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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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Superconductivity in the presence of disorder in skutterudite-related La 3 Co 4 Sn 13 and La 3 Ru 4 Sn 13 compounds: Electrical transport and magnetic studies

La 3 Co 4 Sn 13 and La 3 Ru 4 Sn 13 were categorized as BCS superconductors. In a plot of the critical field H c2 vs T, La 3 Ru 4 Sn 13 displays a second superconducting phase at the higher critical temperature T c *, characteristic of inhomogeneous superconductors, while La 3 Co 4 Sn 13 shows bulk superconductivity below T c . We observe a decrease in critical temperatures with external pressure and magnetic field for both compounds. Additionally, for La 3 Ru 4 Sn 13 we find that T c */dP>dT c /dP. The pressure dependences of T c are interpreted according to the McMillan theory and understood to be a consequence of lattice stiffening. The investigation of the superconducting state of La 3 Co x Ru 4–x Sn 13 shows a T c * that is larger then T c for x < 4. Furthermore, this unique and unexpected observation is discussed as a result of the local disorder and/or the effect of chemical pressure when Ru atoms are partially replaced by smaller Co atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Superconductivity in alkaline earth metal doped boron hydrides

Effects of alkaline earth metal atoms doping in boron hydrides at high pressure are investigated by first-principles calculations. The calculated results showed that doping with Mg, Ca, Ba, and Sr in B8H16 at 50 GPa is thermodynamically favorable and dynamically stable. The doping changes the B 8 H 16 from a semiconductor to a metal with substantial electronic density-of-state around the Fermi level. The superconductivity of the alkaline earth metal doped B 8 H 16 is studied based on electron-phonon coupling mechanism. The calculated critical superconducting transition temperatures (T c ) range from 10 to 25 K at 50 GPa upon doping. These results suggest that doping metal atoms in boron hydrides is an efficient way in designing superconducting materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Superconductivity in layered BiS 2 -based compounds

Here, a novel family of superconductors based on BiS 2 -based superconducting layers were discovered in 2012. In short order, other BiS 2 -based superconductors with the same or related crystal structures were discovered with superconducting critical temperatures T c of up to 10 K. Many experimental and theoretical studies have been carried out with the goal of establishing the basic properties of these new materials and understanding the underlying mechanism for superconductivity. In this selective review of the literature, we distill the central discoveries from this extensive body of work, and discuss the results from different types of experiments on these materials within the context of theoretical concepts and models.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Predicting the superconducting critical temperature in transition metal carbides and nitrides using machine learning

Transition metal carbides and nitrides have unique mechanical and chemical characteristics. At low temperatures many of them also exhibit superconductivity, which can be controlled by substitutions into both the transition metal and carbon/nitrogen sites. To investigate the factors governing the superconducting state, we apply machine learning methods. Here we collected a dataset containing 147 materials, which was used to create a pipeline for predicting their superconducting critical temperature. When this pipeline is applied to a randomly selected test set, it shows a good performance, with of 0.82 and RMSE of 1.9 K. To explore the limits of the machine learning approach, we also use it to predict entire substitution series within the dataset. This represents a realistic test for the predictive models, which can be extremely useful when applied to new substitutions in materials systems. The performance of the pipeline in this case is much more uneven, with good predictions for some series, while for others the model shows minimal predictive power. We discuss possible reasons for these results, as well as methods to estimate the performance of machine learning on new substitution series.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

La$_{2-x}$Ba$_x$CuO$_4$ as a superconducting Rosetta Stone

The high-temperature superconductivity in layered cuprates discovered by Bednorz and Müller arrived as a shock. Gradually, the presence of competing orders, such as antiferromagnetism and charge order, were discovered; however, the relationship to the superconductivity has been confusing. Importantly, it so happens that the original cuprate superconductor family La$_{2-x}$Ba$_x$CuO$_4$ contains all of the relevant phases, with extreme competition among them, and analysis of these phases provides strong clues to the nature of the superconductivity in cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Superconducting magnesium diboride films for levitation of laser targets

