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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Enhanced spin-triplet pairing in magnetic junctions with s -wave superconductors

A common path to superconducting spintronics, Majorana fermions, and topologically protected quantum computing relies on spin-triplet superconductivity. While naturally occurring spin-triplet pairing is elusive, and even common spin-triplet candidates, such as Sr 2 RuO 4 , support alternative explanations, proximity effects in heterostructures can overcome these limitations. It is expected that robust spin-triplet superconductivity in magnetic junctions should rely on highly spin-polarized magnets or complex magnetic multilayers. Instead, we predict that the interplay of interfacial spin-orbit coupling and the barrier strength in simple magnetic junctions, with only a small spin polarization and s -wave superconductors, can lead to nearly complete spin-triplet superconducting proximity effects. This peculiar behavior arises from an effective perfect transparency: interfacial spin-orbit coupling counteracts the native potential barrier for states of a given spin and wave vector. We demonstrate that the enhanced spin-triplet regime is characterized by a huge increase in conductance magnetoanisotropy, orders of magnitude larger than in the normal state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Spin-electron acoustic soliton and exchange interaction in separate spin evolution quantum plasmas

Separate spin evolution quantum hydrodynamics is generalized to include the Coulomb exchange interaction, which is considered as interaction between the spin-down electrons being in quantum states occupied by one electron. The generalized model is applied to study the non-linear spin-electron acoustic waves. Existence of the spin-electron acoustic soliton is demonstrated. Contributions of concentration, spin polarization, and exchange interaction to the properties of the spin electron acoustic soliton are studied.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Crossing resonance of wave fields in a medium with an inhomogeneous coupling parameter

The dynamic susceptibilities (Green’s functions) of the system of two coupled wave fields of different physical natures in a medium with an arbitrary relation between the mean value ɛ and rms fluctuation Δɛ of the coupling parameter have been examined. The self-consistent approximation involving all diagrams with noncrossing correlation lines has been developed for the case where the initial Green’s function of the homogeneous medium describes the system of coupled wave fields. The analysis has been performed for spin and elastic waves. Expressions have been obtained for the diagonal elements G{sub mm} and G{sub uu} of the matrix Green’s function, which describe spin and elastic waves in the case of magnetic and elastic excitations, and for the off-diagonal elements G{sub mu} and G{sub um}, which describe these waves in the case of cross excitation. Change in the forms of these elements has been numerically studied for the case of one-dimensional inhomogeneities with an increase in Δɛ and with a decrease in ɛ under the condition that the sum of the squares of these quantities is conserved: two peaks in the frequency dependences of imaginary parts of G{sub mm} and G{sub uu} are broadened and then joined into one broad peak; a fine structure appears in the form of narrow resonance at the vertex of the Green’s function of one wave field and narrow antiresonance at the vertex of the Green’s function of the other field; peaks of the fine structure are broadened and then disappear with an increase in the correlation wavenumber of the inhomogeneities of the coupling parameter; and the amplitudes of the off-diagonal elements vanish in the limit ɛ → 0.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Extracting off-diagonal order from diagonal basis measurements

Quantum gas microscopy has developed into a powerful tool to explore strongly correlated quantum systems. However, discerning phases with topological or off-diagonal long range order requires the ability to extract these correlations from site-resolved measurements. Here, we show that a multiscale complexity measure can pinpoint the transition to and from the bond ordered wave phase of the one-dimensional extended Hubbard model with an off-diagonal order parameter, sandwiched between diagonal charge and spin density wave phases, using only diagonal descriptors. We study the model directly in the thermodynamic limit using the recently developed variational uniform matrix product states algorithm, and draw our samples from degenerate ground states related by global spin rotations, emulating the projective measurements that are accessible in experiments. Our results will have important implications for the study of exotic phases using optical lattice experiments. Published by the American Physical Society 2024

1-dimensional systems↗
DOE OSTI2014

Nonlinear multiferroic phase shifters for microwave frequencies

A nonlinear microwave phase shifter based on a planar multiferroic composite has been studied. The multiferroic structure is fabricated in the form of a bilayer consisting of yttrium iron garnet and barium strontium titanate. The principle of operation of the device is based on the linear and nonlinear control of the phase shift of the hybrid spin-electromagnetic waves propagating in the bilayer. The linear control is realized with magnetic and electric fields. The nonlinear control is provided by the input power of microwave signal. The device showed a nonlinear phase shift up to 250°, electric field induced phase shift up to 330°, and magnetic field induced phase shift of more than 180°.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Symmetry-projected wave functions in quantum Monte Carlo calculations

