arXiv subjects

G. Bernard

Publications and source records attributed to G. Bernard.

4 recordsLinked to original sources

arXiv2021

Classical Spin Liquid State in the $S=\frac{5}{2}$ Heisenberg Kagomé Antiferromagnet Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$

We investigate the low temperature magnetic properties of a $S=\frac{5}{2}$ Heisenberg kagomé antiferromagnet, the layered monodiphosphate Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$, using magnetization measurements and $^{31}$P nuclear magnetic resonance. An antiferromagnetic-type order sets in at $T_{\rm N}=1.3$ K and a characteristic magnetization plateau is observed at 1/3 of the saturation magnetization below $T^* \sim 5$ K. A moderate $^{31}$P NMR line broadening reveals the development of anisotropic short-range correlations within the plateau phase concomitantly with a gapless spin-lattice relaxation time $T_1 \sim k_B T / \hbar S$, which both point to the presence of a semiclassical nematic spin liquid state predicted for the Heisenberg kagomé antiferromagnetic model or to the persistence of the zero-energy modes of the kagome lattice under large magnetic fields.

cond-mat.str-el↗

arXiv2012

Variance of the Galactic nuclei cosmic ray flux

Measurements of cosmic ray fluxes by the PAMELA and CREAM experiments show unexpected spectral features between 200 GeV and 100 TeV. They could be due to the presence of nearby and young cosmic ray sources. This can be studied in the myriad model, in which cosmic rays diffuse from point-like instantaneous sources located randomly throughout the Galaxy. To test this hypothesis, one must compute the flux due to a catalog of local sources, but also the error bars associated to this quantity. This turns out not to be as straightforward as it seems, as the standard deviation is infinite when computed for the most general statistical ensemble. The goals of this paper are to provide a method to associate error bars to the flux measurements which has a clear statistical meaning, and to explore the relation between the myriad model and the more usual source model based on a continuous distribution. To this end, we show that the quantiles of the flux distribution are well-defined, even though the standard deviation is infinite. They can be used to compute 68% confidence levels, for instance. We also use the fact that local sources have known positions and ages to reduce the statistical ensemble from which random sources are drawn in the myriad model. We present a method to evaluate meaningful error bars for the flux obtained in the myriad model. In this context, we also discuss the status of the spectral features observed in the proton flux by CREAM and PAMELA.

astro-ph.HE↗

arXiv2010

Directional detection of galactic Dark Matter

Directional detection of galactic Dark Matter is a promising search strategy for discriminating geniune WIMP events from background ones. We present technical progress on gaseous detectors as well as recent phenomenological studies, allowing the design and construction of competitive experiments.

astro-ph.CO↗

arXiv2009

A prototype of a directional detector for non-baryonic dark matter search: MIMAC (Micro-TPC Matrix of Chambers)

We have developed a micro-tpc using a pixelized bulk micromegas coupled to dedicated acquisition electronics as a read-out allowing to reconstruct the three dimensional track of a few keV recoils. The prototype has been tested with the Amande facility at the IRSN-Cadarache providing monochromatic neutrons. The first results concerning discrimination of a few keV electrons and proton recoils are presented.

astro-ph.IM↗