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DOE OSTI2022

Revisiting excitation gaps in the fractional quantum Hall effect

Recent systematic measurements of the quantum well width dependence of the excitation gaps of fractional quantum Hall states in high mobility samples open the possibility of a better quantitative understanding of this important issue. Here we present what we believe to be accurate theoretical gaps including the effects of finite width and Landau level (LL) mixing. While theory captures the width dependence, there still remains a deviation between the calculated and the measured gaps, presumably caused by disorder. It is customary to model the experimental gaps of the n/(2n ± 1) states as Δ n/(2n+1) = Ce 2 /[(2n ± 1)ϵl]-Γ, where ϵ is the dielectric constant of the background semiconductor and l is the magnetic length; the first term is interpreted as the cyclotron energy of composite fermions and Γ as a disorder-induced broadening of composite-fermion LLs. Fitting the gaps for various fractional quantum Hall states, we find that Γ can be nonzero even in the absence of disorder.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Gaps in topological magnon spectra: Intrinsic versus extrinsic effects

Determining and explaining the presence of a gap at a magnon crossing point is a critical step to characterize the topological properties of a material. An inelastic neutron scattering study of a single crystal is a powerful experimental technique to probe the magnetic excitation spectra of topological materials. Here, we show that when the scattering intensity rapidly disperses in the vicinity of a crossing point, such as a Dirac point, the apparent topological gap size is extremely sensitive to experimental conditions including sample mosaic, resolution, and momentum integration range. In this work, we demonstrate these effects using comprehensive neutron scattering measurements of CrCl 3 . Our measurements confirm the gapless nature of the Dirac magnon in CrCl 3 , but also reveal an artificial, i.e., extrinsic, magnon gap unless the momentum integration range is carefully controlled. Our study provides an explanation of the discrepancies between spectroscopic and first-principles estimates of Dirac magnon gap sizes and provides guidelines for accurate measurement of topological magnon gaps.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Equation of state, phase transitions, and band-gap closure in Pb Cl 2 and Sn Cl 2

The equations of state and band-gap closures for Pb Cl 2 and Sn Cl 2 were studied using both experimental and theoretical methods. Here we measured the volume of both materials to a maximum pressure of 70 GPa using synchrotron-based angle-dispersive powder x-ray diffraction. The lattice parameters for both compounds showed anomalous changes between 16–32 GPa, providing evidence of a phase transition from the cotunnite structure to the related Co 2 Si structure, in contrast to the postcotunnite structure as previously suggested. First-principles calculations confirm this finding and predict a second phase transition to a Co 2 Si-like structure between 75– 110 GPa in Pb Cl 2 and 60–75 GPa in Sn Cl 2 . Band gaps were measured under compression to ~70 GPa for Pb Cl 2 and ~66 GPa for Sn Cl 2 and calculated up to 200 GPa for Pb CI 2 and 120 GPa for Sn Cl 2 . We find an excellent agreement between our experimental and theoretical results when using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, which suggests that this functional could reliably be used to calculate the band gap of similar AX 2 compounds. Experimental and calculated band-gap results show discontinuous decreases in the band gap corresponding to phase changes to higher-coordinated crystal structures, giving insight into the relationship between interatomic geometry and metallicity.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Energy Gap Closure of Crystalline Molecular Hydrogen with Pressure

Here, we study the gap closure with pressure of crystalline molecular hydrogen. The gaps are obtained from grand-canonical quantum Monte Carlo methods properly extended to quantum and thermal crystals, simulated by coupled electron ion Monte Carlo methods. Nuclear zero point effects cause a large reduction in the gap (~2 eV). Depending on the structure, the fundamental indirect gap closes between 380 and 530 GPa for ideal crystals and 330–380 GPa for quantum crystals. Beyond this pressure the system enters into a bad metal phase where the density of states at the Fermi level increases with pressure up to ~450–500 GPa when the direct gap closes. Our work partially supports the interpretation of recent experiments in high pressure hydrogen.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2021

Hybrid density functional study of band gap engineering of SrTi O 3 photocatalyst via doping for water splitting

