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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Quantum anomalous Hall effect in perfectly compensated collinear antiferromagnetic thin films

Here, we show that the quantum anomalous Hall effect almost always occurs in magnetic topological insulator thin films whenever the top and bottom surface layer magnetizations are parallel at some temperature, independent of the interior layer magnetization configuration. Using this criterion, we identify structures that have a quantum anomalous Hall effect even though they have collinear magnetic structures with no net magnetization, and discuss strategies for realizing these interesting magnetic states experimentally.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Induced anomalous Hall effect of massive Dirac fermionsin ZrTe 5 and HfTe 5 thin flakes

Research on the anomalous Hall effect (AHE) has been lasting for a century to make clear the underlying physical mechanism. Generally, the AHE appears in magnetic materials, in which the extrinsic process related to scattering effects and intrinsic contribution connected with Berry curvature are crucial. Recently, AHE has been counterintuitively observed in nonmagnetic topological materials and attributed to the existence of Weyl points. However, the Weyl point scenario would lead to unsaturated AHE even in large magnetic fields and contradicts the saturation of AHE in several tesla (T) in experiments. In this work, we investigate the Hall effect of ZrTe 5 and HfTe 5 thin flakes in static ultrahigh magnetic fields up to 33 T. We find the AHE saturates to 55(70)Ω 1 cm 1 for ZrTe 5 (HfTe 5 ) thin flakes above ~10T. Combining detailed magnetotransport experiments and Berry curvature calculations, we clarify that the splitting of massive Dirac bands without Weyl points can be responsible for AHE in nonmagnetic topological materials ZrTe 5 and HfTe 5 thin flakes. This model can identify our thin flake samples to be weak topological insulators and serve as a tool to probe the band structure topology in topological materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRSDate not supplied

Sub-kW Class Hall-Effect Thruster Power Processing Unit for Wide Output Range Applications

The National Aeronautics and Space Administration (NASA) Small Spacecraft Electric Propulsion (SSEP) project is maturing high-propellant throughput sub-kilowatt Hall-effect thruster technologies to enable small spacecraft deep space science and exploration missions with high delta-v requirements. In support of this effort, development of a power processing unit (PPU) capable of providing discharge power of up to 1 kW continues to be pursued at the NASA Glenn Research Center (GRC). Previous reported work included a successful integrated test of a scalable, modular breadboard discharge power supply with the NASA-H64M laboratory model Hall-effect thruster and presentation of notional designs for the various auxiliary power supplies needed for thruster operation. Since that time, auxiliary power supply designs have been completed and fabricated, with the cathode heater and keeper power supplies being successfully tested with a hollow cathode assembly (HCA) in the NASA GRC Vacuum Facility 56 (VF-56). The desire for a lower mass, higher efficiency, and more versatile PPU to maximize performance of power and mass-limited small spacecraft has led to the exploration of a discharge power supply based on a series-parallel (LCC) resonant topology. This topology has enabled the discharge power supply to operate over a wider output range at switching frequencies 4-5 times higher than previous design iterations. Simulation models of the topology have been developed and a breadboard of the topology has been fabricated and evaluated on both resistive loads and an integrated Hall thruster test. This paper will present collected performance and integrated test data from both the fabricated auxiliary and resonant discharge power supplies. Advantages of the resonant converter architecture over more traditional pulse-width modulated (PWM) techniques in Hall-effect thruster discharge power supply applications will also be described.

electric propulsion↗
NASA NTRSDate not supplied

Sub-kW Class Hall-Effect Thruster Power Processing Unit for Wide Output Range Applications

