232 research outputs found
Infrared signatures of the spin-Peierls transition in CuGeO3
We investigated the infrared reflectivity of several Mg- and Si-substituted
CuGeO3 single crystals. The temperature dependent b-axis and c-axis optical
response is reported. For T<Tsp we detected the activation of zone-boundary
phonons along the b axis of the crystal on the pure sample and for 1% Mg and
0.7% Si concentrations. From a detailed analysis of the phonon parameters the
redshift of the B2u mode at 48 cm^-1 is observed and discussed in relation to
the soft mode expected to drive the spin-Peierls phase transition in CuGeO3.
Moreover, the polarization dependence of a magnetic excitation measured in
transmission at 44 cm^-1 has been investigated.Comment: Revtex, 3 pages, 5 postscript pictures, submitted to PRB Rapid
Communication
The Butcher-Oemler Effect at Moderate Redshift
We present the results of Butcher-Oemler-style analysis of three moderate-
redshift (0.1<z<0.2) clusters which have bimodal X-ray surface brightness
profiles. We find that at least two of these clusters exhibit unusually high
fractions of blue galaxies as compared to clusters at comparable redshifts
studied by Butcher and Oemler (1984). This implies that star formation is
occurring in a high fraction of the galaxies in the two clusters. Our results
are consistent with hierarchical clustering models in which subcluster-
subcluster mergers create shocks in the intracluster medium. The shocks, in
turn, induce simultaneous starbursts in a large fraction of cluster galaxies.
Our study therefore lends weight to the hypothesis that the Butcher-Oemler
effect is an environmental, as well as evolutionary, phenomenon.Comment: 22 pages, 8 figures; accepted for publication in A
Lattice and Magnetic structures of PrFeAsO, PrFeAsO0.85F0.15 and PrFeAsO0.85
We use powder neutron diffraction to study the spin and lattice structures of
polycrystalline samples of nonsuperconducting PrFeAsO and superconducting
PrFeAsO0.85F0.15 and PrFeAsO0.85. We find that PrFeAsO exhibits an abrupt
structural phase transitions at 153 K, followed by static long range
antiferromagnetic order at 127 K. Both the structural distortion and magnetic
order are identical to other rare-earth oxypnictides. Electron-doping the
system with either Fluorine or oxygen deficiency suppresses the structural
distortion and static long range antiferromagnetic order, therefore placing
these materials into the same class of FeAs-based superconductors.Comment: 14 pages, 3 figures, 1 tabl
Magnetic order in orbital models of the iron pnictides
We examine the appearance of the experimentally-observed stripe
spin-density-wave magnetic order in five different orbital models of the iron
pnictide parent compounds. A restricted mean-field ansatz is used to determine
the magnetic phase diagram of each model. Using the random phase approximation,
we then check this phase diagram by evaluating the static spin susceptibility
in the paramagnetic state close to the mean-field phase boundaries. The momenta
for which the susceptibility is peaked indicate in an unbiased way the actual
ordering vector of the nearby mean-field state. The dominant orbitally resolved
contributions to the spin susceptibility are also examined to determine the
origin of the magnetic instability. We find that the observed stripe magnetic
order is possible in four of the models, but it is extremely sensitive to the
degree of the nesting between the electron and hole Fermi pockets. In the more
realistic five-orbital models, this order competes with a strong-coupling
incommensurate state which appears to be controlled by details of the
electronic structure below the Fermi energy. We conclude by discussing the
implications of our work for the origin of the magnetic order in the pnictides.Comment: 19 pages, 19 figures; published version, typos corrected, references
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