Perovskite SrTiO 3 (STO) is an attractive photocatalyst for solar water splitting but suffers from a limited photoresponse in the visible spectral range due to its wide band gap. By means of hybrid density functional theory calculations, we systematically study engineering its band gap via doping 4$\textit{d}$ and 5$\textit{d}$ transition-metals $\textit{M}$ (M = Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, and Pt) and chalcogen elements $\textit{Y}$ ($\textit{Y}$ = S and Se).We find that transition-metal dopant M either has no effect on the STO band gap or introduces detrimental midgap states except for Pd and Pt that are able to reduce the STO band gap. In contrast, doping S and Se significantly reduces STO’s direct band gap, thus, leading to appreciable optical absorption transitions in the visible spectral range. In this work, our findings provide that Pd-, S-, and Se-doped STO are potential promising photocatalysts for water splitting under visible light irradiation thereby providing insightful theoretical guides for experiments to improve the photocatalytic activity of STO.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Rise and fall of Mott insulating gaps in YNiO 3 paramagnets as a reflection of symmetry breaking and remaking

The YNiO 3 nickelate is a paradigm d-electron oxide that manifests the intriguing temperature-mediated sequence of three phases transitions from (i) magnetically ordered insulator to (ii) paramagnetic (PM) insulator and then to (iii) PM metal. Such phenomena raised the question of the nature of the association of magnetism and structural symmetry breaking with the appearance in (i) and (ii) and disappearance in (iii) of insulating band gaps. It is demonstrated here that first-principles mean-field–like density-functional theory (DFT), driven by molecular dynamics temperature evolution, can describe not only the origin of the magnetically long-range ordered insulating phase (i), but also the creation of an insulating paramagnet (ii) that lacks spin- long-range order, and of a metallic paramagnet (iii) as temperature rises. This approach provides the patterns of structural and magnetic symmetry breaking at different temperatures, in parallel with band gaps obtained when the evolving geometries are used as input to DFT electronic band-structure calculations. This disentangles the complex interplay among spin, charge, and orbital degrees of freedom. Analysis shows that the success in describing the rise and fall of the insulating band gaps along the phase transition sequence is enabled by allowing sufficient flexibility in describing diverse local structural and magnetic motifs as input to DFT. This entails the use of sufficiently large supercells that allow expressing structural disproportionation of octahedra, as well as a description of PM phases as a distribution of local magnetic moments (rather than using a single averaged moment). It appears that the historic dismissal of mean-field–like DFT as being unable to describe such Mott-like transitions was premature, as it was based on consideration of averaged crystallographic unit cells, a description that washes out local symmetry-breaking motifs. The magnetically ordered insulating YNiO 3 phase (i) and the PM insulating phase (ii) result in DFT from allowing symmetry breaking, evident already by considering the athermal internal energy. In contrast, the PM metallic phase (iii) is formed thermally by smearing out thus weakening symmetry breaking. Analysis of snapshots of the different forms of structural vs magnetic symmetry breaking shows that only the loss of the polymorphous distribution of magnetic moments existing in (ii) causes the fall of the band gap, resulting in the metallic state in (iii). Finally, the interesting conclusion is that such a description of the rise [in phases (i) and (ii)] and fall [in phase (iii)] of the insulating gap does not rely on the traditional Mott-like strong correlation understanding, but on breaking and remaking of magnetic and structural symmetries reflected in energy lowering.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Corroborating Channel Gap Probe Measurements with an Optical Profiler

As nuclear fuel plates get irradiated, the fuel plate thickness changes. How the fuel plate changes with irradiation is a significant parameter that is important to predict fuel performance in new fuel systems. The channel gap probe is used to indirectly infer changes in fuel plate thicknesses by measuring the gaps between the fuel plates contained within capsules after each irradiation cycle. Independent measures should be used to corroborate salient measurements such as changes in fuel thickness which help ascertain fuel performance. The use of traditional metrological tools to validate channel gap probe measurements are not practical due to the capsule geometry. The accuracy of the channel gap probe inserted within a capsule has not been verified. Since there is no “gold” standard measurement to verify CGP measurements, an optical profilometer has been used to corroborate the CGP measurements. The optical profilometer has been able to corroborate the capsules channel gaps to within 0.04 mm for both ends of the capsule. The optical profiler is also able to measure plate thickness that matched micrometer measurements to within 0.025 mm. In conclusion, the profilometer corroborated the CGP measurements but a discrepancy was found at one end of the capsule for the Inside Capsule Width from the profilometer when compared to a caliper measurement.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2024

