambiguously investigated. By switching on and off the pure spin current contribution, we demonstrate that the ISHE in Co is independent of the direction of the Co magnetization, which clearly suggests that the ISHE in Co is dominated not by the extrinsic impurity scatterings, but from the intrinsic origin.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Possibility of realizing quantum spin Hall effect at room temperature in stanene / A l 2 O 3 ( 0001 )

Two-dimensional quantum spin Hall (QSH) insulators with reasonably wide band gaps are imperative for the development of various innovative technologies. Through systematic density functional calculations and tight-binding simulations, we found that stanene on an α -alumina surface may possess a sizeable topologically nontrivial band gap ( ~ 0.25 eV ) at the Γ point. Furthermore, stanene is atomically bonded to but electronically decoupled from the substrate, providing high structural stability and isolated QSH states to a large extent. We conclude the underlying physical mechanism is rather general, and this finding may lead to the opening of a new vista for the exploration of QSH insulators for room temperature device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Interplay between intercalated oxygen superstructures and monolayer h -BN on Cu(100)

The confinement effect of intercalated atoms in van der Waals heterostructures can lead to interesting interactions between the confined atoms or molecules and the overlaying two-dimensional (2D) materials. In this paper, we report the formation of ordered Cu(100) p(2×2) oxygen superstructures by oxygen intercalation under the monolayer hexagonal boron nitride (h-BN) on Cu after annealing. By using scanning tunneling microscopy and x-ray photoelectron spectroscopy, we identify the superstructure and reveal its roles in passivating the exposed Cu surfaces, decoupling h-BN and Cu, and disintegrating h-BN monolayers. The oxygen superstructure appears as a 2D pattern on the exposed Cu surface or quasi-1D stripes of paired oxygen intercalated in the interface of h-BN and Cu predominantly oriented along the moiré modulations. The oxygen superstructure is shown to etch the overlaying h-BN monolayer in a thermal annealing process. After extended annealing, the h-BN monolayer disintegrates into nanoislands with zigzag edges. Finally, we discuss the implications of these findings on the stability and oxidation resistance of h-BN and relate them to challenges in process integration and 2D heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Coupling of magnetic orders in La 2 CuO 4 + x

High transverse magnetic field and zero field muon spin rotation and relaxation measurements have been carried out in a lightly oxygen-doped high-T c parent compound La 2 CuO 4 in a temperature range from 2 K to 300 K. As in the stoichiometric compound, muon spin rotation spectra reveal, along with the antiferromagnetic local field, the presence of an additional source of magnetic field at the muon. The results indicate that this second magnetic order is driven by the antiferromagnetism at low temperature but the two magnetic orders decouple at higher temperature. Furthermore, the ability of μ+SR to detect this additional magnetism deteriorates with doping, thus rendering the technique impotent to reveal time-reversal symmetry breaking in superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Multiple charge density wave states at the surface of TbT e 3

We studied TbTe 3 using scanning tunneling microscopy (STM) in the temperature range of 298–355 K. Our measurements detect a unidirectional charge density wave (CDW) state in the surface Te layer with a wave vector consistent with that of the bulk q CDW = 0.30 ± 0.01c*. However, unlike previous STM measurements, and differing from measurements probing the bulk, we detect two perpendicular orientations for the unidirectional CDW with no directional preference for the in-plane crystal axes (a or c axis) and no noticeable difference in wave vector magnitude. In addition, we find regions in which the bidirectional CDW states coexist. We propose that observation of two unidirectional CDW states indicates a decoupling of the surface Te layer from the rare-earth block layer below, and that strain variations in the Te surface layer drive the local CDW direction to the specific unidirectional or, in rare occurrences, bidirectional CDW orders observed. This indicates that similar driving mechanisms for CDW formation in the bulk, where anisotropic lattice strain energy is important, are at play at the surface. Furthermore, the wave vectors for the bidirectional order we observe differ from those theoretically predicted for checkerboard order competing with stripe order in a Fermi-surface nesting scenario, suggesting that factors beyond Fermi-surface nesting drive CDW order in TbTe 3 . As a result, our temperature-dependent measurements provide evidence for localized CDW formation above the bulk transition temperature T CDW .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Auxiliary-field-based trial wave functions in quantum Monte Carlo calculations

