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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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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Three interaction energy scales in the single-layer high-$T_c$ cuprate HgBa 2 CuO 4+δ

The lamellar cuprate superconductors exhibit the highest ambient-pressure superconducting transition temperatures ($T_c$), and after more than three decades of extraordinary research activity, continue to pose formidable scientific challenges. A major experimental obstacle has been to distinguish universal phenomena from materials- or technique-dependent ones. Angle-resolved photoemission spectroscopy (ARPES) measures momentum-dependent single-particle electronic excitations and has been invaluable in the endeavor to determine the anisotropic momentum-space properties of the cuprates. HgBa 2 CuO 4+δ (Hg1201) is a single-CuO 2 -layer cuprate with a particularly high optimal Tc and a simple crystal structure, yet there exists little information from ARPES about the electronic properties of this model system. In this paper we present an ARPES study of doping-, temperature-, and momentum-dependent systematics of near-nodal dispersion anomalies in Hg1201. The data reveal a hierarchy of three distinct energy scales: a subgap low-energy kink, an intermediate-energy kink near 55 meV, and a peak-dip-hump structure. The first two features are attributed to the coupling of electrons to Ba-derived optical phonons and in-plane bond-stretching phonons, respectively. The nodal peak-dip-hump structure appears to have a common doping dependence in several single-layer cuprates and is interpreted as a manifestation of pseudogap physics at the node. These results establish several universal phenomena, both in terms of connecting multiple experimental techniques for a single material and in terms of connecting comparable spectral features in multiple structurally similar cuprates.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Crystal structures and formation mechanisms of boron-rich tungsten borides

Boron-rich tungsten borides tend to adopt mechanically unfavorable layered structures but experimentally exhibit excellent mechanical properties rivalling traditional superhard solids. Unravelling the contraindicated structure-property relationship, however, has been impeded by their structural ambiguities because of the difficulty in probing boron and atomic deficiency of these borides. Here, we study crystal structures of boron-rich tungsten borides WB 3+x and WB 2+x by neutron diffraction based on high-quality samples prepared by a high-pressure method, leading to definitive structural resolutions for both borides with unique compositions of WB 5.14 and WB 2.34 . Combined with theoretical calculations, their structural stability is revealed to be closely related to atomic deficiency, which is governed by the valence-band filling with an optimal valence-electron concentration of ~10 per cell. The presence of interstitial boron trimers at the vacant W : 2b sites in WB 5.14 alters the crystal symmetry, making the Wyckoff 2d site more favorably occupied by W, rather than the 2c site, as previously misassigned. Here, the staggered planar boron layers and wrinkled boron bonding in WB 2.34 are identified to be crucial for stabilizing its structure. These findings unveil the longstanding structural mysteries of boron-rich tungsten borides and offer powerful insights for rational design of borides by defect chemistry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Atom-by-atom imaging of moiré transformations in 2D transition metal dichalcogenides

Understanding the atomic-scale mechanisms that govern the structure of interfaces is critical across materials systems but particularly so for two-dimensional (2D) moiré materials. Here, we image, atom-by-atom, the thermally induced structural evolution of twisted bilayer transition metal dichalcogenides using in situ transmission electron microscopy. We observe low-temperature, local conversion of moiré superlattice into nanoscale aligned domains. Unexpectedly, this process occurs by nucleating a new grain within one monolayer, whose crystal orientation is templated by the other. The aligned domains grow through collective rotation of moiré supercells and hopping of 5|7 defect pairs at moiré boundaries. This provides mechanistic insight into the atomic-scale interactions controlling moiré structures and illustrates the potential to pattern interfacial structure and properties of 2D materials at the nanoscale.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

High-pressure structural systematics of dysprosium metal compressed in a neon pressure medium to 182 GPa