Here, we consider symmetry-projected Hartree-Fock trial wave functions in constrained-path Monte Carlo (CPMC) calculations. Previous CPMC calculations have mostly employed Hartree-Fock (HF) trial wave functions, restricted or unrestricted. The symmetry-projected HF approach results in a hierarchy of wave functions with increasing quality: the more symmetries that are broken and restored in a self-consistent manner, the higher the quality of the trial wave function. This hierarchy is approximately maintained in CPMC calculations: the accuracy in the energy increases and the statistical variance decreases when further symmetries are broken and restored. Significant improvement is achieved in CPMC with the best symmetry-projected trial wave functions over those from simple HF. We analyze and quantify the behavior using the two-dimensional repulsive Hubbard model as an example. In the sign-problem-free region, where CPMC can be made exact but a constraint is deliberately imposed here, spin-projected wave functions remove the constraint bias. Away from half filling, spatial symmetry restoration in addition to that of the spin leads to highly accurate results from CPMC. Since the computational cost of symmetry-projected HF trial wave functions in CPMC can be made to scale algebraically with system size, this provides a potentially general approach for accurate calculations in many-fermion systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Probing Majorana Wave Functions in Kitaev Honeycomb Spin Liquids with Second-Order Two-Dimensional Spectroscopy

Two-dimensional coherent terahertz spectroscopy (2DCS) emerges as a valuable tool to probe the nature, couplings, and lifetimes of excitations in quantum materials. It thus promises to identify unique signatures of spin liquid states in quantum magnets by directly probing properties of their exotic fractionalized excitations. Here, in this study, we calculate the second-order 2DCS of the Kitaev honeycomb model and demonstrate that distinct spin liquid fingerprints appear already in this lowest-order nonlinear response $\chi^{(2)}_{yzx}$⁡($ω_1$, $ω_2$) when using crossed light polarizations. We further relate the off-diagonal 2DCS peaks to the localized nature of the matter Majorana excitations trapped by $\mathbb{Z}_2$ flux excitations and show that 2DCS thus directly probes the inverse participation ratio of Majorana wave functions. By providing experimentally observable features of spin liquid states in the 2D spectrum, our Letter can guide future 2DCS experiments on Kitaev magnets.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Atomic-scale visualization of a cascade of magnetic orders in the layered antiferromagnet GdTe3

Abstract GdTe 3 is a layered antiferromagnet which has attracted attention due to its exceptionally high mobility, distinctive unidirectional incommensurate charge density wave (CDW), superconductivity under pressure, and a cascade of magnetic transitions between 7 and 12 K, with as yet unknown order parameters. Here, we use spin-polarized scanning tunneling microscopy to directly image the charge and magnetic orders in GdTe 3 . Below 7 K, we find a striped antiferromagnetic phase with twice the periodicity of the Gd lattice and perpendicular to the CDW. As we heat the sample, we discover a spin density wave with the same periodicity as the CDW between 7 and 12 K; the viability of this phase is supported by our Landau free energy model. Our work reveals the order parameters of the magnetic phases in GdTe 3 and shows how the interplay between charge and spin can generate a cascade of magnetic orders.

36 MATERIALS SCIENCE↗
NASA NTRS2012

Effects of Accretion Disks on Spins and Eccentricities of Binaries, and Implications for Gravitational Waves

Effects of accretion disks on spins and eccentricities of binaries, and implications for gravitational waves. John Baker Space-based gravitational wave observations will allow exquisitely precise measurements of massive black hole binary properties. Through several recently suggested processes, these properties may depend on interactions with accretion disks through the merger process. I will discuss ways that accretion may influence those binary properties which may be probed by gravitational-wave observations.

Baker, John↗
DOE OSTI2014

Twofold Andreev reflections and coherent transport in ferromagnetic semiconductor/d-wave superconductor/ferromagnetic semiconductor double tunneling junctions

Coherent transport in a ferromagnetic semiconductor (FS)/d-wave superconductor (SC)/FS structure with (110) interfaces is studied by extending Bogoliubov-de Gennes equation into eight components, in which the interband coupling of heavy and light hole bands in the FS, the strengths of potential scattering at the interfaces, and the mismatches in the effective mass and Fermi vector between the FS and SC are taken into account. Twofold Andreev reflections exist due to the existence of two bands in the FS, in which the incident hole and the two Andreev-reflected electrons, belonging to the different spin subbands, form twofold spin-singlet pairing states near the FS/SC interface. It is shown that due to the interplay of the SC with unconventional d-wave pairing symmetry and FS, the differential conductance and tunneling magnetoresistance exhibit an abundant dependence on not only the interband coupling in the FS but also the strengths of potential scattering at the interfaces. More importantly, the properties are found to be quite different from those in the FS/s-wave SC/FS structure with conventional pairing symmetry for the SC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Singlet and triplet instability theorems