Anomalous normal-state gap in an electron-doped cuprate

In the underdoped n-type cuprate Nd 2-x Ce x CuO 4 , long-range antiferromagnetic order reconstructs the Fermi surface, resulting in a putative antiferromagnetic metal with small Fermi pockets. Using angle-resolved photoemission spectroscopy, here we observe an anomalous energy gap, an order of magnitude smaller than the antiferromagnetic gap, in a wide portion of the underdoped regime and smoothly connecting to the superconducting gap at optimal doping. After considering all the known ordering tendencies in tandem with the phase diagram, we hypothesize that the normal-state gap in the underdoped n-type cuprates originates from Cooper pairing. In conclusion, the high temperature scale of the normal-state gap raises the prospect of engineering higher transition temperatures in the n-type cuprates comparable to those of the p-type cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Survey and Gap Prioritization of U.S. Electric Vehicle Charge Management Deployments

The goal of this study was to survey and characterize the scope of current technical and programmatic knowledge pertaining to EV charge management technologies and practices in the US and relevant international jurisdictions. This characterization of existing field demonstrations and knowledge derived were used to determine gaps in the SCM demonstration landscape. Addressing these gaps through research and demonstration could increase confidence in the U.S. that load management and EV charge control could achieve overarching societal benefits. A survey of charge management deployments and input from stakeholders was completed to determine the state-of-the-art of smart charge management (SCM) where SCM is defined as controlling the amount of power exchanged between chargers and EVs to meet customers' charging needs while also responding to external power demand or pricing signals to provide load management, resilience, or other benefits to the customer and electric grid. The survey was the basis of the gap analysis in this report and determines which areas are well understood, with high confidence, and which areas need further investigation. Existing examples of EV charge management are characterized here to determine aspects that are ready for widespread deployment and have been demonstrated in the field. These include demonstration studies, pilots, programs, and EV-specific tariffs. In all, 110 examples of charge management were characterized. The data sources were public literature and utility filings as well as targeted interviews. In addition, 43 interviews with stakeholders were conducted with a consistent set of questions used in each interview. This study prioritized gaps in demonstrated SCM capabilities based on 1) Urgency of the particular use-case to offset traditional grid assets, 2) Impact, extensibility, and scaling of results across the entire spectrum of 3000+ utility service territories including projected technical and market potential for a given grid service, and 3) Value of federal funding in addressing the gap, including potential to leverage and/or add scope to existing field demonstrations funded by other non-federal funding mechanisms.

33 ADVANCED PROPULSION SYSTEMS↗

DOE OSTI2024

Hydropower Supply Chain Gap Analysis

In 2022, DOE conducted supply chain "deep dives" for renewable energy technologies, including hydropower (Uria-Martinez, Hydropower Industry Supply Chain Deep Dive Assessment 2022). The deep dive identified several challenges in the current hydropower supply chain. In addition, Nguyen et. al (2022) conducted an analogous deep-dive assessment on large (> 100-MW) power transformers (LPTs), a critical component of hydropower installations, and concluded that the LPTs as well as several upstream components and materials also have domestic supply chain challenges. These deep dives were the initial high-level assessments of these supply chains and were focused on identifying the biggest issues. Both recommended further investigation. In the two years since the deep dives were published, the Water Power Technologies Office (WPTO) has focused on improving our understanding of the hydropower supply chain and developing strategies for addressing these challenges. Because the challenges outlined above are most acute for large hydropower systems, most of the report and specifically, this report concentrates on the larger > 100-MW hydropower systems. Early in 2023, DOE's Secretary of Energy asked the Water Power Technologies Office (WPTO) to engage the hydropower community and seek input on strategies to secure and encourage domestic manufacturing. WPTO has established three focus areas for engagement: 1) Define the market for planned rehabilitations and new construction of the domestic fleet, 2) Provide insights for policies, incentives, loan programs, and technology investments to encourage domestic content, and 3) Define the existing and required domestic hydropower manufacturing capabilities and workforce. This report summarizes these efforts and complements the earlier work by further exploring the identified challenges and identifying potential actions to address these challenges. Furthermore, we conducted a detailed gap analysis of the domestic hydropower supply chain, down to the component level. From this analysis, we then make specific, actionable recommendations for closing these gaps. Section 2 of the report summarizes recent (i.e., since 2021) legislation impacting hydropower deployment and/or its supply chain. It then describes the efforts of WPTO to assess and improve the hydropower supply chain since the publication of the deep-dive assessments. In Section 3, the report updates the earlier supply chain and market studies, identifying specific capabilities by company and location. Section 4 outlines the hydropower demand signal for both new builds due to clean energy goals as well as refurbishments and upgrading of the current domestic fleet. Section 5 is a detailed gap analysis while Section 6 provides actionable recommendations for closing the gaps. Section 7 concludes the report by linking the recommendations to the identified gaps and discusses future efforts.

