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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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Competition between Kondo effect and RKKY physics in graphene magnetism

The cooperative behavior of quantum impurities on two-dimensional (2D) materials, such as graphene and bilayer graphene, is characterized by a nontrivial competition between screening (Kondo effect) and Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetism. In addition, due to the small density of states at the Fermi level, impurities may not couple to the conduction electrons at all, behaving as free moments. Employing a recently developed exact numerical method to study multi-impurity lattice systems, we obtain nonperturbative results that dramatically depart from expectations based on the conventional RKKY theory. At half filling and for weak coupling, impurities remain in the local moment regime when they are on opposite sublattices, up to a critical value of the interactions when they start coupling antiferromagnetically with correlations that decay very slowly with interimpurity distance. At finite doping, away from half filling, ferromagnetism is completely absent and the physics is dominated by a competition between antiferromagnetism and Kondo effect. In bilayer graphene, impurities on opposite layers behave as free moments, unless the interaction is of the order of the hopping or larger.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Revisiting the origin of satellites in core-level photoemission of transparent conducting oxides: The case of $\mathcal{n}$-doped SnO 2

In this work, the longstanding problem of interpretation of satellite structures in core-level photoemission spectra of metallic systems with a low density of conduction electrons is addressed using the specific example of Sb-doped SnO 2 . Comparison of ab initio many-body calculations with experimental hard x-ray photoemission spectra of the Sn 4d states shows that strong satellites are produced by coupling of the Sn core hole to the plasma oscillations of the free electrons introduced by doping. Within the same theoretical framework, spectral changes of the valence band spectra are also related to dynamical screening effects. These results demonstrate that, for the interpretation of electron correlation features in the core-level photoelectron spectra of such narrow-band materials, going beyond the homogeneous electron gas electron-plasmon coupling model is essential.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anomalous Hall effect in the van der Waals bonded ferromagnet Fe 3 - x GeTe 2

Here, we report the anomalous Hall effect (AHE) in single crystals of a quasi-two-dimensional Fe 3–x GeTe 2 (x ≈ 0.36) ferromagnet grown by the flux method which induces defects on the Fe site and bad metallic resistivity. Fe K-edge x-ray absorption spectroscopy was measured to provide information on the local atomic environment in such crystals. The dc and ac magnetic susceptibility measurements indicate a second-stage transition below 119 K in addition to the paramagnetic to ferromagnetic transition at 153 K. A linear scaling behavior between the modified anomalous Hall resistivity ρxy/μ0Heff and longitudinal resistivity ρ 2 xx M/μ 0 H eff implies that the AHE in Fe 3–x GeTe 2 should be dominated by the intrinsic Karplus-Luttinger mechanism rather than the extrinsic skew-scattering and side-jump mechanisms. The observed deviation in the linear- M Hall conductivity σ A xy below 30 K is in line with its transport characteristic at low temperatures, implying the scattering of conduction electrons due to magnetic disorder and the evolution of the Fermi surface induced by a possible spin-reorientation transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Photonic spin Hall effect in bilayer graphene moiré superlattices

The formation of a superstructure—with a related moiré pattern—plays a crucial role in the extraordinary optical and electronic properties of twisted bilayer graphene, including the recently observed unconventional superconductivity. Here we present an interdisciplinary approach to determine the moiré angle in twisted bilayer graphene based on the photonic spin Hall effect. Here, we show that the photonic spin Hall effect exhibits clear fingerprints of the underlying moiré pattern, and the associated light beam shifts are well beyond current experimental sensitivities in the near-infrared and visible ranges. By discovering the dependence of the frequency position of the maximal photonic spin Hall effect shift on the moiré angle, we argue that the latter could be unequivocally accessed via all-optical far-field measurements. We also disclose that, when combined with the Goos-Hänchen effect, the spin Hall effect of light enables the complete determination of the electronic conductivity of the bilayer. Altogether our findings demonstrate that subwavelength spin-orbit interactions of light provide an unprecedented toolset for investigating optoelectronic properties of multilayer two-dimensional van der Waals materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nontrivial topology and localization in the double exchange model with possible applications to perovskite manganites

