490 research outputs found
Electron concentration effects on the Shastry-Sutherland phase stability in Ce_{2-x}Pd_{2+y}In_{1-z} solid solutions
The stability of a Shastry-Sutherland ShSu phase as a function of electron
concentration is investigated through the field dependence of thermal and
magnetic properties of the solid solution Ce_{2-x}Pd_{2+y}In_{1-z} in the
antiferromagnetic branch. In these alloys the electronic (holes) variation is
realized by increasing concentration. The AF transition T_M decreases from
3.5K to 2.8K as concentration increases from y=0.2 to y=0.4. By applying
magnetic field, the ShSu phase is suppressed once the field induced
ferromagnetic polarization takes over at a critical field B_{cr} which
increases with content. A detailed analysis around the critical point
reveals a structure in the maximum of the dM/dB derivative, which is related
with incipient steps in the magnetization M(B) as predicted by the theory for
the ShSu lattice. The crossing of M(B) isotherms, observed in ShSu prototype
compounds, is also analyzed. The effect of substitution by is
interpreted as an increase of the number of 'holes' in the conduction band and
results in a unique parameter able to describe the variation of the magnetic
properties along the studied range of concentration.Comment: 8 pages, 11 figure
Exploring high temperature magnetic order in CeTi_1-xSc_xGe
Most of magnetic transitions related to Ce ordering are found below
T_ord~12K. Among the few cases exceeding that temperature, two types of
behaviors can be distinguished. One of them is related to the rare cases of Ce
binary compounds formed in BCC structures, with a quartet ground state, whose
degeneracy is reduced by undergoing different types of transitions mostly
structural. The other group shows evidences of itinerant character with the
outstanding example of CeRh_3B_2 showing the highest T_ord=115K. The second
highest ordering temperature has been reported for CeScGe with T_ord=47K, but
the nature of this magnetic state has not been investigated very deeply. In
order to shed more light into this unusual high temperature ordering we studied
the structural, magnetic, transport and thermal properties of CeTi_1-xSc_xGe
alloys in the stability range of the CeScSi-type structure 0.25<x<1 This system
presents a rich variety of magnetic behaviors along this concentration range,
with the magnetic ordering growing from ferromagnetic (FM) T_C~7K up to an
antiferromagnetic (AFM) transition at T_N=47K. The different regions show the
following characteristics: i) on the Ti rich side (0.25<x<0.50) it exhibits a
FM ground state (GS) with large saturation magnetization values M_sat up to
~1.15 mu_B. ii) Around x=0.60, the first crystal electric field excited doublet
starts to contribute to the GS magnetic properties. Furthermore an AFM
component with a connected metamagnetic transition appears. iii) At x=0.65 a
clear change in the GS nature is associated to a critical point above which the
GS properties can be described like for an itinerant system (with decreasing
M_sat) and an effective GS degeneracy N_eff=4. iv) For x>0.65, the magnetic
phase boundary splits into two transitions, with an intermediate phase
presenting incommensurate spin density waves features.Comment: 8 pages, 10 figure
Underground Muon Counters as a Tool for Composition Analyses
The transition energy from galactic to extragalactic cosmic ray sources is
still uncertain, but it should be associated either with the region of the
spectrum known as the second knee or with the ankle. The baseline design of the
Pierre Auger Observatory was optimized for the highest energies. The surface
array is fully efficient above eV and, even if the hybrid
mode can extend this range below eV, the second knee and a
considerable portion of the wide ankle structure are left outside its operating
range. Therefore, in order to encompass these spectral features and gain
further insight into the cosmic ray composition variation along the transition
region, enhancements to the surface and fluorescence components of the baseline
design are being implemented that will lower the full efficiency regime of the
Observatory down to eV. The surface enhancements consist of a
graded infilled area of standard Auger water Cherenkov detectors deployed in
two triangular grids of 433 m and 750 m of spacing. Each surface station inside
this area will have an associated muon counter detector. The fluorescence
