1,401 research outputs found
Do nebular fractionations, evaporative losses, or both, influence chondrule compositions?
We have made observations and performed heating experiments to determine the relative importance of several processes which may have influenced the compositions of chondrules. As heating destroys nuclei, the number density of olivine and pyroxene crystals gives an indication of the extent of melting. We determined number densities of Semarkona type I chondrules and converted them to nominal grain size, for use as a measure of intensity of heating. Bulk compositions of the chondrules show correlations with nominal grain size. Na, K, Fe, Ni, P and S decrease as grain size (degree of melting) increases, and we interpret this as evidence of evaporative loss. The evidence is less clear for Mn, Cr and Si. SiO_2/MgO ratios show very large variations even in fine-grained type I chondrules containing FeS, and we interpret those variations as due to nebular fractionations affecting precursors. Experiments show that Na and S losses increase with higher temperatures and lower cooling rates. It is hard to preserve any sulfide at all, without flash heating. Na, however, can be retained at close to chondritic levels (as in type II chondrules) with flash heating and high cooling rate, provided also that the oxygen fugacity is high. Type II chondrules can retain much more Na than type I under identical thermal conditions, because of higher fO_2 (either due to non-nebular gas or possibly internal buffering by FeO content) and melt structure (higher SiO_2/MgO). Gas reduction experiments show that type II compositions can be converted to IB by Fe loss, but evaporative loss of SiO_2 (so as to approach IA composition) is not achieved without prolonged isothermal heating. Precursors of type I and II chondrules were probably close to chondritic in composition, but with higher Fa in the type II case. They consisted of olivine, pyroxene, plagioclase, Fe (Ni) S and carbon compounds, probably with insignificant metal. Sulfur loss generated much chondrule metal in ordinary chondrites. C is a possible alternative to gas reduction to explain dusty relict grains and the lower olivine Fa in the more melted type I chondrules. We agree with J. N. GROSSMAN and J. T. WASSON (Geochim. Cosmochim. Acta, 47,759,1983) that variations in Mg/Si are due to nebular fractionations and with S. HUANG et al. (Icarus, 122,316,1996) that variations in Na and Fe in type I chondrules are mainly due to evaporative losses
Adiabatic Transfer of Electrons in Coupled Quantum Dots
We investigate the influence of dissipation on one- and two-qubit rotations
in coupled semiconductor quantum dots, using a (pseudo) spin-boson model with
adiabatically varying parameters. For weak dissipation, we solve a master
equation, compare with direct perturbation theory, and derive an expression for
the `fidelity loss' during a simple operation that adiabatically moves an
electron between two coupled dots. We discuss the possibility of visualizing
coherent quantum oscillations in electron `pump' currents, combining quantum
adiabaticity and Coulomb blockade. In two-qubit spin-swap operations where the
role of intermediate charge states has been discussed recently, we apply our
formalism to calculate the fidelity loss due to charge tunneling between two
dots.Comment: 13 pages, 8 figures, to appear in Phys. Rev.
Anomalous exponents in the rapid-change model of the passive scalar advection in the order
Field theoretic renormalization group is applied to the Kraichnan model of a
passive scalar advected by the Gaussian velocity field with the covariance
. Inertial-range
anomalous exponents, related to the scaling dimensions of tensor composite
operators built of the scalar gradients, are calculated to the order
of the expansion. The nature and the convergence of
the expansion in the models of turbulence is are briefly discussed.Comment: 4 pages; REVTeX source with 3 postscript figure
Calculation of the anomalous exponents in the rapid-change model of passive scalar advection to order
The field theoretic renormalization group and operator product expansion are
applied to the model of a passive scalar advected by the Gaussian velocity
field with zero mean and correlation function \propto\delta(t-t')/k^{d+\eps}.
Inertial-range anomalous exponents, identified with the critical dimensions of
various scalar and tensor composite operators constructed of the scalar
gradients, are calculated within the expansion to order
(three-loop approximation), including the exponents in
anisotropic sectors. The main goal of the paper is to give the complete
derivation of this third-order result, and to present and explain in detail the
corresponding calculational techniques. The character and convergence
properties of the expansion are discussed; the improved
``inverse'' expansion is proposed and the comparison with the
existing nonperturbative results is given.Comment: 34 pages, 5 figures, REVTe
Fano resonances and Aharonov-Bohm effects in transport through a square quantum dot molecule
We study the Aharonov-Bohm effect in a coupled 22 quantum dot array
with two-terminals. A striking conductance dip arising from the Fano
interference is found as the energy levels of the intermediate dots are
mismatched, which is lifted in the presence of a magnetic flux. A novel five
peak structure is observed in the conductance for large mismatch. The
Aharonov-Bohm evolution of the linear conductance strongly depends on the
configuration of dot levels and interdot and dot-lead coupling strengths. In
addition, the magnetic flux and asymmetry between dot-lead couplings can induce
the splitting and combination of the conductance peak(s).Comment: 15 pages, 7 figures, Revtex, to be published in Phys. Rev.
