223 research outputs found
Structure and properties of a novel fulleride Sm6C60
A novel fulleride Sm6C60 has been synthesized using high temperature solid
state reaction. The Rietveld refinement on high resolution synchrotron X-ray
powder diffraction data shows that Sm6C60 is isostructural with body-centered
cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60,
and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of
Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules
and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak
T-linear behavior above 180 K, the transport at low temperature is mainly
dominated by granular-metal theory.Comment: 9 pages, 3 figures, submitted to Phys. Rev. B (March 12, 1999
Suppression of the structural phase transition and lattice softening in slightly underdoped Ba(1-x)K(x)Fe2As2 with electronic phase separation
We present x-ray powder diffraction (XRPD) and neutron diffraction
measurements on the slightly underdoped iron pnictide superconductor
Ba(1-x)K(x)Fe2As2, Tc = 32K. Below the magnetic transition temperature Tm =
70K, both techniques show an additional broadening of the nuclear Bragg peaks,
suggesting a weak structural phase transition. However, macroscopically the
system does not break its tetragonal symmetry down to 15 K. Instead, XRPD
patterns at low temperature reveal an increase of the anisotropic microstrain
proportionally in all directions. We associate this effect with the electronic
phase separation, previously observed in the same material, and with the effect
of lattice softening below the magnetic phase transition. We employ density
functional theory to evaluate the distribution of atomic positions in the
presence of dopant atoms both in the normal and magnetic states, and to
quantify the lattice softening, showing that it can account for a major part of
the observed increase of the microstrain.Comment: 7 pages, 4 figure
Effective-Medium Theory for the Normal State in Orientationally Disordered Fullerides
An effective-medium theory for studying the electronic structure of the
orientationally disordered A3C60 fullerides is developed and applied to study
various normal-state properties. The theory is based on a cluster-Bethe-lattice
method in which the disordered medium is modelled by a three-band Bethe
lattice, into which we embed a molecular cluster whose scattering properties
are treated exactly. Various single-particle properties and the
frequency-dependent conductivity are calculated in this model, and comparison
is made with numerical calculations for disordered lattices, and with
experiment.Comment: 12 pages + 2 figures, REVTeX 3.
The polymer phase of the TDAE-C organic ferromagnet
The high-pressure Electron Spin Resonance (ESR) measurements were preformed
on TDAE-C single crystals and stability of the polymeric phase was
established in the parameter space. At 7 kbar the system undergoes a
ferromagnetic to paramagnetic phase transition due to the pressure-induced
polymerization. The polymeric phase remains stable after the pressure release.
The depolymerization of the pressure-induced phase was observed at the
temperature of 520 K. Below room temperature, the polymeric phase behaves as a
simple Curie-type insulator with one unpaired electron spin per chemical
formula. The TDAE donor-related unpaired electron spins, formerly
ESR-silent, become active above the temperature of 320 K and the Curie-Weiss
behavior is re-established.Comment: Submitted to Phys. Rev.
NaIrO3 - A pentavalent post-perovskite
Sodium iridium(V) oxide, NaIrO3, was synthesized by a high pressure solid
state method and recovered to ambient conditions. It is found to be
isostructural with CaIrO3, the much-studied structural analogue of the
high-pressure post-perovskite phase of MgSiO3. Among the oxide
post-perovskites, NaIrO3 is the first example with a pentavalent cation. The
structure consists of layers of corner- and edge-sharing IrO6 octahedra
separated by layers of NaO8 bicapped trigonal prisms. NaIrO3 shows no magnetic
ordering and resistivity measurements show non-metallic behavior. The crystal
structure, electrical and magnetic properties are discussed and compared to
known post-perovskites and pentavalent perovskite metal oxides.Comment: 22 pages, 5 figures. Submitted to Journal of Solid State Chemistr
Mott Transition in Degenerate Hubbard Models: Application to Doped Fullerenes
The Mott-Hubbard transition is studied for a Hubbard model with orbital
degeneracy N, using a diffusion Monte-Carlo method. Based on general arguments,
we conjecture that the Mott-Hubbard transition takes place for U/W \propto
\sqrt{N}, where U is the Coulomb interaction and W is the band width. This is
supported by exact diagonalization and Monte-Carlo calculations. Realistic
