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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2

Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe 2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K-valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Unconventional polaronic ground state in superconducting LiTi 2 O 4

Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel LiB 2 O 4 materials. On the B-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi 2 O 4 and heavy fermion behavior in LiV 2 O 4 . To date, however, LiTi 2 O 4 has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic ground states. Here, we present a multimodal investigation of LiTi 2 O 4 , combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi 2 O 4 from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi 2 O 4 . Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Atomic-scale frustrated Josephson coupling and multicondensate visualization in FeSe

In a Josephson junction involving multiband superconductors, competition between interband and interjunction Josephson couplings gives rise to frustration and spatial disjunction of superfluid densities among superconducting condensates. Such frustrated coupling manifests as the quantum interference of Josephson currents from different tunnelling channels and becomes tunable if channel transparency can be varied. To explore these unconventional effects in the prototypical s ± -wave superconductor FeSe, we use atomic-resolution scanned Josephson tunnelling microscopy for condensate-resolved imaging and junction tuning—capabilities unattainable in macroscopic Josephson devices with fixed characteristics. We quantitatively demonstrate frustrated Josephson tunnelling by examining two tunnelling inequalities. The relative transparency of two parallel tunnelling pathways is found tunable, revealing a tendency towards a 0–π transition with decreasing scanned Josephson tunnelling microscopy junction resistance. Here, the simultaneous visualization of both superconducting condensates reveals anticorrelated superfluid modulations, highlighting the role of interband scattering. Our study establishes scanned Josephson tunnelling microscopy as a powerful tool enabling new research frontiers of multicondensate superconductivity.

Scanning probe microscopy↗
DOE OSTI2018

Strain-tunable magnetism at oxide domain walls

Applying stress to a ferroelastic material results in a nonlinear strain response as domains of different orientations mechanically switch. The ability to write, erase and move domain walls between such ferroelastic domains suggests a method for making nanoelectronics where the domain wall is the device. However, little is known about the magnetic properties of such domain walls. A fascinating model system is SrTiO 3 , where the ferroelastic domain walls display strain-tunable polarity and enhanced conductivity. Here, we reveal a long-range magnetic order with modulations along the ferroelastic domain walls in SrTiO 3 and SrTiO 3 -based heterointerfaces, which manifests itself as a striped pattern in scanning superconducting quantum interference device maps of the magnetic landscape. In conducting interfaces, the magnetism is coupled to itinerant electrons with clear signatures in magnetotransport measurements. The magnetic state is also coupled dynamically to the lattice and can be reversibly tuned by applying local external forces. As a result, this study raises the possibility of designing nanoscale devices based on domain walls where strain-tunable ferroelectric, ferroelastic and ferromagnetic orders may coexist.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Non-Fermi liquid behaviour in a correlated flat-band pyrochlore lattice

Electronic correlation effects are manifested in quantum materials when either the on-site Coulomb repulsion is large or the electron kinetic energy is small. The former is the dominant effect in cuprate superconductors and heavy-fermion systems whereas it is the latter in twisted bilayer graphene and geometrically frustrated metals. However, the simultaneous cooperation of both effects in the same quantum material remains rare. The design aim is to produce correlated topological flat bands pinned at the Fermi level. Here, we observe a flat band at the Fermi level in a 3d pyrochlore metal CuV 2 S 4 . Our angle-resolved photoemission spectroscopy data reveal that destructive quantum interference associated with the V pyrochlore sublattice and further renormalization to the Fermi level by electron interactions induce this flat band. Consequently, we discover transport signatures that evidence a deviation from Fermi liquid behaviour as well as an enhanced Sommerfeld coefficient. Our work illustrates the combined cooperation of local Coulomb interactions and geometric frustration in a pyrochlore lattice system to induce correlated topology by constructing and pinning correlated flat bands near the Fermi level.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Imaging real-space flat band localization in kagome magnet FeSn