The National Aeronautics and Space Administration (NASA) Small Spacecraft Electric Propulsion (SSEP) project is maturing high-propellant throughput sub-kilowatt Hall-effect thruster technologies to enable small spacecraft deep space science and exploration missions with high delta-v requirements. In support of this effort, development of a power processing unit (PPU) capable of providing discharge power of up to 1 kW continues to be pursued at the NASA Glenn Research Center (GRC). Previous reported work included a successful integrated test of a scalable, modular breadboard discharge power supply with the NASA-H64M laboratory model Hall-effect thruster and presentation of notional designs for the various auxiliary power supplies needed for thruster operation. Since that time, auxiliary power supply designs have been completed and fabricated, with the cathode heater and keeper power supplies being successfully tested with a hollow cathode assembly (HCA) in the NASA GRC Vacuum Facility 56 (VF-56). The desire for a lower mass, higher efficiency, and more versatile PPU to maximize performance of power and mass-limited small spacecraft has led to the exploration of a discharge power supply based on a series-parallel (LCC) resonant topology. This topology has enabled the discharge power supply to operate over a wider output range at switching frequencies 4-5 times higher than previous design iterations. Simulation models of the topology have been developed and a breadboard of the topology has been fabricated and evaluated on both resistive loads and an integrated Hall thruster test. This paper will present collected performance and integrated test data from both the fabricated auxiliary and resonant discharge power supplies. Advantages of the resonant converter architecture over more traditional pulse-width modulated (PWM) techniques in Hall-effect thruster discharge power supply applications will also be described.

electric propulsion↗
NASA NTRS1992

Automated High-Temperature Hall-Effect Apparatus

Automated apparatus takes Hall-effect measurements of specimens of thermoelectric materials at temperatures from ambient to 1,200 K using computer control to obtain better resolution of data and more data points about three times as fast as before. Four-probe electrical-resistance measurements taken in 12 electrical and 2 magnetic orientations to characterize specimens at each temperature. Computer acquires data, and controls apparatus via three feedback loops: one for temperature, one for magnetic field, and one for electrical-potential data.

Parker, James B.↗
NASA NTRS1994

Metal-Film Hall-Effect Devices

Large positive and negative Hall coefficients achievable. Family of Hall-effect devices made from multilayer metal films instead of semiconductor materials. Metal films easier to fabricate; formed by deposition on variety of substrates, and leads readily attached to them. Fabricated with larger areas, potentially more reliable, and less affected by impurities. Also used to measure magnetic fields. Devices especially useful at low temperatures.

Peters, Palmer N.↗
DOE OSTI2025

Quantum Nonlinear Acoustic Hall Effect and Inverse Acoustic Faraday Effect in Dirac Insulators

Here, we propose to realize the quantum nonlinear Hall effect and the inverse Faraday effect through the acoustic wave in a time-reversal invariant but inversion broken Dirac insulator. We focus on the acoustic frequency much lower than the Dirac gap such that the interband transition is suppressed and these effects arise solely from the intrinsic valley-contrasting band topology. The corresponding acoustoelectric conductivity and magnetoacoustic susceptibility are both proportional to the quantized valley Chern number and independent of the quasiparticle lifetime. The linear and nonlinear components of the longitudinal and transverse topological currents can be tuned by adjusting the polarization and propagation directions of the surface acoustic wave. The static magnetization generated by a circularly polarized acoustic wave scales linearly with the acoustic frequency as well as the strain-induced charge density. Our results unveil a quantized nonlinear topological acoustoelectric response of gapped Dirac materials, like hexagonal boron nitride and transition-metal dichalcogenide, paving the way toward room-temperature acoustoelectric devices due to their large band gaps.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Controlling the orbital Hall effect in gapped bilayer graphene in the terahertz regime

