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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 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Finite field regime for a quantum spin liquid in α – RuCl 3

An external magnetic field can cause a transition in α – RuCl 3 from an ordered zigzag state to a disordered state that is possibly related to the Kitaev quantum spin liquid. Here, we present field-dependent inelastic neutron scattering and magnetocaloric effect measurements implying the existence of an additional transition out of the quantum spin-liquid phase at an upper field limit B u . The neutron scattering reveals three distinct regimes of magnetic response. In the low-field ordered state the response shows magnon peaks; the intermediate-field regime shows only continuum scattering, and above B u the response shows sharp magnon peaks at the lower bound of a strong continuum. Measurable dispersion of magnon modes along the (0,0,L) direction implies non-negligible interplane interactions. Combining the magnetocaloric effect measurements with other data, a T–B phase diagram is constructed. The results constrain the range where one might expect to observe quantum spin-liquid behavior in α – RuCl 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Local imaging of diamagnetism in proximity-coupled niobium nanoisland arrays on gold thin films.

In this work, we study the effect of engineered disorder on the local magnetic response of proximity-coupled superconducting island arrays by comparing scanning superconducting quantum interference device (SQUID) susceptibility measurements to a model in which we treat the system as a network of one-dimensional (1D) superconductor-normal-metal-superconductor Josephson junctions, each with a Josephson coupling energy E-J determined by the junction length or distance between islands. We find that the disordered arrays exhibit a spatially inhomogeneous diamagnetic response which, for low local applied magnetic fields, is well described by this junction network model, and we discuss these results as they relate to inhomogeneous 2D superconductors. Our model of the static magnetic response of the arrays does not fully capture the onset of nonlinearity and dissipation with increasing applied field, as these effects are associated with vortex motion due to the dynamic nature of the scanning SQUID susceptometry measurement. This work demonstrates a model 2D superconducting system with engineered disorder, and it highlights the impact of dissipation on the local magnetic properties of 2D superconductors and Josephson junction arrays.

Bishop-Van Horn, Logan↗
DOE OSTI2023

Improved quasiparticle self-consistent electronic band structure and excitons in β – LiGaO 2

The band structure of β–LiGaO 2 is calculated using the quasiparticle self-consistent QSGWˆ method where the screened Coulomb interaction Wˆ is evaluated including electron-hole interaction ladder diagrams and G is the one-electron Green's function. Improved convergence compared to previous calculations leads to a significantly larger band gap of about 7.0 eV. However, exciton binding energies are found to be large and lead to an exciton gap of about 6.0 eV if also a zero-point-motion correction of about –0.4 eV is included. Furthermore, these results are in excellent agreement with recent experimental results on the onset of absorption. Besides the excitons observed thus far, the calculations indicate the existence of a Rydberg-like series of exciton excited states, which is however modified from the classical Wannier exciton model by the anisotropies of the material and the more complex mixing of Bloch states in the excitons resulting from the Bethe-Salpeter equation. The exciton fine structure and the exciton wave functions are visualized and analyzed in various ways.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Vortex dynamics induced by scanning SQUID susceptometry

In this work, we measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a steplike nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Magnetotropic susceptibility

The magnetotropic susceptibility is the thermodynamic coefficient associated with the rotational anisotropy of the free energy in an external magnetic field and is closely related to the magnetic susceptibility. It emerges naturally in frequency-shift measurements of oscillating mechanical cantilevers, which are becoming an increasingly important tool in the quantitative study of the thermodynamics of modern condensed-matter systems. Here, in this work, we discuss the basic properties of the magnetotropic susceptibility as they relate to the experimental aspects of frequency-shift measurements, as well as to the interpretation of those experiments in terms of the intrinsic properties of the system under study.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Experimental evidence for nonspherical magnetic form factor in Ru 3 +

