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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Large electron-phonon drag asymmetry and reverse heat flow in the topological semimetal θ-TaN

A broad range of unusual transport behaviors have been discovered in topological semimetals. However, to date, the effect on the thermopower from intrinsic momentum exchange between electrons and phonons has received little attention. Here we report that huge electron-phonon drag enhancements of the thermopower of the to- pological semimetal, θ-phase tantalum nitride (θ-TaN), can occur that persist even up to room temperature. Our first principles calculations also identify a surprising asymmetry in which the large drag-enhanced thermopowers found slightly above the material’s chemical potential disappear just below it. The large thermopower en- hancements result from anomalous drag contributions from high frequency acoustic phonons with unusually small decay rates. The apparent vanishing drag results from (i) the emergence of an exceptionally high electrical conductivity promoted by the steep linear electronic dispersions extending below one of the topological nodal points; (ii) a remarkable cancellation in which momentum transferred from a charge current creates oppositely directed phonon heat currents of nearly equal magnitude, thereby masking the drag contributions. This extraordinary transport behavior is a consequence of an unusual interplay between intrinsic electron and phonon material properties in θ-TaN. Overall, our work gives new insights into the fundamental physical properties of coupled electron-phonon systems and motivates further exploration of drag effects in semimetals.

36 MATERIALS SCIENCE↗
DOE OSTI2021

CMIP6 Models Underestimate the Holton‐Tan Effect

Abstract The teleconnection between the Quasi‐Biennial Oscillation (QBO) and the Arctic polar vortex is investigated using Coupled Model Intercomparison Project 6 (CMIP6) models. Output from 14 CMIP6 models is compared with reanalysis, three experiments with prescribed QBOs, one of which has no free polar stratospheric variability, and transient experiments in which a QBO is prescribed in runs previously devoid of a QBO. Each CMIP6 model underestimates the Holton‐Tan effect (HTE), the weakening of the polar vortex expected with QBO easterlies in the tropical lower stratosphere. To establish why, potential vorticity maps are used to investigate longitudinal variations in the teleconnection. Prescribing easterly QBO in the transient experiments promotes more high‐latitude planetary wave breaking by influencing the mid‐latitude stratospheric circulation, particularly over Asia. CMIP6 models that better simulate this response over Asia better simulate the HTE. These models also have stronger 10 hPa QBO westerlies.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. there are two inequivalent Ta3+ sites. In the first Ta3+ site, Ta3+ is bonded in a trigonal planar geometry to three equivalent N3- atoms. All Ta–N bond lengths are 2.05 Å. In the second Ta3+ site, Ta3+ is bonded to six equivalent N3- atoms to form a mixture of distorted corner, edge, and face-sharing TaN6 pentagonal pyramids. All Ta–N bond lengths are 2.18 Å. N3- is bonded to five Ta3+ atoms to form a mixture of distorted corner and edge-sharing NTa5 trigonal bipyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN is Tungsten Carbide structured and crystallizes in the hexagonal P-6m2 space group. The structure is three-dimensional. Ta3+ is bonded to six equivalent N3- atoms to form a mixture of distorted edge, corner, and face-sharing TaN6 pentagonal pyramids. All Ta–N bond lengths are 2.24 Å. N3- is bonded to six equivalent Ta3+ atoms to form a mixture of distorted edge, corner, and face-sharing NTa6 pentagonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. there are two inequivalent Ta3+ sites. In the first Ta3+ site, Ta3+ is bonded in a hexagonal planar geometry to six equivalent N3- atoms. All Ta–N bond lengths are 2.63 Å. In the second Ta3+ site, Ta3+ is bonded to six equivalent N3- atoms to form a mixture of distorted corner, edge, and face-sharing TaN6 pentagonal pyramids. All Ta–N bond lengths are 2.11 Å. N3- is bonded to six Ta3+ atoms to form a mixture of distorted corner, edge, and face-sharing NTa6 octahedra. The corner-sharing octahedra tilt angles range from 0–60°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Ta3+ is bonded to six equivalent N3- atoms to form a mixture of edge and corner-sharing TaN6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Ta–N bond lengths are 2.21 Å. N3- is bonded to six equivalent Ta3+ atoms to form a mixture of edge and corner-sharing NTa6 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Ta3+ is bonded to six N3- atoms to form a mixture of face, edge, and corner-sharing TaN6 octahedra. The corner-sharing octahedra tilt angles range from 0–60°. There are four shorter (2.20 Å) and two longer (2.68 Å) Ta–N bond lengths. There are two inequivalent N3- sites. In the first N3- site, N3- is bonded to six equivalent Ta3+ atoms to form a mixture of distorted face, edge, and corner-sharing NTa6 pentagonal pyramids. In the second N3- site, N3- is bonded in a hexagonal planar geometry to six equivalent Ta3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN is Tungsten Carbide-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Ta3+ is bonded to six equivalent N3- atoms to form a mixture of edge, face, and corner-sharing TaN6 octahedra. The corner-sharing octahedral tilt angles are 44°. All Ta–N bond lengths are 2.24 Å. N3- is bonded to six equivalent Ta3+ atoms to form a mixture of distorted edge and corner-sharing NTa6 pentagonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN is Tetraauricupride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Ta3+ is bonded in a body-centered cubic geometry to eight equivalent N3- atoms. All Ta–N bond lengths are 2.39 Å. N3- is bonded in a body-centered cubic geometry to eight equivalent Ta3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TaN by Materials Project