The double exchange model describing the coupling between conduction electrons and localized magnetic moments is relevant for a large family of physical systems including manganites. Here we reveal that the one-dimensional double exchange model with an incommensurate magnetic elliptical spiral is a topological insulator with a Chern number 2Z in the two-dimensional space with one physical dimension and one ancillary dimension spanned by the Goldstone mode of the spiral. Moreover, the electronic states can be localized for a strong local exchange coupling. The topological protected edge states are responsible for the pumping of electron charge, and give rise to multiferroic response. Furthermore, our work uncovers hitherto undiscovered nontrivial topology and Anderson localization in the double exchange model with possible applications to perovskite manganites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Orientation of the ground-state orbital in CeCoIn 5 and CeRhIn 5

We present core level nonresonant inelastic x-ray scattering (NIXS) data of the heavy-fermion compounds CeCoIn 5 and CeRhIn 5 measured at the Ce N 4,5 edges. The higher than dipole transitions in NIXS allow determining the orientation of the Γ 7 crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the Ce M In 5 compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wave function is the Γ + 7 (x 2 – y 2 ) or Γ – 7 ( xy orientation) remained undetermined. We show that the Γ – 7 doublet with lobes along the (110) direction forms the ground state in CeCoIn 5 and CeRhIn 5 . For CeCoIn 5 , however, we find also some contribution of the first excited state crystal-field state in the ground state due to the stronger hybridization of 4 f and conduction electrons, suggesting a smaller α 2 value than originally anticipated from x-ray absorption. In conclusion, a comparison is made to the results of existing density functional theory plus dynamical mean-field theory calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Universality in Higher Order Spin Noise Spectroscopy

Higher order time correlators of spin fluctuations reveal considerable information about spin interactions. In this paper, we argue that in a broad class of spin systems, one can justify a phenomenological approach to explore such correlators. We predict that the third and fourth order spin cumulants are described by a universal function that can be parametrized by a small set of parameters. We show that the fluctuation theorem constrains this function so that such correlators are fully determined by lowest nonlinear corrections to the free energy and the mean and variance of microscopic spin currents. We also provide an example of microscopic calculations for conduction electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Resistivity Minimum in Highly Frustrated Itinerant Magnets

We study here the transport properties of frustrated itinerant magnets comprising localized classical moments, which interact via exchange with the conduction electrons. Strong frustration stabilizes a liquidlike spin state, which extends down to temperatures well below the effective Ruderman-Kittel-Kasuya-Yosida interaction scale. The crossover into this state is characterized by spin structure factor enhancement at wave vectors smaller than twice the Fermi wave vector magnitude. Finally, the corresponding enhancement of electron scattering generates a resistivity upturn at decreasing temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Three-Dimensional Non-Fermi-Liquid Behavior from One-Dimensional Quantum Critical Local Moments

We study the temperature dependence of the electrical resistivity in a system composed of critical spin chains interacting with three dimensional conduction electrons and driven to criticality via an external magnetic field. The relevant experimental system is Yb 2 Pt 2 Pb, a metal where itinerant electrons coexist with localized moments of Yb-ions which can be described in terms of effective S = 1/2 spins with dominantly one-dimensional exchange interaction. The spin subsystem becomes critical in a relatively weak magnetic field, where it behaves like a Luttinger liquid. We theoretically examine a Kondo lattice with different effective space dimensionalities of the two interacting subsystems. Lastly, we characterize the corresponding non-Fermi liquid behavior due to the spin criticality by calculating the electronic relaxation rate and the dc resistivity and establish its quasi linear temperature dependence.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Giant Anisotropic Magnetoresistance due to Purely Orbital Rearrangement in the Quadrupolar Heavy Fermion Superconductor PrV 2 Al 20

In this report, we discuss the discovery of giant and anisotropic magnetoresistance due to the orbital rearrangement in a non magnetic correlated metal. In particular, we measured the magnetoresistance under fields up to 31.4 T in the cubic Pr-based heavy fermion superconductor PrV 2 Al 20 with a non magnetic Γ 3 doublet ground state, exhibiting antiferro quadrupole ordering below 0.7 K. For the [100] direction, we find that the high field phase appears between 12 and 25 T, accompanied by a large jump at 12 T in the magnetoresistance (ΔMR ~ 100%) and in the anisotropic magnetoresistivity ratio by ~20%. These observations suggest that the strong hybridization between the conduction electrons and anisotropic quadrupole moments leads to the Fermi surface reconstruction upon crossing the field-induced antiferroquadrupole (orbital) rearrangement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Symmetric Kondo Lattice States in Doped Strained Twisted Bilayer Graphene