We study the orbital Hall effect (OHE) in the AC regime using bilayer graphene (BLG) as a prototypical material platform. While the unbiased BLG has gapless electronic spectra, applying a perpendicular electric field creates an energy band gap that can be continuously tuned from zero to high values. By exploiting this flexibility, we demonstrate the ability to control the behavior of AC orbital Hall conductivity. Particularly, we demonstrate that the orbital Hall conductivity at the neutrality point changes its signal at a critical frequency, the value of which is proportional to the perpendicular electric field. For BLG with narrow band gaps, the active frequency region for the AC OHE may extend to a few terahertz, which is experimentally accessible with current technologies. We also consider the introduction of a perpendicular magnetic field in the weak-coupling regime using first-order perturbation theory to illustrate how the breaking of time-reversal symmetry enables the emergence of the AC charge Hall effect in the charge-doped situation and modifies the AC orbital Hall conductivity. Our calculations suggest that BLG with narrow band gaps is a practical candidate for investigating time-dependent orbital angular momentum transport.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Topological Hall Effect in Antiferromagnetic Co-Doped Fe 3 GaTe 2

Fe 3 GaTe 2 is van der Waals (vdW) ferromagnet with a Curie temperature T C ranging from 350 K to 380 K, followed upon cooling by a ferrimagnetic transition near room temperature. Substituting Fe with Co was previously reported to induce antiferromagnetism (AFM) at a Co fraction dependent Néel temperature TN. In this work, we confirm the overall phase diagram of the Fe 3-x Co x GaTe 2 series as a function of x and temperature via magnetization and electrical transport measurements. For x ≥ 0.6 the Hall effect is observed to mimic the magnetization as the AF ground state is suppressed by the external magnetic field via a metamagnetic transition, thus displaying an anomalous Hall response. At low temperatures, we also observe a pronounced topological Hall signal peaking at μ 0 H = 4 T, or within the metamagnetic transition region of fields. This observation points to the presence of magnetic field-induced chiral spin textures, such as skyrmions upon approaching magnetization saturation. Remarkably, magnetic force microscopy (MFM) reveals the emergence of nearly circular magnetic domains, with diameters on the order of 100–200 nm, within the antiferromagnetic phase. Here, a detailed analysis of the MFM images indicates that the topological Hall effect is closely linked to the field-induced stabilization of magnetic domain structures, likely exhibiting chiral textures. This observation suggests the possible formation of skyrmions already in the AFM phase, i.e., AFM skyrmions, that evolve into ferromagnetic (FM) ones upon increasing the magnetic field. Consequently, Co-doped Fe 3 GaTe 2 might provide a platform to investigate the transformation of skyrmions, initially coupled antiferromagnetically into ferromagnetic skyrmions, and to explore its impact on the topological and skyrmion Hall effects.

anomalous Hall effect↗
DOE OSTI2021

Giant Topological Hall Effect in van der Waals Heterostructures of CrTe 2 /Bi 2 Te 3

Discoveries of the interfacial topological Hall effect (THE) provide an ideal platform for exploring the physics arising from the interplay between topology and magnetism. The interfacial topological Hall effect is closely related to the Dzyaloshinskii-Moriya interaction (DMI) at an interface and topological spin textures. However, it is difficult to achieve a sizable THE in heterostructures due to the stringent constraints on the constituents of THE heterostructures, such as strong spin-orbit coupling (SOC). Here, we report the observation of a giant THE signal of 1.39 mu Omega.cm in the van der Waals heterostructures of CrTe 2 /Bi 2 Te 3 fabricated by molecular beam epitaxy, a prototype of two-dimensional (2D) ferromagnet (FM)/topological insulator (TI). This large magnitude of THE is attributed to an optimized combination of 2D ferromagnetism in CrTe 2 , strong SOC in Bi 2 Te 3 , and an atomically sharp interface. Furthermore, our work reveals CrTe 2 /Bi 2 Te 3 as a convenient platform for achieving large interfacial THE in hybrid systems, which could be utilized to develop quantum science and high-density information storage devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator

