Helicoidal ordering in iron perovskites

We consider magnetic ordering in materials with negative charge transfer energy, such as iron perovskite oxides. We show that for a large weight of oxygen holes in conduction bands, the double exchange mechanism favors a helicoidal rather than ferromagnetic spin ordering both in metals, e.g. SrFeO_3 and insulators with a small gap, e.g. CaFeO_3. We discuss the magnetic excitation spectrum and effects of pressure on magnetic ordering in these materials.Comment: 4 pages, 5 figure

Subgap optical absorption in conjugated polymers

We study the quantum lattice motion in conjugated polymers using the Su-Schrieffer-Heeger (SSH) model. We find a strong reduction of the number of relevant lattice degrees of freedom for short chains (up to N = 70 for polyacetylene), which allows us to calculate fully quantum mechanically the linear optical absorption spectrum