1,494 research outputs found
First evidence for charge ordering in NaVO from Raman spectroscopy
We argue on the basis of symmetry selection rules and Raman scattering
spectra that NaVO undergoes a charge ordering phase transition at
T=34 K. Such a transition is characterized by the redistribution of the
charges at the phase transition and corresponds to the change of the vanadium
ions, from uniform V to two different V and V states. In
the low temperature phase the V ions are forming a "zig-zag" ladder
structure along the {\bf b}-axis, consistent with the symmetry of the P2/b
space group.Comment: to be published in solid state communication
Optical properties of NaxV2O5
The optical properties of sodium-deficient NaxV2O5 (0.85 < x <1) single
crystals are analyzed in the wide energy range, from 0.012 to 4.5 eV, using
ellipsometry, infrared reflectivity, and Raman scattering techniques. The
material remains insulating up to the maximal achieved hole concentration of
about 15%. In sodium deficient samples the optical absorption peak associated
to the fundamental electronic gap develops at about 0.44 eV. It corresponds to
the transition between vanadium dxy and the impurity band, which forms in the
middle of the pure NaV2O5 gap. Raman spectra measured with incident photon
energy larger then 2 eV show strong resonant behavior, due to the presence of
the hole-doping activated optical transitions, peaked at 2.8 eV.Comment: 7 pages, 4 fugures, to be published in PR
Infrared and Raman spectra of LiV2O5 single crystals
The phonon dynamics of LiV2O5 single crystals is studied using infrared and
Raman spectroscopy techniques. The infrared-active phonon frequencies and
dielectric constants are obtained by oscillator fitting procedure of the
reflectivity data measured at room temperature. The Raman scattering spectra
are measured at room temperature and at T=10 K in all nonequivalent polarized
configurations. The assignment of the phonons is done by comparing the infrared
and Raman spectra of LiV2O5 and NaV2O5. The factor-group-analysis of the LiV2O5
crystal symmetry and of its constituent layers is performed to explain the
symmetry properties of the observed modes. We concluded that layer symmetry
dominates in the vibrational properties of this compound.Comment: 10 pages, 5 figure
Charge-ordering and optical transitions of LiV2O5 and NaV2O5
We present the measurements of the polarized optical spectra of NaV2O5 and
LiV2O5. In an energy range from 0.5 to 5.5 eV we observe similar peaks in the E
parallel a spectra of LiV2O5 and NaV2O5, which suggests similar electronic
structure along the a axis in both materials. On the other hand, we find an
almost complete suppression of the peaks in sigma_b of LiV2O5 around 1 and 5
eV. We attribute this suppression to the charge localization originating from
the existence of double-chain charge-ordering patterin in LiV2O5.Comment: 7 pages, 3 figures final version, to appear in PR
Deep Observation of the Giant Radio Lobes of Centaurus A with the Fermi Large Area Telescope
The detection of high energy (HE) {\gamma}-ray emission up to about 3 GeV
from the giant lobes of the radio galaxy Centaurus A has been recently reported
by the Fermi-LAT Collaboration based on ten months of all-sky survey
observations. A data set more than three times larger is used here to study the
morphology and photon spectrum of the lobes with higher statistics. The larger
data set results in the detection of HE {\gamma}-ray emission (up to about 6
GeV) from the lobes with a significance of more than 10 and 20 {\sigma} for the
North and the South lobe, respectively. Based on a detailed spatial analysis
and comparison with the associated radio lobes, we report evidence for a
substantial extension of the HE {\gamma}-ray emission beyond the WMAP radio
image in the case of the Northern lobe of Cen A. We reconstruct the spectral
energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data
from the same integration region. The implications are discussed in the context
of hadronic and leptonic scenarios
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