A topological insulator (TI) interfaced with a magnetic insulator (MI) may host an anomalous Hall effect (AHE), a quantum AHE, and a topological Hall effect (THE). Recent studies, however, suggest that coexisting magnetic phases in TI/MI heterostructures may result in an AHE-associated response that resembles a THE but in fact is not. Here, we report a genuine THE in a TI/MI structure that has only one magnetic phase. The structure shows a THE in the temperature range of T = 2–3 K and an AHE at T = 80–300 K. Over T = 3–80 K, the two effects coexist but show opposite temperature dependencies. Control measurements, calculations, and simulations together suggest that the observed THE originates from skyrmions, rather than the coexistence of two AHE responses. The skyrmions are formed due to a Dzyaloshinskii–Moriya interaction (DMI) at the interface; the DMI strength estimated is substantially higher than that in heavy metal-based systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2019

Completing the development of the 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS)

The Hall Effect Rocket with Magnetic Shielding (HERMeS) is a 12.5 kW Hall thruster co-developed by NASA Glenn Research Center and the Jet Propulsion Laboratory. HERMeS incorporates magnetic shielding to eliminate discharge channel erosion in order to reach its design lifetime of 50 kh at specific impulses up to 3000 s. The capabilities of the HERMeS thruster technology transferred to Aerojet Rocketdyne under the Advanced Electric Propulsion System (AEPS) program are described. HERMeS hardware testing is now focused at reducing risk and supporting the qualification of the AEPS thruster. These includes a series of progressively longer wear tests, plasma characterization and modeling supporting life qualification, magnetic field optimization, and environmental testing. Initial results from AEPS thruster testing show operation consistent with the HERMeS thrusters and steady-state operation has been achieved at 600 V, 12.5 kW.

Herman, Daniel↗
NASA NTRS1993

Nulling Hall-Effect Current-Measuring Circuit

Circuit measures electrical current via combination of Hall-effect-sensing and magnetic-field-nulling techniques. Known current generated by feedback circuit adjusted until it causes cancellation or near cancellation of magnetic field produced in toroidal ferrite core by current measured. Remaining magnetic field measured by Hall-effect sensor. Circuit puts out analog signal and digital signal proportional to current measured. Accuracy of measurement does not depend on linearity of sensing components.

Sullender, Craig C.↗
DOE OSTI2021

Quantum kinetics of anomalous and nonlinear Hall effects in topological semimetals

Highlights: • Linear and photogalvanic anomalous Hall responses are systematically derived. • Extrinsic mechanisms of AHE include Gaussian, diffractive, hybrid skew scattering. • Diagrammatic calculations are matched to semiclassical picture of AHE. • The Pancharatnam phase of multifold fermions determines the skew scattering amplitude. • Photon-induced interband scattering are accompanied by coordinate shifts. We present a systematic microscopic derivation of the semiclassical Boltzmann equation for band structures with the finite Berry curvature based on Keldysh technique of nonequilibrium systems. In the analysis, an AC electrical driving field is kept up to quadratic order, and both cases of small and large frequencies corresponding to intra- and interband transitions are considered. In particular, this formulation is suitable for the study of nonlinear Hall effect and photogalvanic phenomena. The role of impurity scattering is carefully addressed. Specifically, in addition to previously studied side-jump and skew-scattering processes, quantum interference diffractive contributions are now explicitly incorporated within the developed framework. This theory is applied to multifold fermions in topological semimetals, for which the generic formula for the skew scattering rate from the Pancharatnam phase is obtained along with the corresponding anomalous Hall conductivity.

36 MATERIALS SCIENCE↗
NASA NTRS1989

Hall-Effect Current Sensors For Integrated Circuits

Built-in devices measure direct or alternating currents. Hall-effect sensors placed near constriction in conductor strip. Differential configuration reduces effects of stray magnetic fields, nonlinearities, and changes in temperature. Preliminary design studies and experiments with macroscopic commercial Hall-effect sensors conducted to assess feasibility, limitations, and need for further research and development of this concept. Potential applications include programmable power supplies and protective circuitry.

