2,842 research outputs found
Distribution of parallel vortices studied by spin-polarized neutron reflectivity and magnetization
We present the studies of non-uniformly distributed vortices in Nb/Al
multilayers at applied field near parallel to film surface by using
spin-polarized neutron reflectivity (SPNR) and DC magnetization measurements.
We have observed peaks above the lower critical field, Hc1, in the M-H curves
from the multilayers.
Previous works with a model calculation of minimizing Gibbs free energy have
suggested that the peaks could be ascribed to vortex line transitions for
spatial commensuration in a thin film superconductor. In order to directly
determine the distribution of vortices, we performed SPNR measurements on the
multilayer and found that the distribution and density of vortices are
different at ascending and descending fields. At ascending 2000 Oe which is
just below the first peak in the M-H curve, SPNR shows that vortices are mostly
localized near a middle line of the film meanwhile the vortices are distributed
in broader region at the descending 2000 Oe. That is related to the observation
of more vortices trapped at the descending field. As the applied field is
sightly tilted (< 3.5degree), we observe another peak at a smaller field. The
peak position is consistent with the parallel lower critical field (Hc1||). We
discuss that the vortices run along the applied field below Hc1|| and rotate
parallel to the surface at Hc1||.Comment: 17 pages, 9 figure
The extraordinary Hall effect in coherent epitaxial tau (Mn,Ni)Al thin films on GaAs
Ultrathin coherent epitaxial films of ferromagnetic tau(Mn,Ni)0.60Al0.40 have been grown by molecular beam epitaxy on GaAs substrates. X-ray scattering and cross-sectional transmission electron microscopy measurements confirm that the c axis of the tetragonal tau unit cell is aligned normal to the (001) GaAs substrate. Measurements of the extraordinary Hall effect (EHE) show that the films are perpendicularly magnetized, exhibiting EHE resistivities saturating in the range of 3.3-7.1 muOMEGA-cm at room temperature. These values of EHE resistivity correspond to signals as large as +7 and -7 mV for the two magnetic states of the film with a measurement current of 1 mA. Switching between the two magnetic states is found to occur at distinct field values that depend on the previously applied maximum field. These observations suggest that the films are magnetically uniform. As such, tau(Mn,Ni)Al films may be an excellent medium for high-density storage of binary information
Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars
(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.Comment: 11 pages, 11 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pd
Random Matrix Theory and Fund of Funds Portfolio Optimisation
The proprietary nature of Hedge Fund investing means that it is common
practise for managers to release minimal information about their returns. The
construction of a Fund of Hedge Funds portfolio requires a correlation matrix
which often has to be estimated using a relatively small sample of monthly
returns data which induces noise. In this paper random matrix theory (RMT) is
applied to a cross-correlation matrix C, constructed using hedge fund returns
data. The analysis reveals a number of eigenvalues that deviate from the
spectrum suggested by RMT. The components of the deviating eigenvectors are
found to correspond to distinct groups of strategies that are applied by hedge
fund managers. The Inverse Participation ratio is used to quantify the number
of components that participate in each eigenvector. Finally, the correlation
matrix is cleaned by separating the noisy part from the non-noisy part of C.
This technique is found to greatly reduce the difference between the predicted
and realised risk of a portfolio, leading to an improved risk profile for a
fund of hedge funds.Comment: 17 Page
Temperature and orientation dependence of kinetic roughening during homoepitaxy: A quantitative x-ray-scattering study of Ag
URL:http://link.aps.org/doi/10.1103/PhysRevB.54.17938
DOI:10.1103/PhysRevB.54.17938Kinetic roughening during homoepitaxial growth was studied for Ag(111) and Ag(001). For Ag(111), from 150 to 500 K, the rms roughness exhibits a power law, σ∝tβ over nearly three decades in thickness. β≈1/2 at low temperatures, and there is an abrupt transition to smaller values above 300 K. In contrast, Ag(001) exhibits layer-by-layer growth with a significantly smaller β. These results are the first to establish the evolution of surface roughness quantitatively for a broad thickness and temperature range, as well as for the case where growth kinetics are dominated by a step-ledge diffusion barrier.Support is acknowledged from the University of Missouri Research Board, the NSF under Contract Nos. DMR-9202528 and DMR-9623827, and the Midwest Superconductivity Consortium ~MISCON! under DOE Grant No. DE-FG02-90ER45427. The SUNY X3 beamline is supported by the DOE under Contract No. DE-FG02-86ER45231, and the NSLS is supported by the DOE, Div. of Materials Sciences and Div. of Chemical Sciences. One of us
~W.C.E.! acknowledges support from the GAANN program of the U.S. Department of Education. We thank Ian Robinson for the Ag~111! crystal
Against the Odds: Psychomotor Development of Children Under 2 years in a Sudanese Orphanage.
Providing abandoned children the necessary medical and psychological care as possible after their institutionalization may minimize developmental delays. We describe psychomotor development in infants admitted to an orphanage in Khartoum, Sudan, assessed at admission and over an 18-month follow-up. Psychological state and psychomotor quotients were determined using a simplified Neonatal Behavior Assessment Scale (NBAS), the Brunet-Lezine and Alarm distress baby (ADBB) scale. From May-September 2005, 151 children were evaluated 2, 4, 9, 12 and 18 months after inclusion. At admission, ∼15% of children ≤1 month had a regulation impairment according to the NBAS, and 33.8% presented a distress state (ADBB score >5). More than 85% (129/151) recovered normal psychomotor development. The results of the program reinforce the importance of early detection of psychological disorders followed by rapid implementation of psychological case management to improve the development of young children in similar institutions and circumstances
Wavelet multiscale analysis for hedge funds: scaling and strategies
The wide acceptance of Hedge Funds by Institutional Investors and Pension Funds has led to an explosive growth in assets under management. These investors are drawn to Hedge Funds due to the seemingly low correlation with traditional investments and the attractive returns.
The correlations and market risk (the Beta in the Capital Asset Pricing Model) of Hedge Funds are generally calculated using monthly returns data, which may produce misleading results as Hedge Funds often hold illiquid exchange-traded securities or difficult to price over-the-
counter securities. In this paper, the Maximum Overlap Discrete Wavelet Transform (MODWT) is applied to measure the scaling properties of Hedge Fund correlation and market risk with respect to the S&P 500. It is found that the level of correlation and market risk varies greatly
according to the strategy studied and the time scale examined. Finally, the effects of scaling properties on the risk profile of a portfolio made up of Hedge Funds is studied using correlation matrices calculated over different time horizons
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