Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered

We present a comparative study of the low-dimensional compounds α-and β-TeVO 4 . Our data clearly show that the change in the local coordination geometry of V 4+ ions comparing α-and β-TeVO 4 leads to a drastic differences in the magnetic properties. Despite sharing the same crystal structure, the two compounds realize different magnetic exchange topologies. Both compounds exhibit a transition from ferro-to antiferromagnetic correlations with decreasing temperature. This effect however is driven by different mechanisms for both compounds. Additionally, a dimensional crossover is found in β-TeVO 4 . PACS

The Raman-active phonons and magnetic excitations in the orbitally ordered, quasi-one-dimensional Heisenberg antiferromagnet KCuF 3 are studied as a function of temperature in the range between 3 and 290 K in various scattering configurations. The low-energy E g and B 1g phonon modes show an anomalous softening (~25% and ~13%) between room temperature and the characteristic temperature T S = 50 K. In this temperature range a freezing-in of F -ion dynamic displacements is proposed to occur. In addition, the E g mode at about 260 cm -1 clearly splits below T S . The widths of the phonon lines above T S follow an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the lattice symmetry below T S and indicate a strong coupling of lattice, spin, and orbital fluctuations for T > T S . A strongly polarization dependent quasielastic scattering is observed at temperatures above T N = 39 K due to magnetic-energy fluctuations. For temperatures below T N a rich spectrum of additional modes is observed, with different lineshape, polarization and temperatures dependence. We attribute them to longitudinal and transverse magnetic modes as well as to a continuum of magnetic excitations.