5,289 research outputs found
Field-Induced Quantum Critical Point in CeCoIn5
The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a
function of temperature, down to 25 mK and in magnetic fields of up to 16 T
applied perpendicular to the basal plane. With increasing field, we observe a
suppression of the non-Fermi liquid behavior, rho ~ T, and the development of a
Fermi liquid state, with its characteristic rho = rho_0 + AT^2 dependence. The
field dependence of the T^2 coefficient shows critical behavior with an
exponent of 1.37. This is evidence for a field-induced quantum critical point
(QCP), occuring at a critical field which coincides, within experimental
accuracy, with the superconducting critical field H_c2. We discuss the relation
of this field-tuned QCP to a change in the magnetic state, seen as a change in
magnetoresistance from positive to negative, at a crossover line that has a
common border with the superconducting region below ~ 1 K.Comment: 4 pages, 3 figures (published version
Heat Conduction in the Vortex State of NbSe_2: Evidence for Multi-Band Superconductivity
The thermal conductivity kappa of the layered s-wave superconductor NbSe_2
was measured down to T_c/100 throughout the vortex state. With increasing
field, we identify two regimes: one with localized states at fields very near
H_c1 and one with highly delocalized quasiparticle excitations at higher
fields. The two associated length scales are naturally explained as multi-band
superconductivity, with distinct small and large superconducting gaps on
different sheets of the Fermi surface. This behavior is compared to that of the
multi-band superconductor MgB_2 and the conventional superconductor V_3Si.Comment: 5 pages, 4 figure
La modélisation d'accompagnement : une méthode de recherche participative et adaptative
Ce chapitre vise à présenter la diversité dans la mise en ½uvre d'un processus de modélisation d'accompagnement, mais aussi les points communs qui en émergent. L'objectif est de décrire pour mieux comprendre, sans intention normative. Nous nous appuyons sur l'analyse des cas d'études et des documents listés dans l'introduction. Notre analyse rassemble des cas concrets et des pratiques qui se réclament de la modélisation d'accompagnement et qui seront donc considérés comme tels dans notre analyse. La compatibilité de la diversité observée avec le cadre d'une adhésion aux principes initiaux de la charte sort du cadre de ce chapitre, elle est traitée dans la conclusion générale de l'ouvrage
Hamiltonian of a spinning test-particle in curved spacetime
Using a Legendre transformation, we compute the unconstrained Hamiltonian of
a spinning test-particle in a curved spacetime at linear order in the particle
spin. The equations of motion of this unconstrained Hamiltonian coincide with
the Mathisson-Papapetrou-Pirani equations. We then use the formalism of Dirac
brackets to derive the constrained Hamiltonian and the corresponding
phase-space algebra in the Newton-Wigner spin supplementary condition (SSC),
suitably generalized to curved spacetime, and find that the phase-space algebra
(q,p,S) is canonical at linear order in the particle spin. We provide explicit
expressions for this Hamiltonian in a spherically symmetric spacetime, both in
isotropic and spherical coordinates, and in the Kerr spacetime in
Boyer-Lindquist coordinates. Furthermore, we find that our Hamiltonian, when
expanded in Post-Newtonian (PN) orders, agrees with the Arnowitt-Deser-Misner
(ADM) canonical Hamiltonian computed in PN theory in the test-particle limit.
