Thermal spin-transfer in Fe-MgO-Fe tunnel junctions

We compute thermal spin transfer torques (TST) in Fe-MgO-Fe tunnel junctions using a first principles wave function-matching method. At room temperature, the TST in a junction with 3 MgO monolayers amounts to 10^-7J/m^2/K, which is estimated to cause magnetization reversal for temperature differences over the barrier of the order of 10 K. The large TST can be explained by multiple scattering between interface states through ultrathin barriers. The angular dependence of the TST can be very skewed, possibly leading to thermally induced high-frequency generation.Comment: 5 pages, 5 figure

Gas Dynamics of the Nickel-56 Decay Heating in Pair-Instability Supernovae

Very massive 140-260 Msun stars can die as highly-energetic pair-instability supernovae (PI SNe) with energies of up to 100 times those of core-collapse SNe that can completely destroy the star, leaving no compact remnant behind. These explosions can synthesize $0.1-30$ Msun of radioactive Ni56, which can cause them to rebrighten at later times when photons due to Ni56 decay diffuse out of the ejecta. However, heat from the decay of such large masses of Ni56 could also drive important dynamical effects deep in the ejecta that are capable of mixing elements and affecting the observational signatures of these events. We have now investigated the dynamical effect of Ni56 heating on PI SN ejecta with high-resolution two-dimensional hydrodynamic simulations performed with the CASTRO code. We find that expansion of the hot Ni56 bubble forms a shell at the base of the silicon layer of the ejecta about 200 days after the explosion but that no hydrodynamical instabilities develop that would mix Ni56 with the Si/O-rich ejecta. However, while the dynamical effects of Ni56 heating may be weak they could affect the observational signatures of some PI SNe by diverting decay energy into internal expansion of the ejecta at the expense of rebrightening at later times.Comment: Accepted to ApJ, 14 page

Shot noise in magnetic tunnel junctions from first principles

We compute the shot noise in ballistic and disordered Fe/MgO/Fe tunnel junctions by a wave function-matching method. For tunnel barriers with no more than 5 atomic layers we find a suppression of the Fano factor as a function of the magnetic configuration. For thicker MgO barriers the shot noise is suppressed up to a threshold bias indicating the onset of resonant tunneling. We find excellent agreement with recent experiments when interface disorder is taken into accountComment: 5 pages,5 figure

Radiation Transport Simulations of Pulsational Pair-Instability Supernovae

Massive stars of helium cores of 35-65 Msun eventually encounter the electron/positron creation instability, and it triggers explosive carbon or oxygen burning that produces several thermonuclear eruptions. The resulting catastrophe collisions of eruptive shells sometimes produce luminous transients with peak luminosity of $10^{43} - 10^{44}$ erg/sec, known as pulsational pair-instability supernovae (PPISNe). Previous 2D simulations of colliding shells show the development of Rayleigh-Taylor (RT) instabilities and mixing. Here we present radiation hydrodynamic PPISNe simulations of a 110 Msun solar-metallicity star that was promising to produce a superluminous transit in the early work. Our comprehensive study contains a suite of one-, two-, and three-dimensional models. We discuss the impact of dimensionality and fluid instabilities on the resulting light curves. The results show the RT mixing found in previous multidimensional hydro studies transforms into a thin and distorted shell due to radiative cooling. Radiation from the wiggly shell peaks at its bolometric light curve of $\sim 2\times10^{43}$ erg/sec, lasting about 150 days and following with a plateau of $\sim 3\times10^{42}$ erg/sec for another two hundred days before it fades away. The total radiation energy emitted from colliding shells is $\sim 1.8 \times 10^{50}$ erg, which is $\sim 27\%$ of the kinetic energy of the major eruption. The dimensional effects also manifest on the physical properties, such as irregularity and thickness of the shell. Our study suggests PPISNe is a promising candidate of luminous SNe, the radiation of which originates from colliding shells with a homogeneous mixing of ejecta.Comment: Submitted to ApJ, 16 pages, comments are welcom

Direct reconstruction of dynamical dark energy from observational Hubble parameter data

Reconstructing the evolution history of the dark energy equation of state parameter $w(z)$ directly from observational data is highly valuable in cosmology, since it contains substantial clues in understanding the nature of the accelerated expansion of the Universe. Many works have focused on reconstructing $w(z)$ using Type Ia supernova data, however, only a few studies pay attention to Hubble parameter data. In the present work, we explore the merit of Hubble parameter data and make an attempt to reconstruct $w(z)$ from them through the principle component analysis approach. We find that current Hubble parameter data perform well in reconstructing $w(z)$; though, when compared to supernova data, the data are scant and their quality is worse. Both $\Lambda$CDM and evolving $w(z)$ models can be constrained within $10\%$ at redshifts $z \lesssim 1.5$ and even $5\%$ at redshifts 0.1 $\lesssim$ z $\lesssim$ 1 by using simulated $H(z)$ data of observational quality.Comment: 25 pages, 11 figure