Inertial confinement fusion targets would benefit from being levitated inside hohlraums to avoid capsule support-related implosion perturbations. Levitation inside a magnetic trap requires coating the capsule with a thin film that is superconducting at 20 K. Such non-epitaxial film growth on non-planar substrates is challenging. Here, we study Mg vapor annealing and solid-phase reactive inter-diffusion methods to form superconducting magnesium diboride (MgB 2 ) films on different planar and spherical carbon substrates, evaluating glassy carbon, polycrystalline diamond made by chemical vapor deposition, and carbon deposited by magnetron sputtering of graphite targets. Thin films of B and Mg are produced by magnetron sputtering onto stationary planar or rolling spherical substrates and annealed at either 600 or 850 °C in Mg vapor. The films are characterized by a combination of high-energy ion scattering, electron microscopy, and magnetometry. Results show that the critical superconducting temperature of resultant films depends on film microstructure and oxygen impurity content. The formation of MgB 2 films is also strongly substrate dependent, even in this case of non-epitaxial growth. Important factors to consider are oxygen outgassing of the substrate during thermal processing, substrate surface roughness, and the matching of thermal expansion coefficients of different layers in the multilayer structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Atomic-Scale Andreev Probe of Unconventional Superconductivity

Recent emergence of low-dimensional unconventional superconductors and their exotic interface properties calls for new approaches to probe the pairing symmetry, a fundamental and frequently elusive property of the superconducting condensate. Here, we introduce the unique capability of tunneling Andreev reflection (TAR) to probe unconventional pairing symmetry, utilizing the sensitivity of this technique to specific Andreev reflections. Specifically, suppression of the lowest-order Andreev reflection due to quantum interference but emergence of the higher-order Andreev processes provides direct evidence of the sign-changing order parameter in the paradigmatic FeSe superconductor. Furthermore, TAR spectroscopy also reveals two superconducting gaps, points to a possibility of a nodal gap structure, and directly confirms that superconductivity is locally suppressed along the nematic twin boundary, with preferential and near-complete suppression of the larger energy gap. Furthermore, our findings therefore enable new, atomic-scale insight into microscopic, inhomogeneous, and interfacial properties of emerging quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Theoretical Discovery of a Superconducting Two-Dimensional Metal–Organic Framework

Superconductivity is an interesting quantum phenomenon characterized by zero electrical resistance and the Meissner effect. To date, several distinct families of superconductors (SCs) have been discovered. These include three-dimensional (3D) bulk SCs in both inorganic and organic materials as well as two-dimensional (2D) thin film SCs but only in inorganic materials. In this work, we report on superconductivity in 2D and 3D organic metal–organic frameworks by using first-principles calculations. We show that the highly conductive and recently synthesized Cu-benzenehexathial (BHT) is a Bardeen–Cooper–Schrieffer SC. Remarkably, the monolayer Cu-BHT has a critical temperature (T c ) of 4.43 K, while T c of bulk Cu-BHT is 1.58 K. Different from the enhanced T c in 2D inorganic SCs which is induced by interfacial effects, the T c enhancement in this 2D organic SC is revealed to be the out-of-plane soft-mode vibrations, analogous to surface mode enhancement originally proposed by Ginzburg. Our findings not only shed new light on better understanding 2D superconductivity but also open a new direction to search for SCs by interface engineering with organic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Local Atomic Configuration Control of Superconductivity in the Undoped Pnictide Parent Compound BaFe 2 As 2

