503 research outputs found
Superconductivity in metal-semiconductor eutectic alloys
In an eutectic alloy obtained by melting the constituents, alternating domains of metal and semiconductor can exist in a well-defined microscopic array. The possibility of superconductivity arising from an interface interaction between the metal and semiconductor has been investigated. Superconductivity has been observed as a bulk property of some eutectic alloys (e.g., Al-Si and Al-Ge). Transition temperatures well in excess of those for the pure metal were found. The effect of rapid cooling the alloys from the liquid state on the microstructure and on the superconducting properties has been studied. It was found that a decrease in the characteristic domain sizes of the metal and semiconductor was accompanied by an increase of the superconducting transition temperature of the alloy. The results suggest that the enhancement of Tc depends on the Fermi energy of the metal. In metal-metal eutectic systems with comparable microstructure, no increase of Tc was observed. Several explanations of the experimental findings are considered. The exciton mechanism discussed by Ginzburg and by Allender, Bray, and Bardeen is considered as a possible means to account for the enhancements of the metal Tc's
Multi-dimensional combustor flowfield analyses in gas-gas rocket engine
The objectives of the present research are to improve design capabilities for low thrust rocket engines through understanding of the detailed mixing and combustions processes. Of particular interest is a small gaseous hydrogen-oxygen thruster which is considered as a coordinated part of an on-going experimental program at NASA LeRC. Detailed computational modeling requires the application of the full three-dimensional Navier Stokes equations, coupled with species diffusion equations. The numerical procedure is performed on both time-marching and time-accurate algorithms and using an LU approximate factorization in time, flux split upwinding differencing in space. The emphasis in this paper is focused on using numerical analysis to understand detailed combustor flowfields, including the shear layer dynamics created between fuel film cooling and the core gas in the vicinity on the nearby combustor wall; the integrity and effectiveness of the coolant film; three-dimensional fuel jets injection/mixing/combustion characteristics; and their impacts on global engine performance
CFD analyses of combustor and nozzle flowfields
The objectives of the research are to improve design capabilities for low thrust rocket engines through understanding of the detailed mixing and combustion processes. A Computational Fluid Dynamic (CFD) technique is employed to model the flowfields within the combustor, nozzle, and near plume field. The computational modeling of the rocket engine flowfields requires the application of the complete Navier-Stokes equations, coupled with species diffusion equations. Of particular interest is a small gaseous hydrogen-oxygen thruster which is considered as a coordinated part of an ongoing experimental program at NASA LeRC. The numerical procedure is performed on both time-marching and time-accurate algorithms, using an LU approximate factorization in time, flux split upwinding differencing in space. The integrity of fuel film cooling along the wall, its effectiveness in the mixing with the core flow including unsteady large scale effects, the resultant impact on performance and the assessment of the near plume flow expansion to finite pressure altitude chamber are addressed
Contiguous 3d and 4f magnetism: towards strongly correlated 3d electrons in YbFe2Al10
We present magnetization, specific heat, and 27Al NMR investigations on
YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic
susceptibility at low temperatures is strongly enhanced at weak magnetic
fields, accompanied by a ln(T0/T) divergence of the low-T specific heat
coefficient in zero field, which indicates a ground state of correlated
electrons. From our hard X-ray photo emission spectroscopy (HAXPES) study, the
Yb valence at 50 K is evaluated to be 2.38. The system displays valence
fluctuating behavior in the low to intermediate temperature range, whereas
above 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of
about 3.1 mB. The enhanced value of the Sommerfeld Wilson ratio and the dynamic
scaling of spin-lattice relaxation rate divided by T [27(1/T1T)] with static
susceptibility suggests admixed ferromagnetic correlations. 27(1/T1T)
simultaneously tracks the valence fluctuations from the 4f -Yb ions in the high
temperature range and field dependent antiferromagnetic correlations among
partially Kondo screened Fe 3d moments at low temperature, the latter evolve
out of an Yb 4f admixed conduction band.Comment: To appear in Phys. Rev. Let
c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions
Within a microscopic two band model of planes and chains with a pairing
potential in the planes and off diagonal pairing between planes and chains we
find that the chains make the largest contribution to the Josephson tunnelling
current and that through them the d-wave part of the gap contributes to the
current. This is contrary to the usual assumption that for a d-wave tetragonal
superconductor the c-axis Josephson current for incoherent tunnelling into an
s-wave superconductor is zero while that of a d-wave orthorhombic
superconductor with a small s-wave component to its gap it is small but
non-zero. Nevertheless it has been argued that the effect of twins in YBCO
would lead to cancellation between pairs of twins and so the observation of a
current in c-axis YBCO-Pb experiments is evidence against a d-wave type order
