585 research outputs found
Magnetization reversal of an individual exchange biased permalloy nanotube
We investigate the magnetization reversal mechanism in an individual
permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a
nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is
affixed to the tip of a Si cantilever and positioned in order to optimally
couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the
NT's volume magnetization by dynamic cantilever magnetometry and its stray flux
using the nanoSQUID. We observe a training effect and temperature dependence in
the magnetic hysteresis, suggesting an exchange bias. We find a low blocking
temperature K, indicating the presence of a thin
antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy
on similar samples. Furthermore, we measure changes in the shape of the
magnetic hysteresis as a function of temperature and increased training. These
observations show that the presence of a thin exchange-coupled native oxide
modifies the magnetization reversal process at low temperatures. Complementary
information obtained via cantilever and nanoSQUID magnetometry allows us to
conclude that, in the absence of exchange coupling, this reversal process is
nucleated at the NT's ends and propagates along its length as predicted by
theory.Comment: 8 pages, 4 figure
Threshold Electrodisintegration of ^3He
Cross sections were measured for the near-threshold electrodisintegration of
^3He at momentum transfer values of q=2.4, 4.4, and 4.7 fm^{-1}. From these and
prior measurements the transverse and longitudinal response functions R_T and
R_L were deduced. Comparisons are made against previously published and new
non-relativistic A=3 calculations using the best available NN potentials. In
general, for q<2 fm^{-1} these calculations accurately predict the threshold
electrodisintegration of ^3He. Agreement at increasing q demands consideration
of two-body terms, but discrepancies still appear at the highest momentum
transfers probed, perhaps due to the neglect of relativistic dynamics, or to
the underestimation of high-momentum wave-function components.Comment: 9 pages, 7 figures, 1 table, REVTEX4, submitted to Physical Review
JAMI: a Java library for molecular interactions and data interoperability.
BACKGROUND: A number of different molecular interactions data download formats now exist, designed to allow access to these valuable data by diverse user groups. These formats include the PSI-XML and MITAB standard interchange formats developed by Molecular Interaction workgroup of the HUPO-PSI in addition to other, use-specific downloads produced by other resources. The onus is currently on the user to ensure that a piece of software is capable of read/writing all necessary versions of each format. This problem may increase, as data providers strive to meet ever more sophisticated user demands and data types. RESULTS: A collaboration between EMBL-EBI and the University of Cambridge has produced JAMI, a single library to unify standard molecular interaction data formats such as PSI-MI XML and PSI-MITAB. The JAMI free, open-source library enables the development of molecular interaction computational tools and pipelines without the need to produce different versions of software to read different versions of the data formats. CONCLUSION: Software and tools developed on top of the JAMI framework are able to integrate and support both PSI-MI XML and PSI-MITAB. The use of JAMI avoids the requirement to chain conversions between formats in order to reach a desired output format and prevents code and unit test duplication as the code becomes more modular. JAMI's model interfaces are abstracted from the underlying format, hiding the complexity and requirements of each data format from developers using JAMI as a library
Photo- and Electro-Disintegration of 3He at Threshold and pd Radiative Capture
The present work reports results for: pd radiative capture observables
measured at center-of-mass (c.m.) energies in the range 0--100 keV and at 2 MeV
by the TUNL and Wisconsin groups, respectively; contributions to the
Gerasimov-Drell-Hearn (GDH) integral in 3He from the two- up to the three-body
breakup thresholds, compared to experimental determinations by the TUNL group
in this threshold region; longitudinal, transverse, and interference response
functions measured in inclusive polarized electron scattering off polarized 3He
at excitation energies below the threshold for breakup into ppn, compared to
unpolarized longitudinal and transverse data from the Saskatoon group. The
calculations are based on a realistic Hamiltonian with two- and three-nucleon
interactions and a realistic current operator, including one- and two-body
components. The theoretical predictions obtained by including only one-body
currents are in violent disagreement with data. These differences between
theory and experiment are, to a large extent, removed when two-body currents
are taken into account, although some rather large discrepancies remain in the
c.m. energy range 0--100 keV, particularly for the pd differential cross
section and tensor analyzing power at small angles, and contributions to the
GDH integral. A rather detailed analysis indicates that these discrepancies
have, in large part, a common origin, and can be traced back to an excess
strength obtained in the theoretical calculation of the E1 reduced matrix
element associated with the pd channel having L,S,J=1,1/2,3/2. It is suggested
that this lack of E1 strength observed experimentally might have implications
for the nuclear interaction at very low energies. Finally, the validity of the
long-wavelength approximation for electric dipole transitions is discussed.Comment: 47 pages RevTex file, 10 PostScript figures, submitted to Phys. Rev.
Automated derivation of the adjoint of high-level transient finite element programs
In this paper we demonstrate a new technique for deriving discrete adjoint
and tangent linear models of finite element models. The technique is
significantly more efficient and automatic than standard algorithmic
differentiation techniques. The approach relies on a high-level symbolic
representation of the forward problem. In contrast to developing a model
directly in Fortran or C++, high-level systems allow the developer to express
the variational problems to be solved in near-mathematical notation. As such,
these systems have a key advantage: since the mathematical structure of the
problem is preserved, they are more amenable to automated analysis and
manipulation. The framework introduced here is implemented in a freely
available software package named dolfin-adjoint, based on the FEniCS Project.
Our approach to automated adjoint derivation relies on run-time annotation of
the temporal structure of the model, and employs the FEniCS finite element form
compiler to automatically generate the low-level code for the derived models.
The approach requires only trivial changes to a large class of forward models,
including complicated time-dependent nonlinear models. The adjoint model
automatically employs optimal checkpointing schemes to mitigate storage
requirements for nonlinear models, without any user management or intervention.
Furthermore, both the tangent linear and adjoint models naturally work in
parallel, without any need to differentiate through calls to MPI or to parse
OpenMP directives. The generality, applicability and efficiency of the approach
are demonstrated with examples from a wide range of scientific applications
Model Calculations for the Two-Fragment Electro-Disintegration of He
Differential cross sections for the electro-disintegration process are calculated, using a model in which
the final state interaction is included by means of a nucleon-nucleus (3+1)
potential constructed via Marchenko inversion. The required bound-state wave
functions are calculated within the integrodifferential equation approach
(IDEA). In our model the important condition that the initial bound state and
the final scattering state are orthogonal is fulfilled. The sensitivity of the
cross section to the input interaction in certain kinematical regions
is investigated. The approach adopted could be useful in reactions involving
few cluster systems where effective interactions are not well known and exact
methods are presently unavailable. Although, our Plane-Wave Impulse
Approximation results exhibit, similarly to other calculations, a dip in the
five-fold differential cross-section around a missing momentum of , it is argued that this is an artifact of the omission of re-scattering
four-nucleon processes.Comment: 16 pages, 6 figures, accepted for publication by Phys.Rev.
Variational methods
International audienceThis contribution presents derivative-based methods for local sensitivity analysis, called Variational Sensitivity Analysis (VSA). If one defines an output called the response function, its sensitivity to inputs variations around a nominal value can be studied using derivative (gradient) information. The main issue of VSA is then to provide an efficient way of computing gradients. This contribution first presents the theoretical grounds of VSA: framework and problem statement, tangent and adjoint methods. Then it covers pratical means to compute derivatives, from naive to more sophisticated approaches, discussing their various 2 merits. Finally, applications of VSA are reviewed and some examples are presented, covering various applications fields: oceanography, glaciology, meteorology
Review and Unification of Methods for Computing Derivatives of Multidisciplinary Systems
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97061/1/AIAA2012-1589.pd
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