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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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9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Chiral edge state control of thermoelectric effects

Thermoelectric responses in two-dimensional electron gases subjected to magnetic fields have the potential to provide unique information about quasiparticle statistics. In this study, we show that chiral edge states play a key role in thermoelectric Hall bar measurements by completely controlling the direction of the internal thermal gradient. To this end, we perform measurements of the magnetothermoelectric responses of cadmium arsenide quantum wells. The magnetothermoelectric responses in the quantum Hall regime agree with theoretical predictions if one considers the role of chiral edge states, which flow in opposite directions on either side of the Hall bar and establish an internal temperature gradient that is perpendicular to the externally applied thermal gradient. We show that the results are self-consistent within this picture under different measurement conditions. We discuss potential applications of the findings, such as in nanoscale control of local temperature gradients and thermoelectric effects along with the characterization of other topological systems with chiral edges states.

Science & Technology - Other Topics↗
DOE OSTI2026

Non-Altermagnetic Origin of Exchange Bias Behaviors in Incoherent RuO 2 /Fe Bilayer Heterostructures

Initially identified as a promising altermagnetic (AM) candidate, rutile RuO 2 has since become embroiled in controversy due to contradictory findings of modeling and measurements of the magnetic properties of bulk crystals and thin films. For example, despite observations of a bulk nonmagnetic state using density functional theory, neutron scattering, and muon spin resonance measurements, patterned RuO 2 Hall bars and film heterostructures display magnetotransport signatures of magnetic ordering. Among the characteristics routinely cited as evidence for AM is the observation of exchange bias (EB) in an intimately contacted Fe-based ferromagnetic (FM) layer, which can arise due to interfacial coupling with a compensated antiferromagnet. Here, within this work, the origins of this EB coupling in Ru-capped RuO 2 /Fe bilayers are investigated using polarized neutron diffraction, polarized neutron reflectometry, cross-sectional transmission electron microscopy, and super conducting quantum interference device measurements. These experiments reveal that the EB behavior is driven by the formation of an iron oxide interlayer containing Fe 3 O 4 that undergoes a magnetic transition and pins interfacial moments within Fe at low temperature. These findings are confirmed by comparable measurements of Ni-based heterostructures, which do not display EB coupling, as well as magnetometry of additional Fe/Ru bilayers that display oxide-driven EB coupling despite the absence of the epitaxial RuO 2 layer. While these results do not directly refute the possibility of AM ordering in RuO 2 thin films, they reveal that EB, and related magnetotransport phenomena, cannot alone be considered evidence of this characteristic in the rutile structure due to interfacial chemical disorder.

RuO2↗
DOE OSTI2025

Enhancement to the conductivity of surface transfer-doped (111) diamond through thermochemical surface etching

The use of a transition metal catalyzed thermochemical etching method for improving the carrier transport properties of the near-surface two-dimensional (2D) hole gas in surface transfer-doped hydrogen-terminated (111) diamond is demonstrated. Using Ni 0.8 Cr 0.2 films deposited and annealed to a temperature of 900 °C, with up to three etch cycles, preferential (111) surface etching produces large terraces exceeding 10 μm in size with a surface microroughness, σ 2 RMS λ, that is two orders of magnitude lower than for the pre-etched (111) surface. Magnetotransport measurements on hydrogen-terminated Hall bars engineered on the pre- and post-etched surfaces and rendered conductive by the adsorbed water layer formed on exposure to ambient conditions demonstrate that this etching causes an improvement in the hole mobility by an order of magnitude, resulting in a measured sheet resistivity of 1.04 kΩ/sq at a temperature of 4.2 K without gating.

