317 research outputs found
Modular Compilation of a Synchronous Language
Synchronous languages rely on formal methods to ease the development of applications in an efficient and reusable way. Formal methods have been advocated as a means of increasing the reliability of systems, especially those which are safety or business critical. It is still difficult to develop automatic specification and verification tools due to limitations like state explosion, undecidability, etc... In this work, we design a new specification model based on a reactive synchronous approach. Then, we benefit from a formal framework well suited to perform compilation and formal validation of systems. In practice, we design and implement a special purpose language (LE) and its two semantics~: the ehavioral semantics helps us to define a program by the set of its behaviors and avoid ambiguousness in programs' interpretation; the execution equational semantics allows the modular compilation of programs into software and hardware targets (c code, vhdl code, fpga synthesis, observers). Our approach is pertinent considering the two main requirements of critical realistic applications~: the modular compilation allows us to deal with large systems, the model-based approach provides us with formal validation
Orbital and Spin Excitations in Cobalt Oxide
By means of neutron scattering we have determined new branches of magnetic
excitations in orbitally active CoO (TN=290 K) up to 15 THz and for
temperatures from 6 K to 450 K. Data were taken in the (111) direction in six
single-crystal zones. From the dependence on temperature and Q we have
identified several branches of magnetic excitation. We describe a model for the
coupled orbital and spin states of Co2+ subject to a crystal field and
tetragonal distortion.Comment: To be published in Physica B (Proceedings of SCES07 conference in
Houston
Magnetic frustration in an iron based Cairo pentagonal lattice
The Fe3+ lattice in the Bi2Fe4O9 compound is found to materialize the first
analogue of a magnetic pentagonal lattice. Due to its odd number of bonds per
elemental brick, this lattice, subject to first neighbor antiferromagnetic
interactions, is prone to geometric frustration. The Bi2Fe4O9 magnetic
properties have been investigated by macroscopic magnetic measurements and
neutron diffraction. The observed non-collinear magnetic arrangement is related
to the one stabilized on a perfect tiling as obtained from a mean field
analysis with direct space magnetic configurations calculations. The
peculiarity of this structure arises from the complex connectivity of the
pentagonal lattice, a novel feature compared to the well-known case of
triangle-based lattices
Switching of the magnetic order in CeRhInSn in the vicinity of its quantum critical point
We report neutron diffraction experiments performed in the tetragonal
antiferromagnetic heavy fermion system CeRhInSn in its (, )
phase diagram up to the vicinity of the critical concentration
0.40, where long range magnetic order is suppressed. The propagation vector of
the magnetic structure is found to be =(1/2, 1/2, ) with
increasing from =0.298 to =0.410 when increases from =0
to =0.26. Surprisingly, for =0.30, the order has changed drastically and
a commensurate antiferromagnetism with =(1/2, 1/2, 0) is found.
This concentration is located in the proximity of the quantum critical point
where superconductivity is expected.Comment: 5 pages, 5 figures, submitted to Phys. Rev.
Kagom\'{e} ice state in the dipolar spin ice Dy_{2}Ti_{2}O_{7}
We have investigated the kagom\'{e} ice behavior of the dipolar spin-ice
compound Dy_{2}Ti_{2}O_{7} in magnetic field along a [111] direction using
neutron scattering and Monte Carlo simulations. The spin correlations show that
the kagom\'{e} ice behavior predicted for the nearest-neighbor (NN) interacting
model, where the field induces dimensional reduction and spins are frustrated
in each two-dimensional kagom\'{e} lattice, occurs in the dipole interacting
system. The spins freeze at low temperatures within the macroscopically
degenerate ground states of the NN model.Comment: 5 pages, 3 figures, submitted to PR
Phase Diagram of the Dzyaloshinskii-Moriya Helimagnet Ba2CuGe2O7 in Canted Magnetic Fields
The evolution of different magnetic structures of non-centrosymmetric
Ba2CuGe2O7 is systematically studied as function of the orientation of the
magnetic field H. Neutron diffraction in combination with measurements of
magnetization and specific heat show a virtually identical behaviour of the
phase diagram of Ba2CuGe2O7 for H confined in both the (1,0,0) and (1,1,0)
plane. The existence of a recently proposed incommensurate double-k AF-cone
phase is confirmed in a narrow range for H close to the tetragonal c-axis. For
large angles enclosed by H and the c-axis a complexely distorted non-sinusoidal
magnetic structure has recently been observed. We show that its critical field
Hc systematically increases for larger canting. Measurements of magnetic
susceptibility and specific heat finally indicate the existence of an
incommensurate/commensurate transition for H /sim 9 T applied in the basal
(a,b)-plane and agree with a non-planar, distorted cycloidal magnetic
structure.Comment: 14 pages, 13 figure
Half-ordered state in the anisotropic Haldane-gap antiferromagnet NDMAP
Neutron diffraction experiments performed on the Haldane gap material NDMAP
in high magnetic fields applied at an angle to the principal anisotropy axes
reveal two consecutive field-induced phase transitions. The low-field phase is
the gapped Haldane state, while at high fields the system exhibits
3-dimensional long-range Neel order. In a peculiar phase found at intermediate
fields only half of all the spin chains participate in the long-range ordering,
while the other half remains disordered and gapped.Comment: 4 pages, 2 figures, submitted to Phys. Rev.
Cooperative ordering of gapped and gapless spin networks in CuFeGeO
The unusual magnetic properties of a novel low-dimensional quantum
ferrimagnet CuFeGeO are studied using bulk methods, neutron
diffraction and inelastic neutron scattering. It is shown that this material
can be described in terms of two low-dimensional quantum spin subsystems, one
gapped and the other gapless, characterized by two distinct energy scales.
Long-range magnetic ordering observed at low temperatures is a cooperative
phenomenon caused by weak coupling of these two spin networks.Comment: 4 pages, 4 figure
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