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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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

The study of contact properties in edge-contacted graphene–aluminum Josephson junctions

Transparent contact interfaces in superconductor–graphene hybrid systems are critical for realizing superconducting quantum applications. Here, we examine the effect of the edge contact fabrication process on the transparency of the superconducting aluminum–graphene junction. We show significant improvement in the transparency of our superconductor–graphene junctions by promoting the chemical component of the edge contact etch process. Our results compare favorably with state-of-the-art graphene Josephson junctions. The findings of our study contribute to advancing the fabrication knowledge of edge-contacted superconductor–graphene junctions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Phase tuning of multiple Andreev reflections of Dirac fermions and the Josephson supercurrent in Al–MoTe 2 –Al junctions

When an electron is incident on a superconductor from a metal, it is reflected as a hole in a process called Andreev reflection. If the metal N is sandwiched between two superconductors S in an SNS junction, multiple Andreev reflections (MARs) occur. We have found that, in SNS junctions with high transparency (τ → 1) based on the Dirac semimetal MoTe 2 , the MAR features are observed with exceptional resolution. By tuning the phase difference φ between the bracketing Al superconductors, we establish that the MARs coexist with a Josephson supercurrent I s =I A sin φ. As we vary the junction voltage V, the supercurrent amplitude I A varies in step with the MAR order n, revealing a direct relation between them. Two successive Andreev reflections serve to shuttle a Cooper pair across the junction. If the pair is shuttled coherently, it contributes to I s . The experiment measures the fraction of pairs shuttled coherently vs. V. Surprisingly, superconductivity in MoTe 2 does not affect the MAR features.

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

A Josephson junction with h -BN tunnel barrier: observation of low critical current noise

Decoherence in quantum bits (qubits) is a major challenge for realizing scalable quantum computing. One of the primary causes of decoherence in qubits and quantum circuits based on superconducting Josephson junctions is the critical current fluctuation. Many efforts have been devoted to suppressing the critical current fluctuation in Josephson junctions. Nonetheless, the efforts have been hindered by the defect-induced trapping states in oxide-based tunnel barriers and the interfaces with superconductors in the traditional Josephson junctions. Motivated by this, along with the recent demonstration of 2D insulator h-BN with exceptional crystallinity and low defect density, we fabricated a vertical NbSe 2 /h-BN/Nb Josephson junction consisting of a bottom NbSe 2 superconductor thin layer and a top Nb superconductor spaced by an atomically thin h-BN layer. Furthermore, we characterized the superconducting current and voltage (I–V) relationships and Fraunhofer pattern of the NbSe 2 /h-BN/Nb junction. Notably, we demonstrated the critical current noise (1/f noise power) in the h-BN-based Josephson device is at least a factor of four lower than that of the previously studied aluminum oxide-based Josephson junctions. Our work offers a strong promise of h-BN as a novel tunnel barrier for high-quality Josephson junctions and qubit applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Chiral superconductivity from a parent Chern band and its non-Abelian generalization

Here, we propose a minimal model starting from a parent Chern band with quartic dispersion that can describe the spin-valley polarized electrons in rhombohedral tetralayer graphene. The interplay between repulsive and attractive interactions on top of that parent Chern band is studied. We conduct standard self-consistent mean-field calculations, and find a rich phase diagram that consists of metal, quantum anomalous Hall crystal, chiral topological superconductor, as well as trivial gapped Bose-Einstein condensate. In particular, there exists a topological phase transition from the chiral superconductor to the Bose-Einstein condensate at zero temperature. Motivated by the recent experimental and theoretical studies of composite Fermi liquid in rhombohedral stacked multilayer graphene, we further generalize the physical electron model to its composite fermion counterpart based on a field theory analysis. The chiral superconductor phase of the composite fermion becomes the non-abelian Moore-Read quantum Hall phase. We argue that a chiral (pseudo-)spin liquid phase can emerge in the vicinity of this Moore-Read quantum Hall phase. Our work suggests rhombohedral multilayer graphene as a potential platform for rich correlated topological phases.

