544 research outputs found
Emerging frustration effects in ferromagnetic Ce_2[Pd_{1-x}Ag_x]_2In alloys
Magnetic and thermal properties of Ferromagnetic (FM)
Ce_{2.15}(Pd_{1-x}Ag_x)_{1.95}In_{0.9} alloys were studied in order to
determine the Quantum Critical Point (QCP) at T_C => 0. The increase of band
electrons produced by Pd/Ag substitution depresses T_C(x) from 4.1K down to
T_C(x=0.5)=1.1K, with a QCP extrapolated to x_{QCP}~ 0.6. Magnetic
susceptibility from T>30K indicates an effective moment slightly decreasing
from \mu_{eff}=2.56\mu_B to 2.4\mu_B at x=0.5. These values and the
paramagnetic temperature \theta_P~ -10K exclude significant Kondo screening
effects. The T_C(x) reduction is accompanied by a weakening of the FM
magnetization and the emergence of a specific heat C_m(T) anomaly at T*~ 1K,
without signs of magnetism detected from AC-susceptibility. The magnetic
entropy collected around 4K (i.e. the T_C of the x=0 sample) practically does
not change with Ag concentration: S_m(4K)~ 0.8 Rln2, suggesting a progressive
transfer of FM degrees of freedom to the non-magnetic (NM) component. No
antecedent was found concerning any NM anomaly emerging from a FM system at
such temperature. The origin of this anomaly is attributed to an 'entropy
bottleneck' originated in the nearly divergent power law dependence for T>T*.Comment: 5 pages, 4 figures, Int. Conf. ICM 201
A search for point sources of EeV neutrons
A thorough search of the sky exposed at the Pierre Auger Cosmic Ray Observatory reveals no statistically significant excess of events in any small solid angle that would be indicative of a flux of neutral particles from a discrete source. The search covers from -90° to +15° in declination using four different energy ranges above 1 EeV (1018 eV). The method used in this search is more sensitive to neutrons than to photons. The upper limit on a neutron flux is derived for a dense grid of directions for each of the four energy ranges. These results constrain scenarios for the production of ultrahigh energy cosmic rays in the Galaxy.Fil: Abreu, P.. Technical University of Lisbon; PortugalFil: Aglietta, M.. Università di Torino; ItaliaFil: Ahlers, M.. University of Wisconsin; Estados UnidosFil: Ahn, E. J.. Fermi National Accelerator Laboratory; Estados UnidosFil: Albuquerque, I. F. M.. Universidade de Sao Paulo; BrasilFil: Gomez Berisso, Mariano. Comisión Nacional de Energía Atómica. Gerencia de Área de Investigaciones y Aplicaciones No Nucleares. Gerencia de Física (CAB); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Zhou, J.. University of Chicago; Estados UnidosFil: Zhu, Y.. NorthInstitut für Prozessdatenverarbeitung und Elektronik; AlemaniaFil: Zimbres Silva, M.. Universidade Estadual de Campinas; Brasil. NYU Abu Dhabi; Emiratos Arabes UnidosFil: Ziolkowski, M.. Universitat Siegen; Alemani
The Large Aperture GRB Observatory
The Large Aperture GRB Observatory (LAGO) is aiming at the detection of the
high energy (around 100 GeV) component of Gamma Ray Bursts, using the single
particle technique in arrays of Water Cherenkov Detectors (WCD) in high
mountain sites (Chacaltaya, Bolivia, 5300 m a.s.l., Pico Espejo, Venezuela,
4750 m a.s.l., Sierra Negra, Mexico, 4650 m a.s.l). WCD at high altitude offer
a unique possibility of detecting low gamma fluxes in the 10 GeV - 1 TeV range.
The status of the Observatory and data collected from 2007 to date will be
presented.Comment: 4 pages, proceeding of 31st ICRC 200
Water Cherenkov Detectors response to a Gamma Ray Burst in the Large Aperture GRB Observatory
In order to characterise the behaviour of Water Cherenkov Detectors (WCD)
under a sudden increase of 1 GeV - 1 TeV background photons from a Gamma Ray
Burst (GRB), simulations were conducted and compared to data acquired by the
WCD of the Large Aperture GRB Observatory (LAGO). The LAGO operates arrays of
WCD at high altitude to detect GRBs using the single particle technique. The
LAGO sensitivity to GRBs is derived from the reported simulations of the gamma
initiated particle showers in the atmosphere and the WCD response to
secondaries.Comment: 5 pages, proceeding of the 31st ICRC 200
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