Kagome lattices host flat bands due to their frustrated lattice geometry, which leads to destructive quantum interference of electron wave functions. Here, we report imaging of the kagome flat band localization in real-space using scanning tunneling microscopy. We identify both the Fe 3 Sn kagome lattice layer and the Sn 2 honeycomb layer with atomic resolution in kagome antiferromagnet FeSn. On the Fe 3 Sn lattice, at the flat band energy determined by the angle resolved photoemission spectroscopy, tunneling spectroscopy detects an unusual state localized uniquely at the Fe kagome lattice network. We further show that the vectorial in-plane magnetic field manipulates the spatial anisotropy of the localization state within each kagome unit cell. Our results are consistent with the real-space flat band localization in the magnetic kagome lattice. We further discuss the magnetic tuning of flat band localization under the spin–orbit coupled magnetic kagome lattice model.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A graph-theoretical representation of multiphoton resonance processes in superconducting quantum circuits

We propose a graph-theoretical formalism to study generic circuit quantum electrodynamics systems consisting of a two level qubit coupled with a single-mode resonator in arbitrary coupling strength regimes beyond rotating-wave approximation. We define colored-weighted graphs, and introduce different products between them to investigate the dynamics of superconducting qubits in transverse, longitudinal, and bidirectional coupling schemes. In conclusion, the intuitive and predictive picture provided by this method, and the simplicity of the mathematical construction, are demonstrated with some numerical studies of the multiphoton resonance processes and quantum interference phenomena for the superconducting qubit systems driven by intense ac fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Synthesis, characterization, and growth mechanism of motifs of ultrathin cobalt-substituted NaFeSi 2 O 6 nanowires

In this report, we have synthesized and structurally characterized nanowire bundles of cobalt-substituted pyroxenes, similar to the crystal structure of aegirine (i.e. Co-substituted XYSi 2 O 6 with X and Y referring to metallic elements such as but not limited to Co, Na, and Fe), using a readily scalable hydrothermal technique. We then propose a growth mechanism for these bundles, based on detailed time and temperature dependent studies as well as complementary control experiments, particularly reactions in the absence of either 3-aminopropyltriethoxysilane (APTES) or sodium hydroxide (NaOH), via a transmission electron microscopy visualization study. Moreover, these nanowire bundles were probed for their magnetic properties and chemical composition using superconducting quantum interference device (SQUID) measurements, X-ray diffraction, and pair distribution function analysis, respectively. Specifically, SQUID measurement observations highlighted that these bundles evince (i) unique and interesting super-paramagnetic properties at 5 K that are consistent with that of our previously published ~2 nm ultra-small nanoparticles as well as (ii) paramagnetic behavior at 300 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Synthesis and characterization of low viscosity hexafluoroacetylacetonate-based hydrophobic magnetic ionic liquids

Magnetic ionic liquids (MILs) are distinguished from traditional ionic liquids (ILs) by the incorporation of a paramagnetic component within their chemical structure. Hydrophobic MILs are novel solvents that can be used in many applications, including liquid–liquid extraction (LLE) and catalysis. Low viscosity and low water solubility are essential features that determine their feasibility in LLE. Here, we synthesized extremely hydrophobic MILs by using transition and rare earth metal hexafluoroacetylacetonate chelated anions paired with the trihexyl(tetradecyl)phosphonium ([P 66614 + ]) cation. Hydrophobic MILs exhibiting water solubilities less than 0.01% (v/v) were synthesized in a rapid two-step procedure. Furthermore, the viscosities of the MILs are among some of the lowest ever reported for hydrophobic MILs (276.5–927.9 centipoise (cP) at 23.7 °C) dramatically improving the ease of handling these liquids. For the first time, the magnetic properties of MILs possessing hexafluoroacetylacetonate chelated metal anions synthesized in this study are reported using a superconducting quantum interference device (SQUID) magnetometer. We also achieved an effective magnetic moments (μ eff ) as high as 9.7 and 7.7 Bohr magnetons (μ B ) by incorporating high spin dysprosium and gadolinium ions, respectively, into the anion component of the MIL. The low viscosity, high hydrophobicity, and large magnetic susceptibility of these MILs make them highly attractive and promising solvents for separations and purification, liquid electrochromic materials, catalytic studies, as well as microfluidic applications.