Wang, Yan-Qi [University of Maryland, College Park↗
DOE OSTI2020

Route to high-$T_c$ superconductivity via CH 4 -intercalated H 3 S hydride perovskites

While exploring potential superconductors in the C-S-H ternary system using first-principles crystal structure prediction methods, we uncovered a class of hydride perovskites based on the intercalation of methane into an H 3 S framework. These intriguing H 3 S–CH 4 structures emerge as metastable at ~100 GPa. Further, electron-phonon coupling calculations indicate that phases with CSH 7 stoichiometry are potential superconductors with $T_c$ values ranging from 100 K to 190 K at megabar pressures. The results are expected to guide the experimental search for new high-$T_c$ superconductors, including those stable at lower pressures than previously documented superconducting hydrides such as H 3 S and LaH 10 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Thermodynamic signatures of an antiferromagnetic quantum critical point inside a superconducting dome

Recent experiments in unconventional superconductors, and in particular iron-based materials, have reported evidence of an antiferromagnetic quantum critical point (AFM-QCP) emerging inside the superconducting dome of the phase diagram. Fluctuations associated with such an AFM-QCP are expected to promote unusual temperature dependencies of thermodynamic quantities. Here, we compute the T dependence of the specific heat C(T) deep inside a fully gapped s +– superconducting state as the AFM-QCP is approached. We find that at the AFM-QCP, the specific heat C(T) vanishes quadratically with temperature, as opposed to the typical exponential suppression seen in fully gapped BCS superconductors. This robust result is due to a nonanalytic contribution to the free energy arising from the general form of the bosonic (AFM) propagator in the SC state. Away from the AFM-QCP, as temperature is lowered, C(T) shows a crossover from a T2 behavior to an exponential behavior, with the crossover temperature scale set by the value of the superconducting gap and the distance to the QCP. As a result, we argue that these features in the specific heat can be used to unambiguously determine the existence of AFM-QCPs inside the superconducting domes of iron-based and other fully gapped unconventional superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Orbital selective spin waves in detwinned NaFeAs

The existence of orbital-dependent electronic correlations has been recognized as an essential ingredient to describe the physics of iron-based superconductors. NaFeAs, a parent compound of iron-based superconductors, exhibits a tetragonal-to-orthorhombic lattice distortion below T s ≈ 60 K, forming an electronic nematic phase with two 90° rotated (twinned) domains, and orders antiferromagnetically below T N ≈ 42 K. We use inelastic neutron scattering to study spin waves in uniaxial pressure-detwinned NaFeAs. By comparing the data with combined density functional theory and dynamical mean-field theory calculations, we conclude that spin waves up to an energy scale of E crossover ≈ 100 meV are dominated by d yz -d yz intraorbital scattering processes, which have the twofold (C 2 ) rotational symmetry of the underlying lattice. On the other hand, the spin wave excitations above E crossover , which have approximately fourfold (C 4 ) rotational symmetry, arise from the d xy -d xy intraorbital scattering that controls the overall magnetic bandwidth in this material. In addition, we find that the low-energy (E ≈ 6 meV) spin excitations change from approximate C 4 to C 2 rotational symmetry below a temperature T* (>T s ), while spin excitations at energies above E crossover have approximate C 4 rotational symmetry and are weakly temperature dependent. Here, these results are consistent with angle-resolved photoemission spectroscopy measurements, where the presence of a uniaxial strain necessary to detwin NaFeAs also raises the onset temperature T* of observable orbital-dependent band splitting to above T s , thus supporting the notion of orbital selective spin waves in the nematic phase of iron-based superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Prediction of superconductivity and topological aspects in single-layer β -Bi 2 Pd

Topological superconductors, characterized by topologically nontrivial states residing in a superconducting gap, are a recently discovered class of materials having Majorana fermions. The interplay of superconductivity and topological states gives rise to opportunities for achieving such topological superconductors in condensed matter systems. Up to now, several single-material topological superconductors in this form have been theoretically predicted and experimentally confirmed. Here, using the first-principles calculations, we study the superconducting single-layer β-Bi 2 Pd. In this work, the electronic density of states near Fermi level of this monolayer are dominated by the Bi-p and Pd-d orbitals, forming a two-band Fermi surface with multiclass sheets. The presence of soft phonon bands, in cooperation with the electron susceptibility, accounts for electron-phonon superconductivity of single-layer β-Bi 2 Pd. With the centrosymmetric structure, single-layer β-Bi 2 Pd possesses a continuous gap over the whole Brillouin zone and topological Dirac-like states at its one-dimensional boundary. The present findings would lead to the expectation of one-dimensional topological superconductivity and Majorana bound states in a monolayer candidate of β-Bi 2 Pd with intrinsic full-gap superconductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Multigap electron-phonon superconductivity in the quasi-one-dimensional pnictide K 2 Mo 3 As 3

In this work, we show, using density functional calculations, that quasi-one-dimensional K 2 Mo 3 As 3 , which is closely related to K 2 Cr 3 As 3 and has very similar superconducting properties, is not close to magnetism and has conventional s wave electron-phonon superconductivity. This superconductivity is of multigap character due to different coupling on different Fermi surface sheets. This is discussed in relation to the properties of this family of quasi-one-dimensional pnictide superconductors. The results show that this family of superconductors provides a unique opportunity for studying the interplay of spin fluctuations and electron-phonon superconductivity in transition metal pnictides, and offer a path for sorting out the different proposed superconducting scenarios in this fascinating family of pnictide superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Spectroscopic evidence of highly correlated electrons in $\mathrm{VSe_2}$

