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Comment: 4 pages, 4 figures
Comment: 14 pages 4 figures
Comment: 12 pages, 15 figures, 2 references added
Comment: 4 pp, 5 figs, 2 col; final version, with various minor changes
Comment: 4 pages, 3 figures, REVTeX 4
Comment: 6 pages, 4 figures
Ferroelectric spiral magnets DyMnO3 and TbMnO3 show similar behavior of
electric polarization in applied magnetic fields. Studies of the field
dependence of lattice modulations on the contrary show a completely different
picture. Whereas in TbMnO3 the polarization flop from P||c to P||a is
accompanied by a sudden change from incommensurate to commensurate wave vector
modulation, in DyMnO3 the wave vector varies continuously through the flop
transition. This smooth behavior may be related to the giant magnetocapacitive
effect observed in DyMnO3.
Short-range lattice superstructures have been studied with high-energy x-ray
diffuse scattering in underdoped, optimally doped, and overdoped $\rm
(Y,Ca)Ba_2 Cu_3 O_{6+x}$. A new four-unit-cell superstructure was observed in
compounds with $x\sim 0.95$. Its temperature, doping, and material dependence
was used to attribute its origin to short-range oxygen vacancy ordering, rather
than electronic instabilities in the $\rm CuO_2$ layers. No significant diffuse
scattering is observed in YBa$_2$Cu$_4$O$_{8}$. The oxygen superstructures must
be taken into account when interpreting spectral anomalies in $\rm (Y,Ca)Ba_2
Cu_3 O_{6+x}$.
We unambiguously demonstrate, based on high-energy x-ray diffraction data and
LDA calculations, that sodium-density stripes are formed in Na0.75CoO2 at low
temperatures and rule out the previously proposed Na-ordering models. The LDA
calculations prove, that the sodium-density stripes lead to a sizeable dip in
the density of the Co-states at the Fermi level, pointing to band structure
effects as a driving force for the stripe formation. This indicates that the
sodium ordering is connected to stripe-like charge correlations within the CoO2
layers, leading to an astonishing similarity between the doped cuprates and the
NaxCoO2 compounds.
