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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Rotational bands beyond the Elliott model

Rotational bands are commonplace in the spectra of atomic nuclei. Inspired by early descriptions of these bands by quadrupole deformations of a liquid drop, Elliott constructed discrete nucleon representations of SU(3) from fermionic creation and annihilation operators. Ever since, Elliott's model has been foundational to descriptions of rotation in nuclei. Later work, however, suggested the symplectic extension Sp(3, R) provides a more unified picture. We decompose no-core shell-model nuclear wave functions into symmetry-defined subspaces for several beryllium isotopes, as well as 20 Ne, using the quadratic Casimirs of both Elliott's SU(3) and Sp(3, R). The band structure, delineated by strong B(E2) values, has a more consistent description in Sp(3, R) rather than SU(3). In particular, we confirm previous work finding in some nuclides strongly connected upper and lower bands with the same underlying symplectic structure.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Three series of quaternary rare-earth transition-metal pnictides with CaAl{sub 2}Si{sub 2}-type structures: RECuZnAs{sub 2}, REAgZnP{sub 2}, and REAgZnAs{sub 2}

Three series of quaternary rare-earth transition-metal pnictides REMM′Pn{sub 2} (M=Cu, Ag; M′=Zn; Pn=P, As) have been prepared by reaction of the elements at 800 °C, with crystal growth promoted through the addition of iodine. The extent of RE substitution is broad in these series: RECuZnAs{sub 2} (RE=Y, La-Nd, Sm, Gd–Lu), REAgZnP{sub 2} (RE=La–Nd, Sm, Gd–Dy), and REAgZnAs{sub 2} (RE=La-Nd, Sm, Gd-Dy). Powder and single-crystal X-ray diffraction analysis revealed that they adopt the trigonal CaAl{sub 2}Si{sub 2}-type structure (space group P3{sup ¯}m1, Z=1), in which Cu or Ag atoms are disordered with Zn atoms over the unique tetrahedrally coordinated transition-metal site. Magnetic measurements indicated Curie–Weiss behavior for several members of the RECuZnAs{sub 2} and REAgZnP{sub 2} series. Core-line X-ray photoelectron spectra (XPS) collected on some RECuZnAs{sub 2} members corroborate the charge assignment deduced by the Zintl concept for these compounds, (RE{sup 3+})(M{sup 1+})(Zn{sup 2+})(Pn{sup 3−}){sub 2}. Optical diffuse reflectance spectra and valence band XPS spectra established that these compounds are small band-gap semiconductors (up to ∼0.8 eV in REAgZnP{sub 2}) or semimetals (RECuZnAs{sub 2}). Band structure calculations also support this electronic structure and indicate that the band gap can be narrowed through appropriate chemical substitution (RE=smaller atoms, M=Cu, and Pn=As). - Graphical abstract: Cu or Ag atoms are disordered with Zn atoms over the tetrahedral site within relatively rigid [M{sub 2}Pn{sub 2}] slabs in three series of quaternary pnictides adopting the CaAl{sub 2}Si{sub 2}-type structure. - Highlights: • Three series (comprising 25 compounds) of pnictides REMM'Pn{sub 2} were prepared. • Cu or Ag atoms are disordered with Zn atoms within relatively rigid [M{sub 2}Pn{sub 2}] slabs. • They are semimetals or small band-gap semiconductors. • RECuZnAs{sub 2} and REAgZnP{sub 2} are generally paramagnetic.

36 MATERIALS SCIENCE↗
NASA NTRS1997

Electronic Structure of TlBa2CaCu2O(7-Delta)

The core levels of TlBa2CaCu2O(7-delta) (Tl-1212) epitaxial films have been measured with X-ray photoelectron spectroscopy (XPS). The valence electronic structure has been determined using the full-potential linear muffin-tin-orbital band-structure method and measured with XPS. The calculations show that a van Hove singularity (VHS) lies above the Fermi level (E(sub F)) for the stoichiometric compound (delta = 0.5), while for 50% oxygen vacancies in the Tl-O layer (delta = 0.5) E(sub F) is in close proximity to the VHS. Samples annealed in nitrogen (to reduce the hole overdoping by the removal of oxygen) exhibit higher core-level binding energies and a higher T(sub c), consistent with a shift of E(sub F) closer to the VHS. Comparisons are made to the core levels and valence bands of Tl2Ba2CaCu2O(8 + delta)(Tl-2212) and HgBa2CaCu2O)6 + delta) (Hg- 1212). The similarity of the Cu 2p(sub 3/2) spectra for Tl-1212 and Tl-2212 indicates that the number of Tl-O layers has little effect on the Cu-O bonding. However, the Tl-1212 and Hg-1212 Cu 2p(sub 3/2) signals exhibit differences which suggest that the replacement of T(sup 3+) with Hg(sup 2+) results in a decrease in the O 2p right arrow Cu 3d charge-transfer energy and differences in the probabilities of planar vs apical oxygen charge transfer and/or Zhang-Rice singlet-state formation. Differences between the Tl-1212 and the Tl-2212 and Hg-1212 measured valence bands are consistent with the calculated Cu 3d and (Tl,Hg) 6s/5d partial densities of states.

