Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “SUPERFLUIDITY”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Observing the Suppression of Superconductivity in RbEuFe 4 As 4 by Correlated Magnetic Fluctuations

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in RbEuFe 4 As 4 in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. Furthermore, these results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in RbEuFe 4 As 4 , with important implications for future investigations of physical phenomena arising from the interplay between them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Scanning SQUID study of ferromagnetism and superconductivity in infinite-layer nickelates

Infinite-layer nickelates R 1-x Sr x NiO 2 (R = La, Pr, Nd) are a class of superconductors with structural similarities to cuprates. Although long-range antiferromagnetic order has not been observed for these materials, magnetic effects such as antiferromagnetic spin fluctuations and spin-glass behavior have been reported. Different experiments have drawn different conclusions about whether the pairing symmetry is s or d wave. In this paper, we applied a scanning superconducting quantum interference device (SQUID) to probe the magnetic behavior of film samples of three infinite-layer nickelates (La 0.85 Sr 0.15 NiO 2 , Pr 0.8 Sr 0.2 NiO 2 , and Nd 0.775 Sr 0.225 NiO 2 ) grown on SrTiO 3 (STO), each with a nominal thickness of 20 unit cells. In all three films, we observed a ferromagnetic background. We also measured the magnetic susceptibility above the superconducting critical temperature in Pr 0.8 Sr 0.2 NiO 2 and La 0.85 Sr 0.15 NiO 2 and identified a non-Curie-Weiss dynamic susceptibility. Both magnetic features are likely due to NiO x nanoparticles. We investigated superconductivity in Pr 0.8 Sr 0.2 NiO 2 and Nd 0.775 Sr 0.225 NiO 2 , which exhibited inhomogeneous diamagnetic screening. The superfluid density inferred from the diamagnetic susceptibility in relatively homogeneous regions shows T-linear behavior in both samples. Finally, we observed superconducting vortices in Nd 0.775 Sr 0.225 NiO 2 . We determined a Pearl length of 330 µm for Nd 0.775 Sr 0.225 NiO 2 at 300 mK, both from the strength of the diamagnetism and from the size and shape of the vortices. These results highlight the importance of considering NiO x particles when interpreting experimental results for these films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Landau’s Nobel Prize in Physics

The work of Lev Landau had a profound impact on the physics in 20th century. Landau had created the paradigms that had framed the conversations on the outstanding problems in physics for decades. He had laid foundations for our understanding of quantum matter like superfluidity, superconductivity and the theory of Fermi liquid. In this paper we present some Nobel Archive data on the winning nomination that led to the Nobel Prize in Physics in 1962.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

On quantum effects on the surface of solid hydrogen

The low-frequency spectrum of hypothetical superfluidity on the free surface of a quantum crystal of hydrogen is determined. In the quantum-rough state of the surface, crystallization waves with a quadratic spectrum should propagate. In the atomically smooth state, the spectrum is linear. Crystallization waves propagating along elementary steps are also considered.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Andreev bound states. Some quasiclassical reflections

We discuss a very simple and essentially exactly solvable model problem which illustrates some nice features of Andreev bound states, namely, the trapping of a single Bogoliubov quasiparticle in a neutral s-wave BCS superfluid by a wide and shallow Zeeman trap. In the quasiclassical limit, the ground state is a doublet with a splitting which is proportional to the exponentially small amplitude for “normal” reflection by the edges of the trap. We comment briefly on a prima facie paradox concerning the continuity equation and conjecture a resolution to it.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Quasi-Stable Configurations of Torus Vortex Knots and Links

The dynamics of torus vortex configurations V{sub n, p, q} in a superfluid liquid at zero temperature (n is the number of quantum vortices, p is the number of turns of each filament around the symmetry axis of the torus, and q is the number of turns of the filament around its central circle; radii R{sub 0} and r{sub 0} of the torus at the initial instant are much larger than vortex core width ξ) has been simulated numerically based on the regularized Biot–Savart law. The lifetime of vortex systems till the instant of their substantial deformation has been calculated with a small step in parameter B{sub 0} = r{sub 0}/R{sub 0} for various values of parameter Λ = ln(R{sub 0}/ξ). It turns out that for certain values of n, p, and q, there exist quasi-stability regions in the plane of parameters (B{sub 0}, Λ), in which the vortices remain almost invariable during dozens and even hundreds of characteristic lengths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Universal Phase Diagram and Scaling Functions of Imbalanced Fermi Gases