Emergent superconductivity is strongly correlated with the symmetry of local atomic configuration in the parent compounds of iron-based superconductors. While chemical doping or hydrostatic pressure can change the local geometry, conventional approaches do not provide a clear pathway in predictably tuning the detailed atomic arrangement due to the parent compound’s complicated structural deformation in the presence of the tetragonal-to-orthorhombic phase transition. Here, we demonstrate a systematic approach to manipulate local structural configurations in BaFe 2 As 2 epitaxial thin films by controlling two independent structural factors, orthorhombicity (in-plane anisotropy) and tetragonality (out-of-plane/in-plane balance), from lattice parameters. We tune superconductivity without doping utilizing both structural factors separately and controlling local tetrahedral coordination in the designed thin film heterostructures with substrate clamping and biaxial strain. We further show this allows quantitative control of the structural phase transition, the associated magnetism, and superconductivity in parent material BaFe 2 As 2 . Furthermore, this approach will advance the development of tunable thin film superconductors in a reduced dimension.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Interfacial-Redox-Induced Tuning of Superconductivity in YBa 2 Cu 3 O 7-δ

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa 2 Cu 3 O 7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. Finaly, this work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Observation of Persistent Zero Modes and Superconducting Vortex Doublets in UTe 2

Superconducting vortices can reveal electron pairing details and nucleate topologically protected states. Yet, vortices of bulk spin-triplet superconductors have never been visualized at the atomic scale. Recently, UTe 2 has emerged as a prime spin-triplet superconductor, but its superconducting order parameter is elusive, and whether time-reversal symmetry (TRS) is broken remains unsettled. Here, in this work, we visualize vortices on the (011) surface of ultraclean UTe 2 single crystals (T c = 2.1 K) using scanning tunneling microscopy (STM). We introduce $\frac{d^{2}𝐼}{d𝑉^{2}}$ imaging as an effective technique for vortex visualization in superconductors with substantial residual zero-energy density of states (DOS), as in UTe 2 . Anisotropic single-flux-quantum vortices, with coherence lengths of ∼12 nm (4 nm) parallel (perpendicular) to the a -axis, form a triangular vortex lattice (VL) under a small out-of-plane magnetic field. The invariance of vortex structures and VL under changes of field polarity and cooling history strongly supports time-reversal invariant superconductivity under zero field. At vortex cores (VCs), nonsplit, spectrally sharp zero-bias conductance peaks (ZBPs) persist up 8 T that are consistent with symmetry-protected Majorana zero modes (MZMs) in a topological vortex line. Close examination of vortex structures reveals a mirror-asymmetric doublet─one with ZBPs and another with an enhanced apparent gap, possibly originating from a field-induced multicomponent order parameter.

UTe2↗
DOE OSTI2016

Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (T c ) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of T c has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to ~15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above ~6 GPa the sudden enhancement of superconductivity (T c ≤ 38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-T c phase above 6 GPa. In conclusion, the obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-T c cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Superconductivity up to 243 K in the yttrium-hydrogen system under high pressure

The discovery of superconducting H 3 S with a critical temperature T c ~200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted T c s among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH 3 , YH 4 , YH 6 and YH 9 in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH 6 and P6 3 /mmc YH 9 phases with maximal T c s of ~220 K at 183 GPa and ~243 K at 201 GPa, respectively. Fm-3m YH 10 with the highest predicted T c > 300 K was not observed in our experiments, and instead, YH 9 was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Monte Carlo study of the pseudogap and superconductivity emerging from quantum magnetic fluctuations

Abstract The origin of the pseudogap behavior, found in many high- T c superconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows pair formation but suppresses superconductivity. Employing quantum Monte Carlo simulations of a model of itinerant fermions coupled to ferromagnetic spin fluctuations, represented by a quantum rotor, we report numerical evidence of pseudogap behavior, emerging from pairing fluctuations in a quantum-critical non-Fermi liquid. Specifically, we observe enhanced pairing fluctuations and a partial gap opening in the fermionic spectrum. However, the system remains non-superconducting until reaching a much lower temperature. In the pseudogap regime the system displays a “gap-filling" rather than “gap-closing" behavior, similar to the one observed in cuprate superconductors. Our results present direct evidence of the pseudogap state, driven by superconducting fluctuations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Superconductivity from energy fluctuations in dilute quantum critical polar metals

