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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Imaging moiré flat bands in three-dimensional reconstructed WSe 2 /WS 2 superlattices

Moiré superlattices in transition metal dichalcogenide (TMD) heterostructures can host novel correlated quantum phenomena due to the interplay of narrow moiré flat bands and strong, long-range Coulomb interactions. However, microscopic knowledge of the atomically reconstructed moiré superlattice and resulting flat bands is still lacking, which is critical for fundamental understanding and control of the correlated moiré phenomena. Here we quantitatively study the moiré flat bands in three-dimensional (3D) reconstructed WSe 2 /WS 2 moiré superlattices by comparing scanning tunnelling spectroscopy (STS) of high-quality exfoliated TMD heterostructure devices with ab initio simulations of TMD moiré superlattices. A strong 3D buckling reconstruction accompanied by large in-plane strain redistribution is identified in our WSe 2 /WS 2 moiré heterostructures. STS imaging demonstrates that this results in a remarkably narrow and highly localized K-point moiré flat band at the valence band edge of the heterostructure. A series of moiré flat bands are observed at different energies that exhibit varying degrees of localization. Our observations contradict previous simplified theoretical models but agree quantitatively with ab initio simulations that fully capture the 3D structural reconstruction. Furthermore, our results reveal that the strain redistribution and 3D buckling in TMD heterostructures dominate the effective moiré potential and the corresponding moiré flat bands at the Brillouin zone K points.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Hourglass-type bulk Ni 3 d band and Ce 4 f Kondo resonance states in the potential topological Kondo semimetal CeNiSn via angle-resolved photoemission spectroscopy

The electronic structure of CeNiSn, which is a potential topological Kondo insulator and a Dirac nodal-loop semimetal, has been investigated by employing temperature (T) dependent angle-resolved photoemission spectroscopy (ARPES). The Fermi surfaces (FSs) and the band structures of CeNiSn for three orthogonal crystallographic planes are measured, in which both the very dispersive bands and the flat bands are observed, having mainly the Ni 3d character and the Ce 4f character, respectively. The measured FSs and ARPES bands agree reasonably well with the density functional theory (DFT) calculations. The Fermi-edge (E F ) photon energy (hν) map along k b (=k (010) ) shows that the metallic E F -crossing states on the (010) surface have the three-dimensional character, suggesting that the observed E F -crossing metallic states do not correspond to the topological surface states of the two-dimensional character. On the other hand, albeit weak, the features of the hourglass-type bulk band crossings are observed along SXS, with the energies and the slopes being similar to those predicted by the DFT calculations, supporting the Dirac semimetallic nature of CeNiSn. In T-dependent ARPES, the Ce 4f Kondo resonance states are clearly revealed at low T, which become much suppressed above ~80 K. This feature is consistent with the Kondo temperature of CeNiSn, estimated from its ρ(T) data. This work demonstrates the importance of the coherent Kondo states in determining the topological properties of CeNiSn.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Materials Data on K2S by Materials Project

K2S is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. K1+ is bonded to four equivalent S2- atoms to form a mixture of edge and corner-sharing KS4 tetrahedra. All K–S bond lengths are 3.23 Å. S2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K2Te by Materials Project

K2Te is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. K1+ is bonded to four equivalent Te2- atoms to form a mixture of edge and corner-sharing KTe4 tetrahedra. All K–Te bond lengths are 3.56 Å. Te2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on KTeAu by Materials Project

KAuTe crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. K1+ is bonded to six equivalent Au1+ and six equivalent Te2- atoms to form a mixture of edge and face-sharing KTe6Au6 cuboctahedra. All K–Au bond lengths are 3.73 Å. All K–Te bond lengths are 3.73 Å. Au1+ is bonded in a distorted trigonal planar geometry to six equivalent K1+ and three equivalent Te2- atoms. All Au–Te bond lengths are 2.75 Å. Te2- is bonded in a distorted trigonal planar geometry to six equivalent K1+ and three equivalent Au1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K2Ce(PO4)2 by Materials Project

K2Ce(PO4)2 crystallizes in the tetragonal I4_1/amd space group. The structure is three-dimensional. K1+ is bonded to four equivalent O2- atoms to form distorted KO4 hexagonal bipyramids that share corners with four equivalent PO4 tetrahedra and edges with two equivalent KO4 hexagonal bipyramids. All K–O bond lengths are 2.82 Å. Ce4+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are four shorter (2.26 Å) and four longer (2.56 Å) Ce–O bond lengths. P5+ is bonded to four O2- atoms to form PO4 tetrahedra that share corners with four equivalent KO4 hexagonal bipyramids. All P–O bond lengths are 1.55 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Ce4+ and one P5+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to two equivalent K1+, one Ce4+, and one P5+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K2Se by Materials Project

K2Se is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. K1+ is bonded to four equivalent Se2- atoms to form a mixture of edge and corner-sharing KSe4 tetrahedra. All K–Se bond lengths are 3.36 Å. Se2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K8NO3 by Materials Project

K8NO3 is Fluorite-derived structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. K1+ is bonded to one N2- and three equivalent O2- atoms to form a mixture of edge and corner-sharing KNO3 tetrahedra. The K–N bond length is 2.88 Å. All K–O bond lengths are 2.83 Å. N2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms. O2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K8PO3 by Materials Project

K8PO3 crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. K1+ is bonded to one P2- and three equivalent O2- atoms to form a mixture of distorted edge and corner-sharing KPO3 tetrahedra. The K–P bond length is 3.13 Å. All K–O bond lengths are 2.92 Å. P2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms. O2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K8N3O by Materials Project

