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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Josephson detection of time-reversal symmetry broken superconductivity in SnTe nanowires

A Josephson junction (JJ) couples the supercurrent flowing between two weakly linked superconductors to the phase difference between them via a current-phase relation (CPR). While a sinusoidal CPR is expected for conventional junctions with insulating weak links, devices made from some exotic materials may give rise to unconventional CPRs and unusual Josephson effects. In this work, we present such a case: we investigate the proximity-induced superconductivity in SnTe nanowires by incorporating them as weak links in JJs and observe a deviation from the standard CPR. We report on indications of an unexpected breaking of time-reversal symmetry in these devices, detailing the unconventional characteristics that reveal this behavior. These include an asymmetric critical current in the DC Josephson effect, a prominent second harmonic in the AC Josephson effect, and a magnetic diffraction pattern with a minimum in critical current at zero magnetic field. The analysis examines how multiband effects and the experimentally visualized ferroelectric domain walls give rise to this behavior, giving insight into the Josephson effect in materials that possess ferroelectricity and/or multiband superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Pseudogap behavior in charge density wave kagome material ScV 6 Sn 6 revealed by magnetotransport measurements

Over the last few years, significant attention has been devoted to studying the kagome materials A V 3 Sb 5 (A = K, Rb, Cs) due to their unconventional superconductivity and charge density wave (CDW) ordering. Recently $ScV_{6}Sn_{6}$ was found to host a CDW below ≈ 90 K, and, like A V 3 Sb 5 , it contains a kagome lattice comprised only of V ions. Here we present a comprehensive magnetotransport study on $ScV_{6}Sn_{6}$. We discovered several anomalous transport phenomena above the CDW ordering temperature, including insulating behavior in interlayer resistivity, a strongly temperature-dependent Hall coefficient, and a violation of Kohler’s rule. All these anomalies can be consistently explained by a progressive decrease in carrier densities with decreasing temperature, suggesting the formation of a pseudogap. Our findings suggest that high-temperature CDW fluctuations play a significant role in determining the normal state electronic properties of $ScV_{6}Sn_{6}$.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Nanoscale visualization and spectral fingerprints of the charge order in ScV 6 Sn 6 distinct from other kagome metals

Charge density waves (CDWs) in kagome metals have been tied to many exotic phenomena. Here, using spectroscopic-imaging scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we study the charge order in kagome metal ScV 6 Sn 6 . The similarity of electronic band structures of ScV 6 Sn 6 and TbV 6 Sn6 (where charge ordering is absent) suggests that charge ordering in ScV 6 Sn 6 is unlikely to be primarily driven by Fermi surface nesting of the Van Hove singularities. In contrast to the CDW state of cousin kagome metals, we find no evidence supporting rotation symmetry breaking. Differential conductance dI/dV spectra show a partial gap Δ 1 CO ≈ 20 meV at the Fermi level. Interestingly, dI/dV maps reveal that charge modulations exhibit an abrupt phase shift as a function of energy at energy much higher than Δ 1 CO , which we attribute to another spectral gap. Our experiments reveal a distinctive nature of the charge order in ScV 6 Sn 6 with fundamental differences compared to other kagome metals.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ramification of complex magnetism in Nd 2 Ir 2 O 7 observed by Raman scattering spectroscopy

Using Raman scattering spectroscopy, we uncover a complex magnetic behavior of Nd2Ir2O7, which stands out among magnetic pyrochlores by the lowest temperature of the all-in-all-out (AIAO) Ir moments ordering (${T}_{{\rm{Ir}}}^{{\rm{N}}} = 33$ K) and the highest temperature at which AIAO order of rare-earth Nd ions is detected (${T}_{{\rm{Nd}}}^{{\rm{* }}}$ = 15 K). Detected magnetic Raman scattering and calculations of expected response allow us to demonstrate that the ordering of Ir magnetic moments is accompanied by an appearance of one-magnon Raman modes at 26.3 and 29.6 meV compatible with the AIAO order and allowing to estimate the energies of Ir-Ir interactions. An additional two-magnon excitation of the AIAO Nd order at around 33 meV appears in the spectra below the ordering temperature of Nd moments ${T}_{{\rm{Nd}}}^{{\rm{* }}}$ = 15 K. In the temperature range between 15 K and 33 K we observe a broad mode, which demonstrates strong temperature dependence and shifts on cooling below 20 K from 14 meV to higher frequencies, and disappears at 5 K, when two-magnon excitation of Nd moments becomes prominent. We suggest an interpretation of this excitation in terms of continuum arising from collective fluctuations of Nd moments above the transition. This complex behavior emerges from the interplay of strong spin-orbit coupling, electronic correlations, and geometric frustration on two magnetic pyrochlore sublattices of Nd and Ir ions.

