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Results for “SOMMERFELD CONSTANT”

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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

First-principles study of electronic properties of FeSe{sub 1-x}S{sub x} alloys

We have studied the electronic and superconducting properties of FeSe{sub 1-x}S{sub x} (x = 0.0, 0.04) alloys by first-principles calculations using the Korringa-Kohn-Rostoker Atomic Sphere Approximation within the coherent potential approximation (KKR-ASA-CPA). The electronic structure calculations show the ground states of S-doped FeSe to be nonmagnetic. We present the results of our unpolarized calculations for these alloys in terms of density of states (DOS), band structures, Fermi surfaces and the superconducting transition temperature of FeSe and FeSe{sub 0.96}S{sub 0.04} alloys. We find that the substitution of S at Se site into FeSe exhibit the subtle changes in the electronic structure with respect to the parent FeSe. We have also estimated bare Sommerfeld constant (γ{sub b}), electron-phonon coupling constant (λ) and the superconducting transition temperature (T{sub c}) for these alloys, which were found to be in good agreement with experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Robust superconductivity with large upper critical field in Nb{sub 2}PdS{sub 5}

We report synthesis, structural details, and complete superconducting characterization of very recently discovered [Q. Zhang, Sci. Rep. 3, 1446 (2013)] Nb{sub 2}PdS{sub 5} new superconductor. The synthesized compound is crystallized in monoclinic structure with C2/m (#12) space group. Bulk superconductivity is seen in both ac/dc magnetic susceptibility and electrical resistivity measurements with superconducting transition temperature (T{sub c}) at 6 K. The upper critical field (H{sub c2}) being estimated from high field magneto-transport [ρ(T)H] measurements is above 240 kOe. The estimated H{sub c2}(0) is clearly above the Pauli paramagnetic limit of ∼1.84T{sub c}. Heat capacity (C{sub p}) measurements show clear transition with well defined peak at T{sub c}, but with lower jump than as expected for a Bardeen–Cooper–Schrieffer type superconductor. The Sommerfeld constant (γ) and Debye temperature (Θ{sub D}) as determined from low temperature fitting of C{sub P}(T) data are 32 mJ/mole-K{sup 2} and 263 K, respectively. Hall coefficients and resistivity in conjugation with electronic heat capacity indicates multiple gap superconductivity signatures in Nb{sub 2}PdS{sub 5}. We also studied the impact of hydrostatic pressure (0–1.97 Gpa) on superconductivity of Nb{sub 2}PdS{sub 5} and found nearly no change in T{sub c} for the given pressure range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The σ-phase superconductors Nb 20.4 Rh 5.7 Ge 3.9 and Nb 20.4 Rh 5.7 Si 3.9

Here, we show that the previously unreported ternary σ phases Nb 20.4 Rh 5.7 Ge 3.9 and Nb 20.4 Rh 5.7 Si 3.9 are both superconductors with T c values of approximately 1.9 K. The superconducting transitions were characterized through temperature-dependent magnetic susceptibility, electrical resistance, and specific heat measurements. The Sommerfeld constants, γ, for Nb 20.4 Rh 5.7 Ge 3.9 and Nb 20.4 Rh 5.7 Si 3.9 are 89(1) mJ mol-f.u. –1 K –2 and 86(1) mJ mol-f.u. –1 K –2 (about 3.0 and 2.9 mJ mol-atom –1 K –2 ), respectively. Here, the normalized specific heat jumps at T c , ΔC/γT c , are approximately 1.39 for Nb 20.4 Rh 5.7 Ge 3.9 and 1.30 for Nb 20.4 Rh 5.7 Si 3.9 , consistent with both materials being weakly coupled BCS type superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

U 8 Al 19 Si 6 , A Uranium Aluminide Silicide with a Stuffed Supercell Grown from Aluminum Flux

In this work, U 8 Al 19 Si 6 is formed from the reaction of uranium oxide and silicon in aluminum flux. Growth of this material is dependent on the presence of surface hydroxyl groups on alumina in the crucible. The compound forms as large cuboid crystals with a complex new structure that is a cubic stuffed supercell of UAl 3 (AuCu 3 parent structure type). It features a 4-fold expansion of the unit cell axis and the addition of atoms on two extra positions resulting in Pm$\bar{3n}$ symmetry. The ordered distribution of aluminum and silicon was determined using single crystal neutron diffraction. The magnetic susceptibility shows temperature independent paramagnetic behavior, while heat capacity measurements are consistent with Fermi liquid characteristics at low temperatures where the Sommerfeld constant is enhanced in comparison to that of a conventional metal. Lastly, these bulk thermodynamic properties indicate that U 8 Al 19 Si 6 is an itinerant electron Pauli paramagnet with moderately enhanced mass charge carrier quasiparticles.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Superconductivity in the superhard boride WB 4.2