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

Structure and magnetism in strained Ge{sub 1-x-y}Sn{sub x}Mn{sub y} films grown on Ge(001) by low temperature molecular beam epitaxy

In this letter, we study the structural and magnetic properties of Ge{sub 1-x-y}Sn{sub x}Mn{sub y} films grown on Ge(001) by low temperature molecular beam epitaxy using X-ray diffraction, high resolution transmission electron microscopy, and superconducting quantum interference device. Like in Mn doped Ge films, Mn atoms diffuse during the growth and aggregate into vertically aligned Mn-rich nanocolumns of a few nanometers in diameter. Transmission electron microscopy observations in plane view clearly indicate that the Sn incorporation is not uniform with concentration in Mn rich vertical nanocolumns lower than the detection limit of electron energy loss spectroscopy. The matrix exhibits a GeSn solid solution while there is a Sn-rich GeSn shell around GeMn nanocolumns. The magnetization in Ge{sub 1-x-y}Sn{sub x}Mn{sub y} layers is higher than in Ge{sub 1-x}Mn{sub x} films. This magnetic moment enhancement in Ge{sub 1-x-y}Sn{sub x}Mn{sub y} is probably related to the modification of the electronic structure of Mn atoms in the nanocolumns by the Sn-rich shell, which is formed around the nanocolumns.

36 MATERIALS SCIENCE↗
DOE OSTI2013

High-resolution gamma-ray spectroscopy with a microwave-multiplexed transition-edge sensor array

We demonstrate very high resolution photon spectroscopy with a microwave-multiplexed two-pixel transition-edge sensor (TES) array. We measured a {sup 153}Gd photon source and achieved an energy resolution of 63 eV full-width-at-half-maximum at 97 keV and an equivalent readout system noise of 86 pA/√(Hz) at the TES. The readout circuit consists of superconducting microwave resonators coupled to radio-frequency superconducting-quantum-interference-devices and transduces changes in input current to changes in phase of a microwave signal. We use flux-ramp modulation to linearize the response and evade low-frequency noise. This demonstration establishes one path for the readout of cryogenic X-ray and gamma-ray sensor arrays with more than 10{sup 3} elements and spectral resolving powers R=λ/Δλ>10{sup 3}.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Tunable microwave impedance matching to a high impedance source using a Josephson metamaterial

We report the efficient coupling of a 50 Ω microwave circuit to a high impedance conductor. We use an impedance transformer consisting of a λ/4 co-planar resonator whose inner conductor contains an array of superconducting quantum interference devices (SQUIDs), providing it with a tunable lineic inductance L∼80 μ{sub 0}, resulting in a characteristic impedance Z{sub C}∼1 kΩ. The impedance matching efficiency is characterized by measuring the shot noise power emitted by a dc biased tunnel junction connected to the resonator. We demonstrate matching to impedances in the 15 to 35 kΩ range with bandwidths above 100 MHz around a resonant frequency tunable between 4 and 6 GHz.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of low-temperature annealing on the electronic- and band-structures of (Ga,Mn)As epitaxial layers

The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial layers, caused by post-growth low-temperature annealing, on their electronic- and band-structure properties has been investigated by modulation photoreflectance (PR) spectroscopy. The annealing-induced changes in structural and magnetic properties of the layers were examined with high-resolution X-ray diffractometry and superconducting quantum interference device magnetometry, respectively. They confirmed an outdiffusion of Mn interstitials from the layers and an enhancement in their hole concentration, which were more efficient for the layer covered with a Sb cap acting as a sink for diffusing Mn interstitials. The PR results demonstrating a decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers, with respect to that in the reference GaAs one, are interpreted by assuming a merging of the Mn-related impurity band with the GaAs valence band. Whereas an increase in the band-gap-transition energy caused by the annealing treatment of the (Ga,Mn)As layers is interpreted as a result of annealing-induced enhancement of the free-hole concentration and the Fermi level location within the valence band. The experimental results are consistent with the valence-band origin of itinerant holes mediating ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for ferromagnetic semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Transport properties of individual C{sub 60}-molecules

Electrical and thermal transport properties of C{sub 60} molecules are investigated with density-functional-theory based calculations. These calculations suggest that the optimum contact geometry for an electrode terminated with a single-Au atom is through binding to one or two C-atoms of C{sub 60} with a tendency to promote the sp{sup 2}-hybridization into an sp{sup 3}-type one. Transport in these junctions is primarily through an unoccupied molecular orbital that is partly hybridized with the Au, which results in splitting the degeneracy of the lowest unoccupied molecular orbital triplet. The transmission through these junctions, however, cannot be modeled by a single Lorentzian resonance, as our results show evidence of quantum interference between an occupied and an unoccupied orbital. The interference results in a suppression of conductance around the Fermi energy. Our numerical findings are readily analyzed analytically within a simple two-level model.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Magnetic susceptibility and magnetic resonance measurements of the moisture content and hydration condition of a magnetic mixture material