enhancement, on the other hand, consists of three additional fluorescence
telescopes with higher elevation angle () than the ones in
operation at present. The aim of this paper is threefold. We study the effect
of the segmentation of the muon counters and find an analytical expression to
correct for the under counting due to muon pile-up. We also present a detailed
method to reconstruct the muon lateral distribution function for the 750 m
spacing array. Finally, we study the mass discrimination potential of a new
parameter, the number of muons at 600 m from the shower axis, obtained by
fitting the muon data with the above mentioned reconstruction method.Comment: Astroparticle Physics 29 (2008) 461-47
Enhancing the Pierre Auger Observatory to the 10^{17} to 10^{18.5} eV Range: Capabilities of an Infill Surface Array
The Pierre Auger Observatory has been designed to study the highest-energy
cosmic rays in nature (E > 10^{18.5} eV). The determination of their arrival
direction, energy and composition is performed by the analysis of the
atmospheric showers they produce. The Auger Surface Array will consist of 1600
water Cerenkov detectors placed in an equilateral triangular grid of 1.5 km
spacing. The aim of this paper is to show that the addition of a "small" area
of surface detectors at half or less the above mentioned spacing would allow a
dramatic increase of the physical scope of this Observatory, reaching lower
energies at which the transition from galactic to extragalactic sources is
expected.Comment: 21 pages, 5 figures, accepted for publication in Nucl. Instr. & Meth.
in Phys. Res.
Low temperature magnetic phase diagram of the cubic non-Fermi liquid system CeIn_(3-x)Sn_x
In this paper we report a comprehensive study of the magnetic susceptibility
(\chi), resistivity (\rho), and specific heat (C_P), down to 0.5 K of the cubic
CeIn_(3-x)Sn_x alloy. The ground state of this system evolves from
antiferromagnetic (AF) in CeIn_3(T_N=10.2 K) to intermediate-valent in CeSn_3,
and represents the first example of a Ce-lattice cubic non-Fermi liquid (NFL)
system where T_N(x) can be traced down to T=0 over more than a decade of
temperature. Our results indicate that the disappearance of the AF state occurs
near x_c ~ 0.7, although already at x ~ 0.4 significant modifications of the
magnetic ground state are observed. Between these concentrations, clear NFL
signatures are observed, such as \rho(T)\approx \rho_0 + A T^n (with n<1.5) and
C_P(T)\propto -T ln(T) dependencies. Within the ordered phase a first order
phase transition occurs for 0.25 < x < 0.5. With larger Sn doping, different
weak \rho(T) dependencies are observed at low temperatures between x=1 and x=3
while C_P/T shows only a weak temperature dependence.Comment: 7 pages, 7 figures. Accepted in Eur. J. Phys.
Tri-critical point and suppression of the Shastry-Sutherland phase in CePdSn by Ni doping
Structural, magnetization and heat capacity measurements were performed on
Ce(PdNi)Sn () alloys, covering the full
range of the MoFeB structure stability. In this system, the two
transitions observed in CePdSn (at \,K and \,K
respectively) converge into a tri-critical point at \,K for
, where the intermediate antiferromagnetic AF phase is
suppressed. The phase boundary decrease is due to an incipient Kondo
screening of the Ce-4f moments and local atomic disorder in the alloy. Both
mechanisms affect the formation of Ce-magnetic dimers on which the
Shastry-Sutherland lattice (SSL) builds up. On the contrary, the
transition to the ferromagnetic ground state increases as a consequence of the
weakening of the AF-SSL phase. Applied magnetic field also suppresses the AF
phase like in the stoichiometric compound.Comment: 6 pages, 8 figure
Aperture calculation of the Pierre Auger Observatory surface detector
We determine the instantaneous aperture and integrated exposure of the
surface detector of the Pierre Auger Observatory, taking into account the
trigger efficiency as a function of the energy, arrival direction (with zenith
angle lower than 60 degrees) and nature of the primary cosmic-ray. We make use
of the so-called Lateral Trigger Probability function (or LTP) associated with
an extensive air shower, which summarizes all the relevant information about
the physics of the shower, the water tank Cherenkov detector, and the triggers.Comment: Proc. of the 29th ICRC (Pune, India), LaTeX, 4 pages, 5 figure
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