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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Critical fields of superconductors with magnetic impurities

The upper critical field H c2 , the field H c3 for nucleation of the surface superconductivity, and the thermodynamic field H c are evaluated within the weak-coupling theory for the isotropic s-wave case with arbitrary transport, and pair-breaking scattering. In this study, we find that, for the standard geometry of a half-space sample in a magnetic field parallel to the surface, the ratio $\mathscr{R}$=H c3 /H c2 is within the window 1.55≲$\mathscr{R}$≲2.34 , regardless of temperature or the scattering type. While the nonmagnetic impurities tend to flatten the $\mathscr{R}$($\mathrm{T}$) variation, magnetic scattering merely shifts the maximum of $\mathscr{R}$($\mathrm{T}$) to lower temperatures. Surprisingly, while reducing the transition temperature, magnetic scattering has a milder impact on $\mathscr{R}$ than nonmagnetic scattering. The surface superconductivity is quite robust; in fact, the ratio $\mathscr{R}$≈1.7 even in the gapless state. We used Eilenberger's energy functional to evaluate the condensation energy F c and the thermodynamic critical field H c for any temperature and scattering parameters. By comparing H c2 and H c , we find that, unlike transport scattering, the pair-breaking pushes materials toward type-I behavior. Furthermore, we find a peculiar behavior of F c as a function of the pair-breaking scattering parameter at the low-$\mathrm{T}$ transition from gapped to gapless phases, which has recently been associated with the topological transition in the superconducting density of states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1988

Planar thin film SQUID with integral flux concentrator

A thin film SQUID is disclosed having improved flux concentration combined with simplicity of design and fabrication. The SQUID starts with a wafer like substrate having simple planar geometry. A large area of superconducting film is coated on the substrate, with a small open or uncoated area remaining at its center to define a SQUID loop, and a gap in the film formed, beginning at the outer circumferential edge of the substrate and extending radially inward to the open area. A Josephson junction is formed across the gap near the open area to interrupt the electrical continuity of the SQUID loop. A coil is attached to the surface of the substrate, electrically insulated from the superconducting film, and is energized to induce flux within the SQUID which is concentrated within the open area.

Peters, Palmer N.↗
DOE OSTI2024

Quasiparticle Poisoning of Superconducting Qubits from Resonant Absorption of Pair-Breaking Photons

The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1 μ m - 3 , tens of orders of magnitude greater than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and can induce dephasing. Here we show that a dominant mechanism for quasiparticle poisoning is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity of the qubit. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticles. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning.

Liu, Chuan-Hong↗
DOE OSTI2025

Robust Spin-Moiré-Superlattice-Driven Gap Opening in EuAg 4 Sb 2 under in-Plane Magnetic Field

Moiré superlattices introduce new length and energy scales, enabling discoveries, such as unconventional superconductivity, in 2D systems. This concept has recently been extended to bulk materials with multiple-q spin textures, opening exciting opportunities for spin moiré physics. A notable example is EuAg 4 S b2 , where a spin moiré superlattice (SMS), manifested as a double-q spin modulation, induces a superzone gap opening. In this work, we investigate the tunability of this gap by an in-plane magnetic field in EuAg 4 Sb 2 by using neutron scattering, magnetization, and transport measurements. We reveal a cycloidal ground state and multiple spin-reoriented phases induced by the in-plane field, highlighting the critical role of in-plane components in driving magnetic transitions. Moreover, we demonstrate that a robust gap opening persists in the double-q phase, regardless of in-plane field orientation. Model calculations attribute this robustness to the stability of the SMS under tilted fields. Here, our results establish EuAg 4 Sb 2 as a tunable platform for exploring the spin-texture-driven superzone gap opening in electronic states.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Possible topological superconductivity in the topological crystalline insulator $\mathrm{(Pb_{1-x} Sn_x)_{1-y}In_yTe}$

