3,892 research outputs found
Phase transition in Pr0.5Ca0.5CoO3 and related cobaltites
We present an extensive investigation (magnetic, electric and thermal
measurements and X-ray absorption spectroscopy) of the Pr0.5Ca0.5CoO3 and
(Pr1-yYy)0.7Ca0.3CoO3 (y=0.0625-0.15) perovskites, in which a peculiar
metal-insulator (M-I) transition, accompanied with pronounced structural and
magnetic anomalies, occurs at 76 K and 40-132 K, respectively. The inspection
of the M-I transition using the XANES data of Pr L3-edge and Co K-edge proofs
the presence of Pr4+ ions at low temperatures and indicates simultaneously the
intermediate spin to low spin crossover of Co species on lowering the
temperature. The study thus definitively confirms the synchronicity of the
electron transfer between Pr3+ ions and Co^(3+/4+)O3 subsystem and the
transition to the low-spin, less electrically conducting phase. The large
extent of the transfer is evidenced by the good quantitative agreement of the
determined amount of the Pr4+ species, obtained either from the temperature
dependence of the XANES spectra or via integration of the magnetic entropy
change over the Pr4+ related Schottky peak in the low-temperature specific
heat. These results show that the average valence of Pr3+/Pr4+ ions increases
(in concomitance with the decrease of the formal Co valence) below TMI for
(Pr0.925Y0.075)0.7Ca0.3CoO3 up to 3.16+ (the doping level of the CoO3 subsystem
decreases from 3.30+ to 3.20+), for (Pr0.85Y0.15)0.7Ca0.3CoO3 up to 3.28+ (the
decrease of doping level from 3.30+ to 3.13+) and for Pr0.5Ca0.5CoO3 up to
3.46+ (the decrease of doping level from 3.50+ to 3.27+).Comment: 19 pages, 11 figure
On the peak in the far-infrared conductivity of strongly anisotropic cuprates
We investigate the far-infrared and submillimeter-wave conductivity of
electron-doped La_(2-x)Ce_xCuO_4 tilted 1 degree off from the ab-plane. The
effective conductivity measured for this tilt angle reveals an intensive peak
at finite frequency (\nu ~ 50 cm{-1}) due to a mixing of the in-plane and
out-of-plane responses. The peak disappears for the pure in-plane response and
transforms to the Drude-like contribution. Comparative analysis of the mixed
and the in-plane contributions allows to extract the c-axis conductivity which
shows a Josephson plasma resonance at 11.7 cm{-1} in the superconducting state.Comment: 4 pages, 4 figures include
Capture on High Curvature Region: Aggregation of Colloidal Particle Bound to Giant Phospholipid Vesicles
A very recent observation on the membrane mediated attraction and ordered
aggregation of colloidal particles bound to giant phospholipid vesicles (I.
Koltover, J. O. R\"{a}dler, C. R. Safinya, Phys. Rev. Lett. {\bf 82},
1991(1999)) is investigated theoretically within the frame of Helfrich
curvature elasticity theory of lipid bilayer fluid membrane. Since the concave
or waist regions of the vesicle possess the highest local bending energy
density, the aggregation of colloidal beads on these places can reduce the
elastic energy in maximum. Our calculation shows that a bead in the concave
region lowers its energy . For an axisymmetrical dumbbell
vesicle, the local curvature energy density along the waist is equally of
maximum, the beads can thus be distributed freely with varying separation
distance.Comment: 12 pages, 2 figures. REVte
Metal-insulator transition and the Pr/Pr valence shift in (PrY)CaCoO
The magnetic, electric and thermal properties of the
(Y)CaCoO perovskites (~=~Pr, Nd) were
investigated down to very low temperatures. The main attention was given to a
peculiar metal-insulator transition, which is observed in the praseodymium
based samples with and 0.15 at and 132~K, respectively.
The study suggests that the transition, reported originally in
PrCaCoO, is not due to a mere change of cobalt ions from
the intermediate- to the low-spin states, but is associated also with a
significant electron transfer between Pr and Co/Co sites,
so that the praseodymium ions occur below in a mixed
Pr/Pr valence. The presence of Pr ions in the insulating
phase of the yttrium doped samples (PrY)CaCoO
is evidenced by Schottky peak originating in Zeeman splitting of the ground
state Kramers doublet. The peak is absent in pure PrCaCoO
in which metallic phase, based solely on non-Kramers Pr ions, is
retained down to the lowest temperature.Comment: 10 figure
Evidence for a quantum phase transition in the electron-doped cuprate Pr2-xCexCuO4+d from Hall and resistivity measurements
The doping and temperature dependence of the Hall coefficient, RH, and
ab-plane resistivity in the normal state down to 350mK is reported for oriented
films of the electron-doped high-Tc superconductor Pr2-xCexCuO4+d. The doping
dependence of b (r=r0+AT^b) and R_sub_H (at 350 mK) suggest a quantum phase
transition at a critical doping near x=0.165.Comment: 11 pages 4 figures Phys. Rev. Lett. 92, 167001 (2004
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