Quantum Monte Carlo (QMC) algorithms have long relied on Jastrow factors to incorporate dynamic correlation into trial wave functions. While Jastrow-type wave functions have been widely employed in real-space algorithms, they have seen limited use in second-quantized QMC methods, particularly in projection methods that involve a stochastic evolution of the wave function in imaginary time. Here we propose a scheme for generating Jastrow-type correlated trial wave functions for auxiliary-field QMC methods. The method is based on decoupling the two-body Jastrow into one-body projectors coupled to auxiliary fields, which then operate on a single determinant to produce a multideterminant trial wave function. We demonstrate that intelligent sampling of the most significant determinants in this expansion can produce compact trial wave functions that reduce errors in the calculated energies. Lastly, our technique may be readily generalized to accommodate a wide range of two-body Jastrow factors and applied to a variety of model and chemical systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Locking of length scales in two-band superconductors

Here, a model of a clean two-band s-wave superconductor with cylindrical Fermi surfaces, different Fermi velocities v 1,2 , and a general 2×2 coupling matrix V αβ is used to study the order parameter distribution in vortex lattices. The Eilenberger weak coupling formalism is used to calculate numerically the spatial distributions of the pairing amplitudes Δ 1 and Δ 2 of the two bands for vortices parallel to the Fermi cylinders. For generic values of the interband coupling V 12 , it is shown that, independently of the couplings V αβ , of the ratio v 1 /v 2 , of the temperature, and the applied field, the length scales of spatial variation of Δ 1 and of Δ 2 are the same within the accuracy of our calculations. The only exception from this single length-scale behavior is found for V 12 << V 11 , i.e., for nearly decoupled bands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Excitation and characterization of image potential state electrons on quasi-free-standing graphene

We investigate the band structure of image potential states in quasi-free-standing graphene (QFG) monolayer islands using angle-resolved two-photon-photoemission spectroscopy. Direct probing by low-energy electron diffraction shows that QFG is formed following oxygen intercalation into the graphene-Ir(111) interface. Despite the apparent decoupling of the monolayer graphene from the Ir substrate, we find that the binding energy of the n = 1 image potential state on these QFG islands increases by 0.17 eV, as compared to the original Gr/Ir(111) interface. We use calculations based on density-functional theory to construct an empirical, one-dimensional potential that quantitatively reproduces the image potential state binding energy and links the changes in the interface structure to the shift in energy. Specifically, two factors contribute comparably to this energy shift: a deeper potential well arising from the presence of intercalated oxygen adatoms and a wider potential well associated with the increase in the graphene-Ir distance. While image potential states have not been observed previously on QFG by photoemission, our paper now demonstrates that they may be strongly excited in a well-defined QFG system produced by oxygen intercalation. Finally, this opens an opportunity for studying the surface electron dynamics in QFG systems, beyond those found in typical nonintercalated graphene-on-substrate systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Doping effects of Cr on the physical properties of BaFe 1.9 - x Ni 0.1 Cr x As 2

We present a systematic study on the heavily Cr doped iron pnictides BaFe1.9_xNi0.1CrxAs2 by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from x = 0 to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Neel temperature T-n coincides with the tetragonal-to-orthorhombic structural transition temperature T-s, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing x until x = 0.5, and then decreases with further increasing x. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the FeAs4 tetrahedron along the c axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around x = 0.5. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.