Vasquez, R. P.↗
DOE OSTI2016

Electronic properties of hexagonal gallium phosphide: A DFT investigation

A detail density functional investigation is performed to develop hexagonal 2D gallium phosphide material. The geometry, band structure and density of states (total and projected) of 2D hexagonal GaP are reported in detail. It is heartening to note that the developed material is identified as an indirect band gap semiconductor. The indirect gap for this material is predicted as 1.97 eV at K-Γ, and a direct gap of 2.28 eV at K point is achieved, which is very close to the reported direct band gap for zinc blende and buckled structures of GaP.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Elastic constants and Fermi surface topology change in Calaverite AuTe{sub 2}: A density functional study

Structural, elastic, electronic and Fermi surface studies of AuTe{sub 2} have been carried out by means of first principles calculations based on density functional theory. The calculated ground state properties agree well with the experiment. Fermi surface and elastic constants are predicted for the first time and from the calculated elastic constants we find the compound to be mechanically stable satisfying the stability criteria of monoclinic structure. In addition, we also find the c-axis to be more compressible than the other two which is also speculated from the present work. The metallic behaviour of this compound is confirmed from the electronic band structure calculation as we find the bands to cross the Fermi level (E{sub F}). In addition, we also observe a FS topology change under pressure which is also explained in the present work.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Preference of the Ferromagnetic Ordering for the Novel Heusler Rh{sub 2}MnTi Compound

Rh{sub 2}MnTi is a novel magnetic material at potentially high speed in modern technologies. Rh{sub 2}MnTi is susceptible to structural distortion of the ferromagnetic tetragonal Heusler. We tried to provide additional information on the magnetic order for this material through a study by means of first-principle calculations. The numerical work on the electronic and magnetic structural parameters of Rh{sub 2}MnTi crystal was carried out within the density functional theory level as implemented in all-electron full-potential linearized augmented plane wave (FP-LAPW) method. The dependence of the elastic constants C{sub ij} for cubic structure, the aggregate elastic moduli B and G, and the related mechanical properties are investigated. The calculated density of states (DOS) and band structure for Rh{sub 2}MnTi show the absence of energy band gap in their minority-spin channel which moves away half-metallic character, the magnetic moments, and Curie temperature in both the ferromagnetic and antiferromagnetic ordering are also investigated. The dependence of Seebeck coefficient, electrical conductivity, and power factor on the Fermi level are investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural, electronic, vibrational, and dielectric properties of LaBGeO{sub 5} from first principles

Structural, electronic, vibrational, and dielectric properties of LaBGeO{sub 5} with the stillwellite structure are determined based on ab initio density functional theory. The theoretically relaxed structure is found to agree well with the existing experimental data with a deviation of less than 0.2%. Both the density of states and the electronic band structure are calculated, showing five distinct groups of valence bands. Furthermore, the Born effective charge, the dielectric permittivity tensors, and the vibrational frequencies at the center of the Brillouin zone are all obtained. Compared to existing model calculations, the vibrational frequencies are found in much better agreement with the published experimental infrared and Raman data, with absolute and relative rms values of 6.04 cm{sup −1}, and 1.81%, respectively. Consequently, numerical values for both the parallel and perpendicular components of the permittivity tensor are established as 3.55 and 3.71 (10.34 and 12.28), respectively, for the high-(low-)frequency limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhancement of photoluminescence from defect states in ZnS random photonic crystal: An effect of electronic and photonic mode coupling