parameters for the doped fullerenes lead to the conclusion that stoichiometric
A_3 C_60 (A=K, Rb) are near the Mott-Hubbard transition, in a correlated
metallic state.Comment: 4 pages, revtex, 1 eps figure included, to be published in Phys.Rev.B
Rapid Com
Electrical resistivity at large temperatures: Saturation and lack thereof
Many transition metal compounds show saturation of the resistivity at high
temperatures, T, while the alkali-doped fullerenes and the high-Tc cuprates are
usually considered to show no saturation. We present a model of transition
metal compounds, showing saturation, and a model of alkali-doped fullerenes,
showing no saturation. To analyze the results we use the f-sum rule, which
leads to an approximate upper limit for the resistivity at large T. For some
systems and at low T, the resistivity increases so rapidly that this upper
limit is approached for experimental T. The resistivity then saturates. For a
model of transition metal compounds with weakly interacting electrons, the
upper limit corresponds to a mean free path consistent with the Ioffe-Regel
condition. For a model of the high Tc cuprates with strongly interacting
electrons, however, the upper limit is much larger than the Ioffe-Regel
condition suggests. Since this limit is not exceeded by experimental data, the
data are consistent with saturation also for the cuprates. After "saturation"
the resistivity usually grows slowly. For the alkali-doped fullerenes,
"saturation" can be considered to have happened already for T=0, due to
orientational disorder. For these systems, however, the resistivity grows so
rapidly after "saturation" that this concept is meaningless. This is due to the
small band width and to the coupling to the level energies of the important
phonons.Comment: 22 pages, RevTeX, 19 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/fullerene
Evidence for and phases in the morphotropic phase boundary region of : A Rietveld study
We present here the results of the room temperature dielectric constant
measurements and Rietveld analysis of the powder x-ray diffraction data on
(PMN-PT) in the composition range
to show that the morphotropic phase boundary (MPB)
region contains two monoclinic phases with space groups Cm (or type) and
Pm (or type) stable in the composition ranges and
, respectively. The structure of PMN-PT in the
composition ranges 0.26, and is found to be
rhombohedral (R3m) and tetragonal (P4mm), respectively. These results are
compared with the predictions of Vanderbilt & Cohen's theory.Comment: 20 pages, 11 pdf figure
High pressure phases in highly piezoelectric Pb(Zr0.52Ti0.48)O3
Two novel room-temperature phase transitions are observed, via synchrotron
x-ray diffraction and Raman spectroscopy, in the Pb(Zr0.52Ti0.48)O3 alloy under
hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase
transition takes place around 2-3 GPa, while this R1 phase transforms into
another rhombohedral phase, R2, at about 6-7 GPa. First-principles calculations
assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2
acts as a pressure-induced structural bridge between the polar R3m and a
predicted antiferrodistortive R-3c phase.Comment: REVTeX, 4 pages with 3 figures embedded. Figs 1 and 3 in colo
Far-infrared vibrational properties of high-pressure-high-temperature C60 polymers and the C60 dimer
We report high-resolution far-infrared transmission measurements of the 2 + 2 cycloaddition C-60 dimer and two-dimensional rhombohedral and one-dimensional orthorhombic high-pressure high-temperature C60 polymers. In the spectral region investigated(20-650 cm(-1)), we see no low-energy interball modes, but symmetry breaking of the linked C-60 balls is evident in the complex spectrum of intramolecular modes. Experimental features suggest large splittings or frequency shifts of some IhC60-derived modes that are activated by symmetry reduction, implying that the balls are strongly distorted in these structures. We have calculated the vibrations of all three systems by first-principles quantum molecular dynamics and use them to assign the predominant IhC60 symmetries of observed modes. Pur calculations show unprecedentedly large downshifts of T-1u(2)-derived modes and extremely large splittings of other modes, both of which are consistent with the experimental spectra. For the rhombohedral and orthorhombic polymers, the T-1u(2)-derived mode that is polarized along the bonding direction is calculated to downshift below any T-1u(1)-derived modes. We also identify a previously unassigned feature near 610 cm(-1) in all three systems as a widely split or shifted mode derived from various silent IhC60 vibrations, confirming a strong perturbation model for these linked fullerene structures
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