Here we present an experimental and theoretical study of dysprosium metal compressed in the soft pressure transmitting medium Ne up to 182 and 300 GPa, respectively. Angle-dispersive x-ray powder diffraction data from each of the high-pressure polymorphs shows anisotropic compression behavior indicating changes to the electron density distribution throughout its polymorphic landscape. We compare the monoclinic (mC4) and orthorhombic (oF16) structures for the collapsed structure for Dy above 82 GPa and verify that the oF16 structure offers a better fit to our data than the previously reported mC4 structure. Further, we have found that the oF16 structure undergoes similar anisotropic compression of its lattice parameters, with a turning point above 160 GPa; suggesting a potential phase transition at pressures much higher than achieved in this study. Density functional theory calculations show the likely candidate for this new high-pressure phase is the isosymmetric oF8 structure, which is predicted to be lower in energy than the oF16 structure above 275 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Revealing the Origin and Nature of the Buried Metal‐Substrate Interface Layer in Ta/Sapphire Superconducting Films

Abstract Despite constituting a smaller fraction of the qubit's electromagnetic mode, surfaces and interfaces can exert significant influence as sources of high‐loss tangents, which brings forward the need to reveal properties of these extended defects and identify routes to their control. Here, we examine the structure and composition of the metal‐substrate interfacial layer that exists in Ta/sapphire‐based superconducting films. Synchrotron‐based X‐ray reflectivity measurements of Ta films, commonly used in these qubits, reveal an unexplored interface layer at the metal‐substrate interface. Scanning transmission electron microscopy and core‐level electron energy loss spectroscopy identified an intermixing layer (≈0.65 ± 0.05 nm) at the metal‐substrate interface containing Al, O, and Ta atoms. Density functional theory modeling reveals that the structure and properties of the Ta/sapphire heterojunctions are determined by the oxygen content on the sapphire surface prior to Ta deposition for two atomic terminations of sapphire. Using a multimodal approach, we gained deeper insights into the interface layer between the metal and substrate, which suggests that the orientation of deposited Ta films depend on the surface termination of sapphire. The observed elemental intermixing at the metal‐substrate interface influences the thermodynamic stability and electronic behavior of the film, which may also affect qubit performance.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Orbital structure of the effective pairing interaction in the high-temperature superconducting cuprates

The nature of the effective interaction responsible for pairing in the high-temperature superconducting cuprates remains unsettled. This question has been studied extensively using the simplified single-band Hubbard model, which does not explicitly consider the orbital degrees of freedom of the relevant CuO 2 planes. Here, we use a dynamical cluster quantum Monte Carlo approximation to study the orbital structure of the pairing interaction in the three-band Hubbard model, which treats the orbital degrees of freedom explicitly. We find that the interaction predominately acts between neighboring copper orbitals, but with significant additional weight appearing on the surrounding bonding molecular oxygen orbitals. By explicitly comparing these results to those from the simpler single-band Hubbard model, our study provides strong support for the single-band framework for describing superconductivity in the cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Differentiated roles of Lifshitz transition on thermodynamics and superconductivity in La 2- x Sr x CuO 4

The effect of Lifshitz transition on thermodynamics and superconductivity in hole-doped cuprates has been heavily debated but remains an open question. In particular, an observed peak of electronic specific heat is proposed to originate from fluctuations of a putative quantum critical point p* (e.g., the termination of pseudogap at zero temperature), which is close to but distinguishable from the Lifshitz transition in overdoped La-based cuprates where the Fermi surface transforms from hole-like to electron-like. Here we report an in situ angle-resolved photoemission spectroscopy study of three-dimensional Fermi surfaces in La 2- x Sr x CuO 4 thin films ( x = 0.06 to 0.35). With accurate k z dispersion quantification, the said Lifshitz transition is determined to happen within a finite range around x = 0.21. Normal state electronic specific heat, calculated from spectroscopy-derived band parameters, reveals a doping-dependent profile with a maximum at x = 0.21 that agrees with previous thermodynamic microcalorimetry measurements. The account of the specific heat maximum by underlying band structures excludes the need for additionally dominant contribution from the quantum fluctuations at p* . A d -wave superconducting gap smoothly across the Lifshitz transition demonstrates the insensitivity of superconductivity to the dramatic density of states enhancement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Invariant discovery of features across multiple length scales: Applications in microscopy and autonomous materials characterization

