256 research outputs found
Estimating body-fixed frame velocity and attitude from inertial measurements for a quadrotor vehicle
© 2014 IEEE. A key requirement for effective control of quadrotor vehicles is estimation of both attitude and linear velocity. Recent work has demonstrated that it is possible to measure horizontal velocities of a quadrotor vehicle from strap-down ac-celerometers along with a system model. In this paper we extend this to full body-fixed-frame velocity measurement by exploiting recent work in aerodynamic modeling of rotor performance and measurements of mechanical power supplied to the rotor hub. We use these measurements in a combined attitude and velocity nonlinear observer design to jointly estimate attitude and body-fixed-frame linear velocity. Almost global asymptotic stability of the resulting system is demonstrated using Lyapunov analysis of the resulting error system. In the current work, we ignore bias and leave it for future work. The performance of the observer is verified by simulation results
Pressure dependence of the Shubnikov-de Haas oscillation pectrum of beta''-(BEDT-TTF)4(NH4)[Cr(C2O4)3].DMF
The Shubnikov-de Haas (SdH) oscillation spectra of the
beta''-(BEDT-TTF)4(NH4)[Cr(C2O4)\_3].DMF organic metal have been studied in
pulsed magnetic fields of up to either 36 T at ambient pressure or 50 T under
hydrostatic pressures of up to 1 GPa. The ambient pressure SdH oscillation
spectra can be accounted for by up to six fundamental frequencies which points
to a rather complex Fermi surface (FS). A noticeable pressure-induced
modification of the FS topology is evidenced since the number of frequencies
observed in the spectra progressively decreases as the pressure increases.
Above 0.8 GPa, only three compensated orbits are observed, as it is the case
for several other isostructural salts of the same family at ambient pressure.
Contrary to other organic metals, of which the FS can be regarded as a network
of orbits, no frequency combinations are observed for the studied salt, likely
due to high magnetic breakdown gap values or (and) high disorder level
evidenced by Dingle temperatures as large as about 7 K.Comment: To be published in European Physical Journal
Fermiology and electronic homogeneity of the superconducting overdoped cuprate Tl-2201 revealed by quantum oscillations
We report an angular quantum oscillation study of Tl_2Ba_2CuO_{6+delta} for
two different doping levels (Tc = 10K and 26 K) and determine the Fermi surface
size and topology in considerable detail. Our results show that Fermi liquid
behavior is not confined to the edge of the superconducting dome and is robust
up to at least T_c^{max}/3.5. Superconductivity is found to survive up to a
larger doping p_c = 0.31 than in La_{2-x}Sr_xCuO_4. Our data imply that
electronic inhomogeneity does not play a significant role in the loss of
superconductivity and superfluid density in overdoped cuprates, and point
towards a purely magnetic or electronic pairing mechanismComment: 4 page
On the transport and thermodynamic properties of quasi-two-dimensional purple bronzes AMoO (A=Na, K)
We report a comparative study of the specific heat, electrical resistivity
and thermal conductivity of the quasi-two-dimensional purple bronzes
NaMoO and KMoO, with special emphasis on
the behavior near their respective charge-density-wave transition temperatures
. The contrasting behavior of both the transport and the thermodynamic
properties near is argued to arise predominantly from the different
levels of intrinsic disorder in the two systems. A significant proportion of
the enhancement of the thermal conductivity above in
NaMoO, and to a lesser extent in KMoO, is
attributed to the emergence of phason excitations.Comment: 8 pages, 6 figures, To appear in Physical Review
Fermi-surface reconstruction and two-carrier model for the Hall effect in YBa2Cu4O8
Pulsed field measurements of the Hall resistivity and magnetoresistance of
underdoped YBa2Cu4O8 are analyzed self-consistently using a simple model based
on coexisting electron and hole carriers. The resultant mobilities and Hall
numbers are found to vary markedly with temperature. The conductivity of the
hole carriers drops by one order of magnitude below 30 K, explaining the
absence of quantum oscillations from these particular pockets. Meanwhile the
Hall coefficient of the electron carriers becomes strongly negative below 50 K.
The overall quality of the fits not only provides strong evidence for
Fermi-surface reconstruction in Y-based cuprates, it also strongly constrains
the type of reconstruction that might be occurring.Comment: 5 pages, 4 figures, updated after publication in Physical Review B
(Rapid Communication
Topological change of the Fermi surface in ternary iron-pnictides with reduced c/a ratio: A dHvA study of CaFe2P2
We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2
using low temperature torque magnetometry up to 45 T. This system is a close
structural analogue of the collapsed tetragonal non-magnetic phase of CaFe2As2.
We find the Fermi surface of CaFe2P2 to differ from other related ternary
phosphides in that its topology is highly dispersive in the c-axis, being
three-dimensional in character and with identical mass enhancement on both
electron and hole pockets (~1.5). The dramatic change in topology of the Fermi
surface suggests that in a state with reduced (c/a) ratio, when bonding between
pnictogen layers becomes important, the Fermi surface sheets are unlikely to be
nested
Indication for the coexistence of closed orbit and quantum interferometer with the same cross section in the organic metal (ET)4(H3O)[Fe(C2O4)3].C6H4Cl2: Persistence of SdH oscillations above 30 K
Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) oscillations spectra of
the quasi-two dimensional charge transfer salt
"-(ET)(HO)[Fe(CO)]CHCl have been
investigated in pulsed magnetic fields up to 54 T. The data reveal three basic
frequencies F, F and F, which can be interpreted on the basis
of three compensated closed orbits at low temperature. However a very weak
thermal damping of the Fourier component F, with the highest amplitude, is
evidenced for SdH spectra above about 6 K. As a result, magnetoresistance
oscillations are observed at temperatures higher than 30 K. This feature, which
is not observed for dHvA oscillations, is in line with quantum interference,
pointing to a Fermi surface reconstruction in this compound.Comment: published in Eur. Phys. J. B 71 203 (2009
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Isolation of Angola-like Marburg virus from Egyptian rousette bats from West Africa.
Marburg virus (MARV) causes sporadic outbreaks of severe Marburg virus disease (MVD). Most MVD outbreaks originated in East Africa and field studies in East Africa, South Africa, Zambia, and Gabon identified the Egyptian rousette bat (ERB; Rousettus aegyptiacus) as a natural reservoir. However, the largest recorded MVD outbreak with the highest case-fatality ratio happened in 2005 in Angola, where direct spillover from bats was not shown. Here, collaborative studies by the Centers for Disease Control and Prevention, Njala University, University of California, Davis USAID-PREDICT, and the University of Makeni identify MARV circulating in ERBs in Sierra Leone. PCR, antibody and virus isolation data from 1755 bats of 42 species shows active MARV infection in approximately 2.5% of ERBs. Phylogenetic analysis identifies MARVs that are similar to the Angola strain. These results provide evidence of MARV circulation in West Africa and demonstrate the value of pathogen surveillance to identify previously undetected threats
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