Body composition, IGF1 status, and physical functionality in nonagenarians: implications for osteosarcopenia

OBJECTIVES: Body composition alterations occur during aging. The purpose of the present analysis was to explore the functional consequences of the overlap of sarcopenia and osteoporosis, and the potential role of insulin-like growth factor 1 (IGF1) in their development in the oldest old. SETTING AND PARTICIPANTS: Eighty-seven nonagenarians from the Louisiana Healthy Aging Study were included. MEASURES: The definition of sarcopenia was based on appendicular lean mass (ALM). Osteoporosis was diagnosed based on bone mineral density (BMD) T score. Four phenotypes were compared: (1) healthy body composition, that is, nonosteoporotic nonsarcopenic (CO, control group), (2) osteoporotic (O, low BMD T score), (3) sarcopenic (S, low ALM), and (4) osteosarcopenic (OS, low BMD T score and low ALM). Sex- and age-specific IGF1-Standard Deviation Scores (SDS) were calculated. The Continuous Scale-Physical Functional Performance (CS-PFP) test was performed. RESULTS: In OS men, IGF1-SDS values (-0.61 ±0.37 vs -0.04 ± 0.52, P = .02) were lower than those in CO males (control group), whereas IGF1-SDS were similar in the 4 body composition phenotypes in women. In men only, ALM was positively associated with IGF1-SDS values (P = .01) independent of age and C-reactive protein concentration. Regarding bone health, we found no association between IGF1-SDS values and BMD. IGF1-SDS was not associated with functional performance (CS-PFP) in men and women. CONCLUSIONS/IMPLICATIONS: IGF1 sensitivity in skeletal muscle and bone may differ by sex in the oldest old. IGF1 status did not appear to affect physical functionality. Determinants and clinical and functional characteristics of osteosarcopenia need to be further investigated in order to define conclusive diagnostic criteria

Selective ethylene trimerization by titanium complexes bearing phenoxy-imine ligands: NMR and EPR Spectroscopic studies of the reaction intermediates

The catalyst systems (FI)TiCl₃/MAO (FI = phenoxyimine ligand with an additional aryl–O–CH₃ donor) display exceptionally high activity in selective ethylene trimerization. By means of NMR and EPR spectroscopy, the nature of the Ti species formed in the catalyst systems (FI)TiCl₃/MAO, (FI)TiCl₃/MMAO, and (FI)TiCl₃/AlR₃/[Ph₃C]⁺[B(C₆F₅)₄]⁻ (R = Me, Et, ⁱBu) has been studied. It was shown that outer-sphere ion pairs of the type [(FI)TiIVMe₂]⁺[A]⁻ ([A]− = [MeMAO]⁻, [MeMMAO]⁻, [B(C₆F₅)₄]⁻) are formed at the initial stage of the reaction of (FI)TiCl₃ with MAO, MMAO, and AlMe₃/[Ph₃C]⁺[B(C₆F₅)₄]⁻. These ion pairs further partially convert into TiIII and TiII species. In the systems (FI)TiCl₃/MAO and (FI)TiCl₃/AlMe₃/[Ph₃C]⁺[B(C₆F5)₄]⁻, complexes with the proposed structures (FI)TiIIIMe₂, (FI)TiIICl, and [(FI)TiII(S)]⁺[A]⁻ ([A]− = [MeMAO]⁻, [B(C₆F₅)4)]⁻, S = solvent, vacancy) were observed (concentrations of TiIII species was lower than those of the TiII congeners). In contrast, in the system (FI)TiCl₃/MMAO, the concentrations of TiIII species (ion pairs of the type [(FI)TiIII(μ-H)(μ-Cl)AlⁱBu₂]⁺[MeMMAO]⁻) were higher than those of the TiII counterparts (ion pairs [(FI)TiII(S)]⁺[MeMMAO]⁻). The system (FI)TiCl₃/MMAO displays lower activity and selectivity in 1-hexene formation, in comparison to (FI)TiCl₃/MAO, due to undesirable PE generation. Probably, TiII and TiIV ion pairs are those participating in ethylene trimerization