We discuss the phase diagram and the universal scaling functions of attractive Fermi gases at finite imbalance. The existence of a quantum multicritical point for the unitary gas at vanishing chemical potential μ and effective magnetic field h, first discussed by Nikolić and Sachdev, gives rise to three different phase diagrams, depending on whether the inverse scattering length 1/a is negative, positive or zero. Within a Luttinger–Ward formalism, the phase diagram and pressure of the unitary gas is calculated as a function of the dimensionless scaling variables T/μ and h/μ. The results indicate that beyond the Clogston–Chandrasekhar limit at (h/μ){sub c} ≃ 1.09, the unitary gas exhibits an inhomogeneous superfluid phase with FFLO order that can reach critical temperatures near unitarity of ≃0.03T{sub F }.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Tetrads in Solids: from Elasticity Theory to Topological Quantum Hall Systems and Weyl Fermions

Theory of elasticity in topological insulators has many common features with relativistic quantum fields interacting with gravitational fields in the tetrad form. Here we discuss several issues in the effective topological (pseudo)electromagnetic response in three-dimensional weak crystalline topological insulators with no time-reversal symmetry that feature elasticity tetrads, including a mixed “axial-gravitational” anomaly. This response has some resemblance to “quasitopological” terms proposed for massless Weyl quasiparticles with separate, emergent fermion tetrads. As an example, we discuss the chiral/axial anomaly in superfluid {sup 3}He-A. We demonstrate the principal difference between the elasticity tetrads and the Weyl fermion tetrads in the construction of the topological terms in the action. In particular, the topological action expressed in terms of the elasticity tetrads cannot be expressed in terms of the Weyl fermion tetrads since in this case the gauge invariance is lost.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Energy dependence and systematics of level-density parameters in nuclei of mass number in the range of A = 20–60

Existing direct and indirect experimental data on level densities in excited nuclei of mass and charge number in the ranges of A = 20–60 and Z = 11–27, respectively, were compiled and analyzed. Contradictions between values extracted from the results of measurements performed by different methods were revealed. Consistent input data were selected, and a systematics of level-density parameters was created on this basis within the generalized model of superfluid nuclei. The effect of the first discrete vibrational levels on extracted parameters was studied.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Photon Induced Reaction Cross-Section Calculations of Several Structural Fusion Materials

In this paper; photo-neutron cross-sections have been computed for {sup 27}Al, {sup 28,29}Si, {sup 52}Cr, {sup 54,56}Fe isotopes by using nuclear reaction codes TALYS 1.6 and EMPIRE 3.1 with different level density models. Constant Temperature Fermi Gas, Back Shifted Fermi Gas and Generalized Superfluid Models have been employed from TALYS 1.6 whereas Generalized Super Fluid Model was the only model employed from EMPIRE 3.1 reaction codes. Obtained results have been analyzed with experimental values taken from the EXFOR database.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

First and second sound in a two-dimensional harmonically trapped Bose gas across the Berezinskii–Kosterlitz–Thouless transition

We theoretically investigate first and second sound of a two-dimensional (2D) atomic Bose gas in harmonic traps by solving Landau’s two-fluid hydrodynamic equations. For an isotropic trap, we find that first and second sound modes become degenerate at certain temperatures and exhibit typical avoided crossings in mode frequencies. At these temperatures, second sound has significant density fluctuation due to its hybridization with first sound and has a divergent mode frequency towards the Berezinskii–Kosterlitz–Thouless (BKT) transition. For a highly anisotropic trap, we derive the simplified one-dimensional hydrodynamic equations and discuss the sound-wave propagation along the weakly confined direction. Due to the universal jump of the superfluid density inherent to the BKT transition, we show that the first sound velocity exhibits a kink across the transition. These predictions might be readily examined in current experimental setups for 2D dilute Bose gases with a sufficiently large number of atoms, where the finite-size effect due to harmonic traps is relatively weak.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Single-particle spectral density of the unitary Fermi gas: Novel approach based on the operator product expansion, sum rules and the maximum entropy method

Making use of the operator product expansion, we derive a general class of sum rules for the imaginary part of the single-particle self-energy of the unitary Fermi gas. The sum rules are analyzed numerically with the help of the maximum entropy method, which allows us to extract the single-particle spectral density as a function of both energy and momentum. These spectral densities contain basic information on the properties of the unitary Fermi gas, such as the dispersion relation and the superfluid pairing gap, for which we obtain reasonable agreement with the available results based on quantum Monte-Carlo simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Phase diagrams and Hofstadter butterflies in the strongly correlated bosonic systems on the lattices with Dirac points