Superconductivity in topological insulators is expected to show very unconventional features such as a $p+ip$ order parameter, Majorana fermions, etc. However, intrinsic superconductivity has been observed in a very limited number of materials in which the pairing symmetry is still a matter of debate. Here, we study the topological crystalline insulator (TCI) $\mathrm{(Pb_{1-x} Sn_x)_{1-y}In_yTe}$, for which a peculiar insulator to superconductor transition was previously reported near the gap inversion transition, where the system is nearly a three-dimensional Dirac semimetal. Both the existence of superconductivity near the three-dimensional Dirac semimetal and the occurrence of an insulator to superconductor transition in an isotropic material are highly unusual. We suggest that the observed phenomena are related to the intrinsic instability of a three-dimensional Dirac semimetal state in $\mathrm{(Pb_{1-x} Sn_x)_{1-y}In_yTe}$ and “flattening” of the bulk valence and conduction bands as they acquire a Mexican-hat-like dispersion on the inverted side of the phase diagram. Importantly, this favors the pairing instability if the chemical potential is pinned to these flat regions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Transient gap generation in BaFe2As2 driven by coherent lattice vibrations

Abstract Iron-based superconductors provide a rich platform to investigate the interplay between unconventional superconductivity, nematicity, and magnetism. The electronic structure and the magnetic properties of iron-based superconductors are highly sensitive to the pnictogen height. Coherent excitation of the A1g phonon by femtosecond laser directly modulates the pnictogen height, which has been used to control the physical properties of iron-based superconductors. Previous studies show that the driven A1g phonon resulted in a transient increase of the pnictogen height in BaFe2As2, favoring an enhanced Fe magnetic moment. However, there are no direct observations on either the enhanced Fe magnetic moments or the enhanced spin-density wave (SDW) gap. Here, we use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFe2As2 in the A1g phonon-driven state. Below the SDW transition temperature, we observe a transient gap generation at early-time delays. A similar transient feature is observed in the normal state up to room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Structural transition and possible pressure-induced superconductivity in a suboxide La 5 Pb 3 O

Here, in this study, we report a structural phase transition and its interplay with superconductivity in the suboxide La 5⁢ Pb 3 ⁢O. Upon cooling through 𝑇 𝑡 = 225K, La 5⁢ Pb 3⁢ O transforms from a high-temperature 𝐼⁢4/mcm to a low-temperature 𝑃⁢4/ncc structure in which La-Pb dimerization along the 𝑐 axis occurs. This transition is accompanied by anomalies in the temperature dependence of electrical resistivity and specific heat. High-pressure electrical transport measurements reveal that hydrostatic pressure suppresses the structural transition and possibly induces superconductivity with a maximum superconducting temperature of 10 K. Density functional theory calculations show minimal changes in the electronic density of states and no gap opening at 𝐸 𝐹 across 𝑇 𝑡 , suggesting that the transition is driven by bonding effects rather than Fermi surface instability. These findings establish La 5⁢ Pb 3 ⁢O as a promising platform for exploring the interplay between weak structural transitions and superconductivity.

Yan, Jiaqiang [Oak Ridge National Laboratory (ORNL↗
DOE OSTI2024

Pseudogap in electron-doped cuprates: Strong correlation leading to band splitting

The pseudogap phenomena have been a long-standing mystery of the cuprate high-temperature superconductors. The pseudogap in the electron-doped cuprates has been attributed to band folding due to antiferromagnetic (AFM) long-range order or short-range correlation. We performed an angle-resolved photoemission spectroscopy study of the electron-doped cuprates Pr 1.3−x La 0.7 Ce x CuO 4 showing spin-glass, disordered AFM behaviors, and superconductivity at low temperatures and, by measurements with fine momentum cuts, found that the gap opens on the unfolded Fermi surface rather than the AFM Brillouin zone boundary. The gap did not show a node, following the full symmetry of the Brillouin zone, and its magnitude decreased from the zone-diagonal to (π,0) directions, opposite to the hole-doped case. These observations were reproduced by cluster dynamical-mean-field-theory calculation, which took into account electron correlation precisely within a (CuO 2 ) 4 cluster. In conclusion, the present experimental and theoretical results are consistent with the mechanism that electron or hole doping into a Mott insulator creates an in-gap band that is separated from the upper or lower Hubbard band by the pseudogap.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Linearly dispersive bands at the onset of correlations in K 𝑥 ⁢C 60 films

