160 research outputs found
Neutron monitors and muon detectors for solar modulation studies: 2. time series
The level of solar modulation at different times (related to the solar
activity) is a central question of solar and galactic cosmic-ray physics. In
the first paper of this series, we have established a correspondence between
the uncertainties on ground-based detectors count rates and the parameter
(modulation level in the force-field approximation) reconstructed from
these count rates. In this second paper, we detail a procedure to obtain a
reference time series from neutron monitor data. We show that we can
have an unbiased and accurate reconstruction (). We also discuss the potential of Bonner spheres spectrometers and muon
detectors to provide time series. Two by-products of this calculation
are updated values for the cosmic-ray database and a web interface to
retrieve and plot from the 50's to today
(\url{http://lpsc.in2p3.fr/crdb}).Comment: 15 pages, 5 figures, 2 tables. AdSR, in press. Web interface to get
modulation parameter phi(t): new tab in http://lpsc.in2p3.fr/crd
Dielectric anomalies and spiral magnetic order in CoCr2O4
We have investigated the structural, magnetic, thermodynamic, and dielectric
properties of polycrystalline CoCrO, an insulating spinel exhibiting
both ferrimagnetic and spiral magnetic structures. Below = 94 K the
sample develops long-range ferrimagnetic order, and we attribute a sharp phase
transition at 25 K with the onset of long-range spiral magnetic
order. Neutron measurements confirm that while the structure remains cubic at
80 K and at 11 K; there is complex magnetic ordering by 11 K. Density
functional theory supports the view of a ferrimagnetic semiconductor with
magnetic interactions consistent with non-collinear ordering. Capacitance
measurements on CoCrO, show a sharp decrease in the dielectric constant
at , but also an anomaly showing thermal hysteresis falling between
approximately = 50 K and = 57 K. We tentatively attribute the
appearance of this higher temperature dielectric anomaly to the development of
\textit{short-range} spiral magnetic order, and discuss these results in the
context of utilizing dielectric spectroscopy to investigate non-collinear
short-range magnetic structures.Comment: & Figure
Determining conductivity and mobility values of individual components in multiphase composite Cu_(1.97)Ag_(0.03)Se
The intense interest in phase segregation in thermoelectrics as a means to reduce the lattice thermal conductivity and to modify the electronic properties from nanoscale size effects has not been met with a method for separately measuring the properties of each phase assuming a classical mixture. Here, we apply effective medium theory for measurements of the in-line and Hall resistivity of a multiphase composite, in this case Cu_(1.97) Ag_(0.03)Se. The behavior of these properties with magnetic field as analyzed by effective medium theory allows us to separate the conductivity and charge carrier mobility of each phase. This powerful technique can be used to determine the matrix properties in the presence of an unwanted impurity phase, to control each phase in an engineered composite, and to determine the maximum carrier concentration change by a given dopant, making it the first step toward a full optimization of a multiphase thermoelectric material and distinguishing nanoscale effects from those of a classical mixture
Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAlGaO: Lattice expansion and site disorder
The spinels CoBO with magnetic Co ions on the diamond lattice
A site can be frustrated because of competing near-neighbor () and
next-near neighbor () interactions. Here we describe attempts to tune the
relative strengths of these interactions by substitution on the non-magnetic
B-site. The system we employ is CoAlGaO, where Al is
systematically replaced by the larger Ga, ostensibly on the B site. As
expected, Ga substitution expands the lattice, resulting in Co atoms on the
A-site being pushed further from one other and thereby weakening magnetic
interactions. In addition, Ga distributes between the B and the A site in a
concentration dependent manner displacing an increasing amount of Co from the A
site with increasing . This increased inversion, which is confirmed by
neutron diffraction studies carried out at room temperature, affects magnetic
ordering very significantly, and changes the nature of the ground state.
