3,755 research outputs found
Role of magnetic and orbital ordering at the metal-insulator transition in NdNiO3
Soft x-ray resonant scattering at the Ni L2,3 edges is used to test models of
magnetic and orbital-ordering below the metal-insulator transition in NdNiO3.
The large branching ratio of the L3 to L2 intensities of the (1/2,0,1/2)
reflection and the observed azimuthal angle and polarization dependence
originates from a non collinear magnetic structure. The absence of an orbital
signal and the non collinear magnetic structure show that the nickelates are
materials for which orbital ordering is absent at the metal-insulator
transition.Comment: 10 pages, 4 figures, Physical Review B rapid communication, to be
publishe
Infrared Resonance Enhanced Multiphoton Ionization of Fullerenes
Gas-phase fullerenes are resonantly heated by a train of high power subpicosecond pulses from a free electron laser (FEL) to internal energies at which they efficiently undergo delayed ionization. When the laser is tuned from 6–20μm while the amount of laser produced parent ions is recorded, resonant absorption of 200–600 IR photons, resulting in almost fragmentation-free ion spectra, is observed. Infrared resonance enhanced multiphoton ionization with a FEL is shown to enable extremely sensitive IR spectroscopy with mass selective detection of gas-phase fullerenes
Asymptotics and zeros of Sobolev orthogonal polynomials on unbounded supports
In this paper we present a survey about analytic properties of polynomials
orthogonal with respect to a weighted Sobolev inner product such that the
vector of measures has an unbounded support. In particular, we are focused in
the study of the asymptotic behaviour of such polynomials as well as in the
distribution of their zeros. Some open problems as well as some new directions
for a future research are formulated.Comment: Changed content; 34 pages, 41 reference
Dynamic stability control in younger and older adults during stair descent.
The purpose of this study was to examine dynamic stability control in older and younger adults while descending stairs. Thirteen older (aged 64-77years) and 13 younger (aged 22-29years) adults descended a staircase at their preferred speed. A motion capture system and three force plates were used to determine locomotion mechanics. Dynamic stability was investigated by using the margin of stability, calculated as the instantaneous difference between anterior boundary of the base of support and extrapolated centre of mass. At the initiation of the single support phase, older adults demonstrated a more negative (p<.05) margin of stability value. The component responsible for the lower margin of stability in the elderly was the higher velocity of the centre of mass (p<.05). Before the initiation of the single support phase, the older adults showed a lower (p<.05) ankle and knee joint angular impulse compared to the younger ones. We found a significant correlation (r=.729, p<.05) between centre of mass velocity and joint angular impulse. These results indicate that older adults are at greater risk of falls while descending stairs potentially due to a reduced ability to generate adequate leg-extensor muscular output to safely control the motion of the body's centre of mass while stepping down
Effects of hole-doping on the magnetic ground state and excitations in the edge-sharing CuO chains of CaYCuO
Neutron scattering experiments were performed on the undoped and hole-doped
CaYCuO, which consists of ferromagnetic edge-sharing
CuO chains. It was previously reported that in the undoped
CaYCuO there is an anomalous broadening of spin-wave
excitations along the chain, which is caused mainly by the antiferromagnetic
interchain interactions [Matsuda , Phys. Rev. B 63, 180403(R)
(2001)]. A systematic study of temperature and hole concentration dependencies
of the magnetic excitations shows that the magnetic excitations are softened
and broadened with increasing temperature or doping holes irrespective of
direction. The broadening is larger at higher . A characteristic feature is
that hole-doping is much more effective to broaden the excitations along the
chain. It is also suggested that the intrachain interaction does not change so
much with increasing temperature or doping although the anisotropic interaction
and the interchain interaction are reduced. In the spin-glass phase (=1.5)
and nearly disordered phase (=1.67) the magnetic excitations are much
broadened in energy and . It is suggested that the spin-glass phase
originates from the antiferromagnetic clusters, which are caused by the hole
disproportionation.Comment: 8 pages, submitted to Phys. Rev.
Collisional and thermal ionization of sodium Rydberg atoms I. Experiment for nS and nD atoms with n=8-20
Collisional and thermal ionization of sodium nS and nD Rydberg atoms with
n=8-20 has been studied. The experiments were performed using a two-step pulsed
laser excitation in an effusive atomic beam at atom density of about 2 10^{10}
cm^{-3}. Molecular and atomic ions from associative, Penning, and thermal
ionization processes were detected. It has been found that the atomic ions were
created mainly due to photoionization of Rydberg atoms by photons of blackbody
radiation at the ambient temperature of 300K. Blackbody ionization rates and
effective lifetimes of Rydberg states of interest were determined. The
molecular ions were found to be from associative ionization in Na(nL)+Na(3S)
collisions. Rate constants of associative ionization have been measured using
an original method based on relative measurements of Na_{2}^{+} and Na^{+} ion
signals.Comment: 23 pages, 10 figure
Single-photon-emitting optical centers in diamond fabricated upon Sn implantation
The fabrication of luminescent defects in single-crystal diamond upon Sn
implantation and annealing is reported. The relevant spectral features of the
optical centers (emission peaks at 593.5 nm, 620.3 nm, 630.7 nm and 646.7 nm)
are attributed to Sn-related defects through the correlation of their
photoluminescence (PL) intensity with the implantation fluence. Single
Sn-related defects were identified and characterized through the acquisition of
their second-order auto-correlation emission functions, by means of
Hanbury-Brown-Twiss interferometry. The investigation of their single-photon
emission regime as a function of excitation laser power revealed that
Sn-related defects are based on three-level systems with a 6 ns radiative decay
lifetime. In a fraction of the studied centers, the observation of a blinking
PL emission is indicative of the existence of a dark state. Furthermore,
absorption dependence from the polarization of the excitation radiation with
about 45 percent contrast was measured. This work shed light on the existence
of a new optical center associated with a group-IV impurity in diamond, with
similar photo-physical properties to the already well-known Si-V and Ge-V
emitters, thus providing results of interest from both the fundamental and
applicative points of view.Comment: 10 pages, 4 figure
Electrical current distribution across a metal-insulator-metal structure during bistable switching
Combining scanning electron microscopy (SEM) and electron-beam-induced
current (EBIC) imaging with transport measurements, it is shown that the
current flowing across a two-terminal oxide-based capacitor-like structure is
preferentially confined in areas localized at defects. As the thin-film device
switches between two different resistance states, the distribution and
intensity of the current paths, appearing as bright spots, change. This implies
that switching and memory effects are mainly determined by the conducting
properties along such paths. A model based on the storage and release of charge
carriers within the insulator seems adequate to explain the observed memory
effect.Comment: 8 pages, 7 figures, submitted to J. Appl. Phy
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