magnetic properties and materials↗
DOE OSTI2021

Unusual high-field metal in a Kondo insulator

Strong electronic interactions in condensed-matter systems often lead to unusual quantum phases. One such phase occurs in the Kondo insulator YbB12, the insulating state of which exhibits phenomena that are characteristic of metals, such as magnetic quantum oscillations, a gapless fermionic contribution to heat capacity and itinerant-fermion thermal transport. To understand these phenomena, it is informative to study their evolution as the energy gap of the Kondo insulator state is closed by a large magnetic field. Here we show that clear quantum oscillations are observed in the resulting high-field metallic state in YbB12; this is despite it possessing relatively high resistivity, large effective masses and huge Kadowaki–Woods ratio, a combination that normally precludes quantum oscillations. Both quantum oscillation frequency and cyclotron mass display a strong field dependence. By tracking the Fermi surface area, we conclude that the same quasiparticle band gives rise to quantum oscillations in both insulating and metallic states. These data are understood most simply by using a two-fluid picture in which neutral quasiparticles—contributing little or nothing to charge transport—coexist with charged fermions. Overall, our observations of the complex field-dependent behaviour of the fermion ensemble inhabiting YbB 12 provide strong constraints for existing theoretical models.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Oscillations of the thermal conductivity in the spin-liquid state of α-RuCl 3

In the class of materials called spin liquids a magnetically ordered state cannot be attained even at millikelvin temperatures because of conflicting constraints on each spin; for example, from geometric or exchange frustration. The resulting quantum spin-liquid state is currently of intense interest because it exhibits unusual excitations as well as wave-function entanglement. The layered insulator α-RuCl 3 orders as a zigzag antiferromagnet at low temperature in zero magnetic field. The zigzag order is destroyed when a magnetic field is applied parallel to the zigzag axis. At moderate magnetic field strength, there is growing evidence that a quantum spin-liquid state exists. Here we report the observation of oscillations in its thermal conductivity in that field range. The oscillations, whose amplitude is very large within this field range and strongly suppressed on either side, are periodic. This is analogous to quantum oscillations in metals, even though α-RuCl 3 is an excellent insulator with a large gap. As the temperature is raised above 0.5 K, the oscillation amplitude decreases exponentially, anticorrelating with the emergence of an anomalous planar thermal Hall conductivity above approximately 2 K.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Unconventional gapping behaviour in a kagome superconductor

Determining the types of superconducting order in quantum materials is a challenge. This is especially true when several degrees of freedom contribute to the fermiology and when superconductivity competes with other symmetry-breaking orders. One example is the kagome-lattice superconductor CsV 3 Sb 5 , in which multiband superconductivity coexists with a charge order that substantially reduces the space group symmetries of the compound. Here we demonstrate the presence of two superconducting regimes in CsV 3 Sb 5 that are characterized by distinct transport and thermodynamic properties. Our results reveal a substantial quasiparticle weight in a high-temperature regime. At lower temperatures, this weight is removed through the formation of a second gap. We argue that the band with a gap opening at lower temperatures continues to host low-energy quasiparticles, possibly due to the nodal structure of the gap. Taken together, our results present evidence for band-selective superconductivity with uncoupled superconducting gaps.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

A microscopic perspective on moiré materials

Quantum materials research has experienced a veritable renaissance in the past 6 years, with a surge of experimental reports of nearly every known electronic phase of matter and of several unique and unexpected phases, all found in an emerging class of highly tunable 2D materials, known as moiré materials1,2,3. Here, moiré materials are designed through either the rotational misalignment of identical 2D atomic crystals or the lattice mismatch of dissimilar 2D atomic crystals (Fig. 1). Both these incommensurability conditions cause long-wavelength (~10 nm) interference patterns between the two constituent atomic lattices, forming an enlarged moiré superlattice that generically hosts flat electronic bands, which are highly conducive to correlated, collective phases of matter.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electronic transport properties of a lithium-decorated ZrTe 5 thin film

Through a combination of single crystal growth, experiments involving in situ deposition of surface adatoms, and complimentary modeling, we examine the electronic transport properties of lithiumdecorated ZrTe 5 thin films. We observe that the surface states in ZrTe 5 are robust against Li adsorption. Both the surface electron density and the associated Berry phase are remarkably robust to adsorption of Li atoms. Fitting to the Hall conductivity data reveals that there exist two types of bulk carriers: those for which the carrier density is insensitive to Li adsorption, and those whose density decreases during initial Li depositions and then saturates with further Li adsorption. We propose this dependence is due to the gating effect of a Li-adsorption-generated dipole layer at the ZrTe 5 surface.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Observation of gapped state in rare-earth monopnictide HoSb