13 HYDRO ENERGY↗

DOE OSTI2020

Two Views of the Radius Gap and the Role of Light Curve Fitting

Recently, several groups have resolved a gap that bifurcates planets between the size of Earth and Neptune into two populations. The location and depth of this feature is an important signature of the physical processes that form and sculpt planets. In particular, planets residing in the radius gap are valuable probes of these processes as they may be undergoing the final stages of envelope loss. Here, we discuss two views of the radius gap by Fulton & Petigura (F18) and Van Eylen et al. (V18). In V18, the gap is wider and more devoid of planets. This is due, in part, to V18's more precise measurements of planet radius R{sub p}. Thanks to Gaia, uncertainties in stellar radii R{sub ⋆} are no longer the limiting uncertainties in determining R{sub p} for the majority of Kepler planets; instead, errors in R{sub p}/R{sub ⋆} dominate. V18's analysis incorporated short-cadence photometry along with constraints on mean stellar density that enabled more accurate determinations of R{sub p}/R{sub ⋆}. In the F18 analysis, less accurate R{sub p}/R{sub ⋆} blurs the boundary the radius gap. The differences in R{sub p}/R{sub ⋆} are largest at high impact parameter (b≳0.8) and often exceed 10%. This motivates excluding high-b planets from demographic studies, but identifying such planets from long-cadence photometry alone is challenging. We show that transit duration can serve as an effective proxy, and we leverage this information to enhance the contrast between the super-Earth and sub-Neptune populations.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1972

Enhanced indirect optical absorption in AlAs and GaP.

Measurement and analysis of indirect interband optical absorption in GaP. Deviations from a simple indirect absorption law are shown to be consistent with those given by a model which takes into account both the variation of the energy denominators with photon energy and indirect absorption to higher-energy conduction-band valleys. The analysis of the GaP data indicates that such an analysis on data of limited absorption-constant range can provide reasonably accurate values of the direct band gap. The value of the direct band gap best fitting this model for AlAs at 6 K is 3.13 eV. In GaP and AlAs there is evidence of a higher set of extrema giving rise to an additional indirect absorption component 0.34 and 0.20 eV, respectively, above that due to the lowest-energy conduction-band minima. The strength of the absorption due to these higher-energy valleys indicates that they contain states of X sub 3 symmetry.

Dumke, W. P.↗

NASA NTRS1978

Aerodynamic heating in gaps of thermal protection system tile arrays in laminar and turbulent boundary layers

An experimental heat-transfer investigation was conducted on two staggered arrays of metallic tiles in laminar and turbulent boundary layers. This investigation was conducted for two purposes. The impingement heating distribution where flow in a longitudinal gap intersects a transverse gap and impinges on a downstream blocking tile was defined. The influence of tile and gap geometries was analyzed to develop empirical relationships for impingement heating in laminar and turbulent boundary layers. Tests were conducted in a high temperature structures tunnel at a nominal Mach number of 7, a nominal total temperature of 1800 K, and free-stream unit Reynolds numbers from 1.0 x 10 million to 4.8 x 10 million per meter. The test results were used to assess the impingement heating effects produced by parameters that include gap width, longitudinal gap length, slope of the tile forward-facing wall, boundary-layer displacement thickness, Reynolds number, and local surface pressure.