A useful definition of orbital degeneracy—form-degeneracy—is introduced, which is distinct from the usual energy-degeneracy: Two canonical spatial orbitals are form-degenerate when the energy expectation value in the restricted Hartree–Fock (RHF) wave function is unaltered upon a two-electron excitation from one of these orbitals to the other. Form-degenerate orbitals tend to have isomorphic electron densities and occur in the highest-occupied and lowest-unoccupied molecular orbitals (HOMOs and LUMOs) of strongly correlated systems. Here, we present a mathematical proof of the existence of a triplet instability in a real or complex RHF wave function of a finite system in the space of real or complex unrestricted Hartree–Fock wave functions when HOMO and LUMO are energy- or form-degenerate. We also show that a singlet instability always exists in a real RHF wave function of a finite system in the space of complex RHF wave functions, when HOMO and LUMO are form-degenerate, but have nonidentical electron densities, or are energy-degenerate. These theorems provide Hartree–Fock-theory-based explanations of Hund’s rule, a singlet instability in Jahn–Teller systems, biradicaloid electronic structures, and a triplet instability during some covalent bond breaking. They also suggest (but not guarantee) the spontaneous formation of a spin density wave (SDW) in a metallic solid. The stability theory underlying these theorems extended to a continuous orbital-energy spectrum proves the existence of an oscillating (nonspiral) SDW instability in one- and three-dimensional homogeneous electron gases, but only at low densities or for strong interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Quantum critical point revisited by dynamical mean-field theory

Dynamical mean-field theory is used to study the quantum critical point (QCP) in the doped Hubbard model on a square lattice. We characterize the QCP by a universal scaling form of the self-energy and a spin density wave instability at an incommensurate wave vector. The scaling form unifies the low-energy kink and the high-energy waterfall feature in the spectral function, while the spin dynamics includes both the critical incommensurate and high-energy antiferromagnetic paramagnons. Here, we use the frequency-dependent four-point correlation function of spin operators to calculate the momentum-dependent correction to the electron self-energy. Furthermore, by comparing with the calculations based on the spin-fermion model, our results indicate the frequency dependence of the quasiparticle-paramagnon vertices is an important factor to capture the momentum dependence in quasiparticle scattering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Competing magnetic states in multiferroic BaYFeO 4 : A high magnetic field study

Spin-driven ferroelectricity phenomena have drawn great interest in the scientific community due to potential application in spintronics and their complex physical mechanisms. A noticeable example of this is multiferroic BaYFeO 4 that exhibits an unconventional magnetoelectric (ME) coupling due to the uncorrelated behavior of the ferroelectric and cycloidal states under an applied magnetic field. To shed more light on this spin-driven ME effect, a high-quality sample of BaYFeO 4 was synthesized by a standard solid-state reaction method, and its high-field (up to 9 T) magnetic properties have been systematically investigated by means of magnetometry, magnetocaloric effect, and Mössbauer measurements over a wide temperature range (5–400 K). In addition, its crystal and magnetic structures have been studied using x-ray and neutron powder diffraction. Results obtained in this work indicate that Fe spins form a long-range spin density wave (SDW) antiferromagnetic (AFM) order at $T_{\text{N1}}$ ~ 50K, which transforms into the cycloidal AFM order at $T_{\text{N2}}$ ~ 35K. A spin-glass-like state emerges below $T^\ast$ ~ 17K, and coexists with the long-range cycloidal AFM one in this temperature range. Magnetocaloric and Mössbauer measurements consistently confirm the robustness of both the long-range SDWand cycloidal AFM orders under applied magnetic fields up to 6 T, whereas the spin-glass state is converted into the ferromagnetic (FM) state when the applied magnetic field exceeds 1 T. These findings pinpoint the fact that the magnetic field evolution of spin correlations from the AFM to FM character in the spin-glass state is responsible for the magnetic field dependence of ferroelectricity in BaYFeO 4 .

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evidence of a short-range incommensurate d-wave charge order from a fermionic two-loop renormalization group calculation of a 2D model with hot spots

The two-loop renormalization group (RG) calculation is considerably extended here for the two-dimensional (2D) fermionic effective field theory model, which includes only the so-called “hot spots” that are connected by the spin-density-wave (SDW) ordering wavevector on a Fermi surface generated by the 2D t−t{sup ′} Hubbard model at low hole doping. We compute the Callan–Symanzik RG equation up to two loops describing the flow of the single-particle Green’s function, the corresponding spectral function, the Fermi velocity, and some of the most important order-parameter susceptibilities in the model at lower energies. As a result, we establish that–in addition to clearly dominant SDW correlations–an approximate (pseudospin) symmetry relating a short-range incommensurated-wave charge order to the d-wave superconducting order indeed emerges at lower energy scales, which is in agreement with recent works available in the literature addressing the 2D spin-fermion model. We derive implications of this possible electronic phase in the ongoing attempt to describe the phenomenology of the pseudogap regime in underdoped cuprates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Impact of reduced dimensionality on the correlation length and magnetization dynamics of the spin chain cobaltite Ca 3 Co 2 O 6