parameter. We argue that both theory and experiment give evidence that the two
twin orientations are not necessarily equally abundant and that the ratio of
tunnelling currents in twinned and untwinned materials should be related to the
relative abundance of the two twin orientations.Comment: 6 pages, RevTeX 3.0, 15 PostScript figur
Phase-sensitive Evidence for d-wave Pairing Symmetry in Electron-doped Cuprate Superconductors
We present phase-sensitive evidence that the electron-doped cuprates
Nd1.85Ce0.15CuO4-y (NCCO) and Pr1.85Ce0.15CuO4-y (PCCO) have d-wave pairing
symmetry. This evidence was obtained by observing the half-flux quantum effect,
using a scanning SQUID microscope, in c-axis oriented films of NCCO or PCCO
epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be
frustrated for a d(x2-y2) order parameter. Samples with two other
configurations, designed to b unfrustrated for a d-wave superconductor, do not
show the half-flux quantum effect.Comment: 4 pages, Latex, 4 figure
Quantitative study of valence and configuration interaction parameters of the Kondo semiconductors CeM2Al10 (M = Ru, Os and Fe) by means of bulk-sensitive hard x-ray photoelectron spectroscopy
The occupancy of the 4f^n contributions in the Kondo semiconductors
CeM2Al10(M = Ru, Os and Fe) has been quantitatively determined by means of
bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) on the Ce 3d core
levels. Combining a configuration interaction scheme with full multiplet
calculations allowed to accurately describe the HAXPES data despite the
presence of strong plasmon excitations in the spectra. The configuration
interaction parameters obtained from this analysis -- in particular the
hybridization strength V_eff and the effective f binding energy Delta_f --
indicate a slightly stronger exchange interaction in CeOs2Al10 compared to
CeRu2Al10, and a significant increase in CeFe2Al10. This verifies the
coexistence of a substantial amount of Kondo screening with magnetic order and
places the entire CeM2Al10 family in the region of strong exchange
interactions.Comment: 9 pages, 4 figures, submitted to Physical Review
Anti-ferromagnetic ordering in arrays of superconducting pi-rings
We report experiments in which one dimensional (1D) and two dimensional (2D)
arrays of YBa2Cu3O7-x-Nb pi-rings are cooled through the superconducting
transition temperature of the Nb in various magnetic fields. These pi-rings
have degenerate ground states with either clockwise or counter-clockwise
spontaneous circulating supercurrents. The final flux state of each ring in the
arrays was determined using scanning SQUID microscopy. In the 1D arrays,
fabricated as a single junction with facets alternating between alignment
parallel to a [100] axis of the YBCO and rotated 90 degrees to that axis,
half-fluxon Josephson vortices order strongly into an arrangement with
alternating signs of their magnetic flux. We demonstrate that this ordering is
driven by phase coupling and model the cooling process with a numerical
solution of the Sine-Gordon equation. The 2D ring arrays couple to each other
through the magnetic flux generated by the spontaneous supercurrents. Using
pi-rings for the 2D flux coupling experiments eliminates one source of disorder
seen in similar experiments using conventional superconducting rings, since
pi-rings have doubly degenerate ground states in the absence of an applied
field. Although anti-ferromagnetic ordering occurs, with larger negative bond
orders than previously reported for arrays of conventional rings, long-range
order is never observed, even in geometries without geometric frustration. This
may be due to dynamical effects. Monte-Carlo simulations of the 2D array
cooling process are presented and compared with experiment.Comment: 10 pages, 15 figure
INTERLAYER VORTICES AND EDGE DISLOCATIONS IN HIGH TEMPERATURE SUPERCONDUCTORS
The interaction of an edge dislocation made of half the superconducting plane
with a magnetic interlayer vortex is considered within the framework of the
Lawrence-Doniach model with negative as well as positive Josephson interlayer
coupling. In the first case the binding energy of the vortex and the
dislocation has been calculated by employing a variational procedure. The
current distribution around the bound vortex turns out to be asymmetric. In the
second case the dislocation carries a spontaneous magnetic half-vortex, whose
binding energy with the dislocation turns out to be infinite. The half-vortex
energy has been calculated by the same variational procedure. Implications of
the possible presence of such half-vortices for the properties of high
temperature superconductors are discussed.Comment: 14 Latex pages, 1 figure available upon request
Bulk and surface electronic properties of SmB6: a hard x-ray photoelectron spectroscopy study
We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy
(HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single
crystals. Using the multiplet-structure in the Sm 3d core level spectra, we
determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5
K. Temperature dependent measurements revealed that the Sm valence gradually
increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly
observe that not only the J=0 but also the J=1 state of the Sm 4f 6
configuration becomes occupied at elevated temperatures. Making use of the
polarization dependence, we were able to identify and extract the Sm 4f
spectral weight of the bulk material. Finally, we revealed that the oxidized or
chemically damaged surface region of the ex-situ polished SmB6 single crystal
is surprisingly thin, about 1 nm only.Comment: 11 pages, 8 figure
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