Physical imaging is a foundational characterization method in areas from condensed matter physics and chemistry to astronomy and spans length scales from atomic to universe. Images encapsulate crucial data regarding atomic bonding, materials microstructures, and dynamic phenomena such as microstructural evolution and turbulence, among other phenomena. The challenge lies in effectively extracting and interpreting this information. Variational Autoencoders (VAEs) have emerged as powerful tools for identifying the underlying factors of variation in image data, providing a systematic approach to distilling meaningful patterns from complex data sets. However, a significant hurdle in their application is the definition and selection of appropriate descriptors reflecting local structures. Here, we introduce the scale-invariant VAE approach (SI-VAE) based on the progressive training of the VAE with the descriptors sampled at different length scales. The SI-VAE allows the discovery of the length scale-dependent factors of variation in the system. Here, we illustrate this approach using the ferroelectric domain images and generalize it to the movies of the electron-beam induced phenomena in graphene and topography evolution across combinatorial libraries. This approach can further be used to initialize the decision making in automated experiments including structure–property discovery and can be applied across a broad range of imaging methods. This approach is universal and can be applied to any spatially resolved data including both experimental imaging studies and simulations, and can be particularly useful for exploration of phenomena such as turbulence and scale-invariant transformation fronts.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Experimental observations of large changes in electron density distributions in β - Ge

We report the electron density distributions in β-Ge have been experimentally determined at in situ high-pressure conditions using synchrotron diffraction techniques in a diamond anvil cell. Upon decompression, the electron density along the $c$ axis in tetragonal β-Ge displays a sudden drop at 10–11 GPa close to that of the structural α-β transition, while the β-Ge samples remain as single crystals until transforming to metastable Ge phases around 6.7–8.5 GPa. In contrast to the covalently bonded α-Ge that displays only a weak participation of $d$ electrons in the valence band under compression above 7.7 GPa, our experimental results suggest that a large change in $d$-orbital participation can occur in the β-Ge lattice which has mixed covalent and metallic bonding. The β-Ge below 10 GPa may display large fluctuations in electronic properties, which sheds light for exploring novel materials with intriguing electronic and optical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Band Structure and Charge Ordering of Dirac Semimetal EuAl 4 at Low Temperatures

In this article, EuAl 4 is proposed to host a topological Hall state. This material also undergoes four consecutive antiferromagnetic (AFM) transitions upon cooling below T N1 = 15.4 K in the presence of charge density wave (CDW) order that sets in below T CDW = 140 K. We use angle-resolved photoemission spectroscopy and density-functional-theory calculations to study how magnetic ordering affects the electronic properties in EuAl 4 . Here, we found changes in the band structure upon each of the four consecutive AFM transitions including band splitting, renormalizations, and appearance of new bands forming additional Fermi sheets. In addition we also found significant enhancement of the quasiparticles’ lifetime due to suppression of spin flip scattering, similar to what was previously reported for ferromagnetic EuCd 2 As 2 . Surprisingly, we observe that most significant changes in electronic properties occur not at T N1 , but instead at the AFM3 to AFM4 transition, which coincides with the largest drop in resistivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Prediction of s ± -Wave Superconductivity Enhanced by Electronic Doping in Trilayer Nickelates La 4 Ni 3 O 10 under Pressure

Motivated by the recently reported signatures of superconductivity in trilayer La 4 ⁢Ni 3 ⁢O 10 under pressure, here we comprehensively study this system using ab initio and random-phase approximation techniques. Without electronic interactions, the Ni d 3z 2 –r 2 orbitals show a bonding-antibonding and nonbonding splitting behavior via the O p z orbitals inducing a “trimer” lattice in La 4⁢ Ni 3 ⁢O 10 , analogous to the dimers of La 3 ⁢Ni 2 ⁢O 7 . The Fermi surface consists of three electron sheets with mixed e g orbitals, and a hole and an electron pocket made up of the d 3⁢z 2 –r 2 orbital, suggesting a Ni two-orbital minimum model. In addition, we find that superconducting pairing is induced in the s ± -wave channel due to partial nesting between the M = (π,π) centered pockets and portions of the Fermi surface centered at the Γ = (0,0) point. With changing electronic density n, the s ± instability remains leading and its pairing strength shows a domelike behavior with a maximum around n = 4.2 ( ~6.7% electron doping). The superconducting instability disappears at the same electronic density as that in the new 1313 stacking La 3 ⁢Ni 2 ⁢O 7 , correlated with the vanishing of the hole pocket that arises from the trilayer sublattice, suggesting that the high-T c superconductivity of La 3⁢ Ni 2 ⁢O 7 does not originate from a trilayer and monolayer structure. Furthermore, we confirm the experimentally proposed spin state in La 4 ⁢Ni 3 ⁢O 10 with an in-plane (π, π) order and antiferromagnetic coupling between the top and bottom Ni layers, and spin zero in the middle layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Competing magnetic and nonmagnetic states in monolayer VSe 2 with charge density wave