Modeling of the magnetic coupling illustrates the complexity that arises from
the cation site disorder.Comment: 9 pages, 10 figure
Magnetodielectric coupling in Mn3O4
We have investigated the dielectric anomalies associated with spin ordering
transitions in the tetragonal spinel MnO, using thermodynamic,
magnetic, and dielectric measurements. We find that two of the three magnetic
ordering transitions in MnO lead to decreases in the temperature
dependent dielectric constant at zero applied field. Applying a magnetic field
to the polycrystalline sample leaves these two dielectric anomalies practically
unchanged, but leads to an increase in the dielectric constant at the
intermediate spin-ordering transition. We discuss possible origins for this
magnetodielectric behavior in terms of spin-phonon coupling. Band structure
calculations suggest that in its ferrimagnetic state, MnO corresponds
to a semiconductor with no orbital degeneracy due to strong Jahn-Teller
distortion.Comment: 6 pages, 7 figure
Risks management and cobots. Identifying critical variables
Trabajo presentado en: 29th European Safety and Reliability Conference (ESREL), 22–26 September 2019, HannoverA collaborative robot or a "Cobot" is the name of a robot that can share a workspace with operators
in the absence of a protective fence or with only partial protection. They represent a new and expanding sector of
industrial robotics. This investigation draws from the latest international rules and safety parameters related to
work with collaborative robots. Its detailed research is motivated by the design of a collaborative industrial robot
system, hazard elimination, risk reduction, and different collaborative operations, such as power and force
limiting, collaborative operation design, and end-effector safety requirements, among others. The purpose of our
study is to analyze the most important variables that must be controlled in accordance with the desired use of the
Cobot, according to ISO / TS 15066, ISO / TR 20218-1and some other generic safety regulations on machines and
industrial robots. A series of observations and appreciations on the use of the Cobot will also be presented
Structural and magnetic characterization of the complete delafossite solid solution (CuAlO2){1-x}(CuCrO2){x}
We have prepared the complete delafossite solid solution series between
diamagnetic CuAlO2 and the t2g^3 frustrated antiferromagnet CuCrO2. The
evolution with composition x in CuAl(1-x)Cr(x)O2 of the crystal structure and
magnetic properties has been studied and is reported here. The room-temperature
unit cell parameters follow the Vegard law and increase with x as expected. The
effective moment is equal to the Cr^3+ spin-only S = 3/2 value throughout the
entire solid solution. Theta is negative, indicating that the dominant
interactions are antiferromagnetic, and its magnitude increases with Cr
substitution. For dilute Cr compositions, J_BB was estimated by mean-field
theory to be 2.0 meV. Despite the sizable Theta, long-range antiferromagnetic
order does not develop until very large x, and is preceeded by glassy behavior.
Data presented here, and that on dilute Al-substitution from Okuda et al.,
suggest that the reduction in magnetic frustration due to the presence of
non-magnetic Al does not have as dominant an effect on magnetism as chemical
disorder and dilution of the magnetic exchange. For all samples, the 5 K
isothermal magnetization does not saturate in fields up to 5 T and minimal
hysteresis is observed. The presence of antiferromagnetic interactions is
clearly evident in the sub-Brillouin behavior with a reduced magnetization per
Cr atom. An inspection of the scaled Curie plot reveals that significant
short-range antiferromagnetic interactions occur in CuCrO2 above its Neel
temperature, consistent with its magnetic frustration. Uncompensated
short-range interactions are present in the Al-substituted samples and are
likely a result of chemical disorder
Electronic structure and transport in thermoelectric compounds AZn_2Sb_2 (A = Sr, Ca, Yb, Eu)
The AZn_2Sb_2 (P¯3m1, A = Ca, Sr, Eu, Yb) class of Zintl compounds has shown high thermoelectric efficiency (zT ~ 1) and is an appealing system for the development of Zintl structure–property relationships. High temperature transport measurements have previously been conducted for all known compositions except for SrZn_2Sb_2; here we characterize polycrystalline SrZn_2Sb_2 to 723 K and review the transport behavior of the other compounds in this class. Consistent with the known AZn_2Sb_2 compounds, SrZn_2Sb_2 is found to be a hole-doped semiconductor with a thermal band gap ~ 0.27 eV. The Seebeck coefficients of the AZn2Sb2 compounds are found to be described by similar effective mass (m* ~ 0.6 m_e). Electronic structure calculations reveal similar m* is due to antimony p states at the valence band edge which are largely unaffected by the choice of A-site species. However, the choice of A-site element has a dramatic effect on the hole mobility, with the room temperature mobility of the rare earth-based compositions approximately double that found for Ca and Sr on the A site. This difference in mobility is examined in the context of electronic structure calculations
Magnetic phase evolution in the spinel compounds ZnCoCrO
We present the magnetic properties of complete solid solutions of
ZnCrO and CoCrO: two well-studied oxide spinels with very
different magnetic ground states. ZnCrO, with non-magnetic
cations occupying the A site and magnetic cations on the B site, is a
highly frustrated antiferromagnet. CoCrO, with magnetic cations
(three unpaired electrons) on the A site as well, exhibits both N\'eel
ferrimagnetism as well as commensurate and incommensurate non-collinear
magnetic order. More recently, CoCrO has been studied extensively for
its polar behavior which arises from conical magnetic ordering. Gradually
introducing magnetism on the A site of ZnCrO results in a transition
from frustrated antiferromagnetism to glassy magnetism at low concentrations of
Co, and eventually to ferrimagnetic and conical ground states at higher
concentrations. Real-space Monte-Carlo simulations of the magnetic
susceptibility suggest that the first magnetic ordering transition and features
of the susceptibility across are captured by near-neighbor self- and
cross-couplings between the magnetic A and B atoms. We present as a part of
this study, a method for displaying the temperature dependence of magnetic
susceptibility in a manner which helps distinguish between compounds possessing
purely antiferromagnetic interactions from compounds where other kinds of
ordering are present.Comment: 10 pages, 5 figures, 1 tabl
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