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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Anisotropic upper critical field of pristine and proton-irradiated single crystals of the magnetically ordered superconductor RbEuFe{sub 4}As{sub 4

We present a study of the upper critical field H-c2 of pristine and proton-irradiated RbEuFe4As4 crystals in pulsed magnetic fields of up to 65 T. The data for H-c2 reveal pronounced downwards curvature, particularly for the in-plane field orientation, and a superconducting anisotropy that decreases with decreasing temperature. These features are indicative of Pauli paramagnetic limiting. For the interpretation of these data, we use a model of a clean single-band superconductor with an open Fermi surface in the shape of a warped cylinder, which takes into account Zeeman spin splitting. Fits to the data reveal that the in-plane upper critical field is Pauli paramagnetic limited, while the out-of-plane upper critical field is orbitally limited and that the orbital and paramagnetic fields have opposite anisotropies. A consequence of this particular combination is the unusual inversion of the anisotropy H-c2(ab) < H-c2(c) of the irradiated sample at temperatures below 10 K. The fits also yield an in-plane Maki parameter alpha(110)(M) approximate to 2.6 exceeding the critical value for the formation of the Fulde-Ferrell-Larkin-Ovchinnikov state. Nevertheless, the current measurements did not reveal direct evidence for the occurrence of this state.

Smylie, M. P.↗
DOE OSTI2015

Loss for photoemission versus gain for Auger: Direct experimental evidence of crystal-field splitting and charge transfer in photoelectron spectroscopy

Here, we find a 5 eV satellite in the Ti1s photoelectron spectrum of the transition-metal oxide SrTiO 3 . This satellite appears in addition to the well-studied 13 eV structure that is typically associated with the Ti2p core line. We give direct experimental evidence that the presence of two satellites is due to the crystal-field splitting of the metal 3d orbitals. They originate from ligand 2pt 2 g → metal3dt 2 g and ligand 2pe g → metal 3de g monopole charge-transfer excitations within the sudden approximation of quantum mechanics. This assignment is made by the energetics of the resonant and high-energy threshold behaviors of the TiK–L 2 L 3 Auger decay that follows Ti1s photoionization.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Quenched crystal-field disorder and magnetic liquid ground states in Tb 2 Sn 2 - x Ti x O 7 [Crystal field disorder in the quantum spin ice ground state of Tb2Sn2-xTixO7]

Solid-solutions of the "soft" quantum spin ice pyrochlore magnets Tb 2 B 2 O 7 with B=Ti and Sn display a novel magnetic ground state in the presence of strong B-site disorder, characterized by a low susceptibility and strong spin fluctuations to temperatures below 0.1 K. These materials have been studied using ac-susceptibility and muSR techniques to very low temperatures, and time-of-flight inelastic neutron scattering techniques to 1.5 K. Remarkably, neutron spectroscopy of the Tb 3+ crystal field levels appropriate to at high B-site mixing (0.5 < x < 1.5 in Tb 2 Sn 2-x Ti x O 7 ) reveal that the doublet ground and first excited states present as continua in energy, while transitions to singlet excited states at higher energies simply interpolate between those of the end members of the solid solution. The resulting ground state suggests an extreme version of a random-anisotropy magnet, with many local moments and anisotropies, depending on the precise local configuration of the six B sites neighboring each magnetic Tb 3+ ion.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides.

The equilibrium structure and functional properties exhibited by brownmillerite oxides, a family of perovskitederived structures with alternating layers of BO6 octahedra and BO4 tetrahedra, viz., ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. We use electronic structure calculations to disentangle the complex interactions in two ferrates, Sr2Fe2O5 and Ca2Fe2O5, relating the stability of the equilibrium (strain-free) and thin film structures to both previously identified and herein newly proposed descriptors.We show that cation size and intralayer separation of the tetrahedral chains provide key contributions to the preferred ground state. We show the bulk ground-state structure is retained in the ferrates over a range of strain values; however, a change in the orientation of the tetrahedral chains, i.e., a perpendicular orientation of the vacancies relative to the substrate, is stabilized in the compressive region. The structure stability under strain is largely governed by maximizing the intraplane separation of the dipoles generated from rotations of the FeO4 tetrahedra. Lastly, we find that the electronic band gap is strongly influenced by strain, manifesting as an unanticipated asymmetric-vacancy alignment dependent response. This atomistic understanding establishes a practical route for the design of functional electronic materials in thin film geometries.