Avery, D. E.↗

NASA NTRS1977

Design of single-winding energy-storage reactors for dc-to-dc converters using air-gapped magnetic-core structures

A procedure is presented for designing air-gapped energy-storage reactors for nine different dc-to-dc converters resulting from combinations of three single-winding power stages for voltage stepup, current stepup and voltage stepup/current stepup and three controllers with control laws that impose constant-frequency, constant transistor on-time and constant transistor off-time operation. The analysis, based on the energy-transfer requirement of the reactor, leads to a simple relationship for the required minimum volume of the air gap. Determination of this minimum air gap volume then permits the selection of either an air gap or a cross-sectional core area. Having picked one parameter, the minimum value of the other immediately leads to selection of the physical magnetic structure. Other analytically derived equations are used to obtain values for the required turns, the inductance, and the maximum rms winding current. The design procedure is applicable to a wide range of magnetic material characteristics and physical configurations for the air-gapped magnetic structure.

Ohri, A. K.↗

NASA NTRS1978

Prediction of orbiter RSI tile gap heating ratios from NASA/Ames double wedge model test

In-depth gap heating ratios for Orbiter RSI tile sidewalls were predicted based on near steady state temperature measurements obtained from double wedge model tests. An analysis was performed to derive gap heating ratios which would result in the best fit of test data; provide an assessment of open gap response, and supply the definition of gap filler requirements on the Orbiter. A comparison was made of these heating ratios with previously derived ratios in order to verify the extrapolation of the wing glove data to Orbiter flight conditions. The analysis was performed with the Rockwell TPS Multidimensional Heat Conduction Program for a 3-D, 2.0-inch thick flat RSI tile with 255 nodal points. The data from 14 tests was used to correlate with the analysis. The results show that the best-fit heating ratios at the station farthest upstream on the model for most gap depths were less than the extrapolated values of the wing glove model heating ratios. For the station farthest downstream on the model, the baseline heating ratios adequately predicted or over-predicted the test data.

Source record↗

NASA NTRS1982

Tile-Gap Measurement Tool

Hand-held tool measures small gaps between tiles rapidly and accurately, even when gap is tapered or indented below surface. Tool indicates gap dimensions on calibrated disk. Measurements are accurate within plus or minus 0.003 inch. Tool was developed for determining gap between tiles on Space Shuttle, but may be of use in other applications requiring precise setting of gaps between tiles or other structures.

Helman, D. H.↗

NASA NTRS1980

Aerothermal environment in chordwise gaps between split elevons at Mach 6.8

A large scale model representing a wing elevon junction on a shuttle-type entry vehicle was aerothermally tested in the Langley 8 foot high temperature structures tunnel with a turbulent boundary layer on the wing and elevon. The flow pattern between elevons was studied, and the pressure and heat load were determined within the chordwise gaps formed by adjacent elevons and by the stub fairing which separates the elevons. Model angle, elevon deflection angle, and gap widths were varied. Heating in the gaps was generally proportional to windward surface heating and inversely proportional to gap width. Maximum heating between the elevon and stub was 36 percent of the windward wing surface for a 0.18 cm wide elevon stub gas, and maximum heating between elevons was 30 percent of the heating on the windward elevon surfaces for a 7.7 cm wide elevon gap.

Hunt, L. R.↗

NASA NTRS1982

Flow through the tile gaps in the Space Shuttle Thermal Protection System

The problem of predicting aerodynamic loads on the insulating tiles of the Space Shuttle Thermal Protection System (TPS) is discussed and seen to require a method for predicting pressure and mass flux in the gaps between tiles. A mathematical model of the tile-gap flow is developed based upon a slow viscous (Stokes) flow analysis and is verified against available experimental data. This model derives the tile-gap pressure field from a solution of the two-dimensional Laplace equation; the mass flux vector is then calculated from the pressure gradient. The means for incorporating this model into a lumped-parameter network analogy for porous-media flow is also given. The flow model shows tile-gap mass flux to be very sensitive to the gap width indicating a need for coupling the TPS flow and tile displacement calculations. Finally recommendations are made concerning additional analytical and experimental work to improve TPS flow predictions.

Dwoyer, D. L.↗