The field of two-dimensional ferromagnets has been reinvigorated by the discovery of VSe 2 monolayer grown on van der Waals substrates, which is reported to be ferromagnetic with a Curie point higher than 330 K. However, the ferromagnetic and nonmagnetic states of pristine monolayer VSe 2 are highly debated. Here, employing density functional theory, Wannier function calculations, and the band unfolding method, we explore the electronic structure of monolayer VSe 2 with a $\sqrt{3} × \sqrt{7}$ charge density wave (CDW). Certain qualitative aspects of the calculated unfolded band dispersion and unfolded Fermi surface of monolayer VSe 2 with the $\sqrt{3} × \sqrt{7}$ CDW in the nonmagnetic state agree well with previous angle-resolved photoemission spectroscopy results, albeit with uncertainty about whether these experiments probed single or multiple domains. Specifically, we find that an isolated CDW domain naturally induces a strong breaking of the threefold symmetry of the electronic structure. In addition we find that, relative to the undistorted structure, the CDW structure shows a strong competition between nonmagnetic and various magnetic states, with an energy difference less than 5 meV/f.u. For the CDW structure in the antiferromagnetic state, the band dispersions and Fermi surface are similar to those in the nonmagnetic state, while the unfolded bands of the ferromagnetic CDW state display a sizable exchange splitting. These results indicate the possibility of various antiferromagnetic fluctuations in VSe 2 to coexist and compete with ferromagnetic order and the experimentally reported CDW order. In this work, our calculations build insights for exploring the interplay between magnetism and CDW behaviors more generally in transition metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Superior Piezo-/Ferro-Electricity in Antiferroelectric Ag x NbO 3-δ Thin Films by Nanopillar Local Structure Design

Antiferroelectrics are fundamental mother compounds critical in developing innovative lead-free piezoelectrics and ferroelectrics and hold great promise for wide-ranging applications in energy conversion and electronic devices. However, harnessing their superior properties presents a significant challenge due to the delicate balance required between their various states. Here, in this study, through the unique design of nanopillar structures to alleviate the local polar heterogeneity, we have achieved significantly improved piezo-/ferro-electricity in classic lead-free antiferroelectric Ag x NbO 3-δ (x = 1, 0.9, and 0.8) epitaxial thin films. The effective piezoelectric coefficient reaches 440 pm V –1 , 1 order of magnitude larger than the stoichiometric AgNbO 3 , rivaling classic lead zirconate titanate piezoelectrics. Atomic-scale electron microscopy investigations unravel the underlying mechanisms. The nanopillars, characterized by antisite occupancy of both Ag and Nb atoms and forming out-of-phase boundaries with the matrix, reduce the local crystal symmetry via interphase strain. This leads to the creation of flexible multinanodomain structures that significantly facilitate polarization rotation, thus substantially enhancing the piezoelectric performance. This study demonstrates the feasibility of engineering local heterogeneity through nanopillar design, offering a generally applicable method for property improvement of a wide range of antiferroelectrics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Electronic structure of the Si-containing topological Dirac semimetal CaAl 2 Si 2

There has been an upsurge in the discovery of topological quantum materials, where various topological insulators and semimetals have been theoretically predicted and experimentally observed. However, very few of them contain silicon, the most widely used element in the electronics industry. Recently, ternary compound CaA l 2 S i 2 has been predicted to be a topological Dirac semimetal, hosting Lorentz-symmetry-violating quasiparticles with a strongly tilted conical band dispersion. In this work, by using high-resolution angle-resolved photoemission spectroscopy, we investigated the comprehensive electronic structure of CaA l 2 S i 2 . A pair of topological Dirac crossings is observed along the k z direction, in good agreement with the ab initio calculations, confirming the topological Dirac semimetal nature of the compound. Our study expands the topological material family on Si-containing compounds, which have great application potential in realizing low-cost, nontoxic electronic devices with topological quantum states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Three-dimensional ferromagnetism and magnetotransport in van der Waals Mn-intercalated tantalum disufide