42 ENGINEERING↗
DOE OSTI2025

A15 phase Ta 3⁢ Sb thin films: Direct synthesis, charge transport, and spin-orbit torque

Ta 3 ⁢Sb is one of the A15 compounds that have been predicted to have giant spin Hall conductivities due to the gapped Dirac-like band crossings in their electronic structures. We use co-sputtering to directly synthesize thin films of Ta 3 ⁢Sb and identify a large window of Ta:Sb flux ratio that permits the formation of single-phase A15 structure. These sputtered films have an actual Ta:Sb atomic ratio of 4:1, as determined from Rutherford backscattering spectrometry. Their high resistivity, at the Mott-Ioffe-Regel limit, suggests that the electron mean free path is comparable to interatomic distances. From harmonic Hall and spin-torque ferromagnetic resonance measurements, the intrinsic spin Hall conductivity of thin film Ta 3 ⁢Sb is estimated to be in the range of −526 to −1230⁢(ℏ/𝑒)S/cm at 300 K, lower in magnitude than the predicted value of −1400⁢(ℏ/𝑒)S/cm . Here, first-principles calculations of the electronic structure show that the discrepancy is consistent with an increase of the Fermi level due to the nonideal stoichiometry needed to stabilize the A15 structure.

Density functional theory↗
DOE OSTI2026

Buried Dirac Points in Quantum Spin Hall Insulators: Implications for Majorana Kramers Pair-Based Quantum Computing

Quantum spin Hall insulators (QSHIs) host helical electronic edge states that are protected from backscattering due to time-reversal symmetry (TRS). Despite considerable work investigating QSHI edge states, there is still an open question about their unexpected resilience to large magnetic fields where TRS is undoubtedly broken. In this work, we investigate the transport properties of helical edge states in a QSHI-superconductor (QSHI-SC) junction formed by a In⁢As(15 nm)/Ga⁢Sb(5 nm) double quantum well and a superconducting tantalum (Ta) constriction. We observe a robust conductance plateau up to 2 T, signaling resilient edge-state transport. Using a modified Landauer-Büttiker analysis, we find that the zero-field conductance is consistent with 98% Andreev-reflection probability owing to the high transparency of the (In⁢As/Ga⁢Sb)-Ta interface. Such resilience is consistent with the Dirac point for the edge states being buried in the bulk valence band. We further theoretically show that a buried Dirac point does not affect the robustness of the quasi-one-dimensional topological superconducting phase. We find that a buried Dirac point favors the hybridization of Majorana Kramers pairs (MKPs)—predicted to exist in a QSHI-SC constriction—and fermionic modes in the QSHI vacuum edge resulting in extended MKP states, highlighting the subtle role of buried Dirac points in probing MKPs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Spin polarization engineering in 𝑑-wave altermagnets

Altermagnets host unconventional spin-polarized bands despite zero net magnetization, but controlling their spin structure remains challenging. Here, we theoretically propose a multifield approach to engineer spin polarization in 𝑑-wave altermagnets using gating, optical driving, and in-plane electric fields, which enable tunable and switchable polarizations along multiple directions. Optical driving induces out-of-plane (𝑧) polarization, while gating and in-plane fields generate 𝑥 and 𝑦 polarizations via the Edelstein effect, all of which are experimentally detectable. We further find that spin- and band-selective doping induces chiral optical activity, a feature unique to altermagnets. Our approach offers a versatile means to control the direction of spin polarization in altermagnets.

Floquet systems↗
DOE OSTI2024

Unconventional fieldlike spin torques in CrPt 3

The topological semimetal CrPt 3 has potential for generating unconventional spin torques due to its ferrimagnetic ordering, topological band structure, and high anomalous Hall effect. CrPt 3 exhibits ferrimagnetic behavior only in its chemically ordered phase and is paramagnetic in its chemically disordered phase. By controlling the growth and annealing temperatures, we prepare epitaxial films of both chemically ordered and chemically disordered phases of CrPt 3 , allowing us to investigate the effect of magnetic ordering on unconventional-torque generation. We use angle-dependent spin-torque-ferromagnetic-resonance and second-harmonic Hall measurements to probe the spin torques generated from epitaxial CrPt 3 in CrPt 3 /Cu/Ni 81 ⁢Fe 19 heterostructures. With current applied along specific directions with respect to the crystal order, we reveal unconventional spin torques in both ordered and disordered films. When current flows parallel to the [$1\bar{1}1$] and [ $\bar{1}11$] directions, we observe an unconventional fieldlike torque that is opposite in sign for the two directions. Finally, our calculations reveal that this unconventional torque originates from an indirect nonlocal spin-orbit torque due to spin scattering at the CrPt 3 /Cu interface, in addition to symmetry breaking at this interface.