The rare-earth monopnictide family is attracting an intense current interest driven by its unusual extreme magnetoresistance (XMR) property and the potential presence of topologically non-trivial surface states. The experimental observation of non-trivial surface states in this family of materials are not ubiquitous. Here, using high-resolution angle-resolved photoemission spectroscopy, magnetotransport, and parallel first-principles modeling, we examine the nature of electronic states in HoSb. Although we find the presence of bulk band gaps at the G and X-symmetry points of the Brillouin zone, we do not find these gaps to exhibit band inversion so that HoSb does not host a Dirac semimetal state. Our magnetotransport measurements indicate that HoSb can be characterized as a correlated nearly-complete electron-hole-compensated semimetal. Our analysis reveals that the nearly perfect electron-hole compensation could drive the appearance of non-saturating XMR effect in HoSb.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

A new water-based scintillator for efficient Cherenkov and scintillation separation

Neutrinos offer a unique window into the world around us, allowing us to probe otherwise unreachable regions like the interior of stars and the depths of the earth, as well as potentially offering a mechanism for monitoring nuclear activity. Both pure-water and organic liquid scintillators have been used as the detection medium of large-scale neutrino detectors. Organic liquid scintillators offer higher sensitivity at lower energies, which is desirable for many applications, but detection of the Cherenkov radiation required for directional sensitivity is very difficult. A possible solution is to use water-based liquid scintillators (WbLSs), where some fraction of the water is replaced with a micellar solution of the liquid scintillator. This can enhance the detection sensitivity beyond that of pure water, without significantly affecting the ability to leverage the topological Cherenkov signature. Very specific scintillation properties are required to achieve this goal: the scintillation decay time should be significantly slower than that of the Cherenkov emission, so timing-based discrimination of Cherenkov light from scintillation can be applied. At the same time, the light yield should be high enough to enhance the overall sensitivity of the detector, but without losing too much of the Cherenkov light. In this study, we report a new water-based liquid scintillation cocktail based on a 9-methylcarbazole fluorescent dye and a linear alkylbenzene solvent. Here, the proposed composition offers 13.8 ns scintillation decay time and 368 nm emission that matches the desired properties.

Wolszczak, Weronika W. [Lawrence Berkeley National↗
DOE OSTI2021

Epitaxial growth of monolayer PdTe 2 and patterned PtTe 2 by direct tellurization of Pd and Pt surfaces

Two-dimensional (2D) palladium ditelluride (PdTe 2 ) and platinum ditelluride (PtTe 2 ) are two Dirac semimetals, which have fascinating quantum properties such as superconductivity, magnetism and topological order and show the promising applications in future nanoelectronics and optoelectronics. However, the synthesis of PdTe 2 and PtTe 2 monolayers (MLs) is hindered by a strong interlayer coupling and orbital hybridization. Here, in this study, we demonstrate an efficient synthesis of PdTe 2 and PtTe 2 MLs Large-area and high quality MLs of PdTe 2 and patterned PtTe 2 were epitaxially grown on the Pd(111) and Pt(111) surfaces by direct telurization in ultra-high vacuum. This was confirmed by x-ray photoelectron spectroscopy, low energy electron diffraction and scanning tunnelling microscopy. PdTe 2 ML demonstrated high thermal stability showing no decomposition sign after annealing at 470 °C. A well-ordered (2 × 2) PtTe 2 structure with Kagome lattice was observed on Te/Pt(111) surface following annealing at 200 °C in UHV, where the (2 × 2) pattern was formed by Te atom vacancies. PtTe 2 multilayer film was prepared as well, and it demonstrated the excitation of the Dirac plasmons as measured by high-resolution electron energy loss spectroscopy. The direct tellurization offers the simple and reliable protocol for the preparation of the ML of PdTe 2 and patterned PtTe 2 , and this opens new opportunities for quantum phenomena research and for practical optoelectronics applications.

2D materials↗
DOE OSTI2022

Rare-earth monopnictides: Family of antiferromagnets hosting magnetic Fermi arcs

We report since the discovery of topological insulators a great deal of research effort has been devoted to magnetic topological materials, in which nontrivial spin properties can be controlled by magnetic fields, culminating in a wealth of fundamental phenomena and possible applications. The main focus was on ferromagnetic materials that can host Weyl fermions and therefore spin-textured Fermi arcs. The recent discovery of Fermi arcs and new magnetic band splitting in the antiferromagnet (AFM) NdBi has opened up new avenues for exploration. Here we show that these uncharted effects are not restricted to this specific compound, but also emerge in CeBi and NdSb when they undergo paramagnetic to AFM transition. Our data show that the Fermi arcs in NdSb have twofold symmetry, leading to strong anisotropy that may enhance effects of spin textures on transport properties. Our findings thus demonstrate that the RBi and RSb series are materials that host magnetic Fermi arcs and may be a potential platform for modern spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Giant spin transport anisotropy in magnetic topological insulators