Here, we show that the superhard boride WB 4.2 is a superconductor with a T c of 2.05(5) K. Temperature-dependent magnetic susceptibility, electrical resistivity, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant gamma for WB 4.2 is 2.07(3) mJ mol( -1 ) K -2 and the Δ C/γ T c = 1.56, which is somewhat higher than what is expected for weakly coupled Bardeen-Cooper-Schrieffer type superconductors. The H c2 versus T plot is linear over a wide temperature range but does show signs of flattening by the lowest temperatures studied and therefore the zero temperature upper critical field ( μ 0 H c2 (0)) for WB 4.2 lies somewhere between the linear extrapolation of μ 0 H c2 (T) to 0 K and expectations based on the Werthamer-Helfand-Hohenberg model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Simultaneous metal-insulator and antiferromagnetic transitions in orthorhombic perovskite iridate Sr 0.94 Ir 0.78 O 2.68 single crystals

The orthorhombic perovskite SrIrO 3 is a semimetal, an intriguing exception in iridates where the strong spin-orbit interaction coupled with electron correlations tends to impose an insulating state. Here, we report results of our investigation of bulk single-crystal Sr 0.94 Ir 0.78 O 2.68 or Ir-deficient, orthorhombic perovskite SrIrO 3 . It retains the same crystal structure as stoichiometric SrIrO 3 but exhibits a sharp, simultaneous antiferromagnetic (AFM) and metal-insulator (MI) transition occurring in the basal-plane resistivity at 185 K. Above it, the basal-plane resistivity features an extended regime of almost linear temperature dependence up to 800 K but the strong electronic anisotropy renders an insulating behavior in the out-of-plane resistivity. The Hall resistivity undergoes an abrupt sign change and grows below 40 K, which along with the Sommerfeld constant of 20 mJ/mol K 2 suggests a multiband effect. All results including our first-principles calculations underscore a delicacy of the paramagnetic, metallic state in SrIrO 3 that is in close proximity to an AFM insulating state. The contrasting ground states in isostructural Sr 0.94 Ir 0.78 O 2.68 and SrIrO 3 illustrate a critical role of lattice distortions and Ir deficiency in rebalancing the ground state in the iridates. Finally, the concurrent AFM and MI transitions reveal a direct correlation between the magnetic transition and formation of an activation gap in the iridate, which is conspicuously absent in Sr 2 IrO 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Superconductivity in the Nb-Ru-Ge σ phase

Here, we show that the previously unreported ternary σ-phase material Nb 20.4 Ru 5.7 Ge 3.9 (Nb 0.68 Ru 0.19 Ge 0.13 ) is a superconductor with a critical temperature of 2.2 K. Temperature-dependent magnetic susceptibility, resistance, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant γ for Nb 20.4 Ru 5.7 Ge 3.9 is 91 mJ mol-f.u. -1 K -2 (~3 mJ mol-atom -1 K -2 ) and the specific heat anomaly at the superconducting transition, ΔC/γT c , is approximately 1.38. The zero-temperature upper critical field (µ 0 Hc 2 (0)) was estimated to be 2 T by resistance data. Field-dependent magnetization data analysis estimated µ 0 Hc 1 (0) to be 5.5 mT. Thus, the characterization shows Nb 20.4 Ru 5.7 Ge 3.9 to be a type II BCS superconductor. This material appears to be the first reported ternary phase in the Nb-Ru-Ge system, and the fact that there are no previously reported binary Nb-Ru, Nb-Ge, or Ru-Ge σ-phases shows that all three elements are necessary to stabilize the material. An analogous σ-phase in the Ta-Ru-Ge system did not display superconductivity above 1.7 K, which suggests that electron count cannot govern the superconductivity observed. Preliminary characterization of a possible superconducting σ-phase in the Nb-Ru-Ga system is also reported.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Constraints on alternate universes: stars and habitable planets with different fundamental constants

