P. Scheib’s research while affiliated with Technische Universität Braunschweig and other places

We present a Raman spectroscopic investigation of the herbertsmithite ZnCu3(OH)6Cl2, a realization of a Heisenberg s=1/2 antiferromagnet on a perfect kagome lattice. Focusing on its magnetic excitation spectrum we find two components, a high-temperature quasielastic signal and a low temperature, broad maximum. The latter has a linear low-energy slope and extends to high energy. We have investigated the temperature dependence and symmetry properties of both signals. Our data agree with previous calculations and point to a gapless spin liquid ground state.

We present a Raman spectroscopic investigation of the Herbertsmithite ZnCu3(OH)6Cl2, the first realization of a Heisenberg s=1/2 antiferromagnet on a perfect kagome lattice. The magnetic excitation spectrum of this compound is dominated by two components, a high temperature quasi elastic signal and a low temperature, broad maximum. The latter has a linear low energy slope and extends to high energy. We have investigated the temperature dependence and symmetry properties of both signals. Our data agree with previous calculations and point to a spin liquid ground state.

Raman scattering in the spin-crossover system FepmdH 2 OAuCN 2 2 ·H 2 O reveals a complex three-phase spin-state transition in contrast to earlier observations in magnetization measurements. We observe different spin-state phases as a function of temperature and electromagnetic radiation in the visible spectral range. There exists a fluctuating spin-state phase with an unexpected wipeout of the low-frequency phonon-scattering intensity. Furthermore we observe one phase with reduced symmetry that is attributed to a coopera-tive Jahn-Teller effect. Pronounced electron-phonon interaction manifests itself as a strong Fano resonance of phonons related to FeN 6 and FeN 4 O 2 coordination octahedra. Density-functional theory supports this interpretation.

The effect of chemical treatment on physical and chemical properties, i.e., pore diameter, porosity, specific surface area, and chemical bonding of electrochemically formed mesoporous silicon were investigated by using of nitrogen sorption isotherm, scanning electron microscopy, and Fourier transform infrared spectroscopy. The adsorption isotherms measurements show the general behavior found for the porous materials, but at the same time, they exhibit clear differences following different chemical treatments of porous layer. It was clearly observed from Fourier transform infrared spectroscopy that the chemical environment of porous silicon wall changes significantly after chemical treatment. In scanning electron microscopy images, we see that the rough dendritic structure of the pore walls is modified to smooth tubular pore wall structure on chemical treatment. The changes in nanocrystalline porous silicon were also clearly observed by an asymmetric broadening and shift of the optical silicon phonons in Raman spectra. Furthermore, changes are observed in the multiphonon regime due to surface assisted multiphonon processes, which are enhanced in highly porous silicon. The chemically modified porous silicon samples suggest possibilities of use as a porous matrix for fundamental study and technological application.

We report inelastic light scattering experiments on FeCr2S4, which shows colossal magnetoresistance (CMR) effects around TC≈170 K. Several distinct features have been observed: (i) anomalies in phonon frequencies and linewidths around TC, (ii) the vanishing of one-magnon excitation close to 0.83 TC, and (iii) the presence of an electronic background at low temperature. This highlights the significance of an intricate interplay among electron, spin, orbital, and lattice degrees of freedom in inducing CMR effects.

Raman scattering experiments on CdCr2S4 single crystals show pronounced anomalies in intensity and frequency of optical phonon modes with an onset temperature T*=130 K that coincides with the regime of giant magnetocapacitive effects. A loss of inversion symmetry and Cr off-centering are deduced from the observation of longitudinal optical and formerly infrared active modes for T<T_c=84 K. The intensity anomalies are attributed to the enhanced electronic polarizability of displacements that modulate the Cr-S distance and respective hybridization. Photo doping leads to an annihilation of the symmetry reduction. Our scenario of multiferroic effects is based on the near degeneracy of polar and nonpolar modes and the additional low energy scale due to hybridization.

The effect of surface degradation of the thermolectric cobaltite on Raman spectra is discussed and compared to experimental results from Co3O4 single crystals. We conclude that on NaCl flux grown NaxCoO2 crystals a surface layer of Co3O4 easily forms that leads to the observation of an intense phonon around 700 cm-1 [Phys. Rev. B 70, 052502 (2004)]. Raman spectra on freshly cleaved crystals from optical floating zone ovens do not show such effects and have a high frequency phonon cut-off at approximately 600 cm -1 [Phys. Rev. Lett 96, 167204 (2006)]. We discuss the relation of structural dimensionality, electronic correlations and the high frequency phonon cut-off of the thermolectric cobaltite.