486 research outputs found
Satellite and ground radiotracking of elk
Radiotracking and monitoring of free-living animals in natural environments is providing an effective new technique for acquiring information on biological processes, including animal orientation and navigation. To test the practicability of extending the technique by using satellite systems for tracking animals, a female elk was instrumented with an electronic collar. It contained both the Interrogation Recording Location System (IRLS) transponder and a Craighead-Varney ground-tracking transmitter. The elk was successfully tracked and monitored by satellite during month of April 1970. This was the first time an animal had been tracked by satellite on the surface of the earth
Crystal and magnetic structure of the oxypnictide superconductor LaO(1-x)FxFeAs: evidence for magnetoelastic coupling
High-resolution and high-flux neutron as well as X-ray powder-diffraction
experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with
0<x<0.15 in order to study the crystal and magnetic structure. The magnetic
symmetry of the undoped compound corresponds to those reported for ReOFeAs
(with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered
magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger
than the values previously reported for this compound. A sizable ordered
magnetic moment is observed up to a F-doping of 4.5% whereas there is no
magnetic order for a sample with a F concentration of x=0.06. In the undoped
sample, several interatomic distances and FeAs4 tetrahedra angles exhibit
pronounced anomalies connected with the broad structural transition and with
the onset of magnetism supporting the idea of strong magneto-elastic coupling
in this material.Comment: 8 pages, 7 figures, regular articl
Photoemission induced gating of topological insulator
The recently discovered topological insulators exhibit topologically
protected metallic surface states which are interesting from the fundamental
point of view and could be useful for various applications if an appropriate
electronic gating can be realized. Our photoemission study of Cu intercalated
Bi2Se3 shows that the surface states occupancy in this material can be tuned by
changing the photon energy and understood as a photoemission induced gating
effect. Our finding provides an effective tool to investigate the new physics
coming from the topological surface states and suggests the intercalation as a
recipe for synthesis of the material suitable for electronic applications.Comment: + resistivity data and some discussio
Resistivity and Hall effect of LiFeAs: Evidence for electron-electron scattering
LiFeAs is unique among the broad family of FeAs-based superconductors,
because it is superconducting with a rather large K under
ambient conditions although it is a stoichiometric compound. We studied the
electrical transport on a high-quality single crystal. The resistivity shows
quadratic temperature dependence at low temperature giving evidence for strong
electron-electron scattering and a tendency towards saturation around room
temperature. The Hall constant is negative and changes with temperature, what
most probably arises from a van Hove singularity close to the Fermi energy in
one of the hole-like bands. Using band structure calculations based on angular
resolved photoemission spectra we are able to reproduce all the basic features
of both the resistivity as well as the Hall effect data.Comment: 6 pages, 3 figures included; V2 has been considerably revised and
contain a more detailed analysis of the Hall effect dat
Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2
We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice
relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped
Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1}
is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain.
Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in
the Ca-doped sample evidencing the opening of a spin gap. The data analysis
within the J_1-J_2 Heisenberg model employing density-matrix renormalization
group calculations suggests an impurity driven small alternation of the
J_2-exchange coupling as a possible cause of the spin gap.Comment: 4 pages, 4 figure
Magnetic excitations in two-leg spin 1/2 ladders: experiment and theory
Magnetic excitations in two-leg S=1/2 ladders are studied both experimentally
and theoretically. Experimentally, we report on the reflectivity, the
transmission and the optical conductivity sigma(omega) of undoped La_x Ca_14-x
Cu_24 O_41 for x=4, 5, and 5.2. Using two different theoretical approaches
(Jordan-Wigner fermions and perturbation theory), we calculate the dispersion
of the elementary triplets, the optical conductivity and the momentum-resolved
spectral density of two-triplet excitations for 0.2 <=
J_parallel/J_perpendicular <= 1.2. We discuss phonon-assisted two-triplet
absorption, the existence of two-triplet bound states, the two-triplet
continuum, and the size of the exchange parameters.Comment: 6 pages, 7 eps figures, submitted to SNS 200
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