Crystallography and Chemistry of Perovskites

Despite the simplicity of the original perovskite crystal structure, this family of compounds shows an enormous variety of structural modifications and variants. In the following, we will describe several examples of perovskites, their structural variants and discuss the implications of distortions and non-stoichiometry on their electronic and magnetic properties.Comment: 11 pages, 8 figures, further information http://www.peter-lemmens.d

Magnetism of a tetrahedral cluster-chain

Magnetic properties of a completely frustrated tetrahedral chain are briefly summarized. Using exact diagonalization, and bond-operator theory results for the ground-state phase diagram, the one-triplet excitations and the Raman spectrum are given. The link to novel tellurate materials is clarified.Comment: 2 pages, 3 figure

Interplay of electronic correlations and lattice instabilities in BaVS3

The quasi-one-dimensional metallic system BaVS3 with a metal-insulator transition at T_MI=70 K shows large changes of the optical phonon spectrum, a central peak, and an electronic Raman scattering continuum that evolve in a three-step process. Motivated by the observation of a strongly fluctuating precursor state at high temperatures and orbital ordering and a charge gap at low temperatures we suggest a concerted action of the orbital, electronic, and lattice subsystems dominated by electronic correlations.Comment: 4 pages, 4 figure

Anomalous frequency and intensity scaling of collective and local modes in a coupled spin tetrahedron system

We report on the magnetic excitation spectrum of the coupled spin tetrahedral system Cu$_{2}$Te$_{2}$O$_{5}$Cl$_{2}$ using Raman scattering on single crystals. The transition to an ordered state at T$_{N}^{Cl}$=18.2 K evidenced from thermodynamic data leads to the evolution of distinct low-energy magnetic excitations superimposed by a broad maximum. These modes are ascribed to magnons with different degree of localization and a two-magnon continuum. Two of the modes develop a substantial energy shift with decreasing temperature similar to the order parameter of other Neel ordered systems. The other two modes show only a negligible temperature dependence and dissolve above the ordering temperature in a continuum of excitations at finite energies. These observations point to a delicate interplay of magnetic inter- and intra-tetrahedra degrees of freedom and an importance of singlet fluctuations in describing a spin dynamics.Comment: 7pages, 6figures, 1tabl

Investigation of the oxohalogenide Cu4Te5O12Cl4 with weakly coupled Cu(II) tetrahedra

The crystal structure of the copper(II) tellurium(IV) oxochloride Cu$_{4}$Te$_{5}$O$_{12}$Cl$_{4}$ (Cu-45124) is composed of weakly coupled tetrahedral Cu clusters and shows crystallographic similarities with the intensively investigated compound Cu$_{2}$Te$_{2}$O$_{5}$X$_{2}$, with X~=~Cl, Br (Cu-2252). It differs from the latter by a larger separation of the tetrahedra within the crystallographic ab plane, that allows a more direct assignment of important inter-tetrahedra exchange paths and the existence of an inversion center. Magnetic susceptibility and specific heat evidence antiferromagnetic, frustrated correlations of the Cu spin moments and long range ordering with $T_{c}$=13.6 K. The entropy related to the transition is reduced due to quantum fluctuations. In Raman scattering a well structured low energy magnetic excitation is observed at energies of $\approx$50K (35cm$^{-1})$. This energy scale is reduced as compared to Cu-2252.Comment: 11 pages, 9 figures, further information see http://www.peter-lemmens.d

Doping effects on charge density instability in non-centrosymmetric PbxTaSe2

We report on the investigation of vibrational and electronic properties of the Pb doped dichalcogenide PbxTaSe2 using Raman scattering experiments. We observe a marked variation of the main vibrational modes with Pb concentration x. The concentration dependence of the vibrational modes resembles the dependence of the vibrational modes in TaSe2 on the number of crystallographic layers along the c axis direction [1]. The temperature and polarization dependence of Raman spectra of PbxTaSe2 revealed additional broad modes in the low frequency regime which are discussed in context of remnant charge density wave, induced disorder, or PbSe phase formed in the interface of Pb and TaSe2 layers.Comment: 14 page