Molecular crystals are a flexible platform to induce novel electronic phases. Due to the weak forces between molecules, intermolecular distances can be varied over larger ranges than interatomic distances in atomic crystals. On the other hand, the hopping terms are generally small, which results in narrow bands, strong correlations, and heavy electrons. Here, by growing K x C 60 fullerides on hexagonal layered Bi 2 Se 3 , we show that upon doping the series undergoes a Mott transition from a molecular insulator to a correlated metal and an in-gap state evolves into highly dispersive Dirac-like fermions at half filling, where superconductivity occurs. This picture challenges the commonly accepted description of the low-energy quasiparticles as appearing from a gradual electron doping of the conduction states and suggests an intriguing parallel with the more famous family of the cuprate superconductors. More in general, it indicates that molecular crystals offer a viable route to engineer electron-electron interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

HTS Dipole Magnet With Mechanical Energy Transfer in the Magnetic Field

Several high-temperature superconducting (HTS) model magnets for particle accelerators were designed and successfully tested at Fermilab. Some worked in a persistent current mode by continuously generating magnetic fields in the iron-dominated magnet gap. This paper investigated a novel HTS dipole magnet concept with a mechanical energy transfer in the magnetic field. To pump the energy in the superconducting HTS dipole magnet, a detachable magnetizer was used. The HTS dipole magnet was built and successfully tested at a liquid nitrogen temperature. We discuss the magnet design, test results, the proposed approach limits, and efficiency.

43 PARTICLE ACCELERATORS↗
DOE OSTI2023

Anisotropy and isotope effect in superconducting solid hydrogen

Elucidating the phase diagram of solid hydrogen is a key objective in condensed matter physics. Several decades ago, it was proposed that at low temperatures and high pressures, solid hydrogen would be a metal with a high superconducting transition temperature. This transition to a metallic state can happen through the closing of the energy gap in the molecular solid or through a transition to an atomic solid. Recent experiments have managed to reach pressures in the range of 400–500 GPa, providing valuable insights. There is strong evidence suggesting that metallization via either of these mechanisms occurs within this pressure range. Computational and experimental studies have identified multiple promising crystal phases, but the limited accuracy of calculations and the limited capabilities of experiments prevent us from determining unequivocally the observed phase or phases. Therefore, it is crucial to investigate the superconducting properties of all the candidate phases. Recently, we reported the superconducting properties of the C2/c-24, Cmca-12, Cmca-4 and I41/amd-2 phases, including anharmonic effects. Here, we report the effects of anisotropy on superconducting properties using Eliashberg theory. Then, we investigate the superconducting properties of deuterium and estimate the size of the isotope effect for each phase. We find that the isotope effect on superconductivity is diminished by anharmonicity in the C2/c-24 and Cmca-12 phases and enlarged in the Cmca-4 and I41/amd-2 phases. Our anharmonic calculations of the C2/c-24 phase of deuterium agree closely with the most recent experiment by Loubeyre et al (2022 Phys. Rev. Lett.29 035501), indicating that the C2/c-24 phase remains the leading candidate in this pressure range, and has a strong anharmonic character. These characteristics can serve to distinguish among crystal phases in experiment. Furthermore, expanding our understanding of superconductivity in pure hydrogen holds significance in the study of high-T c hydrides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Suppression of thermal conductivity and electronic correlations in Fe 1– x Ru x Sb 2 (0 ≤ x ≤ 0.6)

We present simultaneous suppression of FeSb 2 thermal conductivity and electronic correlations in Fe 1– x Ru x Sb 2 (0 ≤ x ≤ 0.6) single crystal alloys. Small energy gap Δ 1 in Kondo-insulator-like semiconductor FeSb 2 associated with impurity in-gap state increases whereas the intrinsic bandgap Δ 2 decreases upon Ru substitution on Fe atomic site. Thermopower is suppressed along with the intrinsic bandgap and with the thermal conductivity. The more delocalized 4 d character of atomic orbital of Ru brings suppression of electronic correlations, but also an increase in impurity density which reduces phonon mean free path and surface scattering length. Our results indicate a range of Ru doping x where nanostructuring could be used to suppress thermal conductivity further, potentially toward the amorphous limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Slope of the upper critical field at T c in two-band superconductors with nonmagnetic disorder: s + + superconductivity in Ba 1 − x K x Fe 2 As 2

