8,454 research outputs found
Spin transport and spin dephasing in zinc oxide
The wide bandgap semiconductor ZnO is interesting for spintronic applications
because of its small spin-orbit coupling implying a large spin coherence
length. Utilizing vertical spin valve devices with ferromagnetic electrodes
(TiN/Co/ZnO/Ni/Au), we study the spin-polarized transport across ZnO in
all-electrical experiments. The measured magnetoresistance agrees well with the
prediction of a two spin channel model with spin-dependent interface
resistance. Fitting the data yields spin diffusion lengths of 10.8nm (2K),
10.7nm (10K), and 6.2nm (200K) in ZnO, corresponding to spin lifetimes of 2.6ns
(2K), 2.0ns (10K), and 31ps (200K).Comment: 7 pages, 5 figures; supplemental material adde
Triggered Star Formation in the Environment of Young Massive Stars
Recent observations with the Spitzer Space Telescope show clear evidence that
star formation takes place in the surrounding of young massive O-type stars,
which are shaping their environment due to their powerful radiation and stellar
winds. In this work we investigate the effect of ionising radiation of massive
stars on the ambient interstellar medium (ISM): In particular we want to
examine whether the UV-radiation of O-type stars can lead to the observed
pillar-like structures and can trigger star formation. We developed a new
implementation, based on a parallel Smooth Particle Hydrodynamics code (called
IVINE), that allows an efficient treatment of the effect of ionising radiation
from massive stars on their turbulent gaseous environment. Here we present
first results at very high resolution. We show that ionising radiation can
trigger the collapse of an otherwise stable molecular cloud. The arising
structures resemble observed structures (e.g. the pillars of creation in the
Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of
gravitation we find small regions that can be identified as formation places of
individual stars. We conclude that ionising radiation from massive stars alone
can trigger substantial star formation in molecular clouds.Comment: 4 pages, 2 figures. To appear in: "Triggered Star Formation in a
Turbulent ISM", IAU Symposium 237, Prague, Czech Republic, August 2006; eds.
B.G.Elmegreen & J. Palou
Cold giant planets evaporated by hot white dwarfs
Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extreme ultraviolet (EUV) irradiation plays an important role in the evolution of exoplanets and in shaping their ensemble properties. Intermediate and low mass stars are brightest at EUV wavelengths at the very end of their lives, after they have expelled their envelopes and evolved into hot white dwarfs. Yet the effect of the intense EUV irradiation of giant planets orbiting young white dwarfs has not been assessed. We show that the giant planets in the solar system will experience significant hydrodynamic escape caused by the EUV irradiation from the white dwarf left behind by the Sun. A fraction of the evaporated volatiles will be accreted by the solar white dwarf, resulting in detectable photospheric absorption lines. As a large number of the currently known extrasolar giant planets will survive the metamorphosis of their host stars into white dwarfs, observational signatures of accretion from evaporating planetary atmospheres are expected to be common. In fact, one-third of the known hot single white dwarfs show photospheric absorption lines of volatile elements, which we argue are indicative of ongoing accretion from evaporating planets. The fraction of volatile contaminated hot white dwarfs strongly decreases as they cool. We show that accretion from evaporating planetary atmospheres naturally explains this temperature dependence if more than 50% of hot white dwarfs still host giant planets
SDSS J212531.92–010745.9 : the first definite PG 1159 close binary system
Aims. The archival spectrum of SDSS J212531.92−010745.9 shows not only the typical signature of a PG 1159 star, but also indicates the presence of a companion. Our aim was the proof of the binary nature of this object and the determination of its orbital period.
Methods. We performed time-series photometry of SDSS J212531.92−010745.9. We observed the object during 10 nights, spread over one month, with the Tübingen 80 cm and the Göttingen 50 cm telescopes. We fitted the observed light curve with a sine and simulated the light curve of this system with the nightfall program. Furthermore, we compared the spectrum of SDSS J212531.92−010745.9 with
NLTE models, the results of which also constrain the light curve solution.
Results. An orbital period of 6.95616(33) h with an amplitude of 0.354(3) mag is derived from our observations. A pulsation period could not be detected. For the PG 1159 star we found, as preliminary results from comparison with our NLTE models, T eff ∼ 90 000 K, log g ∼ 7.60, and the abundance ratio C/He ∼ 0.05 by number fraction. For the companion we obtained with a mean radius of 0.4 ± 0.1 R, a mass of 0.4 ± 0.1 M, and a temperature of 8200 K on the irradiated side, good agreement between the observed light curve and the nightfall simulation, but we do not regard those values as final
A universal platform for magnetostriction measurements in thin films
We present a universal nanomechanical sensing platform for the investigation
of magnetostriction in thin films. It is based on a doubly-clamped silicon
nitride nanobeam resonator covered with a thin magnetostrictive film. Changing
the magnetization direction within the film plane by an applied magnetic field
generates a magnetostrictive stress and thus changes the resonance frequency of
the nanobeam. A measurement of the resulting resonance frequency shift, e.g. by
optical interferometry, allows to quantitatively determine the magnetostriction
constants of the thin film. We use this method to determine the
magnetostriction constants of a 10nm thick polycrystalline cobalt film, showing
very good agreement with literature values. The presented technique can be
useful in particular for the precise measurement of magnetostriction in a
variety of (conducting and insulating) thin films, which can be deposited by
e.g. electron beam deposition, thermal evaporation or sputtering
Spin Hall magnetoresistance in antiferromagnet/heavy-metal heterostructures
We investigate the spin Hall magnetoresistance in thin film bilayer
heterostructures of the heavy metal Pt and the antiferromagnetic insulator NiO.
While rotating an external magnetic field in the easy plane of NiO, we record
the longitudinal and the transverse resistivity of the Pt layer and observe an
amplitude modulation consistent with the spin Hall magnetoresistance. In
comparison to Pt on collinear ferrimagnets, the modulation is phase shifted by
90{\deg} and its amplitude strongly increases with the magnitude of the
magnetic field. We explain the observed magnetic field-dependence of the spin
Hall magnetoresistance in a comprehensive model taking into account magnetic
field induced modifications of the domain structure in antiferromagnets. With
this generic model we are further able to estimate the strength of the
magnetoelastic coupling in antiferromagnets. Our detailed study shows that the
spin Hall magnetoresistance is a versatile tool to investigate the magnetic
spin structure as well as magnetoelastic effects, even in antiferromagnetic
multidomain materials
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