Here, we report on exceptionally long spin transport and giant spin lifetime anisotropy in the gapped surface states of three-dimensional (3D) magnetic topological insulators (MTIs). We examine the properties of these states using the three-dimensional Fu-Kane-Mele Hamiltonian model in the presence of a magnetic exchange field. The corresponding spin textures of surface states, which are well reproduced by an effective two-band model, hint at a considerable enhancement of the lifetime of out-of-plane spins compared to in-plane spins. This is confirmed by large-scale spin transport simulations for 3D MTIs with disorder. The energy dependence of the spin lifetime anisotropy arises directly from the nontrivial spin texture of the surface states, and is correlated with the onset of the quantum anomalous Hall phase. Our findings suggest different spin filtering capabilities of the gapped surface MTI states, which could be explored by nonlocal spin valve measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1996

Visualization of 3-D tensor fields

Second-order tensor fields have applications in many different areas of physics, such as general relativity and fluid mechanics. The wealth of multivariate information in tensor fields makes them more complex and abstract than scalar and vector fields. Visualization is a good technique for scientists to gain new insights from them. Visualizing a 3-D continuous tensor field is equivalent to simultaneously visualizing its three eigenvector fields. In the past, research has been conducted in the area of two-dimensional tensor fields. It was shown that degenerate points, defined as points where eigenvalues are equal to each other, are the basic singularities underlying the topology of tensor fields. Moreover, it was shown that eigenvectors never cross each other except at degenerate points. Since we live in a three-dimensional world, it is important for us to understand the underlying physics of this world. In this report, we describe a new method for locating degenerate points along with the conditions for classifying them in three-dimensional space. Finally, we discuss some topological features of three-dimensional tensor fields, and interpret topological patterns in terms of physical properties.

Hesselink, L.↗
NASA NTRS2005

Metrics for More than Two Points at Once

The conventional definition of a topological metric over a space specifies properties that must be obeyed by any measure of "how separated" two points in that space are. Here it is shown how to extend that definition, and in particular the triangle inequality, to concern arbitrary numbers of points. Such a measure of how separated the points within a collection are can be bootstrapped, to measure "how separated" from each other are two (or more) collections. The measure presented here also allows fractional membership of an element in a collection. This means it directly concerns measures of "how spread out" a probability distribution over a space is. When such a measure is bootstrapped to compare two collections, it allows us to measure how separated two probability distributions are, or more generally, how separated a distribution of distributions is.

Wolpert, David H.↗
NASA NTRSDate not supplied

Computational Investigation of Oxidative Etch Pitting in FiberForm and Its Impact on Material Properties

Oxidation-driven carbon erosion does not occur uniformly but rather through the development of localized etch pits at active surface sites. These active sites form due to atomic defects on the carbon surface, making them significantly more reactive than the surrounding, non-defective areas. As a result, these sites are the first to react during ablation, leading to their removal. This process creates new defects in neighboring atoms, increasing their reactivity and causing localized carbon removal around these active sites. In this way, the highly reactive defective areas serve as nucleation points for the formation and growth of etch pits, which can have adverse effects on structural integrity of FiberForm. To better understand how these etch pits impact the material properties of carbon fiber microstructures, we have developed a new capability within the direct simulation Monte Carlo (DSMC) framework to capture the etch pit formation process. This capability, integrated into the DSMC code SPARTA (Stochastic Parallel Rarefied-gas Time-accurate Analyzer), models material removal in the presence of active sites, leading to the formation of etch pits. The current work focuses on studying the effects of these etch pits on the material properties of FiberForm, a widely used base material in thermal protection systems (TPS). The microstructure of virgin FiberForm, obtained via X-ray microtomography, is imported into SPARTA to generate the ablated geometries with etch pits. These modified microstructures are then analyzed using the Porous Microstructure Analysis (PuMA) software to compute various material properties, including elasticity, thermal conductivity, and permeability. We investigate the variation of these properties due to the complex surface topology changes caused by etch pit formation. Additionally, we compare the effects of pitting with the conventional model of shrinking fibers, traditionally used to simulate the ablation of carbon structures. Significant differences emerge between the two approaches. Consequently, this physically realistic model of material removal through etch pit formation offers improved accuracy in predicting the degradation of carbon-based TPS during oxidation. It also provides insights into other mechanisms, such as spallation, where chunks of material are removed into the flow due to etch pit growth. Ultimately, this model enhances our understanding of failure modes in these materials during ablation.

PuMA↗