This paper develops constraints on the values of the fundamental constants that allow universes to be habitable. We focus on the fine structure constant α and the gravitational structure constant α{sub G}, and find the region in the α-α{sub G} plane that supports working stars and habitable planets. This work is motivated, in part, by the possibility that different versions of the laws of physics could be realized within other universes. The following constraints are enforced: [A] long-lived stable nuclear burning stars exist, [B] planetary surface temperatures are hot enough to support chemical reactions, [C] stellar lifetimes are long enough to allow biological evolution, [D] planets are massive enough to maintain atmospheres, [E] planets are small enough in mass to remain non-degenerate, [F] planets are massive enough to support sufficiently complex biospheres, [G] planets are smaller in mass than their host stars, and [H] stars are smaller in mass than their host galaxies. This paper delineates the portion of the α-α{sub G} plane that satisfies all of these constraints. The results indicate that viable universes—with working stars and habitable planets—can exist within a parameter space where the structure constants α and α{sub G} vary by several orders of magnitude. These constraints also provide upper bounds on the structure constants (α,α{sub G}) and their ratio. We find the limit α{sub G}/α ∼< 10{sup −34}, which shows that habitable universes must have a large hierarchy between the strengths of the gravitational force and the electromagnetic force.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Varying alpha and cosmic acceleration in Brans-Dicke-BSBM theory: stability analysis and observational tests

By integration of generalized BSBM and Brans-Dicke cosmological models, in this article, we investigate the theoretical framework of fine structure constant variation and current cosmic acceleration. We first develop a mathematical formalism to analyse the stability of the model. By employing observational data to constrain the model parameters, phase space is performed and the attractor solutions of the model are detected. We then examine the model against observational data such as observational Hubble parameter dataset and quasar absorption spectra. The results confirm current universe acceleration and also predicts fine structure constant variation. Furthermore, extrapolation of the best fitted model in high redshift (z > 15) illustrates a significantly larger variation of fine structure constant in earlier epoch of the universe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Universe acceleration and fine structure constant variation in BSBM theory

In this work we investigate the utility of using SNe Ia observations in constraining the cosmological parameters in BSBM theory where a scalar field is responsible for both fine structure constant variation and late time universe acceleration. The model is discussed in the presence of an exponential self potential for the scalar field. Stability and phase space analysis of the solutions are studied. The model is tested against observational data for Hubble parameter and quasar absorption spectra. With the best fitted model parameters, the theory predicts a good match with the experimental results and exhibits fine structure constant variation. The analysis also predicts the recent universe acceleration and possible phantom crossing in future.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Lamb Shift of n = 1 and n = 2 States of Hydrogen-like Atoms, 1 ≤ Z ≤ 110

Theoretical energy levels of the n = 1 and n = 2 states of hydrogen-like atoms with the nuclear charge numbers 1 ≤ Z ≤ 110 are tabulated. The tabulation is based on ab initio quantum electrodynamics calculations performed to all orders in the nuclear binding strength parameter Zα, where α is the fine structure constant. Theoretical errors due to various effects are critically examined and estimated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Isotope shifts and relativistic shifts of Cr ii for the study of {alpha} variation in quasar absorption spectra

We use the combination of configuration interaction and the many-body perturbation theory method (CI + MBPT) to perform ab initio calculations of the low-energy spectra of Cr ii with high accuracy. It is found that second-order MBPT diagrams should be included in a consistent and complete way for the MBPT to improve the accuracy of calculations in this five-valence-electron system. This contrasts with previous ions with fewer valence electrons, where it was found that single-valence-electron diagrams dominate the corrections. Isotope shifts and relativistic shifts (q values) are calculated for use in astronomical determination of the fine-structure constant in quasar absorption spectra.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Transitions in Zr, Hf, Ta, W, Re, Hg, Ac, and U ions with high sensitivity to variation of the fine-structure constant

We study transitions between ground and low-energy excited states of heavy ions corresponding to s-d single-electron transitions or s{sup 2}-d{sup 2} double-electron transitions. The large nuclear charge Z and significant change in angular momentum of electron orbitals make these transitions highly sensitive to a potential variation in the fine-structure constant {alpha}. The transitions may be considered as candidates for laboratory searches for space-time variation of {alpha}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The Effect of Quantum-Mechanical Interference on Precise Measurements of the n = 2 Triplet P Fine Structure of Helium