TaN is Zincblende, Sphalerite structured and crystallizes in the cubic F-43m space group. The structure is three-dimensional. Ta3+ is bonded to four equivalent N3- atoms to form corner-sharing TaN4 tetrahedra. All Ta–N bond lengths are 2.06 Å. N3- is bonded to four equivalent Ta3+ atoms to form corner-sharing NTa4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Basin-Size Mapping: Prediction of Metastable Polymorph Synthesizability Across TaC–TaN Alloys

The sizes of the basins of attraction on the potential energy surface are helpful indicators in determining the experimental synthesizability of metastable phases. In principle, these basins can be controlled with changes in thermodynamic conditions such as composition, pressure, and surface energy. Herein, we use random structure sampling to computationally study how alloying smoothly perturbs basin of attraction sizes. The TaC 1-x N x pseudobinary is an ideal test system given the structural and polymorphic contrast of its parent compounds and their technological relevance as epitaxial substrates for Al 1-x Ga x N. While we find limited thermodynamic stability across all computationally observed phases, random structure sampling shows a significant composition region where the rocksalt basin dominates. As such, we predict the potential for the nonequilibrium synthesis of metastable rocksalt TaC 1-x N x alloys as substrates for Al 1-x Ga x N. At higher nitrogen concentrations, other low-energy metastable polymorphs emerge that continue to retain the hexagonal close packing suitable for III-N growth. Confidence in these trends was established through uncertainty quantification of the basin sizes and energy distributions; such analysis utilized the Beta and Dirichlet distributions. In conclusion, we also find (a) polymorph basin sizes can be rationalized in terms of energetic preferences for different coordination environments; and (b) basin sizes universally shrink with increasing nitrogen content, making the system more prone to amorphous growth.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Multistate resistance in TaN/(Hf,Zr)O 2 /Ta ferroelectric tunnel junctions

Ferroelectric tunnel junctions (FTJs) utilizing hafnium zirconium oxide (HZO) have emerged as promising non-volatile memory elements for microelectronics, compatible with back end of line (BEOL) complementary–metal–oxide semiconductor fabrication. This study investigates asymmetric electrode TaN/HZO/Ta devices with a 6 nm thick HZO layer as FTJs for multistate resistive memory applications. The individual FTJs exhibit a resistance ratio exceeding 10× when utilized as a binary state device, with pulsing between −1.7 and +1.4 V to set the high resistance state (HRS) and low resistance state (LRS), respectively. Following with reduced write voltage pulses allows the ferroelectric device to operate with a selection of over 32 distinct resistance states (2 5 bits) between the LRS and HRS. This work then explores the stability of the resistance states during write/read pulse cycling, along with the stability of the state after multiple read pulses. Accessing the multibit state shows stability within 50 reads with the binary state remaining stable for more than 4000 reads pulses. With their multistate tunability and versatility, FTJs hold promise as BEOL memory elements for compute-in-memory (CiM) arrays, binary digital memory, or weighted vector matrix multiplication applications with low power consumption during computations.

CMOS↗
DOE OSTI2024

Effect of X-ray Irradiation and Carnauba Wax Coating on Quality of Lime (Citrus latifolia Tan.) Fruit

The quality of Persian (Tahiti) lime (Citrus latifoliaTan.) fruit was determined following coating with carnauba wax and X-ray irradiation at doses suitable for disinfestation of quarantine pests. Fruit with or without carnauba wax coating were treated with irradiation doses of 0, 150, 300, or 450 Gy, and stored for 14 days at 13 °C and 6 days at 20 °C to simulate commercial transportation and marketing conditions from Hawaii to the continental United States. The fruit color, weight loss, total soluble solids (TSS) content, and titratable acidity (TA) were analyzed at 7, 14, and 14 + 6 days post irradiation. Wax coating significantly delayed fruit peel discoloration, and reduced fruit weight loss by more than 7% compared with the unwaxed controls. Irradiation did not affectΔEof the peel for coated fruit at day 14 + 6. Irradiation with or without coating did not affectΔEof flesh color, weight loss, TSS content, or TA. Wax coating combined with irradiation treatment of limes at doses ≤450 Gy ensured marketable visual quality and chemical composition while providing quarantine security.

Agriculture↗