K8N3O is Fluorite-derived structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. K1+ is bonded to three equivalent N2- and one O2- atom to form a mixture of edge and corner-sharing KN3O tetrahedra. All K–N bond lengths are 2.94 Å. The K–O bond length is 2.84 Å. N2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms. O2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K3SO4F by Materials Project

K3SO4F crystallizes in the cubic P-43m space group. The structure is three-dimensional. K1+ is bonded to four equivalent O2- and two equivalent F1- atoms to form distorted KO4F2 pentagonal pyramids that share corners with two equivalent KO4F2 pentagonal pyramids, corners with four equivalent SO4 tetrahedra, and edges with eight equivalent KO4F2 pentagonal pyramids. All K–O bond lengths are 2.78 Å. Both K–F bond lengths are 2.73 Å. S6+ is bonded to four equivalent O2- atoms to form SO4 tetrahedra that share corners with twelve equivalent KO4F2 pentagonal pyramids. All S–O bond lengths are 1.50 Å. O2- is bonded in a distorted rectangular see-saw-like geometry to three equivalent K1+ and one S6+ atom. F1- is bonded to six equivalent K1+ atoms to form corner-sharing FK6 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K8SbO3 by Materials Project

K8SbO3 crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. K1+ is bonded to one Sb2- and three equivalent O2- atoms to form a mixture of distorted edge and corner-sharing KSbO3 tetrahedra. The K–Sb bond length is 3.29 Å. All K–O bond lengths are 2.98 Å. Sb2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms. O2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K4NO by Materials Project

K4NO is Fluorite-derived structured and crystallizes in the tetragonal P4/mmm space group. The structure is three-dimensional. K1+ is bonded to two equivalent N2- and two equivalent O2- atoms to form a mixture of edge and corner-sharing KN2O2 tetrahedra. Both K–N bond lengths are 2.92 Å. Both K–O bond lengths are 2.84 Å. N2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms. O2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K8AsO3 by Materials Project

K8AsO3 crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. K1+ is bonded to one As2- and three equivalent O2- atoms to form a mixture of distorted edge and corner-sharing KAsO3 tetrahedra. The K–As bond length is 3.17 Å. All K–O bond lengths are 2.93 Å. As2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms. O2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K3Tc by Materials Project

KK2Tc is alpha bismuth trifluoride structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional and consists of four potassium molecules and one K2Tc framework. In the K2Tc framework, K1+ is bonded to four equivalent Tc3- atoms to form a mixture of edge and corner-sharing KTc4 tetrahedra. All K–Tc bond lengths are 3.87 Å. Tc3- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2024

The Simons Observatory: Design, Integration, and Testing of the Small Aperture Telescopes

The Simons Observatory (SO) is a cosmic microwave background survey experiment that includes small-aperture telescopes (SATs) observing from an altitude of 5200 m in the Atacama Desert in Chile. The SO SATs will cover six spectral bands between 27 and 280 GHz to search for primordial B-modes to a sensitivity of σ(r) = 0.002, with quantified systematic errors well below this value. Each SAT is a self-contained cryogenic telescope with a 35° field of view, 42 cm diameter optical aperture, 40 K half-wave plate, 1 K refractive optics, and <0.1 K focal plane that holds >12,000 transition edge sensor detectors. We describe the nominal design of the SATs and present details about the integration and testing for one operating at 93 and 145 GHz.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1975

Continued development of abradable gas path seals

Major program objectives were the continued development of NiCrAlY feltmetal and honeycomb systems for knife edge seal applications in the 1144 to 1366 K temperature range, and to initiate abradable seal material evaluation for blade tip seal applications in the 1366 to 1589 K temperature range. Larger fiber size, higher density feltmetal showed greatly improved erosion resistance with a slight reduction in abradability compared to the baseline feltmetal. Pack aluminide coating of the honeycomb extended the oxidation resistance and slightly improved the abradability of this material. Evaluation through selected abradability, erosion and oxidation testing, and pertinent metallography led to selection of a plasma sprayed yttria stabilized zirconia (ZrO2)/CoCrAlY layered system as the system with the most potential to meet the 1589 K requirement for blade tip seals. This system demonstrated structural integrity, erosion resistance, and some degree of abradability.

Shiembob, L. T.↗
NASA NTRS1985

The gaseous component of the disk around Beta Pictoris

Optical spectra of alpha Lyr, alpha PsA, and beta Pic have been obtained at a velocity resolution of 3 km/s. No circumstellar absorption lines of Ca II or Na I are detected toward alpha Lyr or alpha PsA at sensitive limits. In the favorable case of beta Pic, where the circumstellar disk imaged by Smith and Terrile (1984) is seen nearly edge-on, a strong, narrow, circumstellar Ca II K absorption line previously reported by Slettebak (1982) and weaker, still narrower circumstellar Na I D lines are detected. Negative results of high sensitivity also are obtained for the Ca I 4226 A and CH(+) 4232 A lines, along with upper limits on the Zn II 2026, 2062 A doublet from archival IUE spectra. Under assumptions which agree with other well-established observations of the gaseous abundances of calcium and zinc, the total gaseous column density of hydrogen along a radius of the circumstellar disk is between 10 to the 18th and 4 x 10 to the 20th/sq cm. Within the boundaries of the dust disk detected by Smith and Terrile (1984) the total gaseous mass then is less than about 2, or less than 1 percent of the mass of the planetary system. A simplified model of the density distribution in the gaseous disk yields a characteristic total density n(H) of about 100,000/cu cm, which exceeds that of all interplanetary gas at earth's position by a factor of about 10,000.

Hobbs, L. M.↗