197,105 research outputs found
Oxygen Isotope Effect on the Spin State Transition in (PrSm)CaCoO
Oxygen isotope substitution is performed in the perovskite cobalt oxide
(PrSm)CaCoO which shows a sharp spin
state transition from the intermediate spin (IS) state to the low spin (LS)
state at a certain temperature. The transition temperature of the spin state
up-shifts with the substitution of by O from the resistivity
and magnetic susceptibility measurements. The up-shift value is 6.8 K and an
oxygen isotope exponent () is about -0.8. The large oxygen isotope
effect indicates strong electron-phonon coupling in this material. The
substitution of O by O leads to a decrease in the frequency of
phonon and an increase in the effective mass of electron (), so that
the bandwidth W is decreased and the energy difference between the different
spin states is increased. This is the reason why the is shifted to high
temperature with oxygen isotopic exchange.Comment: 4 pages, 3 figure
sigma and f_0(980) substructures from gamma-gamma to pi-pi, J/psi, phi radiative and D_s semi-leptonic decays
Using an improved "analytic K-matrix model", we reconsider the extraction of
the sigma/f_0(600) and f_0(980) gamma-gamma widths from gamma-gamma to pi-pi
scatterings data of Crystal Ball and Belle. Our main results are summarized in
Tables 3 and 4. The averaged sigma "direct width" to gamma-gamma is 0.16(3) keV
which confirms a previous result of [1] and which does neither favour a large
four-quark (diquark-antidiquark) nor a molecule nor a pure \bar qq component.
The "direct width" of the f_0(980) of 0.28(2) keV is much larger than the
four-quark expectation but can be compatible with a \bar ss or gluonium
component. We also found that the rescattering part of the amplitude is
relatively large indicating an important contribution of the meson loops in the
determination of the gamma-gamma total widths. This is mainly due to the large
couplings of the sigma and f_0(980) to pi-pi and/or \bar KK, which can also be
due to a light scalar gluonium with large OZI violating couplings but not
necessary to a four-quark or molecule state. Our average results for the total
(direct+rescattering) gamma-gamma widths: Gamma_sigma^{tot}= 3.08(82) keV,
Gamma_{f_0}^{tot}= 0.16(1) keV} are comparable with the ones from dispersion
relations and PDG values. Using the parameters from QCD spectral sum rules, we
complete our analysis by showing that the production rates of unmixed scalar
gluonia sigma_B(1) and G (1.5-1.6) agree with the data from J/\psi, phi
radiative and D_s semi-leptonic decays.Comment: 11 pages, 6 figures (1 more figure added in Fig 6), 4 tables :
version appeared on-line in doi:10.1016/j.physletb.2010.12.00
Hysteresis and Anisotropic Magnetoresistance in Antiferromagnetic
The out-of-plane resistivity () and magnetoresistivity (MR) are
studied in antiferromangetic (AF) single crystals, which
have three types of noncollinear antiferromangetic spin structures. The
apparent signatures are observed in measured at the zero-field and
14 T at the spin structure transitions, giving a definite evidence for the
itinerant electrons directly coupled to the localized spins. One of striking
feature is an anisotropy of the MR with a fourfold symmetry upon rotating the
external field (B) within ab plane in the different phases, while twofold
symmetry at spin reorientation transition temperatures. The intriguing thermal
hysteresis in and magnetic hysteresis in MR are observed at spin
reorientation transition temperatures.Comment: 4 pages, 4 figure
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