SEARCH · Search NASA

Results for “Layer Hall effect”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9

All sources NASA NTRS DOE OSTI

DOE OSTI2024

Intrinsic exchange biased anomalous Hall effect in an uncompensated antiferromagnet MnBi 2 Te 4

Achieving spin-pinning at the interface of hetero-bilayer ferromagnet/antiferromagnet structures in conventional exchange bias systems can be challenging due to difficulties in interface control and the weakening of spin-pinning caused by poor interface quality. In this work, we propose an alternative approach to stabilize the exchange interaction at the interface of an uncompensated antiferromagnet by utilizing a gradient of interlayer exchange coupling. We demonstrate this exchange interaction through a designed field training protocol in the odd-layer topological antiferromagnet MnBi 2 Te 4 . Our results reveal a remarkable field-trained exchange bias of up to ~ 400 mT, which exhibits high repeatability and can be easily reset by a large training field. Notably, this field-trained exchange bias effect persists even with zero-field initialization, presenting a stark contrast to the traditional field-cooled exchange bias. The highly tunable exchange bias observed in this single antiferromagnet compound, without the need for an additional magnetic layer, provides valuable insight into the exchange interaction mechanism. These findings pave the way for the systematic design of topological antiferromagnetic spintronics.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Electronic transport properties of a lithium-decorated ZrTe 5 thin film

Through a combination of single crystal growth, experiments involving in situ deposition of surface adatoms, and complimentary modeling, we examine the electronic transport properties of lithiumdecorated ZrTe 5 thin films. We observe that the surface states in ZrTe 5 are robust against Li adsorption. Both the surface electron density and the associated Berry phase are remarkably robust to adsorption of Li atoms. Fitting to the Hall conductivity data reveals that there exist two types of bulk carriers: those for which the carrier density is insensitive to Li adsorption, and those whose density decreases during initial Li depositions and then saturates with further Li adsorption. We propose this dependence is due to the gating effect of a Li-adsorption-generated dipole layer at the ZrTe 5 surface.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Imaging the Breakdown and Restoration of Topological Protection in Magnetic Topological Insulator MnBi 2 Te 4

Quantum anomalous Hall (QAH) insulators transport charge without resistance along topologically protected chiral 1D edge states. Yet, in magnetic topological insulators to date, topological protection is far from robust, with zero-magnetic field QAH effect only realized at temperatures an order of magnitude below the Néel temperature T N , though small magnetic fields can stabilize QAH effect. Understanding why topological protection breaks down is therefore essential to realizing QAH effect at higher temperatures. Here a scanning tunneling microscope is used to directly map the size of exchange gap (E g,ex ) and its spatial fluctuation in the QAH insulator 5-layer MnBi 2 Te 4 . Long-range fluctuations of E g,ex are observed, with values ranging between 0 (gapless) and 70 meV, appearing to be uncorrelated to individual surface point defects. The breakdown of topological protection is directly imaged, showing that the gapless edge state, the hallmark signature of a QAH insulator, hybridizes with extended gapless regions in the bulk. Finally, it is unambiguously demonstrated that the gapless regions originate from magnetic disorder, by demonstrating that a small magnetic field restores E g,ex in these regions, explaining the recovery of topological protection in magnetic fields. The results indicate that overcoming magnetic disorder is the key to exploiting the unique properties of QAH insulators.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Janus skyrmion: Interfacial quasiparticle with two-faced helicity

Janus particles are functional particles with at least two surfaces showing asymmetric properties. Here, we show at the interface between two magnetic regions with different antisymmetric exchange interactions, an alternative species of two-dimensional topological quasiparticles can emerge, in which different helicity structures can coexist. We name such an interfacial quasiparticle a “Janus skyrmion,” in analogy to the Janus particle. As the Janus skyrmion shows helicity asymmetry, its size could vary with both the in-plane and out-of-plane magnetic fields. A vertical spin current could drive the Janus skyrmion into one-dimensional motion along the interface without showing the skyrmion Hall effect, at a speed which depends on both the in-plane spin-polarization direction and current density. Thermal fluctuations could also lead to one-dimensional random walk of a Brownian Janus skyrmion. This work uncovers unique dynamics intrinsic to interfacial quasiparticles with exotic helicity, which may be realized in interface-engineered magnetic layers.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Plumbene: a comprehensive review of theoretical and experimental advances