We present detailed high-resolution angle-resolved photoemission experiments on $\mathrm{VSe_2}$ grown under various conditions. The surface electronic structure optimally grown samples can host high-temperature spectral kink, quasiparticle peak, and the Fermi gap. Collective observation of these electronic features refers to the strong electronic correlation and is usually attributed to the characteristics of superconductors. Temperature evolution of the quasiparticle peak and the Fermi gap also follow the trend observed in high-$T_c$ superconductors as progressive quenching of the quasiparticle peak at a lower temperature than the complete Fermi gap closing. As a result, the quasiparticle peak can be observed up to 100 ± 5 K while the Fermi gap persists up to ~ 150 K. Importantly, these realizations can guide future studies to induce high-temperature superconductivity in transition metal dichalcogenides as well as understand the physics behind high-$T_c$ superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Superconducting diode effect in quasi-one-dimensional systems

The recent observations of the superconducting diode effect pose the challenge to fully understand the necessary ingredients for nonreciprocal phenomena in superconductors. In this theoretical work, we focus on the nonreciprocity of the critical current in a quasi-one-dimensional superconductor. We define the critical current as the value of the supercurrent at which the quasiparticle excitation gap closes (depairing). Once the critical current is exceeded, the quasiparticles can exchange energy with the superconducting condensate, giving rise to dissipation. Our minimal model can be microscopically derived as a low-energy limit of a Rashba spin-orbit coupled superconductor in a Zeeman field. Within the proposed model, we explore the nature of the nonreciprocal effects of the critical current both analytically and numerically. Further, our results quantify how system parameters such as spin-orbit coupling and quantum confinement affect the strength of the superconducting diode effect. Our theory provides a complementary description to Ginzburg-Landau theories of the effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Superconductivity at carrier density 10 17 cm – 3 in quasi-one-dimensional Li 0.9 Mo 6 O 17

Superconductivity is a fascinating phenomenon that involves an attractive interaction by which electrons are paired and can move without electrical resistance below a critical temperature T c . A paradigm for understanding superconductive pairing, the successful “BCS” theory, developed more than 65 years ago, is challenged by certain classes of materials for which parameters (e.g. the value of T c , the density of electrons, etc.) fall outside the range for which BCS theory is applicable. The study of such materials is important because new physics may be revealed and because superconductivity has many potential applications in sensing and electronics. Superconductivity at very low carrier density is a rare and fascinating phenomenon that is of considerable importance and interest in condensed matter physics. The conditions of high density of states and effective screening of Coulomb repulsion, both key ingredients of stable Cooper pairing in the BCS theory, may not be met at low carrier density. Thus, such systems may also harbor unconventional electron pairing (e.g. non-phononic) or pairing without phase coherence as a precursor to Bose-Einstein condensation. An intriguing route toward extreme low-density (and possibly unconventional) superconductivity may be found in quasi-one-dimensional metals for which density-wave instabilities compete for the ground state and can lead to gapping of substantial portions of the Fermi surface (FS), leaving a residual (reconstructed) FS comprising a very small carrier density. Our work highlights a novel material in this unusual category: Li 0.9 Mo 6 O 17 (T c =2 K) (also known as "lithium purple bronze" or "LiPB"). We present both electrical and thermoelectric magnetotransport measurements that reveal superconductivity to occur at carrier density 2 x10 17 cm -3 , among the lowest known for any superconductor, and four orders of magnitude lower than is expected from its chemical valence and band structure. With its exceptionally large ratio of superconducting to Fermi temperature, T c /TF ≳ 0.1, LiPB is properly placed at the top of a very small group of superconductors (including cuprates, heavy-fermion and Fe-based superconductors) for which this ratio falls in the BCS-BEC crossover regime and for which pairing is believed to be spin mediated.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Anisotropic, multiband, and strong-coupling superconductivity of the Pb 0.64 ⁢Bi 0.36 alloy