Young, Joshua↗
DOE OSTI2016

Microscopic parameters from high-resolution specific heat measurements on superoptimally substituted BaFe 2 ( As 1 - x P x ) 2 single crystals

We investigate the electronic specific heat of superoptimally substituted BaFe 2 (As 1-x P x ) 2 single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Δ, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump ΔC ∝ to T$3\atop{c}$ , found in many iron-based superconductors, in this system originates from a T c -dependent ratio Delta/k B T c in combination with a substitution-dependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n s yields a penetration depth lambda that decreases with increasing T c without sharp divergence at the quantum critical point. Uemura scaling indicates that T c is governed by the Fermi temperature T F for this multiband system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Geometry-guided colloidal interactions and self-tiling of elastic dipoles formed by truncated pyramid particles in liquid crystals

The progress of realizing colloidal structures mimicking natural forms of organization in condensed matter is inherently limited by the availability of suitable colloidal building blocks. To enable new forms of crystalline and quasicrystalline self-organization of colloids, we develop truncated pyramidal particles that form nematic elastic dipoles with long-range electrostaticlike and geometry-guided low-symmetry short-range interactions. Using a combination of nonlinear optical imaging, laser tweezers, and video microscopy, we characterize colloidal pair interactions and demonstrate unusual forms of self-tiling of these particles into crystalline, quasicrystalline, and other arrays. Here, our findings are explained using an electrostatics analogy along with liquid crystal elasticity and symmetry breaking considerations, potentially expanding photonic and electro-optic applications of colloids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Two-Dimensional Electron Gases at LaAlO 3 /SrTiO 3 Interfaces: Orbital Symmetry and Hierarchy Engineered by Crystal Orientation

Recent findings show the emergence of two-dimensional electron gases (2DEGs) at LaAlO 3 /SrTiO 3 interfaces along different orientations; yet details on band reconstructions have remained so far unknown. In this study, via x-ray linear dichroism spectroscopy, we demonstrate that crystal symmetry imposes distinctive 2DEG orbital hierarchies on (001)-and (110)-oriented quantum wells, allowing selective occupancy of states of different symmetry. Such orientational tuning expands the possibilities for electronic engineering of 2DEGs and opens up enticing opportunities to understand the link between orbital symmetry and complex correlated states at LaAlO 3 /SrTiO 3 quantum wells.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate S.

Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of the material properties, which, along with the interactions, define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate, NdNiO3, combined with a dielectric spacer, LaAlO3, has been layered along the pseudocubic [111] direction. The resulting artificial graphenelike Mott crystal with magnetic 3d electrons has antiferromagnetic correlations. In addition, a combination of resonant X-ray linear dichroism measurements and ab initio calculations reveal the presence of an ordered orbital pattern, which is unattainable in either bulk nickelates or nickelate based heterostructures grown along the [001] direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering.

Middey, Srimanta↗
DOE OSTI2019

Creep‐fatigue behaviour of single‐crystal Ni‐base superalloy CMSX‐8

Abstract The creep‐fatigue behaviour of a lower cost, reduced rhenium Ni‐base superalloy, CMSX‐8, a variant of CMSX‐4, cast in a single crystal was experimentally evaluated over a broad range of conditions, from room temperature to 1100°C, and for two loading orientations: <001> and <111>. The fatigue lives depend on the orientation, cycle type, and temperature. The relative importance of these parameters on influencing the life is identified and discussed. From this understanding, a simple life model is constructed to capture the influence of these broad test conditions. The results are compared with those of the higher Re superalloy, CMSX‐4.

Estrada Rodas, Ernesto A.↗