Abstract Superconductivity in low carrier density metals challenges the conventional electron-phonon theory due to the absence of retardation required to overcome Coulomb repulsion. Here we demonstrate that pairing mediated by energy fluctuations, ubiquitously present close to continuous phase transitions, occurs in dilute quantum critical polar metals and results in a dome-like dependence of the superconducting T c on carrier density, characteristic of non-BCS superconductors. In quantum critical polar metals, the Coulomb repulsion is heavily screened, while the critical transverse optical phonons decouple from the electron charge. In the resulting vacuum, long-range attractive interactions emerge from the energy fluctuations of the critical phonons, resembling the gravitational interactions of a chargeless dark matter universe. Our estimates show that this mechanism may explain the critical temperatures observed in doped SrTiO 3 . We provide predictions for the enhancement of superconductivity near polar quantum criticality in two- and three-dimensional materials that can be used to test our theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Unlikelihood of a phonon mechanism for the high-temperature superconductivity in La 3 Ni 2 O 7

The discovery of ~80 K superconductivity in nickelate La 3 Ni 2 O 7 under pressure has ignited intense interest. Here, we present a comprehensive first-principles study of the electron-phonon (e-ph) coupling in La 3 Ni 2 O 7 and its implications on the observed superconductivity. Our results conclude that the e-ph coupling is too weak (with a coupling constant λ ≲ 0.5) to account for the high T c , albeit interesting many-electron correlation effects exist. While Coulomb interactions (via GW self-energy and Hubbard U) enhance the e-ph coupling strength, electron doping (oxygen vacancies) introduces no major changes. Additionally, different structural phases display varying characteristics near the Fermi level, but do not alter the conclusion. The e-ph coupling landscape of La 3 Ni 2 O 7 is intrinsically different from that of infinite-layer nickelates. These findings suggest that a phonon-mediated mechanism is unlikely to be responsible for the observed superconductivity in La 3 Ni 2 O 7 , pointing instead to an unconventional nature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Evidence for a robust sign-changing s-wave order parameter in monolayer films of superconducting Fe (Se,Te)/Bi 2 Te 3

The Fe-based superconductor Fe (Se,Te) combines non-trivial topology with unconventional superconductivity and may be an ideal platform to realize exotic states such as high-order topological corner modes and Majorana modes. Thin films of Fe (Se,Te) are particularly important for device fabrication and phase sensitive transport measurements. While bulk Fe (Se,Te) has been extensively studied, the nature of the superconducting order parameter in the monolayer limit has not yet been explored. In this work, we study monolayer films of Fe (Se,Te) on Bi 2 Te 3 with scanning tunneling spectroscopy. Monolayer Fe (Se,Te)/Bi 2 Te 3 heterostructures host a multigap superconducting state that strongly resembles the bulk. Analysis of the phase-referenced quasiparticle interference signal reveals a sign-changing s-wave order parameter similar to the bulk as well as a unique pattern of sign changes which have not been observed in the bulk. Our work establishes monolayer Fe (Se,Te)/Bi 2 Te 3 as a robust multi-band unconventional superconductor and sets the stage for explorations of non-trivial topology in this highly-tunable system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

A cryogenic near-field thermal diode leveraging superconducting phase transitions

Control of charge and heat transport is essential for computing and thermal management technologies. Recent work with superconducting materials has shown rectified electrical supercurrents near liquid helium temperatures. However, despite large theoretical interest and expected impact on quantum technologies, no experiments have demonstrated control of nanoscale radiative heat currents at cryogenic temperatures. Here we study photon-mediated thermal transport in nanogaps between niobium and gold. Using novel scanning calorimetric probes and nanofabricated devices, we reveal a ~20-fold suppression of radiative heat transport, when niobium transitions from the metallic to the superconducting state. Taking advantage of this effect, we also demonstrate a niobium-based cryogenic thermal diode with a heat rectification ratio of 70%. As a result, the experimental techniques and advances presented here will enable studying nanoscale thermal transport in quantum materials and advancing thermal management of superconducting devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