5,235 research outputs found
Chiral effective field theory for nuclear matter
We report on the recent developments of a new effective field theory for
nuclear matter [1,2,3]. We present first the nuclear matter chiral power
counting that takes into account both short-- and long--range inter-nucleon
interactions. It also identifies non-perturbative strings of diagrams, related
to the iteration of nucleon-nucleon interactions, which have to be re-summed.
The methods of unitary chiral perturbation theory has been shown to be a useful
tool in order to perform those resummations. Results up to next-to-leading
order for the ground state energy per particle of nuclear matter, the in-medium
chiral quark condensate and pion self-energy are discussed.Comment: Plenary talk at Chiral10 WORKSHOP, 21-24 Jun 2010, Valencia, Spai
Thermal Effects on the Magnetic Field Dependence of Spin Transfer Induced Magnetization Reversal
We have developed a self-aligned, high-yield process to fabricate CPP
(current perpendicular to the plane) magnetic sensors of sub 100 nm dimensions.
A pinned synthetic antiferromagnet (SAF) is used as the reference layer which
minimizes dipole coupling to the free layer and field induced rotation of the
reference layer. We find that the critical currents for spin transfer induced
magnetization reversal of the free layer vary dramatically with relatively
small changes the in-plane magnetic field, in contrast to theoretical
predictions based on stability analysis of the Gilbert equations of
magnetization dynamics including Slonczewski-type spin-torque terms. The
discrepancy is believed due to thermal fluctuations over the time scale of the
measurements. Once thermal fluctuations are taken into account, we find good
quantitative agreement between our experimental results and numerical
simulations.Comment: 14 pages, 4 figures, Submitted to Appl. Phys. Lett., Comparison of
some of these results with a model described by N. Smith in cond-mat/040648
Magnetic domain wall propagation in a submicron spin-valve stripe: influence of the pinned layer
The propagation of a domain wall in a submicron ferromagnetic spin-valve
stripe is investigated using giant magnetoresistance. A notch in the stripe
efficiently traps an injected wall stopping the domain propagation. The authors
show that the magnetic field at which the wall is depinned displays a
stochastic nature. Moreover, the depinning statistics are significantly
different for head to head and tail-to-tail domain walls. This is attributed to
the dipolar field generated in the vicinity of the notch by the pinned layer of
the spin-valve
First direct detection of an exoplanet by optical interferometry. Astrometry and K-band spectroscopy of HR 8799 e
Aims. To date, infrared interferometry at best achieved contrast ratios of a few times 10^(−4) on bright targets. GRAVITY, with its dual-field mode, is now capable of high contrast observations, enabling the direct observation of exoplanets. We demonstrate the technique on HR 8799, a young planetary system composed of four known giant exoplanets.
Methods. We used the GRAVITY fringe tracker to lock the fringes on the central star, and integrated off-axis on the HR 8799 e planet situated at 390 mas from the star. Data reduction included post-processing to remove the flux leaking from the central star and to extract the coherent flux of the planet. The inferred K band spectrum of the planet has a spectral resolution of 500. We also derive the astrometric position of the planet relative to the star with a precision on the order of 100 μas.
Results. The GRAVITY astrometric measurement disfavors perfectly coplanar stable orbital solutions. A small adjustment of a few degrees to the orbital inclination of HR 8799 e can resolve the tension, implying that the orbits are close to, but not strictly coplanar. The spectrum, with a signal-to-noise ratio of ≈5 per spectral channel, is compatible with a late-type L brown dwarf. Using Exo-REM synthetic spectra, we derive a temperature of 1150 ± 50 K and a surface gravity of 10^(4.3 ± 0.3) cm s^2. This corresponds to a radius of 1.17_(−0.11)^(+0.13) R_(Jup) and a mass of 10_(−4)^(+7) M_(Jup), which is an independent confirmation of mass estimates from evolutionary models. Our results demonstrate the power of interferometry for the direct detection and spectroscopic study of exoplanets at close angular separations from their stars
The chiral quark condensate and pion decay constant in nuclear matter at next-to-leading order
Making use of the recently developed chiral power counting for the physics of
nuclear matter [1,2], we evaluate the in-medium chiral quark condensate up to
next-to-leading order for both symmetric nuclear matter and neutron matter. Our
calculation includes the full in-medium iteration of the leading order local
and one-pion exchange nucleon-nucleon interactions. Interestingly, we find a
cancellation between the contributions stemming from the quark mass dependence
of the nucleon mass appearing in the in-medium nucleon-nucleon interactions.
Only the contributions originating from the explicit quark mass dependence of
the pion mass survive. This cancellation is the reason of previous observations
concerning the dominant role of the long-range pion contributions and the
suppression of short-range nucleon-nucleon interactions. We find that the
linear density contribution to the in-medium chiral quark condensate is only
slightly modified for pure neutron matter by the nucleon-nucleon interactions.
For symmetric nuclear matter the in-medium corrections are larger, although
smaller compared to other approaches due to the full iteration of the lowest
order nucleon-nucleon tree-level amplitudes. Our calculation satisfies the
Hellmann-Feynman theorem to the order worked out. Also we address the problem
of calculating the leading in-medium corrections to the pion decay constant. We
find that there are no extra in-medium corrections that violate the
Gell-Mann-Oakes-Renner relation up to next-to-leading order.Comment: 21 pages, 9 figure
Wide range and tunable linear TMR sensor using two exchange pinned electrodes
A magnetic tunnel junction sensor is proposed, with both the detection and
the reference layers pinned by IrMn. Using the differences in the blocking
temperatures of the IrMn films with different thicknesses, crossed anisotropies
can be induced between the detection and the reference electrodes. The pinning
of the sensing electrode ensures a linear and reversible output. It also allows
tuning both the sensitivity and the linear range of the sensor. The authors
show that the sensitivity varies linearly with the ferromagnetic thickness of
the detection electrode. It is demonstrated that an increased thickness leads
to a rise of sensitivity and a reduction of the operating range
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