A recent theory of the disorder-dependent slope of the upper critical field at the superconducting transition temperature T c , defined as S ≡ |dH c2 /dT| T→T c , is extended to multiband superconductors aimed at iron-based superconductors, considering two constant gaps of different magnitudes and, potentially, different signs. Here, we show that the slope S decreases with increasing nonmagnetic scattering rate P in the s ± pairing state and increases in the s ++ superconductor for a reasonable range of parameters. The experiment shows that in a typical iron-based superconductor, Ba 1–x K x Fe 2 As 2 (BaK122), the nonmagnetic disorder induced by electron irradiation leads to an increasing S(P) across the superconducting “dome,” at different x. This implies that Ba 1–x K x Fe 2 As 2 is likely an s ++ superconductor with two effective gaps of different magnitudes, at least at moderate doping levels, x< 0.6. This work reopens a decade-long discussion about the nature of the superconducting order parameter in iron pnictides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. In this work, we combine equilibrium and time-resolved ARPES to track the temperature-dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and bi-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

$\testit{d}$-wave superconductivity and Bogoliubov-Fermi surfaces in Rarita-Schwinger-Weyl semimetals

In this work, we uncover the properties of complex tensor ($\textit{d}$-wave) superconducting order in three-dimensional Rarita-Schwinger-Weyl semimetals that host pseudospin-$\frac{3}{2}$ fermions at a fourfold linear band-crossing point. Although the general theory of $\textit{d}$-wave order was originally developed for materials displaying quadratic band touching, it directly applies to the case of semimetals with linear dispersion, several candidate compounds of which have been discovered experimentally very recently. The spin-$\frac{3}{2}$ nature of the fermions allows for the formation of spin-2 Cooper pairs which may be described by a complex second-rank tensor order parameter. In the case of linear dispersion, for the chemical potential at the Fermi point and at strong coupling, the energetically preferred superconducting state is the uniaxial nematic state, which preserves time-reversal symmetry and provides a full (anisotropic) gap for quasiparticle excitations. In contrast, at a finite chemical potential, we find that the usual weak-coupling instability is toward the “cyclic state,” well known from the studies of multicomponent Bose-Einstein condensates, which breaks time-reversal symmetry maximally, has vanishing average value of angular momentum, and features 16 small Bogoliubov-Fermi surfaces. The Rarita-Schwinger-Weyl semimetals provide therefore the first example of weakly coupled, three-dimensional, isotropic $\textit{d}$-wave superconductors where the $\textit{d}$-wave superconducting phase is uniquely selected by the quartic expansion of the mean-field free energy, and is not afflicted by the accidental degeneracy first noticed by Mermin over 40 years ago. We discuss the appearance and stability of the Bogoliubov-Fermi surfaces in absence of inversion symmetry in the electronic Hamiltonian, as in the case at hand.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Thermal transport and mixed valence in ZrTe3 doped with Hf and Se

Two-dimensional transition metal trichalcogenides (TMTCs) feature covalently bonded metal-chalcogen layers separated by the van der Waals (vdW) gap. Similar to transition metal dichalcogenides (TMDCs), TMTCs often host charge density waves (CDWs) and superconductivity, but unlike TMDCs, atomic chains in the crystal structure give rise to quasi one-dimensional (quasi 1D) conduction. ZrTe3 features the CDW below TCDW = 63 K and filamentary superconductivity below 2 K that can be enhanced by pressure or chemical substitution. Here, we report the presence of mixed valent Zr2+ and Zr4+ atoms in ZrTe3 crystals that are reduced by doping in ZrTe3−xSex and Zr1−yHfyTe3. Superconductivity is enhanced via disorder in Te2-Te3 atomic chains that are associated with CDW formation. Hf substitution on the Zr atomic site enhances TCDW due to unperturbed Te2-Te3 chain periodicity and enhanced electron-phonon coupling. Weak electronic correlations in ZrTe3−xSex are likely governed by the lattice contraction effects.

Liu, Yu (ORCID:0000000188862876)↗
NASA NTRS1992

Large-gap magnetic suspension systems

The classification of magnetic suspension devices into small-gap and large-gap categories is addressed. The relative problems of position sensing, control systems, power supplies, electromagnets, and magnetic field or force analysis are discussed. The similarity of all systems from a controls standpoint is qualified. Some applications where large-gap technology is being applied to systems with a physically small air-gap are mentioned. Finally, the applicability of some other suspension approaches, such as electrodynamic or superconducting are briefly addressed.

Britcher, Colin P.↗