Gauge potentials with different configurations have been recently realized in the optical lattice experiments. It is remarkable that one of the simplest gauge potential can generate particle energy spectrum with the self-similar structure known as a Hofstadter butterfly. We investigate theoretically the impact of strong on-site interaction on such a spectrum in the bosonic Mott insulator within Bose–Hubbard model. In particular, it is shown that the fractal structure is encoded in the quasi-particle and hole bosonic branches for different lattice backgrounds. For example a square lattice and other structures (brick-wall and staggered magnetic flux lattice) which contain Dirac points in energy dispersions are considered. This shows that single-particle physics is still present even in the strong interaction limit for whole Hofstadter spectrum. Additionally we observe, that although in brick-wall and staggered flux lattices the quasi-particle densities of states look qualitatively similar, the corresponding Hofstadter butterfly assumes different forms. In particular, we use a superposition of two different synthetic gauge fields which appears to be a generator of non-trivial phenomena in the optical lattice systems. We also discuss the consequences of these phenomena on the phase diagrams between bosonic Mott insulator and superfluid phase. The analysis is carried out within the strong coupling expansion method on the finite size lattices and also at finite temperatures which are relevant for the currently made experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Direct evidence for a pressure-induced nodal superconducting gap in the Ba 0.65 Rb 0.35 Fe 2 As 2 superconductor

The superconducting gap structure in iron-based high-temperature superconductors (Fe-HTSs) is non-universal. Contrasting with other unconventional superconductors, in the Fe-HTSs both d-wave and extended s-wave pairing symmetries are close in energy. Probing the proximity between these very different superconducting states and identifying experimental parameters that can tune them is of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth in the optimally doped nodeless s-wave Fe-HTS Ba 0.65 Rb 0.35 Fe 2 As 2 . Upon pressure, a strong decrease of the penetration depth in the zero-temperature limit is observed, while the superconducting transition temperature remains nearly constant. More importantly, the low-temperature behaviour of the inverse-squared magnetic penetration depth, which is a direct measure of the superfluid density, changes qualitatively from an exponential saturation at zero pressure to a linear-in-temperature behaviour at higher pressures, indicating that hydrostatic pressure promotes the appearance of nodes in the superconducting gap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Universal lower limit on vortex creep in superconductors

Superconductors are excellent testbeds for studying vortices, topological excitations that also appear in superfluids, liquid crystals and Bose–Einstein condensates. Vortex motion can be disruptive; it can cause phase transitions, glitches in pulsars, and losses in superconducting microwave circuits, and it limits the current-carrying capacity of superconductors4. Understanding vortex dynamics is fundamentally and technologically important, and the competition between thermal energy and energy barriers defined by material disorder is not completely understood. Specifically, early measurements of thermally activated vortex motion (creep) in iron-based superconductors unveiled fast rates (S) comparable to measurements of YBa 2 Cu 3 O 7–δ . This was puzzling because S is thought to somehow correlate with the Ginzburg number (Gi), and Gi is significantly lower in most iron-based superconductors than in YBa 2 Cu 3 O 7–δ . Here, we report very slow creep in BaFe 2 (As 0.67 P 0.33 ) 2 films, and propose the existence of a universal minimum realizable S ~ Gi 1/2 (T/T c ) (T c is the superconducting transition temperature) that has been achieved in our films and few other materials, and is violated by none. Furthermore, this limitation provides new clues about designing materials with slow creep and the interplay between material parameters and vortex dynamics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Parametric amplification of a superconducting plasma wave

Many applications in photonics require all-optical manipulation of plasma waves, which can concentrate electromagnetic energy on sub-wavelength length scales. This is difficult in metallic plasmas because of their small optical nonlinearities. Some layered superconductors support Josephson plasma waves, involving oscillatory tunnelling of the superfluid between capacitively coupled planes. Josephson plasma waves are also highly nonlinear, and exhibit striking phenomena such as cooperative emission of coherent terahertz radiation, superconductor–metal oscillations and soliton formation. In this paper, we show that terahertz Josephson plasma waves can be parametrically amplified through the cubic tunnelling nonlinearity in a cuprate superconductor. Finally, parametric amplification is sensitive to the relative phase between pump and seed waves, and may be optimized to achieve squeezing of the order-parameter phase fluctuations or terahertz single-photon devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Millimetre-long transport of photogenerated carriers in topological insulators

Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi 2-x Sb x Se 3 nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m 2 s -1 , which corresponds to a mobility of 6 × 104 m 2 V -1 s -1 at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor FeTe 0.55 Se 0.45

By using scanning tunneling microscopy (STM) we find and characterize dispersive, energy-symmetric in-gap states in the iron-based superconductor FeTe 0.55 Se 0.45 , a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (YSR) type, and argue that the energy shift is caused by the low superfluid density in FeTe 0.55 Se 0.45 , which allows the electric field of the tip to slightly penetrate the sample. We model the newly introduced tip-gating scenario within the single-impurity Anderson model and find good agreement to the experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