4,723 research outputs found
Neutron inelastic scattering investigation of the magnetic excitations in Cu_2Te_2O_5X_2 (X=Br, Cl)
Neutron inelastic scattering investigations have been performed on the spin
tetrahedral system Cu_2Te_2O_5X_2 (X = Cl, Br). We report the observation of
magnetic excitations with a dispersive component in both compounds, associated
with the 3D incommensurate magnetic order that develops below =18.2
K and =11.4 K. The excitation in Cu_2Te_2O_5Cl_2 softens as the
temperature approaches , leaving diffuse quasi-elastic scattering
above the transition temperature. In the bromide, the excitations are present
well above , which might be attributed to the presence of a degree
of low dimensional correlations above in this compound
Two-photon correlations as a sign of sharp transition in quark-gluon plasma
The photon production arising due to time variation of the medium has been
considered. The Hamilton formalism for photons in time-variable medium (plasma)
has been developed with application to inclusive photon production. The results
have been used for calculation of the photon production in the course of
transition from quark-gluon phase to hadronic phase in relativistic heavy ion
collisions. The relative strength of the effect as well as specific two- photon
correlations have been evaluated. It has been demonstrated that the opposite
side two-photon correlations are indicative of the sharp transition from the
quark-gluon phase to hadrons.Comment: 23 pages, 2 figure
From antiferromagnetism to superconductivity in Fe 1+y(Te1-x,Sex) (0 < x < 0.20): a neutron powder diffraction analysis
The nuclear and magnetic structure of Fe1+y(Te1-x,Sex) (0 < x < 0.20)
compounds was analyzed between 2 K and 300 K by means of Rietveld refinement of
neutron powder diffraction data. Samples with x < 0.075 undergo a tetragonal to
monoclinic phase transition at low temperature, whose critical temperature
decreases with increasing Se content; this structural transition is strictly
coupled to a long range antiferromagnetic ordering at the Fe site. Both the
transition to a monoclinic phase and the long range antiferromagnetism are
suppressed for 0.10 < x < 0.20. The onset of the structural and of the magnetic
transition remains coincident with the increase of Se substitution. The low
temperature monoclinic crystal structure has been revised. Superconductivity
arises for x > 0.05, therefore a significant region where superconductivity and
long range antiferromagnetism coexist is present in the pseudo-binary FeTe -
FeSe phase diagram.Comment: 33 pages, 4 tables, 13 figure
Brown Dwarfs and the Cataclysmic Variable Period Minimum
Using improved, up-to-date stellar input physics tested against observations
of low-mass stars and brown dwarfs we calculate the secular evolution of
low-mass donor cataclysmic variables (CVs), including those which form with a
brown dwarf donor. Our models confirm the mismatch between the calculated
minimum period (Pmin ~ 70 min) and the observed short-period cut-off (~ 80 min)
in the CV period histogram. We find that tidal and rotational corrections
applied to the one-dimensional stellar structure equations have no significant
effect on the period minimum. Theoretical period distributions synthesized from
our model sequences always show an accumulation of systems at the minimum
period, a feature absent from the observed distribution. We suggest that
non-magnetic CVs become unobservable as they are effectively trapped in
permanent quiescence before they reach Pmin, and that small-number statistics
may hide the period spike for magnetic CVs.Comment: 10 pages; accepted for publication in MNRA
Theoretical search for Chevrel phase based thermoelectric materials
We investigate the thermoelectric properties of some semiconducting Chevrel
phases. Band structure calculations are used to compute thermopowers and to
estimate of the effects of alloying and disorder on carrier mobility. Alloying
on the Mo site with transition metals like Re, Ru or Tc to reach a
semiconducting composition causes large changes in the electronic structure at
the Fermi level. Such alloys are expected to have low carrier mobilities.
Filling with transition metals was also found to be incompatible with high
thermoelectric performance based on the calculated electronic structures.
Filling with Zn, Cu, and especially with Li was found to be favorable. The
calculated electronic structures of these filled Chevrel phases are consistent
with low scattering of carriers by defects associated with the filling. We
expect good mobility and high thermopower in materials with the composition
close to (Li,Cu)MoSe, particularly when Li-rich, and recommend this
system for experimental investigation.Comment: 4 two-column pages, 4 embedded ps figure
XSS J00564+4548 and IGR J00234+6141 -- new cataclysmic variables from RXTE and INTEGRAL all sky surveys
We present the results of optical identification of two X-ray sources from
RXTE and INTEGRAL all sky surveys: XSS J00564+4548 and IGR J00234+6141. Using
the optical data from Russian-Turkish 1.5-m Telescope (RTT150) and SWIFT X-ray
observations, we show that these sources most probably are intermediate polars,
i.e. binary systems with accreting white dwarfs with not very strong magnetic
field (<~10 MG). Periodical oscillations of optical emission with periods 480 s
and 570 s were found. We argue that these periods most probably correspond to
the rotating periods of the white dwarfs in these systems. Further optical
observations scheduled at RTT150 will allow to study the parameters of these
systems in more detail.Comment: 5 pages, 8 figures, accepted for publication in Astronomy Letter
Evidence of strong antiferromagnetic coupling between localized and itinerant electrons in ferromagnetic Sr2FeMoO6
Magnetic dc susceptibility () and electron spin resonance (ESR)
measurements in the paramagnetic regime, are presented. We found a Curie-Weiss
(CW) behavior for (T) with a ferromagnetic K and
, this being lower than that expected for
either or ions. The ESR g-factor , is associated with . We obtained an excellent description
of the experiments in terms of two interacting sublattices: the localized
() cores and the delocalized electrons. The coupled equations
were solved in a mean-field approximation, assuming for the itinerant electrons
a bare susceptibility independent on . We obtained
emu/mol. We show that the reduction of for
arises from the strong antiferromagnetic (AFM) interaction between the two
sublattices. At variance with classical ferrimagnets, we found that is
ferromagnetic. Within the same model, we show that the ESR spectrum can be
described by Bloch-Hasegawa type equations. Bottleneck is evidenced by the
absence of a -shift. Surprisingly, as observed in CMR manganites, no
narrowing effects of the ESR linewidth is detected in spite of the presence of
the strong magnetic coupling. These results provide evidence that the magnetic
order in does not originates in superexchange interactions,
but from a novel mechanism recently proposed for double perovskites
Effect of band-filling and structural distortions on the Curie temperature of Fe-Mo double perovkites
By means of high resolution neutron powder diffraction at low temperature we
have characterized the structural details of
() and () series of compounds. This study reveals a similar variation of the mean
bond-angle \FeOMo in both series. In contrast, the mean bond-distance \FeMoO\
increases with La but not with Ca substitution. Both series also present a
different evolution of the Curie temperature (), which raises in the La
series and slightly decreases in the Ca one. We thus conclude that the
enhancement of in the La series is due to the electron filling of the
conduction band and a concomitant rising of the density of states at the Fermi
level.Comment: Revtex, 4 Journal pages, 2 figures, 1 tabl
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