Plumbene, a two-dimensional (2D) material consisting of a single layer of lead (Pb) atoms in a honeycomb lattice, stands as a pioneering addition to the elemental 2D material family. Here, this comprehensive review encapsulates the theoretical predictions and experimental advancements that underscore its potential. With properties shaped by significant spin-orbit coupling and structural buckling, plumbene offers a playground for phenomena like the quantum spin Hall effect, superconductivity, and topological phase transitions. Despite promising theoretical insights, experimental realization has encountered challenges, primarily due to high reactivity of Pb and the need for precise fabrication conditions. This paper critically examines the electronic, thermal, mechanical, and topological attributes of plumbene, alongside synthesis methods, functionalization techniques, and substrate effects. It further proposes directions for addressing synthesis challenges and integrating plumbene into advanced technologies, particularly in electronic, spintronic, and quantum computing applications. This review aims to bridge the gap between theoretical potential and experimental progress, fostering a deeper understanding of this intriguing material.

2D materials↗

NASA NTRS1984

On the stability of an infinite swept attachment line boundary layer

The instability of an infinite swept attachment line boundary layer is considered in the linear regime. The basic three dimensional flow is shown to be susceptible to travelling wave disturbances which propagate along the attachment line. The effect of suction on the instability is discussed and the results suggest that the attachment line boundary layer on a swept wing can be significantly stabilized by extremely small amounts of suction. The results obtained are in excellent agreement with the available experimental observations.

Hall, P.↗

NASA NTRS1984

On the stability of an infinite swept attachment line boundary layer

The instability of an infinite swept attachment line boundary layer is considered in the linear regime. The basic three dimensional flow is shown to be susceptible to travelling wave disturbances which propagate along the attachment line. The effect of suction on the instability is discussed and the results suggest that the attachment in boundary layer on a swept wing can be significantly stabilized by extremely small amounts of suction. The results obtained are in excellent agreement with the available experimental observations.

Hall, P.↗

DOE OSTI2021

Spin wave interference detection via inverse spin Hall effect

Here, in this Letter, we present experimental data that demonstrate a spin wave interference detection using an inverse spin Hall effect (ISHE). Two coherent spin waves are excited in a yttrium-iron garnet waveguide by continuous microwave signals. The initial phase difference between the spin waves is controlled by the external phase shifter. The ISHE voltage is detected at a distance of 2 and 4 mm away from the spin wave generating antennas by an attached Pt layer. Experimental data show ISHE voltage oscillation as a function of the phase difference between the two interfering spin waves. This experiment demonstrates an intriguing possibility of using ISHE in a spin wave logic circuit converting a spin wave phase into an electric signal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

Anomalous Hall Crystals in Rhombohedral Multilayer Graphene. I. Interaction-Driven Chern Bands and Fractional Quantum Hall States at Zero Magnetic Field

Recent experiments on rhombohedral pentalayer graphene with a substrate-induced moiré potential have identified both Chern insulators and fractional quantum Hall states at zero magnetic field. Surprisingly, these states are observed in strong displacement fields where the effects of the moiré lattice are weak, and seem to be readily accessed without fine-tuning. To address these experimental puzzles, we study a model of interacting electrons in this geometry. Within self-consistent Hartree-Fock (SCHF) calculations, we find an isolated Chern band with small bandwidth and good quantum geometry. Exact diagonalization and density-matrix renormalization group calculations both confirm the band hosts fractional quantum Hall states without a magnetic field. Remarkably, the Chern band is stable at a wide range of angles, at four through six rhombohedral layers, at varying rhombohedral hopping parameters, and—most strikingly—survives in SCHF calculations when the moiré potential vanishes. In this limit, the state spontaneously breaks time-reversal and translation symmetry simultaneously, giving a topological crystalline state that we term the “anomalous Hall crystal.” We argue this is a general mechanism to create stable Chern bands in rhombohedral multilayer graphene, opening the door to studying the interplay between electronic topology, fractionalization, and spontaneous translation symmetry breaking.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Resistance of Boron Nitride Nanotubes to Radiation-Induced Oxidation