This paper reports on the enhanced defect state emission from ZnS in the form of a random photonic crystal (RPC) medium. ZnS photonic crystals with varied randomness are fabricated by colloidal self assembly of ZnS nanospheres (215 ± 10 nm). Reflection and transmission studies reveal mid band gap wavelength at ∼435 nm. The band structure calculated for BCC lattice with reduced packing fraction (53%) is in good agreement with experimental results. The reflection due to the photonic band gap diminishes with increased randomness in the nanosphere arrangement. The features of fluorescence from ZnS are modified in the RPC medium, resulting in suppression at wavelengths in the photonic band gap region and an enhancement at band edge wavelengths of 415 and 468 nm. This enhancement becomes less prominent with increasing randomness in the structure. Interestingly these two modes correspond to the electronic defect states of ZnS. Emission enhancement is shown to be due to the strong coupling of electronic defect states and photonic band edge states which is facilitated by randomly scattering slow Bloch modes in the ZnS RPC. Fabrication of RPCs by colloidal self-assembly with specifically designed degrees of randomness (leading to controllable features of emission) provides scope for the design of low threshold random lasing systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of Chemical Composition on Volume and Surface Magnon Creation in Multilayer Co{sub x}Pt{sub 1−x}/Pt

The existence of two types of magnon populations created in volume and in the surfaces of the Co{sub x}Pt{sub 1−x}/Pt multilayer was demonstrated. A band structure of these magnons was evidenced by the shape of the calculated excitation spectra. Two bands of energy allowed for a creation of these magnetic excitations are obtained. “Creation gaps” and associated lifetimes are also calculated. The comparison of the calculated relaxation times with experimental results is more than satisfactory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Sr{sub 7}Ge{sub 6}, Ba{sub 7}Ge{sub 6} and Ba{sub 3}Sn{sub 2} -Three new binary compounds containing dumbbells and four-membered chains of tetrel atoms with considerable Ge-Ge {pi}-bonding character

The germanides Sr{sub 7}Ge{sub 6} and Ba{sub 7}Ge{sub 6} as well as the stannide Ba{sub 3}Sn{sub 2} were prepared by arc melting and annealing in welded tantalum ampoules using induction as well as resistance furnaces. The compounds were investigated by powder and single crystal X-ray diffraction. Sr{sub 7}Ge{sub 6} and Ba{sub 7}Ge{sub 6} crystallize in the Ca{sub 7}Sn{sub 6} structure type (space group Pmna, Z=4: a=7.777(2) A, b=23.595(4) A, c=8.563(2) A, wR{sub 2}=0.081 (all data), 2175 independent reflections, 64 variable parameters for Sr{sub 7}Ge{sub 6} and a=8.0853(6) A, b=24.545(2) A, c=8.9782(8) A, wR{sub 2}=0.085 (all data), 2307 independent reflections, 64 variable parameters for Ba{sub 7}Ge{sub 6}). Ba{sub 3}Sn{sub 2} crystallizes in an own structure type with the space group P4{sub 3}2{sub 1}2, Z=4, a=6.6854(2) A, c=17.842(2) A, wR{sub 2}=0.037 (all data), 1163 independent reflections, 25 variable parameters. In Sr{sub 7}Ge{sub 6} and Ba{sub 7}Ge{sub 6} the Ge atoms are arranged as Ge{sub 2} dumbbells and Ge{sub 4} four-membered atom chains. Their crystal structures cannot be rationalized according to the (8-N) rule. In contrast, Ba{sub 3}Sn{sub 2} presents Sn{sub 2} dumbbells as a main structural motif and thereby can be described as an electron precise Zintl phase. The chemical bonding situation in these structures is discussed on the basis of partial and total Density Of States (DOS) curves, band structures including fatbands, topological analysis of the Electron Localization Function (ELF) as well as Bader analysis of the bond critical points using the programs TB-LMTO-ASA and WIEN2K. While Ba{sub 3}Sn{sub 2} reveals semiconducting behaviour, all germanides Ae{sub 7}Ge{sub 6} (Ae=Ca, Sr, and Ba) show metallic properties and a considerable {pi}-bonding character between the Ge atoms of the four-membered chains and the dumbbells. The {pi}-bonding character of the germanides is best reflected by the resonance hybrid structures {l_brace}[Ge-Ge]{sup 6-}/[Ge-{sup ....}Ge-{sup ....}Ge-{sup ....}Ge]{sup 8-}{r_brace}{r_reversible}{l_brace}[Ge=Ge]{sup 4-}/[Ge-Ge-Ge-Ge]{sup 10-}{r_brace}. - Graphical abstract: The structure of Ba{sub 3}Sn{sub 2} contains Sn{sub 2} dumbbells as a main structural motif and thereby can be described as an electron precise Zintl phase. Ge{sub 2} dumbbells and Ge{sub 4} four-membered atom chains are the predominant features in Sr{sub 7}Ge{sub 6} and Ba{sub 7}Ge{sub 6}. Their crystal structures cannot be rationalized according to the (8-N) rule. While Ba{sub 3}Sn{sub 2} reveals semiconducting behaviour, the germanides Ae{sub 7}Ge{sub 6} (Ae=Ca, Sr, and Ba) show metallic properties and a considerable {pi}-bonding character between the Ge atoms of the four-membered chains and the dumbbells. Highlights: Black-Right-Pointing-Pointer The germanides Sr{sub 7}Ge{sub 6} and Ba{sub 7}Ge{sub 6} as well as the stannide Ba{sub 3}Sn{sub 2} have been synthesized. Black-Right-Pointing-Pointer In Sr{sub 7}Ge{sub 6} and Ba{sub 7}Ge{sub 6} the Ge atoms are arranged as dumbbells and four-membered atom chains. Black-Right-Pointing-Pointer Ba{sub 3}Sn{sub 2} presents Sn{sub 2} dumbbells as a main structural motif. Black-Right-Pointing-Pointer The chemical bonding situation within these structures is discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Two-dimensional titanium carbonitrides and their hydroxylated derivatives: Structural, electronic properties and stability of MXenes Ti{sub 3}C{sub 2−x}N{sub x}(OH){sub 2} from DFTB calculations

