11,867 research outputs found
Damping rate limitations for transverse dampers in large hadron colliders
The paper focuses on two issues important for design and operation of
bunch-by-bunch transverse damper in a very large hadron collider, where fast
damping is required to suppress beam instabilities and noise induced emittance
growth. The first issue is associated with kick variation along a bunch which
affects the damping of head-tail modes. The second issue is associated with
affect of damper noise on the instability threshold
On the nature of change in Ni oxidation state in BaTiO3-SrTiO3 system
XAFS studies of Ni-doped BaSrTiO solid solution reveal that
the Ni oxidation state changes from 4 in SrTiO to 2.5 in BaTiO when
varying . This change is accompanied by a noticeable change in the
interatomic Ni-O distances in the first shell. The first-principles
calculations show that nickel creates an impurity band in the forbidden band
gap of BaTiO and SrTiO, which explains the appearance of intense
absorption of Ni-doped samples in the visible region. The analysis of the
electronic structure of doped crystals and calculations of the oxygen vacancy
formation energy in them show that different oxidation states of Ni in
SrTiO and BaTiO can be explained by different formation energies of the
oxygen vacancies in these compounds.Comment: 5 pages, 2 figures, 1 tabl
An experimental and theoretical study of Ni impurity centers in BaSrTiO
The local environment and the charge state of a nickel impurity in cubic
BaSrTiO are studied by XAFS spectroscopy. According to the
XANES data, the mean Ni charge state is 2.5+. An analysis of the EXAFS
spectra and their comparison with the results of first-principle calculations
of the defect geometry suggest that Ni ions are in a high-spin state at
the sites of the perovskite structure and the difference of the Ni
and Ti charges is mainly compensated by distant oxygen vacancies. In
addition, a considerable amount of nickel in the sample is in a second phase
BaNiO. The measurements of the lattice parameter show a decrease
in the unit cell volume upon doping, which can indicate the existence of a
small amount of Ni ions at the sites.Comment: 8 pages, 3 figures, 3 table
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