For many decades, improvements in both theory and experiment of the fine structure of the n = 2 triplet P levels of helium have allowed for an increasingly precise determination of the fine-structure constant. Recently, it has been observed that quantum-mechanical interference between neighboring resonances can cause significant shifts, even if such neighboring resonances are separated by thousands of natural widths. The shifts depend in detail on the experimental method used for the measurement, as well as the specific experimental parameters employed. Here, we review how these shifts apply for the most precise measurements of the helium 2{sup 3}P fine-structure intervals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Atomic interactions in precision interferometry using Bose-Einstein condensates

We present theoretical tools for predicting and reducing the effects of atomic interactions in Bose-Einstein condensate (BEC) interferometry experiments. To address mean-field shifts during free propagation, we derive a robust scaling solution that reduces the three-dimensional Gross-Pitaevskii equation to a set of three simple differential equations valid for any interaction strength. To model the other common components of a BEC interferometer--condensate splitting, manipulation, and recombination--we generalize the slowly varying envelope reduction, providing both analytic handles and dramatically improved simulations. Applying these tools to a BEC interferometer to measure the fine structure constant, {alpha}[S. Gupta, K. Dieckmann, Z. Hadzibabic, and D. E. Pritchard, Phys. Rev. Lett. 89, 140401 (2002)], we find agreement with the results of the original experiment and demonstrate that atomic interactions do not preclude measurement to better than part-per-billion accuracy, even for atomic species with relatively large scattering lengths. These tools help make BEC interferometry a viable choice for a broad class of precision measurements.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

The variation of the fine-structure constant from disformal couplings

We study a theory in which the electromagnetic field is disformally coupled to a scalar field, in addition to a usual non-minimal electromagnetic coupling. We show that disformal couplings modify the expression for the fine-structure constant, α. As a result, the theory we consider can explain the non-zero reported variation in the evolution of α by purely considering disformal couplings. We also find that if matter and photons are coupled in the same way to the scalar field, disformal couplings itself do not lead to a variation of the fine-structure constant. A number of scenarios are discussed consistent with the current astrophysical, geochemical, laboratory and the cosmic microwave background radiation constraints on the cosmological evolution of α. The models presented are also consistent with the current type Ia supernovae constraints on the effective dark energy equation of state. We find that the Oklo bound in particular puts strong constraints on the model parameters. From our numerical results, we find that the introduction of a non-minimal electromagnetic coupling enhances the cosmological variation in α. Better constrained data is expected to be reported by ALMA and with the forthcoming generation of high-resolution ultra-stable spectrographs such as PEPSI, ESPRESSO, and ELT-HIRES. Furthermore, an expected increase in the sensitivity of molecular and nuclear clocks will put a more stringent constraint on the theory.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

E-ELT constraints on runaway dilaton scenarios

We use a combination of simulated cosmological probes and astrophysical tests of the stability of the fine-structure constant α, as expected from the forthcoming European Extremely Large Telescope (E-ELT), to constrain the class of string-inspired runaway dilaton models of Damour, Piazza and Veneziano. We consider three different scenarios for the dark sector couplings in the model and discuss the observational differences between them. We improve previously existing analyses investigating in detail the degeneracies between the parameters ruling the coupling of the dilaton field to the other components of the universe, and studying how the constraints on these parameters change for different fiducial cosmologies. We find that if the couplings are small (e.g., α{sub b} = α{sub V} ∼ 0) these degeneracies strongly affect the constraining power of future data, while if they are sufficiently large (e.g., α{sub b} ∼> 10{sup −5}−α{sub V} ∼> 0.05, as in agreement with current constraints) the degeneracies can be partially broken. We show that E-ELT will be able to probe some of this additional parameter space.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1986

Interactions of coupled acoustic and vortical instability

In the past, the acoustic combustion instability was studied independently of the hydrodynamic instability induced by vortex motions. This paper is intended to combine the two different sources of energy everywhere within the spatial domain and determine the effect of one upon the other. This can be achieved by calculating the mean flow velocities and vorticities and their fluctuating parts of velocities and vortices, as well as the fluctuating pressure. The Orr-Sommerfeld equation is utilized to determine the wavenumbers and unsteady stream functions from which vortically coupled acoustic instability growth constants are calculated. This process demonstrates that there are two different frequencies, acoustic and hydrodynamic, various combinations of which contribute to either damping or amplification. It is found that stability boundaries for coupled acoustic and vortical oscillations are somewhat similar to the classical hydrodynamic stability boundaries, but they occur in the form of multiple islands.

Chung, T. J.↗