1,026 research outputs found
First and second order magnetic and structural transitions in BaFeCoAs
We present here high resolution magnetization measurements on high-quality
BaFeCoAs, 0x0.046 as-grown single crystals.
The results confirm the existence of a magnetic tricritical point in the
(,) plane at x0.022 and reveal the emergence of the
heat capacity anomaly associated with the onset of the structural transition at
x0.0064. We show that the samples with doping near x
do not show superconductivity, but rather superconductivity emerges at a
slightly higher cobalt doping, x0.0315Comment: 4 pages, 5 figure
Co atoms on BiSe revealing a coverage dependent spin reorientation transition
We investigate Co nanostructures on BiSe by means of scanning
tunneling microscopy and spectroscopy [STM/STS], X-ray absorption spectroscopy
[XAS], X-ray magnetic dichroism [XMCD] and calculations using the density
functional theory [DFT]. In the single adatom regime we find two different
adsorption sites by STM. Our calculations reveal these to be the fcc and hcp
hollow sites of the substrate. STS shows a pronounced peak for only one species
of the Co adatoms indicating different electronic properties of both types.
These are explained on the basis of our DFT calculations by different
hybridizations with the substrate. Using XMCD we find a coverage dependent spin
reorientation transition from easy-plane toward out-of-plane. We suggest
clustering to be the predominant cause for this observation.Comment: 10 pages, 4 figure
The Aschenbach effect: unexpected topology changes in motion of particles and fluids orbiting rapidly rotating Kerr black holes
Newton's theory predicts that the velocity of free test particles on
circular orbits around a spherical gravity center is a decreasing function of
the orbital radius , . Only very recently, Aschenbach (A&A 425,
p. 1075 (2004)) has shown that, unexpectedly, the same is not true for
particles orbiting black holes: for Kerr black holes with the spin parameter
, the velocity has a positive radial gradient for geodesic, stable,
circular orbits in a small radial range close to the black hole horizon. We
show here that the {\em Aschenbach effect} occurs also for non-geodesic
circular orbits with constant specific angular momentum . In Newton's theory it is , with being the cylindrical
radius. The equivelocity surfaces coincide with the surfaces which,
of course, are just co-axial cylinders. It was previously known that in the
black hole case this simple topology changes because one of the ``cylinders''
self-crosses. We show here that the Aschenbach effect is connected to a second
topology change that for the tori occurs only for very highly
spinning black holes, .Comment: 9 pages, 7 figure
Strong out-of-plane magnetic anisotropy of Fe adatoms on BiTe
The electronic and magnetic properties of individual Fe atoms adsorbed on the
surface of the topological insulator BiTe(111) are investigated.
Scanning tunneling microscopy and spectroscopy prove the existence of two
distinct types of Fe species, while our first-principles calculations assign
them to Fe adatoms in the hcp and fcc hollow sites. The combination of x-ray
magnetic circular dichroism measurements and angular dependent magnetization
curves reveals out-of-plane anisotropies for both species with anisotropy
constants of meV/atom and meV/atom. These values are well in line with the results of
calculations.Comment: 6 pages, 3 figure
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