The structural, electronic properties and stability of the new MXene compounds—two-dimensional pristine carbonitrides Ti{sub 3}C{sub 2−x}N{sub x} and their hydroxylated derivatives Ti{sub 3}C{sub 2−x}N{sub x}(OH){sub 2} are studied by means of DFTB calculations. The genesis of the properties is discussed in the sequence: binary MXenes Ti{sub 3}C{sub 2} (Ti{sub 3}N{sub 2})→hydroxylated forms Ti{sub 3}C{sub 2}(OH){sub 2} (Ti{sub 3}N{sub 2}(OH){sub 2})→pristine MXene Ti{sub 3}C{sub 2−x}N{sub x}→hydroxylated Ti{sub 3}C{sub 2−x}N{sub x}(OH){sub 2}. All examined materials are metallic-like. The most favorable type of OH-covering is presented by the occupation of the hollow sites between three neighboring carbon (nitrogen) atoms. Two-dimensional MXene carbonitrides with random distribution of C and N atoms are found to be thermodynamically more favorable. - Graphical abstract: The side views of the optimized atomic structures of some examined hydroxylated derivatives of MXene Ti{sub 3}CN and their electronic band structures. Display Omitted - Highlights: • Very recently 2D titanium carbonitrides have been synthesized. • Structural, electronic properties and stability for these materials were evaluated. • The hydroxylated derivatives of 2D titanium carbonitrides are examined.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Quantum Machine-Learning for Eigenstate Filtration in Two-Dimensional Materials

Quantum machine-learning algorithms have emerged to be a promising alternative to their classical counterparts as they leverage the power of quantum computers. Such algorithms have been developed to solve problems like electronic structure calculations of molecular systems and spin models in magnetic systems. However, the discussion in all these recipes focuses specifically on targeting the ground state. Herein we demonstrate a quantum algorithm that can filter any energy eigenstate of the system based on either symmetry properties or a predefined choice of the user. The workhorse of our technique is a shallow neural network encoding the desired state of the system with the amplitude computed by sampling the Gibbs–Boltzmann distribution using a quantum circuit and the phase information obtained classically from the nonlinear activation of a separate set of neurons. We show that the resource requirements of our algorithm are strictly quadratic. To demonstrate its efficacy, we use state filtration in monolayer transition metal dichalcogenides which are hitherto unexplored in any flavor of quantum simulations. We implement our algorithm not only on quantum simulators but also on actual IBM-Q quantum devices and show good agreement with the results procured from conventional electronic structure calculations. We thus expect our protocol to provide a new alternative in exploring the band structures of exquisite materials to usual electronic structure methods or machine-learning techniques that are implementable solely on a classical computer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Nonlinear optical properties of bulk cuprous oxide using single beam Z-scan at 790 nm