Rippel, Wally E.↗
DOE OSTI2021

Giant topological Hall effect in centrosymmetric tetragonal Mn 2–x Zn x Sb

Topological magnetism typically appears in noncentrosymmetric compounds or compounds with geometric frustration. Here, we report the effective tuning of magnetism in centrosymmetric tetragonal Mn 2–x Zn x Sb by Zn substitution. The magnetism is found to be closely coupled to the transport properties, giving rise to a very large topological Hall effect with fine-tuning of Zn content, which even persists to high temperature (~250K). The further magnetoentropic analysis suggests that the topological Hall effect is possibly associated with topological magnetism. Furthermore, our finding suggests Mn 2–x Zn x S is a candidate material for a centrosymmetric tetragonal topological magnetic system, offering opportunities for studying and tuning spin textures and developing near room temperature spin-based devices.

36 MATERIALS SCIENCE↗
NASA NTRS2018

Optimization of the Magnetic Field Topology in the Hall Effect Rocket with Magnetic Shielding

NASA's Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5kW Technology Demonstration Unit-1 (TDU-1) has been the subject of extensive technology maturation in preparation for flight system development. The TDU-1 thruster implements a magnetically shielded field topology and has demonstrated the elimination of the discharge channel erosion. Extensive wear testing the TDU Hall thrusters has identified the thruster front pole covers as the next life limiting component. This effort aims to explore and investigate alternate magnetic field topologies to assess whether reductions in the front pole cover erosion can be attained while still maintaining very low erosion rates on the discharge channel walls. NASA GRC and JPL have begun a magnetic field topology characterization and optimization study by designing four candidate magnetic field topologies that reduce the effectiveness of the shielding along the discharge channel walls with the intent to also reduce the erosion rates along the pole covers. Three of the four candidate magnetic field topologies have been manufactured subjected to an extensive test campaign that includes performance, plume, and stability characterization. In Phase I of the testing campaign, the thruster's oscillation magnitude and Laser Induced fluorescence (LIF) measurements of the plasma plume were performed for the three candidate topologies. In Phase I, the thruster's oscillation magnitude and LIF measurements were performed for the three candidate topologies. Phase I test results found that the B1 configuration attained lower oscillation levels than B0. Additionally, LIF measurements along the discharge chamber centerline found that upstream retraction of the thruster's peak magnetic field does result in an upstream shift of the acceleration zone but the magnitude of the shift does not correspond one-to-one to the shift in the location of the peak radial magnetic field magnitude. Phase II test segment will include performing performance, stability, plume, and erosion measurements for the various candidate magnetic field topologies.

Kamhawi, Hani↗
DOE OSTI2020

Quantized thermoelectric Hall effect induces giant power factor in a topological semimetal

Abstract Thermoelectrics are promising by directly generating electricity from waste heat. However, (sub-)room-temperature thermoelectrics have been a long-standing challenge due to vanishing electronic entropy at low temperatures. Topological materials offer a new avenue for energy harvesting applications. Recent theories predicted that topological semimetals at the quantum limit can lead to a large, non-saturating thermopower and a quantized thermoelectric Hall conductivity approaching a universal value. Here, we experimentally demonstrate the non-saturating thermopower and quantized thermoelectric Hall effect in the topological Weyl semimetal (WSM) tantalum phosphide (TaP). An ultrahigh longitudinal thermopower $$S_{xx} \sim 1.1 \times 10^3 \, \mu \, {\mathrm{V}} \, {\mathrm{K}}^{ - 1}$$ S x x ~ 1.1 × 1 0 3 μ V K − 1 and giant power factor $$\sim 525 \, \mu \, {\mathrm{W}} \, {\mathrm{cm}}^{ - 1} \, {\mathrm{K}}^{ - 2}$$ ~ 525 μ W cm − 1 K − 2 are observed at ~40 K, which is largely attributed to the quantized thermoelectric Hall effect. Our work highlights the unique quantized thermoelectric Hall effect realized in a WSM toward low-temperature energy harvesting applications.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