We present unprecedented results on the damage thresholds and pathways for boron nitride nanotubes (BNNT) under the influence of energetic electrons in an oxidative gas environment, using an environmental aberration-corrected electron microscope over a range of oxygen pressures. We observe a damage cascade process that resists damage until a higher electron dose, compared with carbon nanotubes, initiating at defect-free BNNT sidewalls and proceeding through the conversion from crystalline nanotubes to amorphous boron nitride (BN), resisting oxidation throughout. We compare with prior results on the oxidation of carbon nanotubes and present a model that attributes the onset of damage in both cases to a physisorbed oxygen layer that reduces the threshold for damage onset. Surprisingly, increased temperatures offer protection against damage, as do electron dose rates that significantly exceed the oxygen dose rates, and our model attributes both effects to a physisorbed oxygen population.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Dataset for "Resistance of Boron Nitride Nanotubes to Radiation-Induced Oxidation" as published in The Journal of Physical Chemistry C

Adsorption↗

DOE OSTI2022

Large Magnetic Gap in a Designer Ferromagnet–Topological Insulator–Ferromagnet Heterostructure

Abstract Combining magnetism and nontrivial band topology gives rise to quantum anomalous Hall (QAH) insulators and exotic quantum phases such as the QAH effect where current flows without dissipation along quantized edge states. Inducing magnetic order in topological insulators via proximity to a magnetic material offers a promising pathway toward achieving the QAH effect at a high temperature for lossless transport applications. One promising architecture involves a sandwich structure comprising two single‐septuple layers (1SL) of MnBi 2 Te 4 (a 2D ferromagnetic insulator) with ultrathin few quintuple layer (QL) Bi 2 Te 3 in the middle, and it is predicted to yield a robust QAH insulator phase with a large bandgap greater than 50 meV. Here, the growth of a 1SL MnBi 2 Te 4 /4QL Bi 2 Te 3 /1SL MnBi 2 Te 4 heterostructure via molecular beam epitaxy is demonstrated and the electronic structure probed using angle‐resolved photoelectron spectroscopy. Strong hexagonally warped massive Dirac fermions and a bandgap of 75 ± 15 meV are observed. The magnetic origin of the gap is confirmed by the observation of the exchange‐Rashba effect, as well as the vanishing bandgap above the Curie temperature, in agreement with density functional theory calculations. These findings provide insights into magnetic proximity effects in topological insulators and reveal a promising platform for realizing the QAH effect at elevated temperatures.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Quantum Size Effects, Multiple Dirac Cones, and Edge States in Ultrathin Bi(110) Films

The presence of inherently strong spin–orbit coupling in bismuth, its unique layer-dependent band topology and high carrier mobility make it an interesting system for both fundamental studies and applications. Theoretically, it has been suggested that strong quantum size effects should be present in the Bi(110) films, with the possibility of Dirac Fermion states in the odd-bilayer (BL) films, originating from dangling p z orbitals and quantum-spin hall (QSH) states in the even-bilayer films. However, the experimental verification of these claims has been lacking. Here, we study the electronic structure of Bi(110) films grown on a high-T c superconductor, Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES). We observe an oscillatory behavior of electronic structure with the film thickness and identify the Dirac-states in the odd-bilayer films, consistent with the theoretical predictions. In the even-bilayer films, we find another Dirac state that was predicted to play a crucial role in the QSH effect. In the low thickness limit, we observe several extremely one-dimensional states, probably originating from the edge-states of Bi(110) islands. Our results provide a much needed experimental insight into the electronic and structural properties of Bi(110) films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Endless Dirac nodal lines in kagome-metal Ni3In2S2

Abstract Topological semimetals are a frontier of quantum materials. In multiband electronic systems, topological band crossings can form closed curves, known as nodal lines. In the presence of spin–orbit coupling and/or symmetry-breaking operations, topological nodal lines can break into Dirac/Weyl nodes and give rise to interesting transport properties, such as the chiral anomaly and giant anomalous Hall effect. Recently, the time-reversal symmetry-breaking induced Weyl fermions are observed in a kagome-metal Co 3 Sn 2 S 2 , triggering interests in nodal-line excitations in multiband kagome systems. Here, using first-principles calculations and symmetry-based indicator theories, we find six endless nodal lines along the stacking direction of kagome layers and two nodal rings in the kagome plane in nonmagnetic Ni 3 In 2 S 2 . The linear dipsersive electronic structure, confirmed by angle-resolved photoemission spectroscopy, induces large magnetoresistance up to 2000% at 9 T. Our results establish a diverse topological landscape of multiband kagome metals.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Terahertz inverse spin Hall effect in spintronic nanostructures with various ferromagnetic materials