This paper presents theoretical and experimental studies on the superconductivity of Pb ⁢0.64 ⁢Bi 0.36 alloy, which is a prototype of strongly coupled superconductors and exhibits one of the strongest coupling under ambient pressure among the materials studied so far. The critical temperature, the specific heat in the superconducting state, and the magnetic critical fields are experimentally determined. Deviations from the single-gap s-wave BCS-like behavior are observed. The electronic structure, phonons, and electron-phonon interactions are analyzed in relation to the metallic Pb, explaining why the Pb-Bi alloy exhibits such a large value of the electron-phonon coupling parameter λ ≃ 2. Superconductivity is studied using the isotropic Eliashberg formalism as well as the anisotropic density functional theory for superconductors. Here, we find that while Pb is a two-gap superconductor with well-defined separate superconducting gaps, in the Pb-Bi alloy an overlapped three-gap-like structure is formed with a strong anisotropy. Furthermore, the chemical disorder, inherent to this alloy, leads to strong electron scattering, which is found to reduce the critical temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Triplet Resonating Valence Bond State and Superconductivity in Hund’s Metals

A central idea in strongly correlated systems is that doping a Mott insulator leads to a superconductor by transforming the resonating valence bonds (RVBs) into spin-singlet Cooper pairs. Here, we argue that a spin-triplet RVB (tRVB) state, driven by spatially, or orbitally anisotropic ferromagnetic interactions can provide the parent state for triplet superconductivity. We apply this idea to the iron-based superconductors, arguing that strong on site Hund’s interactions develop intra-atomic tRVBs between the $t_{2_g}$ orbitals. On doping, the presence of two iron atoms per unit cell allows these interorbital triplets to coherently delocalize onto the Fermi surface, forming a fully gapped triplet superconductor. In this work, this mechanism gives rise to a unique staggered structure of on site pair correlations, detectable as an alternating π phase shift in a scanning Josephson tunneling microscope.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Ultrahigh supercurrent density in a two-dimensional topological material

Ongoing advances in superconductors continue to revolutionize technology thanks to the increasingly versatile and robust availability of lossless supercurrents. In particular, high supercurrent density can lead to more efficient and compact power transmission lines, high-field magnets, as well as high-performance nanoscale radiation detectors and superconducting spintronics. In this work, we report the discovery of an unprecedentedly high superconducting critical current density (17 MA / cm 2 at 0 T and 7 MA / cm 2 at 8 T) in 1T'–WS 2 , exceeding those of all reported two-dimensional superconductors to date. 1T'–WS 2 features a strongly anisotropic (both in- and out-of-plane) superconducting state that violates the Pauli paramagnetic limit signaling the presence of unconventional superconductivity. Spectroscopic imaging of the vortices further substantiates the anisotropic nature of the superconducting state. More intriguingly, the normal state of 1T'–WS 2 carries topological properties. The band structure obtained via angle-resolved photoemission spectroscopy and first-principles calculations points to a Z 2 topological invariant. The concomitance of topology and superconductivity in 1T'–WS 2 establishes it as a topological superconductor candidate, which is promising for the development of quantum computing technology.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Towards 20 T Hybrid Accelerator Dipole Magnets

We report the most effective way to achieve very high collision energies in a circular particle accelerator is to maximize the field strength of the main bending dipoles. In dipole magnets using Nb-Ti superconductor the practical field limit is considered to be 8-9 T. When Nb 3 Sn superconductor material is utilized, a field level of 15-16 T can be achieved. To further push the magnetic field beyond the Nb 3 Sn limits, High Temperature Superconductors (HTS) need to be considered in the magnet design. The most promising HTS materials for particle accelerator magnets are Bi2212 and REBCO. However, their outstanding performance comes with a significantly higher cost. Therefore, an economically viable option towards 20 T dipole magnets could consist in an “hybrid” solution, where both HTS and Nb3Sn materials are used. We discuss in this paper preliminary conceptual designs of various 20 T hybrid magnet concepts. After the definition of the overall design criteria, the coil dimensions and parameters are investigated with finite element models based on simple sector coils. Preliminary 2D cross-section computation results are then presented and three main layouts compared: cos-theta, block, and common-coil. Both traditional designs and more advanced stress-management options are considered.

43 PARTICLE ACCELERATORS↗
DOE OSTI2023

Potential Lifshitz transition at optimal substitution in nematic pnictide Ba 1 x Sr x Ni 2 As 2

BaNi 2 As 2 is a structural analog of the pnictide superconductor BaFe 2 As 2 , which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature (T c ). To understand the mechanisms responsible for enhancement of T c , we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba 1-x Sr x Ni 2 As 2 series, which agree well with our density functional theory (DFT) calculations throughout the substitution range. Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and reasonably nested electron and hole Fermi pockets near optimal substitution where T c is maximum. These nested pockets host Ni d xz /d yz orbital compositions, which we associate with the enhancement of nematic fluctuations, revealing unexpected connections to the iron-pnictide superconductors. This gives credence to a scenario in which nematic fluctuations drive an enhanced T c .

36 MATERIALS SCIENCE↗