Van der Waals (vdW) ferromagnets are an important class of materials for spintronics applications. The recent discovery of atomically vdW magnets CrI 3 and Cr 2 Ge 2 Te 6 has triggered a renaissance in the area of two-dimensional (2D) magnetism. Herein we systematically studied 2H-Mn 0.28 TaS 2 single crystal, a 2D vdW ferromagnet with T c ~ 82.3K, and a large in-plane magnetic anisotropy. Mn K-edge x-ray absorption spectroscopy was measured to provide information on its electronic state and local atomic environment. The detailed magnetic isotherms measured in the vicinity of T c indicates that the spin coupling inside 2H-Mn 0.28 TaS 2 is of a 3D Heisenberg type coupled with the attractive long-range interaction between spins that decay as J(r) ≈ r –4.85 . Both resistivity ρ(T) and thermopower S(T) exhibit anomalies near T c , confirming that the hole-type transport carriers strongly interact with local moments. Furthermore, an unusual angle-dependent magnetoresistance is further observed, suggesting a possible field-induced novel magnetic structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Growth and stability of Pb intercalated phases under graphene on SiC

Graphene intercalation is a novel way to control graphene's band structure and generate two-dimensional quantum materials with unusual spintronic and electronic properties. In spite of its importance, information about the intercalation mechanism is lacking, especially the role of low density domain boundaries between regions of graphene of different thickness. With high resolution surface diffraction we have systematically studied Pb intercalation on epi-graphene grown on SiC, with domain boundaries between buffer and single layer graphene. By examining the evolution of different diffraction spots as a function of tempertature, the location of Pb and stability of the intercalated phases underneath were determined.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Preservation of high-pressure volatiles in nanostructured diamond capsules

High pressure induces dramatic changes and novel phenomena in condensed volatiles(1,2) that are usually not preserved after recovery from pressure vessels. In this study we report a process that pressurizes volatiles into nanopores of type 1 glassy carbon precursors, converts glassy carbon into nanocrystalline diamond by heating and synthesizes free-standing nanostructured diamond capsules (NDCs) capable of permanently preserving volatiles at high pressures, even after release back to ambient conditions for various vacuum-based diagnostic probes including electron microscopy. As a demonstration, we perform a comprehensive study of a high-pressure argon sample preserved in NDCs. Synchrotron X-ray diffraction and high-resolution transmission electron microscopy show nanometre-sized argon crystals at around 22.0 gigapascals embedded in nanocrystalline diamond, energy-dispersive X-ray spectroscopy provides quantitative compositional analysis and electron energy-loss spectroscopy details the chemical bonding nature of high-pressure argon. The preserved pressure of the argon sample inside NDCs can be tuned by controlling NDC synthesis pressure. To test the general applicability of the NDC process, we show that high-pressure neon can also be trapped in NDCs and that type 2 glassy carbon can be used as the precursor container material. Additional experiments on other volatiles and carbon allotropes open the possibility of bringing high-pressure explorations on a par with mainstream condensed-matter investigations and applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Attosecond intra-valence band dynamics and resonant-photoemission delays in W(110)

Time-resolved photoelectron spectroscopy with attosecond precision provides new insights into the photoelectric effect and gives information about the timing of photoemission from different electronic states within the electronic band structure of solids. Electron transport, scattering phenomena and electron-electron correlation effects can be observed on attosecond time scales by timing photoemission from valence band states against that from core states. However, accessing intraband effects was so far particularly challenging due to the simultaneous requirements on energy, momentum and time resolution. Here we report on an experiment utilizing intracavity generated attosecond pulse trains to meet these demands at high flux and high photon energies to measure intraband delays between sp- and d- band states in the valence band photoemission from tungsten and investigate final-state effects in resonant photoemission.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