ferromagnetic resonance↗
DOE OSTI2026

Ammonia Synthesis by a Supported Iron-Lithium Hydride Precatalyst: Silicon Nitride Support Enabled Synthesis and Nitrogen Reservoir Dynamics

Amorphous silicon nitride (Si 3 N 4 ) is an unconventional support for the chemisorption of organometallic complexes and offers potential improvements in active site stability and reactivity through enhanced metal-nitrogen covalency and orbital overlap in bonding interactions with the nitride framework. Here, we show that silicon nitride-supported iron mesityl complexes display divergent reactivity compared to their silica-supported homologues, resisting metallic particle formation under reducing pretreatment conditions (exposure to excess organolithium reagents) and maintaining active iron/lithium speciation under ammonia synthesis conditions that is absent on the oxide support. When the organometallic iron complex on silicon nitride is exposed to excess n-butyllithium, iron remains isolated, catalyzing the conversion of butyllithium to lithium hydride, resulting in a divalent iron site in a polyhydride environment. In contrast, the silica-supported complex is converted to reduced iron clusters without forming persistent isolated hydrides. These structural differences lead to markedly different catalytic behaviors under ammonia synthesis conditions. The Li/Fe/Si 3 N 4 catalyst is highly active (7.5 mol NH 3 /mol Fe/h at 300 °C, 10 bar, or 46 mol NH 3 /mol Fe/h at 400 °C, 10 bar), while both the silica-supported analog and the nonlithiated Si 3 N 4 -supported species are inactive. Notably, this activity is enhanced relative to previously reported iron-lithium hydride composite catalysts (0.43–4.1 mol NH 3 /mol Fe/h at 300 °C, 10 bar) and relative to the industrial benchmark promoted iron catalyst KM1 (3.0 mol NH 3 /mol Fe/h at 400 °C, 10 bar). The catalyst activation and LiH/LiNH x nitrogen reservoir dynamics for Li/Fe/Si 3 N 4 are studied by X-ray Absorption, Mössbauer, and in situ DRIFT spectroscopies and isotopic exchange kinetics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Production of Creep-Resistant FeCrAl-ODS

Four ODS FeCrAl alloys with base alloy composition Fe-16Cr-4Al-2Mo-0.2Y (wt.%) were mechanically alloyed with minor additions of CeO 2 and Y 2 O 3 additions and were successfully extruded into rectangular-shaped bars. Two oxide compositions were explored, which were designated NC with oxide addition of 0.5CeO 2 plus 0.2Yin solution of the base alloy powder and PC with additions of 0.5CeO 2 and 0.7Y 2 O 3 to elevate the O level. The microstructure of both alloys revealed by SEM and EBSD to consist of very small grains with sizes of ~450-520 nm and weak texture components. Sub-size tensile specimens fabricated from both alloys were tensile tested from room temperature to 800ºC. The results for both alloys from room temperature to 400ºC showed very high strengths, but low ductilities, especially uniform strain. From 500ºC to 800ºC, the strengths decreased rapidly, and the ductilities increased significantly. The reason for the rapid decreases in tensile strengths above 500ºC for both alloys is not clear, but will require advanced characterization techniques including atom probe tomography, to determine the size and number density of the nano-size oxide particles, and TEM/EFTEM, for investigating the dispersion of nano-size oxide particles on grain boundaries to assess the Zener grain boundary pinning effect, which will hinder grain coarsening at high temperatures that may enhance the Hall Petch strengthening mechanism.

36 MATERIALS SCIENCE↗