Gong, Dongliang↗
DOE OSTI2018

Fluxon-based quantum simulation in circuit QED

Long-lived fluxon excitations can be trapped inside a superinductor ring, which is divided into an array of loops by a periodic sequence of Josephson junctions in the quantum regime, thereby allowing fluxons to tunnel between neighboring sites. By tuning the Josephson couplings and, implicitly, the fluxon tunneling probability amplitudes, a wide class of one-dimensional tight-binding lattice models may be implemented and populated with a stable number of fluxons. We illustrate the use of this quantum simulation platform by discussing the Su-Schrieffer-Heeger model in the one-fluxon subspace, which hosts a symmetry-protected topological phase with fractionally charged bound states at the edges. Furthermore, this pair of localized edge states could be used to implement a superconducting qubit increasingly decoupled from decoherence mechanisms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effects of lattice geometry on the dynamic properties of dipolar-coupled magnetic nanodisk arrays

Here, we have studied the impact of lattice geometry on the dynamic properties of close-spaced arrays of circular nanomagnets, also known as magnonic crystals. To this end, we prepared 2D nanomagnet arrays with both square and hexagonal lattice symmetries (300-nm disk diameter, 400-nm center-to-center distance) and performed broadband ferromagnetic resonance (FMR) measurements. Micromagnetic simulations were used to interpret distinct features of the measured resonance spectra. The FMR bias field was applied along two distinct principal directions for each lattice, and a sample with well-separated, decoupled disks was measured for reference. We found that the interdisk dipolar coupling has a strong impact on the FMR for these 2D magnonic crystals. Distinctly different oscillation modes were found for the individual nanomagnets, dependent on lattice symmetry and direction of the bias field. Moreover, we find that spectral peak splitting from excitation of edge and center modes, as well as the damping, depends on the lattice symmetry and the orientation of the bias field. These findings demonstrate that lattice geometry has a strong influence on the excited spin-wave spectrum and is a relevant design parameter for spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Analysis of the 18 F g,m (d,p) 19 F reactions in the rotational model

Spectroscopic strengths for the 18 F g , m ( d , p ) F 19 reactions to members of the ground-state band in F 19 are interpreted in the rotational model. The analysis considers two different coupling schemes, namely, the strong and decoupled limits of the particle rotor model to describe the ground and isomeric states of F 18 , as well as the transitions to the ground-state band in F 19 . Our results, obtained using Nilsson amplitudes for the neutron levels 1 2 [ 220 ] , 3 2 [ 211 ] , and 5 2 [ 202 ] calculated at a deformation ε 2 = 0.32 are in agreement with the measurements and USDB shell-model calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Uncertainty quantification of an empirical shell-model interaction using principal component analysis

Recent investigations have emphasized the importance of uncertainty quantification (UQ) in nuclear theory. Here, we carry out UQ for configuration-interaction shell-model calculations in the 1$\textit{s}$–0$\textit{d}$ valence space, investigating the sensitivity of observables to perturbations in the 66 parameters (matrix elements) of a high-quality empirical interaction. The large parameter space makes computing the corresponding Hessian numerically costly, so we compare a cost-effective approximation, using the Feynman-Hellmann theorem, to the full Hessian and find it works well. Diagonalizing the Hessian yields the principal components of the interaction: linear combinations of parameters ordered by sensitivity. This approximately decoupled distribution of parameters facilitates theoretical uncertainty propagation onto structure observables: electromagnetic transitions, Gamow-Teller decays, and dark matter-nucleus scattering matrix elements.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Core of 25 F in the rotational model

In a recent experiment, carried out at the Radioactive Isotope Beam Factory/RIKEN, the 25 F(p,2p) 24 O reaction was studied at 270 MeV/A in inverse kinematics. Derived spectroscopic factors suggest that the effective core of 25 F significantly differs from a free 24 O nucleus. We interpret these results within the particle-rotor model and show that the experimental level scheme of 25 F can be understood in the rotation-aligned coupling scheme with its 5/2$^{+}_{1}$ ground state as the bandhead of a decoupled band. Here, the excitation energies of the observed 1/2$^{+}_{1}$ and 9/2$^{+}_{1}$ states correlate strongly with the rotational energy of the effective core, seen by the odd proton, and allow us to estimate its 2 + energy at ≈3.2MeV and a moderate quadrupole deformation ϵ 2 ≈0.15. The measured fragmentation of the πd 5/2 single-particle strength is discussed, and some further experiments suggested.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