33,655 research outputs found
The Fractal Dimension of SAT Formulas
Modern SAT solvers have experienced a remarkable progress on solving
industrial instances. Most of the techniques have been developed after an
intensive experimental testing process. Recently, there have been some attempts
to analyze the structure of these formulas in terms of complex networks, with
the long-term aim of explaining the success of these SAT solving techniques,
and possibly improving them.
We study the fractal dimension of SAT formulas, and show that most industrial
families of formulas are self-similar, with a small fractal dimension. We also
show that this dimension is not affected by the addition of learnt clauses. We
explore how the dimension of a formula, together with other graph properties
can be used to characterize SAT instances. Finally, we give empirical evidence
that these graph properties can be used in state-of-the-art portfolios.Comment: 20 pages, 11 Postscript figure
Different steady states for spin currents in noncollinear multilayers
We find there are at least two different steady states for transport across
noncollinear magnetic multilayers. In the conventional one there is a
discontinuity in the spin current across the interfaces which has been
identified as the source of current induced magnetic reversal; in the one
advocated herein the spin torque arises from the spin accumulation transverse
to the magnetization of a magnetic layer. These two states have quite different
attributes which should be discerned by current experiments.Comment: 8 pages, no figure. Accepted for publication in Journal of Physics:
Condensed Matte
Correlation between magnetic and transport properties of phase separated LaCaMnO
The effect of low magnetic fields on the magnetic and electrical transport
properties of polycrystalline samples of the phase separated compound
LaCaMnO is studied. The results are interpreted in the
framework of the field induced ferromagnetic fraction enlargement mechanism. A
fraction expansion coefficient af, which relates the ferromagnetic fraction f
with the applied field H, was obtained. A phenomenological model to understand
the enlargement mechanism is worked out.Comment: 3 pages, 3 figures, presented at the Fifth LAW-MMM, to appear in
Physica B, Minor change
Magnetoresistive memory in phase separated LaCaMnO
We have studied a non volatile memory effect in the mixed valent compound
LaCaMnO induced by magnetic field (H). In a previous work
[R.S. Freitas et al., Phys. Rev. B 65 (2002) 104403], it has been shown that
the response of this system upon application of H strongly depends on the
temperature range, related to three well differentiated regimes of phase
separation occurring below 220 K. In this work we compare memory capabilities
of the compound, determined following two different experimental procedures for
applying H, namely zero field cooling and field cooling the sample. These
results are analyzed and discussed within the scenario of phase separation.Comment: 4 pages, 2 figure
Simulation of turbulent transonic separated flow over an airfoil
A code developed for simulating high Reynolds number transonic flow fields of arbitrary configuration is described. This code, in conjunction with laboratory experiments, is used to devise and test turbulence transport models which may be suitable in the prediction of such flow fields, with particular emphasis on regions of flow separation. The solutions describe the flow field, including both the shock-induced and trailing-edge separation regions, in sufficient detail to provide the profile and friction drag
Lateral diffusive spin transport in layered structures
A one dimensional theory of lateral spin-polarized transport is derived from
the two dimensional flow in the vertical cross section of a stack of
ferromagnetic and paramagnetic layers. This takes into account the influence of
the lead on the lateral current underneath, in contrast to the conventional 1D
modeling by the collinear configuration of lead/channel/lead. Our theory is
convenient and appropriate for the current in plane configuration of an
all-metallic spintronics structure as well as for the planar structure of a
semiconductor with ferromagnetic contacts. For both systems we predict the
optimal contact width for maximal magnetoresistance and propose an electrical
measurement of the spin diffusion length for a wide range of materials.Comment: 4 pages, 3 figure
Quasi-Particle Degrees of Freedom versus the Perfect Fluid as Descriptors of the Quark-Gluon Plasma
The hot nuclear matter created at the Relativistic Heavy Ion Collider (RHIC)
has been characterized by near-perfect fluid behavior. We demonstrate that this
stands in contradiction to the identification of QCD quasi-particles with the
thermodynamic degrees of freedom in the early (fluid) stage of heavy ion
collisions. The empirical observation of constituent quark ``'' scaling of
elliptic flow is juxtaposed with the lack of such scaling behavior in
hydrodynamic fluid calculations followed by Cooper-Frye freeze-out to hadrons.
A ``quasi-particle transport'' time stage after viscous effects break down the
hydrodynamic fluid stage, but prior to hadronization, is proposed to reconcile
these apparent contradictions. However, without a detailed understanding of the
transitions between these stages, the ``'' scaling is not a necessary
consequence of this prescription. Also, if the duration of this stage is too
short, it may not support well defined quasi-particles. By comparing and
contrasting the coalescence of quarks into hadrons with the similar process of
producing light nuclei from nucleons, it is shown that the observation of
``'' scaling in the final state does not necessarily imply that the
constituent degrees of freedom were the relevant ones in the initial state.Comment: 9 pages, 7 figures, Updated text and figure
QUASAT: An orbiting very long baseline interferometer program using large space antenna systems
QUASAT, which stands for QUASAR SATELLITE, is the name given to a new mission being studied by NASA. The QUASAT mission concept involves a free flying Earth orbiting large radio telescope, which will observe astronomical radio sources simultaneously with ground radio telescopes. The primary goal of QUASAT is to provide a system capable of collecting radio frequency data which will lead to a better understanding of extremely high energy events taking place in a variety of celestial objects including quasars, galactic nuclei, interstellar masers, radio stars and pulsars. QUASAT's unique scientific contribution will be the increased resolution in the emission brightness profile maps of the celestial objects
Low temperature irreversibility induced by thermal cycles on two prototypical phase separated manganites
We have studied the effect of irreversibility induced by repeated thermal
cycles on the electric transport and magnetization of polycrystalline samples
of La0.5Ca0.5MnO3 and La0.325Pr0.3Ca0.375MnO3. An increase of the resistivity
and a decrease of the magnetization at different temperature ranges after
cycling is obtained in the temperature range between 300 K and 30 K. Both
compounds are known to exhibit intrinsic submicrometric coexistence of phases
and undergo a sequence of phase transitions related to structural changes.
Changes induced by thermal cycling can be partially inhibited by applying
magnetic field and hydrostatic pressure.
Our results suggest that the growth and coexistence of phases with different
structures gives rise to microstructural tracks and strain accommodation,
producing the observed irreversibility. Irrespective of the actual ground state
of each compound, the effect of thermal cycling is towards an increase of the
amount of the insulating phase in both compounds.Comment: to appear in Journal of Alloys and Compounds (2003
Thermodynamic Properties of the Spin-1/2 Antiferromagnetic ladder Cu2(C2H12N2)2Cl4 under Magnetic Field
Specific heat () measurements in the spin-1/2
Cu(CHN)Cl system under a magnetic field up to
are reported and compared to the results of numerical calculations
based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature
dependences of both the susceptibility and the low field specific heat are
accurately reproduced by this model, deviations are observed below the critical
field at which the spin gap closes. In this Quantum High Field phase,
the contribution of the low-energy quantum fluctuations are stronger than in
the Heisenberg ladder model. We argue that this enhancement can be attributed
to dynamical lattice fluctuations. Finally, we show that such a Heisenberg
ladder, for , is unstable, when coupled to the 3D lattice, against a
lattice distortion. These results provide an alternative explanation for the
observed low temperature ( -- ) phase (previously
interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped
state.Comment: Minor changes, list of authors complete
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