Bilayers of ferromagnetic and heavy metal nanolayers excited with femtosecond laser pulses emit subpicosecond bursts of electromagnetic radiation with spectral frequencies up to several THz. We fabricated such spintronic THz emitters containing various ferromagnetic materials with vastly different magnetic remanence, saturation magnetization, and coercive fields. In all cases, the THz amplitude versus magnetic-field dependence follows the independently measured magnetization hysteresis loops and the THz polarization direction is perpendicular to the sample’s magnetization M, both consistent with the inverse spin Hall effect (ISHE) as the physical origin. The mV-level signals of observed THz transient also favor the ISHE over the anomalous Nernst effect (ANE). The M(T) variation governs the temperature dependence of the THz generation. Emitters with weakly remanent ferromagnets are magnetic-field tunable, while moderately and strongly remanent ferromagnets, once magnetized, allow intense THz generation even without an external field, nevertheless exhibiting low susceptibility to magnetic perturbations if the hysteresis loop is square. Hence, exploiting the magnetic properties of the ferromagnetic layer enables tailoring of weakly temperature-dependent spintronic THz emitters. Finally, we explored the applicability of perovskite oxide materials such as La-Sr-Mn-O (LSMO) for THz transient generation. We detected weak (<40-μV amplitude) THz emissions from both LSMO/Au and pure LSMO nanostructures with no sign flip upon the sample reversal. Furthermore, we excluded the ISHE and believe it was likely governed by the ANE in the thick-film regime, although we cannot exclude the transient demagnetization mechanism.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Spin–orbit torques in structures with asymmetric dusting layers

Current-induced spin–orbit torques (SOTs) in heavy metal/ferromagnet heterostructures have emerged as an efficient method for magnetization switching with applications in nonvolatile magnetic memory and logic devices. However, experimental realization of SOT switching of perpendicular magnetization requires an additional inversion symmetry breaking, calling for modifications of the conventional SOT heterostructures. In this work, we study SOTs and deterministic switching of perpendicular magnetization by inserting different asymmetric dusting layers at the heavy metal/ferromagnet interface. Similar to the previous works with lateral structural asymmetry, we study the emergence of current-induced perpendicular effective magnetic fields (H z eff ). By examining three different material combinations of heavy metal/dusting layers (W/IrMn, Pt/IrMn, and W/Ta), we shed light on the origins of H z eff ; we show that H z eff is generically created in all the studied asymmetric structures, has a close correlation with the interfacial magnetic anisotropy, and is independent of the signs of spin Hall angles of the materials. Furthermore, we show that the induction of H z eff enables field-free deterministic SOT switching of perpendicular magnetization. Our results can be used in designing SOT heterostructures for practical applications in nonvolatile technologies.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Crossover behavior in the magnetoresistance of thin flakes of the topological material ZrTe 5

ZrTe 5 is a layered material that exhibits intricate topological effects. Intensive theoretically and experimental efforts have been devoted to try to understand the physics in this materials. In this paper the temperature dependent magneto-transport properties of ZrTe 5 thin flakes are investigated. A characteristic temperature T* is observed in the temperature dependence of three different types of magnetoresistance simultaneously, which are the saturated Hall anomaly, the chiral anomaly and the longitudinal magnetoresistance. Furthermore, the value of T* decreases monotonically from 200K to 160K with increasing thickness of the ZrTe 5 thin flakes from 42nm to 89nm. Temperature induced topological phase transitions are attributed to the cause of such anomaly in the three types of magnetoresistance at T*. Overall, our findings provide a multi-parameter indicator for the emergence of topological phase transition in ZrTe 5 and could be extended to the study of other topological materials. The temperature dependence of the three types of magnetoresistance also shed light on the role of anomalous Hall Effect in the transport properties of ZrTe 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1979

Full-scale aircraft simulation with cryogenic tunnels and status of the National Transonic Facility

The effect of thermal and caloric imperfections in cryogenic nitrogen on boundary layers was determined to indicate that in order to simulate nonadiabatic laminar or turbulent boundary layers in a cryogenic nitrogen wind tunnel, the flight enthalpy ratio, rather than the temperature ratio, should be reproduced. The absence of significant real gas effects on both viscous and inviscid flows makes it unlikely that there will be large real gas effects on the cryogenic tunnel simulation of shock boundary layer interactions or other complex flow conditions encountered in flight. Condensation effects were studied to determine the minimum usable temperature and indicated that under most circumstances free stream Mach number rather than maximum local Mach number determines the onset of condensation effects.

Kilgore, R. A.↗