The Mott insulator ..alpha..-RuCl3 has generated great interest in the community due to its possible field-induced Kitaev quantum spin liquid state. Despite enormous effort spent trying to obtain the form of the low-energy Hamiltonian, there is currently no agreed upon set of parameters which is able to explain all of the data. A key piece of missing information lies in the determination of the magnetic form factor of Ru3+, particularly for a true quantitative treatment of inelastic neutron scattering data. Here we present the experimentally derived magnetic form factor of Ru3+ in the low spin 4d5 state using polarized neutron diffraction within the paramagnetic regime on high-quality single crystals of ..alpha..-RuCl3. We observe strong evidence of an anisotropic form factor, expected of the spin-orbit coupled jeff=12 ground state. We model the static magnetization density in increasing complexity from simple isotropic cases, to a multipolar expansion, and, finally, ab initio calculations of the generalized jeff=12 ground state. Comparison of both single ion models and inclusion of Cl- anions support the presence of hybridization of Ru3+ with the surrounding Cl- ligands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Irradiation damage in Gd{sub 2}Ti{sub 2}O{sub 7} single crystals: Ballistic versus ionization processes

The structural transformations induced in Gd{sub 2}Ti{sub 2}O{sub 7} single crystals irradiated at high energies (870-MeV Xe), where ionization processes (electronic stopping) dominate, and at low energies (4-MeV Au), where ballistic processes (nuclear stopping) dominate, have been studied via the combination of Rutherford backscattering spectrometry and channeling (RBS/C), Raman spectroscopy, and transmission electron microscopy (TEM) experiments. At high energy, amorphization occurs directly in individual ion tracks from the extreme electronic-energy deposition, and full amorphization results from the overlapping of these tracks as described by a direct impact model. The track diameters lie in the range 6-9 nm. At low energy, amorphization occurs via indirect processes, driven by ballistic nuclear energy deposition from the ions, that is accounted for in the framework of both direct-impact/defect-stimulated and multi-step damage accumulation models. The ion fluence for total amorphization of the irradiated layer is much higher at low energy (0.5 ion nm{sup -2}) than at high energy (0.05 ion nm{sup -2}), consistent with the nuclear stopping at low energy (5.2 keV/nm) compared to the electronic stopping at high energy (29 keV/nm).

36 MATERIALS SCIENCE↗
DOE OSTI2011

Diffusive-to-ballistic transition in grain boundary motion studied by atomistic simulations

An adapted simulation method is used to systematically study grain boundary motion at velocities and driving forces across more than five orders of magnitude. This analysis reveals that grain boundary migration can occur in two modes, depending upon the temperature ( T ) and applied driving force ( P ). At low P and T , grain boundary motion is diffusional, exhibiting the kinetics of a thermally activated system controlled by grain boundary self-diffusion. At high P and T , grain boundary migration exhibits the characteristic kinetic scaling behavior of a ballistic process. A rather broad transition range in both P and T lies between the regimes of diffusive and ballistic grain boundary motion, and is charted here in detail. In conclusion, the recognition and delineation of these two distinct modes of grain boundary migration also leads to the suggestion that many prior atomistic simulations might have probed a different kinetic regime of grain boundary motion (ballistic) as compared to that revealed in most experimental studies (diffusional).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Pressure-induced transformations in amorphous Si-Ge alloy

The pressure behavior of an amorphous Si-rich SiGe alloy ({alpha}-Si{sub x}Ge{sub 1-x}, x = 0.75) has been investigated up to about 30 GPa, by a combination of Raman spectroscopy, x-ray absorption spectroscopy, and x-ray diffraction measurements. The trends of microscopic structural properties and of the Raman-active phonon modes are presented in the whole pressure range. Nucleation of nanocrystalline alloy particles and metallization have been observed above 12 GPa, with a range of about 2 GPa of coexistence of amorphous and crystalline phases. Transformations from the amorphous tetrahedral, to the crystalline tetragonal ({beta}-Sn) and to the simple hexagonal structures have been observed around 13.8 and 21.8 GPa. The recovered sample upon depressurization, below about 4 GPa, shows a local structure similar to the as-deposited one. Inhomogeneities of the amorphous texture at the nanometric scale, probed by high-resolution transmission electron microscopy, indicate that the recovered amorphous sample has a different ordering at this scale, and therefore the transformations can not be considered fully reversible. The role of disordered grain boundaries at high pressure and the possible presence of a high-density amorphous phase are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Distinct oxygen hole doping in different layers of Sr₂CuO 4-δ /La₂CuO₄ superlattices