Notably, we recover the known spin-orbit couplings through 2.5PN order and the
spin-spin couplings of type S_Kerr S (and S_Kerr^2) through 3PN order, S_Kerr
being the spin of the Kerr spacetime. Our method allows one to compute the PN
Hamiltonian at any order, in the test-particle limit and at linear order in the
particle spin. As an application we compute it at 3.5PN order.Comment: Corrected typo in the ADM Hamiltonian at 3.5 PN order (eq. 6.20
Light Element Abundance Patterns in the Orion Association: I) HST Observations of Boron in G-dwarfs
The boron abundances for two young solar-type members of the Orion
association, BD -6 1250 and HD 294297, are derived from HST STIS spectra of the
B I transition at 2496.771 A. The best-fit boron abundances for the target
stars are 0.13 and 0.44 dex lower than the solar meteoritic value of log
e(B)=2.78. An anticorrelation of boron and oxygen is found for Orion when these
results are added to previous abundances obtained for 4 B-type stars and the
G-type star BD -5 1317. An analysis of the uncertainties in the abundance
calculations indicates that the observed anticorrelation is probably real. The
B versus O relation observed in the Orion association does not follow the
positive correlation of boron versus oxygen which is observed for the field
stars with roughly solar metallicity. The observed anticorrelation can be
accounted for by a simple model in which two poorly mixed components of gas
(supernova ejecta and boron-enriched ambient medium) contribute to the new
stars that form within the lifetime of the association. This model predicts an
anticorrelation for Be as well, at least as strong as for boron.Comment: 16 pages + 1 table + 7 figures, accepted for publication in Ap
Ciprofloxacin/amikacin combination therapy: evaluation in biofilms of Pseudomonas aeruginosa overproducting efflux pumps
International audienc
Combining dynamic relaxation method with artificial neural networks to enhance simulation of tensegrity structures
Abstract: Structural analyses of tensegrity structures must account for geometrical nonlinearity. The dynamic relaxation method correctly models static behavior in most situations. However, the requirements for precision increase when these structures are actively controlled. This paper describes the use of neural networks to improve the accuracy of the dynamic relaxation method in order to correspond more closely to data measured from a full-scale laboratory structure. An additional investigation evaluates training the network during the service life for further increases in accuracy. Tests showed that artificial neural networks increased model accuracy when used with the dynamic relaxation method. Replacing the dynamic relaxation method completely by a neural network did not provide satisfactory results. First tests involving training the neural network online showed potential to adapt the model to changes during the service life of the structure. DOI: 10.1061/�ASCE�0733-9445�2003�129:5�672
The Spatial Limitations of Current Neutral Models of Biodiversity
The unified neutral theory of biodiversity and biogeography is increasingly accepted as an informative null model of community composition and dynamics. It has successfully produced macro-ecological patterns such as species-area relationships and species abundance distributions. However, the models employed make many unrealistic auxiliary assumptions. For example, the popular spatially implicit version assumes a local plot exchanging migrants with a large panmictic regional source pool. This simple structure allows rigorous testing of its fit to data. In contrast, spatially explicit models assume that offspring disperse only limited distances from their parents, but one cannot as yet test the significance of their fit to data. Here we compare the spatially explicit and the spatially implicit model, fitting the most-used implicit model (with two levels, local and regional) to data simulated by the most-used spatially explicit model (where offspring are distributed about their parent on a grid according to either a radially symmetric Gaussian or a ‘fat-tailed’ distribution). Based on these fits, we express spatially implicit parameters in terms of spatially explicit parameters. This suggests how we may obtain estimates of spatially explicit parameters from spatially implicit ones. The relationship between these parameters, however, makes no intuitive sense. Furthermore, the spatially implicit model usually fits observed species-abundance distributions better than those calculated from the spatially explicit model's simulated data. Current spatially explicit neutral models therefore have limited descriptive power. However, our results suggest that a fatter tail of the dispersal kernel seems to improve the fit, suggesting that dispersal kernels with even fatter tails should be studied in future. We conclude that more advanced spatially explicit models and tools to analyze them need to be developed
Effect of antibiotics, alone and in combination, on Panton–Valentine leukocidin production by a Staphylococcus aureus reference strain
AbstractThe capacity of Staphylococcus aureus strain LUG855 to release Panton–Valentine leukocidin (PVL) in the presence of sub-inhibitory concentrations of anti-staphylococcal drugs was examined. Oxacillin enhanced PVL release 2.5-fold, while clindamycin, linezolid, fusidic acid and rifampicin were inhibitory, and vancomycin, pristinamycin, tetracycline, ofloxacin and co-trimoxazole had no effect. In combination with oxacillin, sub-inhibitory concentrations of clindamycin or rifampicin inhibited PVL induction significantly, linezolid was less inhibitory, and fusidic acid did not inhibit PVL induction by oxacillin. These data support the use of oxacillin in combination with clindamycin, rifampicin or linezolid for the treatment of PVL-positive S. aureus infections
- …