We have recently applied the magnetic field and temperature dependence of magnetic entropy change as a probe of the magnetic states of a single crystal of Ca 3 Co 2 O 6 to establish a more comprehensive magnetic phase diagram for this exotic system. The results are consistent with the spin-density wave description of the ground state and indicate the suppression of the modulated state in favor of a ferrimagnetic up-up-down arrangement of the spin chains with the application of a moderate field. We demonstrate, in this study, that the features of the static magnetic phase diagram are preserved when the grain size of Ca 3 Co 2 O 6 is reduced to 440 nm (below the maximum c-axis correlation length of ~550 nm in Ca 3 Co 2 O 6 ), although the polycrystalline materials show a non-saturating behavior, rounded steps, and reduced magnetization when compared to the single crystal. Our new finding of the multiple low-temperature magnetization steps in the 440 nm sample, together with conflicting reports concerning such steps in nanostructured samples, suggests that correlations in the ab plane are more important than correlations along the c-axis in the appearance of the plateaus. The relaxation processes in the system are studied through the decay of remanent magnetization, ac susceptibility, and Argand diagrams. The introduction of grain boundaries in Ca 3 Co 2 O 6 weakens the slow-dynamic state, widens the low-temperature distribution of relaxation times, and lowers activation energies within the Arrhenius-like relaxation regime.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The cosine-sine decomposition with different-orbitals-for-different-spins determinants

We report the spin decomposition of a spin-contaminated different-orbitals-for-different-spins (DODS) wave function is formulated in terms of the Krylov space of the $\hat{S}^2$ ||operator. The number of determinants that contribute to the various projected spin eigenfunctions vertical | $\psi ; S, M \rangle$ depends on the sparsity of the orbital overlap matrix X between the α and β spatial orbitals. The cosine-sine decomposition (CSD) procedure may be applied to this overlap matrix, and the resulting redundant orbital transformations may be applied to the α and β spatial orbitals. This produces a sparse X' matrix in the transformed basis in which each row or column has either one or two nonzero elements. This sparse X' matrix simplifies the spin-decomposition procedure in three ways:1) it reduces the number of contributing determinants within the Krylov basis functions and within the projected spin functions, 2) it reduces the effective orbital dimension through elimination of the frozen core and frozen virtual orbitals, and 3) it simplifies the spin-decomposition procedure in both the original and transformed bases by limiting the Krylov space dimension. These simplifications all reduce the computational effort for the spin-decomposition process. This procedure is implemented within a string-based DODS determinant formulation and applied to spin projection of unrestricted Hartree-Fock determinants.

unrestricted Hartree-Fock↗
DOE OSTI2023

Fluctuating intertwined stripes in the strange metal regime of the Hubbard model

Strongly correlated electron systems host a variety of poorly understood correlations in their high-temperature normal state. Unlike ordered phases defined by order parameters, regions of the normal state are often defined through unconventional properties such as strange metallic transport or spectroscopic pseudogaps. Characterizing the microscopic correlations in the normal state is necessary to elucidate mechanisms that lead to these properties and their connection to ground-state orders. Here, in this paper, we establish the presence of intertwined charge and spin stripes in the strange metal normal state of the Hubbard model using determinant quantum Monte Carlo calculations. The charge and spin density waves constituting the stripes are fluctuating and short ranged; yet they obey a mutual commensurability relation and remain microscopically interlocked, as evidenced through measurements of three-point spin-spin-hole correlation functions. Our findings demonstrate the ability of many-body numerical simulations to unravel the microscopic correlations that define quantum states of matter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Spin trapping and flipping in FeCO through relativistic electron dynamics

Transition metal compounds are very versatile, and their characteristics can differ profoundly depending on their electronic structure. Compounds in which a spin transition from a low-spin to a high-spin state can be achieved through means of an optical excitation are particularly intriguing, as a controlled spin–flip opens promising avenues in areas such as sensing, information technology, molecular switches and energy technology. The fundamental mechanisms in spin crossover and spin transitions remain unanswered, due to the complexity of electronic structure and interplay of relativistic effects. Presented here is a new approach that allows the first direct study of spin flip dynamics through a mapping of spin-mixed to spin-pure states. The method is applied to FeCO and addresses the spin–flip dynamics during a spin transition. Finally, wave packets that combine different spin states are generated through optical excitation and relevant mechanisms in optically triggered spin transitions are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