X-ray absorption in Sr₂CuO 4-δ /La₂CuO₄ (SCO/LCO) superlattices shows a variable occupation with doping of a hole state different from holes doped for x≲x optimal in bulk La 2-x Sr x CuO₄ and suggests that this hole state is on apical oxygen atoms and polarized in the a-b plane. Considering the surface reflectivity gives a good qualitative description of the line shapes of resonant soft x-ray scattering. The interference between superlattice and surface reflections was used to distinguish between scatterers in the SCO and the LCO layers, with the two hole states maximized in different layers of the superlattice.

Smadici, S.↗
DOE OSTI2012

Probing the connections between superconductivity, stripe order, and structure in La₁.₉₀₅Ba₀.₀₉₅Cu 1-y Zn y O₄

The superconducting system La 2-x Ba x CuO₄ is known to show a minimum in the transition temperature T c at x=1/8 where maximal stripe order is pinned by the anisotropy within the CuO₂ planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For x=0.095, where T c reaches its maximum value of 32 K, there is a roughly coincident structural transition to a phase that is very close to LTT. Here, we present a neutron scattering study of the structural transition, and demonstrate how features of it correlate with anomalies in the magnetic susceptibility, electrical resistivity, thermal conductivity, and thermoelectric power. We also present measurements on a crystal with 1% Zn substituted for Cu, which reduces T c to 17 K, enhances the spin stripe order, but has much less effect on the structural transition. We make the case that the structural transition correlates with a reduction of the Josephson coupling between the CuO₂ layers, which interrupts the growth of the superconducting order. We also discuss evidence for two-dimensional superconducting fluctuations in the normal state, analyze the effective magnetic moment per Zn impurity, and consider the significance of the anomalous thermopower often reported in the stripe-ordered phase.

Wen, Jinsheng↗
DOE OSTI2012

Anomalous grain growth in the surface region of a nanocrystalline CeO2 film under low-temperature heavy ion irradiation

Grain growth and phase stability of nanocrystalline ceria are investigated under ion irradiation at different temperatures. Irradiations at temperatures of 300 and 400 K result in uniform grain growth throughout the film. Anomalous grain growth is observed in thin films of nanocrystalline ceria under 3 MeV Au+ irradiation at 160 K. At this low temperature, significant grain growth is observed within 100 nm from the surface, no obvious growth is detected in the rest of the films. While the grain growth is attributed to a defect-stimulated mechanism at room temperature and above, a defect diffusion-limited mechanism is significant at low temperature with the primary defect responsible being the oxygen vacancy. The nanocrystalline grains remain in the cubic phase regardless of defect kinetics.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2012

Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO₁₋ x F x and Sr(Fe₁₋ x Co x )₂As₂

A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO₁₋ x F x (x = 0, 0.11) and Sr(Fe₁₋ x Co x )2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10⁻¹⁵ s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3μ B in CeFeAsO and 2.1μB in SrFe₂As₂. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9μ B in CeFeAsO₀.₈₉F₀.₁₁ and 1.3μ B in Sr(Fe₀.₉Co₀.₁)₂As₂. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.

Vilmercati, Paolo↗
DOE OSTI2012

Ab Initio Molecular Dynamics Simulations of Low-Energy Recoil Events in ThO2, CeO2, and ZrO2

Ab initio molecular dynamics simulations of low-energy recoil events in ThO2, CeO2, and ZrO2 have been carried out to determine the threshold displacement energies, resulting defect configurations, dynamics of defect generation, and role of charge transfer during the process. The results reveal that, in most cases, these fluorite structure oxides exhibit a similar response to low-energy recoils. A variety of different defect configurations are created, consisting mainly of vacancies and interstitials. Charge transfer occurs during the dynamic displacement process. Local charge redistribution leads to cation and O vacancies being negatively and positively charged, respectively. Likewise, due to charge redistribution, the cation and O interstitials are less positively and negatively charged, respectively, than the ions on lattice sites in perfect MO2.

Environmental Molecular Sciences Laboratory↗