We developed a magnetic measurement method to measure the moisture content and hydration condition of mortar as a magnetic mixture material. Mortar is a mixture of Portland cement, sand, and water, and these materials exhibit different magnetic properties. The magnetization–magnetic field curves of these components and of mortars with different moisture contents were measured, using a specially developed high-temperature-superconductor superconducting quantum interference device. Using the differences in magnetic characteristics, the moisture content of mortar was measured at the ferromagnetic saturation region over 250 mT. A correlation between magnetic susceptibility and moisture content was successfully established. After Portland cement and water are mixed, hydration begins. At the early stage of the hydration/gel, magnetization strength increased over time. To investigate the magnetization change, we measured the distribution between bound and free water in the mortar in the early stage by magnetic resonance imaging (MRI). The MRI results suggest that the amount of free water in mortar correlates with the change in magnetic susceptibility.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magnetic susceptibility measurement of solid oxygen at pressures up to 3.3 GPa

The magnetic susceptibility of solid oxygen had long been observed only in the restricted pressure region below 0.8 GPa. We succeeded in extending the pressure region up to 3.3 GPa by clamping condensed oxygen in the sample chamber of a miniature diamond anvil cell and measuring the dc magnetic susceptibility using a superconducting quantum interference device magnetometer. In this experiment, the well-known α–β and β–γ transitions are observed in the phase diagram, suggesting consistency with the previous results of X-ray and Raman studies. In addition, a new magnetic anomaly is observed in the β phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Study of Gd-doped Bi{sub 2}Te{sub 3} thin films: Molecular beam epitaxy growth and magnetic properties

Incorporation of magnetic dopants into topological insulators to break time-reversal symmetry is a prerequisite for observing the quantum anomalous Hall (QAHE) effect and other novel magnetoelectric phenomena. GdBiTe{sub 3} with a Gd:Bi ratio of 1:1 is a proposed QAHE system, however, the reported solubility limit for Gd doping into Bi{sub 2}Te{sub 3} bulk crystals is between ∼0.01 and 0.05. We present a magnetic study of molecular beam epitaxy grown (Gd{sub x}Bi{sub 1–x}){sub 2}Te{sub 3} thin films with a high Gd concentration, up to x ≈ 0.3. Magnetometry reveals that the films are paramagnetic down to 1.5 K. X-ray magnetic circular dichroism at the Gd M{sub 4,5} edge at 1.5 K reveals a saturation field of ∼6 T, and a slow decay of the magnetic moment with temperature up to 200 K. The Gd{sup 3+} ions, which are substitutional on Bi sites in the Bi{sub 2}Te{sub 3} lattice, exhibit a large atomic moment of ∼7 μ{sub B}, as determined by bulk-sensitive superconducting quantum interference device magnetometry. Surface oxidation and the formation of Gd{sub 2}O{sub 3} lead to a reduced moment of ∼4 μ{sub B} as determined by surface-sensitive x-ray magnetic circular dichroism. Their large atomic moment makes these films suitable for incorporation into heterostructures, where interface polarization effects can lead to the formation of magnetic order within the topological insulators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Intrinsic magnetic properties of hexagonal LuFeO{sub 3} and the effects of nonstoichiometry

We used oxide molecular-beam epitaxy in a composition-spread geometry to deposit hexagonal LuFeO{sub 3} (h-LuFeO{sub 3}) thin films with a monotonic variation in the Lu/Fe cation ratio, creating a mosaic of samples that ranged from iron rich to lutetium rich. We characterized the effects of composition variation with x-ray diffraction, atomic force microscopy, scanning transmission electron microscopy, and superconducting quantum interference device magnetometry. After identifying growth conditions leading to stoichiometric film growth, an additional sample was grown with a rotating sample stage. From this stoichiometric sample, we determined stoichiometric h-LuFeO{sub 3} to have a T{sub N} = 147 K and M{sub s} = 0.018 μ{sub B}/Fe.

36 MATERIALS SCIENCE↗