The two-photon absorption (TPA) coefficient β and the nonlinear index of refraction n{sub 2} for bulk cuprous oxide (Cu{sub 2}O) direct gap semiconductor single crystal have been measured by using a balance-detection Z-scan single beam technique, with an excellent signal to noise ratio. Both coefficients were measured at 790 nm using a 65 fs laser pulse at a repetition rate of 90.9 MHz, generated by a Ti:Sapphire laser oscillator. The experimental values for β were explained by using a model that includes allowed-allowed, forbidden-allowed, and forbidden-forbidden transitions. It was found that the forbidden-forbidden transition is the dominant mechanism, which is consistent with the band structure of Cu{sub 2}O. The low value for β found in bulk, as compared with respect to thin film, is explained in terms of the structural change in thin films that result in opposite parities of the conduction and valence band. The n{sub 2} is also theoretically calculated by using the TPA dispersion curve and the Kramers-Kronig relations for nonlinear optics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

High-Pressure Synthesis of Dirac Materials: Layered van der Waals Bonded BeN 4 Polymorph

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. In this paper, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN 4 . A triclinic phase of beryllium tetranitride tr-BeN 4 was synthesized from elements at similar to 85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN 4 layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated pi systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN 4 layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN 4 layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Giant proximity exchange and flat Chern band in 2D magnet-semiconductor heterostructures

van der Waals (vdW) heterostructures formed by two-dimensional (2D) magnets and semiconductors have provided a fertile ground for fundamental science and spintronics. We present first-principles calculations finding a proximity exchange splitting of 14 meV (equivalent to an effective Zeeman field of 120 T) in the vdW magnet semiconductor heterostructure MoS 2 /CrBr 3 , leading to a 2D spin-polarized half-metal with carrier densities ranging up to 10 13 cm -2 . We consequently explore the effect of large exchange coupling on the electronic band structure when the magnetic layer hosts chiral spin textures such as skyrmions. A flat Chern band is found at a “magic” value of magnetization $\overline{m}$ ~ 0.2 for Schrödinger electrons, and it generally occurs for Dirac electrons. The magnetic proximity–induced anomalous Hall effect enables transport-based detection of chiral spin textures, and flat Chern bands provide an avenue for engineering various strongly correlated states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electronic Properties of Aluminum Doped Carbon Nanotubes with Stone Wales Defects: Density Functional Theory

Al-doped single wall carbon nanotube with Stone Wales defect was theoretically analyzed, two different orientations of chiral (8, 4) carbon nanotubes was doped among the joints of defective carbon rings. Density functional theory was implemented to study structural and electronic properties of Al-doped chiral carbon nanotubes. Doping bond lengths as well as their geometrical structure were determined at the different orientations. The electronic properties were also illustrated by evaluation band of the gap energies at each possible doping site. Our results indicated that not only Al-doping tune the band structure, but also dopant site played a crucial rule on manipulating physical properties of chiral carbon nanotubes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1986

Semiconductor superlattice photodetectors

Technical progress made in the study of superlattice photoconductors is summarized and papers submitted for publication are listed. Since the quantum-well regions may contain several subbands, each of which may be occupied by electrons depending on the doping concentrations, it is important to include the multi-subbands in calculating the impact ionization rate. The electrons occupying the higher subbands require a smaller amount of energy to get out of the quantum well; thus, those higher level subband electrons contribute significantly to the impact ionization rate. The results of the subbands have been calculated. Results concerning the nonparabolicity effect of the band structure, the effect of the quantum-well size, and the effect of the band-edge discontinuity and doping are also summarized.

Chuang, S. L.↗
NASA NTRS1992

Raman spectra of seven interplanetary dust particles

The Raman shift spectra of seven interplanetary dust particles, U2034(F10), U2034(F8), U2022(B1), W7074 18, W7074 C15, W7074 C3 and W7074 A7, were measured with a Spex-1403 Raman spectrograph. The exciting radiations were the 488 nm and 514 nm line of a 5W argon ion laser. All seven spectra exhibit the 1350 and 1600 Delta/cm arbon bands, implying that the Interplanetary dust particles were coated with hydrocarbon and incompletely crystallized carbon, the part of which may be the residue of hydrocarbon contents in the particles after water loss by the heating during their entry into the earth's atmosphere. A weak band structure in the 520-610/cm range could be caused by cyclosilicates, and a weak band at 2900/cm is tentatively identified as due to hydrocarbon molecules.

Xu, Yin-Lin↗