Magnetic Properties And Giant Magnetoresistance Of Magnetic Granular Co10cu90 Alloys Obtained By Direct-current Joule Heating
The direct-current (dc) joule heating technique was exploited to fabricate giant magnetoresistance (GMR) Co10Cu90 granular alloys. The Co cluster precipitation process was investigated by calorimetric and x-ray diffraction measurements. At T=10 K, the largest MR change of 25.0% has been observed for the melt-spun Co10Cu90 ribbon annealed at I=5 A. The magnetoresistance scales approximately as the inverse Co particle size. At room temperature, it was found that the dc joule-heated samples show relatively high GMR in comparison with furnace-annealed samples. Based on the phenomenological GMR model, we assumed that it is a consequence of smaller Co particles formed in dc joule-heated samples. © 1995 American Institute of Physics.7885062506
Non-equilibrium entangled steady state of two independent two-level systems
We determine and study the steady state of two independent two-level systems
weakly coupled to a stationary non-equilibrium environment. Whereas this
bipartite state is necessarily uncorrelated if the splitting energies of the
two-level systems are different from each other, it can be entangled if they
are equal. For identical two-level systems interacting with two bosonic heat
baths at different temperatures, we discuss the influence of the baths
temperatures and coupling parameters on their entanglement. Geometric
properties, such as the baths dimensionalities and the distance between the
two-level systems, are relevant. A regime is found where the steady state is a
statistical mixture of the product ground state and of the entangled singlet
state with respective weights 2/3 and 1/3
Magnetic properties and giant magnetoresistance of magnetic granular Co10 Cu90 alloys obtained by direct‐current joule heating
The direct-current (de) joule heating technique was exploited to fabricate giant magnetoresistance (GEAR) Co10Cu90 granular alloys. The Co cluster precipitation process was investigated by calorimetric and x-ray diffraction measurements. At T=10 K, the largest MR change of 25.0% has been observed for the melt-spun Co10Cu90 ribbon annealed at I=5 A. The magnetoresistance scales approximately as the inverse Co particle size. At room temperature, it was found that the de joule-heated samples show relatively high GMR in comparison with furnace-annealed samples. Based on the phenomenological GMR model, we assumed that it is a consequence of smaller Co particles formed in de joule-heated samples. (C) 1995 American Institute of Physics
Urinary Neutrophil Gelatinase-Associated Lipocalin Measured on Admission to the Intensive Care Unit Accurately Discriminates between Sustained and Transient Acute Kidney Injury in Adult Critically Ill Patients
Background: First we aimed to evaluate the ability of neutrophil gelatinase-associated lipocalin (NGAL) and cystatin-C (CyC) in plasma and urine to discriminate between sustained, transient and absent acute kidney injury (AKI), and second to evaluate their predictive performance for sustained AKI in adult intensive care unit (ICU) patients. Methods: A prospective cohort study of 700 patients was studied. Sample collection was performed over 8 time points starting on admission. Results: After exclusion 510 patients remained for the analysis. All biomarkers showed significant differentiation between sustained and no AKI at all time points (p ≤ 0.0002) except for urine CyC (uCyC) on admission (p = 0.06). Urine NGAL (uNGAL) was the only biomarker significantly differentiating sustained from transient AKI on ICU admission (p = 0.02). Individually, uNGAL performed better than the other biomarkers (area under the curves, AUC = 0.80, 95% confidence interval, CI = 0.72–0.88) for the prediction of sustained AKI. The combination with plasma NGAL (pNGAL) showed a nonsignificant improvement (AUC = 0.83, 95% CI = 0.75–0.91). The combination of individual markers with a model of clinical characteristics (MDRD eGFR, HCO3– and sepsis) did not improve its performance significantly. However, the integrated discrimination improvement showed significant improvement when uNGAL was added (p = 0.04). Conclusions: uNGAL measured on ICU admission differentiates patients with sustained AKI from transient or no-AKI patients. Combining biomarkers such as pNGAL, uNGAL and plasma CyC with clinical characteristics adds some value to the predictive model
Evidence for LineLike Vortex Liquid Phase in TlBaCaCuO Probed by the Josephson Plasma Resonance
We measured the Josephson plasma resonance (JPR) in optimally doped
TlBaCaCuO thin films using terahertz time-domain
spectroscopy in transmission. The temperature and magnetic field dependence of
the JPR frequency shows that the c-axis correlations of pancake vortices remain
intact at the transition from the vortex solid to the liquid phase. In this
respect TlBaCaCuO films, withanisotropy parameter
, are similar to the less anisotropic
YBaCuO rather than to the most
anisotropic BiSrCaCuO single crystals ).Comment: Submitted to Physical Review Letter
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