We use the topological heavy fermion (THF) model and its Kondo lattice (KL) formulation to study the possibility of a symmetric Kondo (SK) state in twisted bilayer graphene. Via a large-N approximation, we find a SK state in the KL model at fillings v = 0,±1,±2 where a KL model can be constructed. In the SK state, all symmetries are preserved and the local moments are Kondo screened by the conduction electrons. At the mean-field level of the THF model at v = 0,±1,±2,±3 we also find a similar symmetric state that is adiabatically connected to the symmetric Kondo state. We study the stability of the symmetric state by comparing its energy with the ordered (symmetry-breaking) states found in [H. Hu et al., Phys. Rev. Lett. 131, 026502 (2023)., Z.-D. Song and B. A. Bernevig, Phys. Rev. Lett. 129, 047601 (2022).] and find the ordered states to have lower energy at v = 0,±1,±2. However, moving away from integer fillings by doping the light bands, our mean-field calculations find the energy difference between the ordered state and the symmetric state to be reduced, which suggests the loss of ordering and a tendency toward Kondo screening. In order to include many-body effects beyond the mean-field approximation, we also performed dynamical mean-field theory calculations on the THF model in the nonordered phase. The spin susceptibility follows a Curie behavior at v = 0,±1,±2 down to ~2 K where the onset of screening of the local moment becomes visible. This hints to very low Kondo temperatures at these fillings, in agreement with the outcome of our mean-field calculations. At noninteger filling v = ±0.5,±0.8,±1.2 dynamical mean-field theory shows deviations from a 1/T susceptibility at much higher temperatures, suggesting a more effective screening of local moments with doping. Lastly, we study the effect of a C 3⁢z -rotational-symmetry-breaking strain via mean-field approaches and find that a symmetric phase (that only breaks C 3⁢z symmetry) can be stabilized at sufficiently large strain at v = 0,±1,±2. Our results suggest that a symmetric Kondo phase is strongly suppressed at integer fillings, but could be stabilized either at noninteger fillings or by applying strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Charge-spin interconversion in epitaxial Pt probed by spin-orbit torques in a magnetic insulator

We measure spin-orbit torques (SOTs) in a model system of all-epitaxial ferrite/Pt bilayers to gain insights into charge-spin interconversion in Pt. With negligible electronic conduction in the insulating ferrite, the crystalline Pt film acts as the sole source of charge-to-spin conversion. A small fieldlike SOT independent of Pt thickness suggests a weak Rashba-Edelstein effect at the ferrite/Pt interface. By contrast, we observe a sizable dampinglike SOT that depends on the Pt thickness, from which we deduce the dominance of an extrinsic spin-Hall effect (skew scattering) and Dyakonov-Perel spin relaxation in the crystalline Pt film. Furthermore, our results point to a large internal spin-Hall ratio of ≈ 0.8 in epitaxial Pt. Furthermore, our experimental work takes an essential step towards understanding the mechanisms of charge-spin interconversion and SOTs in Pt-based heterostructures, which are crucial for power-efficient spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Correlated anharmonicity and dynamic disorder control carrier transport in halide perovskites

Halide pervoskites are an important class of semiconducting materials that hold great promise for optoelectronic applications. In this work we investigate the relationship between vibrational anharmonicity and dynamic disorder in this class of solids. Via a multiscale model parametrized from first-principles calculations, we demonstrate that the non-Gaussian lattice motion in halide perovskites is microscopically connected to the dynamic disorder of overlap fluctuations among electronic states. This connection allows us to rationalize the emergent differences in temperature-dependent mobilities of prototypical MAPbI 3 and MAPbBr 3 compounds across structural phase transitions, in agreement with experimental findings. In conclusion, our analysis suggests that the details of vibrational anharmonicity and dynamic disorder can complement known predictors of electronic conductivity and can provide structure-property guidelines for the tuning of carrier transport characteristics in anharmonic semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Collapsing of spin-split Fermi surfaces in the magnetic Weyl semimetal candidate EuAuSb

EuAuSb is a magnetic Weyl semimetal candidate that exhibits in-plane ferromagnetic ordering and a helical magnetic order across the plane at temperatures below 3.9 K. These magnetic characteristics can drive an interaction-induced reconstruction of the Fermi surface. Here, we report a rare instance of Fermi surface splitting and collapsing driven by exchange coupling, as revealed by strongly temperature-dependent quantum oscillation frequencies in EuAuSb. Shubnikov–de Haas measurements reveal two distinct quantum oscillation frequencies (𝐹 𝛼 = 80 T and 𝐹 𝛽 = 165 T) at 2 K. As the temperature increases toward 40 K, the oscillation peaks at these two frequencies gradually merge into a single peak centered around 128 T. This behavior signifies the collapse of the spin-split Fermi surfaces, driven by the weakening of short-range magnetic correlations and the exchange coupling between conduction electrons and local magnetic moments as temperature increases. Furthermore, our results are further supported by density functional theory calculations, angle-resolved photoemission spectroscopy, and magnetization measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Unconventional magnetotransport and antiferromagnetic order in Eu 2 ⁢InTe 5

Here, we report on the structural characterization and physical properties of Eu 2 ⁢InTe 5 , a distorted square-net motif antiferromagnetic semiconductor. Single crystal x-ray diffraction measurements reveal a distortion of the Te square-net layers such that an orthorhombic supercell is necessary to accurately describe the structure. Magnetization, resistivity, and specific heat measurements confirm antiferromagnetic ordering at 𝑇 𝑁 = 7.3K. Anisotropy in magnetization data suggests the magnetic moments are oriented parallel to the 𝑐 axis. We observe an unconventional transverse magnetoresistance and Hall resistivity in the paramagnetic state (𝑇 ≤ 80K), indicating strong coupling between localized Eu 2+ magnetic moments and conduction electrons. Resistivity and Hall measurements indicate the material is a heavily doped semiconductor, with impurity states related to disorder. Our findings highlight a relatively unexplored Eu-based antiferromagnetic semiconductor with unconventional magnetotransport behavior.

Cook, Matthew S. [Oak Ridge National Laboratory (O↗
DOE OSTI2015

Atomic layer deposited lithium aluminum oxide: (In)dependency of film properties from pulsing sequence

Atomic layer deposition (ALD) holds markedly high potential of becoming the enabling method for achieving the three-dimensional all-solid-state thin-film lithium ion battery (LiB). One of the most crucial components in such a battery is the electrolyte that needs to hold both low electronic conductivity and at least fair lithium ion conductivity being at the same time pinhole free. To obtain these desired properties in an electrolyte film, one necessarily has to have a good control over the elemental composition of the deposited material. The present study reports on the properties of ALD lithium aluminum oxide (Li{sub x}Al{sub y}O{sub z}) thin films. In addition to LiB electrolyte applications, Li{sub x}Al{sub y}O{sub z} is also a candidate low dielectric constant (low-k) etch stop and diffusion barrier material in nanoelectronics applications. The Li{sub x}Al{sub y}O{sub z} films were deposited employing trimethylaluminum-O{sub 3} and lithium tert-butoxide-H{sub 2}O for Al{sub 2}O{sub 3} and Li{sub 2}O/LiOH, respectively. The composition was aimed to be controlled by varying the pulsing ratio of those two binary oxide ALD cycles. The films were characterized by several methods for composition, crystallinity and phase, electrical properties, hardness, porosity, and chemical environment. Regardless of the applied pulsing ratio of Al{sub 2}O{sub 3} and Li{sub 2}O/LiOH, all the studied ALD Li{sub x}Al{sub y}O{sub z} films of 200 and 400 nm in thickness were polycrystalline in the orthorhombic β-LiAlO{sub 2} phase and also very similar to each other with respect to composition and other studied properties. The results are discussed in the context of both fundamental ALD chemistry and applicability of the films as thin-film LiB electrolytes and low-k etch stop and diffusion barriers.

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

Ultrafast magnetization reversal by picosecond electrical pulses

The field of spintronics involves the study of both spin and charge transport in solid-state devices. Ultrafast magnetism involves the use of femtosecond laser pulses to manipulate magnetic order on subpicosecond time scales. Here, we unite these phenomena by using picosecond charge current pulses to rapidly excite conduction electrons in magnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub–10-ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than any other electrically controlled magnetic switching, and demonstrates a fundamentally new electrical switching mechanism that does not require spin-polarized currents or spin-transfer/orbit torques. The energy density required for switching is low, projecting to only 4 fJ needed to switch a (20 nm) 3 cell. This discovery introduces a new field of research into ultrafast charge current–driven spintronic phenomena and devices.

36 MATERIALS SCIENCE↗