M. Sing

High-energy photoemission on Fe3O4: Small polaron physics and the Verwey transition

We have studied the electronic structure and charge ordering (Verwey) transition of magnetite (Fe3O4) by soft X-ray photoemission. Due to the enhanced probing depth and the use of different surface preparations we are able to distinguish surface and volume effects in the spectra. The pseudogap behavior of the intrinsic spectra and its temperature dependence give evidence for the existence of strongly bound small polarons consistent with both dc and optical conductivity. Together with other recent structural and theoretical results our findings support a picture in which the Verwey transition contains elements of a cooperative Jahn-Teller effect, stabilized by local Coulomb interaction.

Technique for bulk Fermiology by photoemission applied to layered ruthenates

We report the Fermi surfaces of the superconductor

Terahertz conductivity at the Verwey transition in magnetite

{mathrm{Sr}}_{2}{mathrm{RuO}}_{4}

One-electron singular branch lines of the Hubbard chain

and the nonsuperconductor

Surface characterization and surface electronic structure of organic quasi-one-dimensional charge transfer salts

{mathrm{Sr}}_{1.8}{mathrm{Ca}}_{0.2}{mathrm{RuO}}_{4}

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

probed by bulk-sensitive high-energy angle-resolved photoemission. It is found that there is one square-shaped hole-like, one square-shaped electron-like, and one circle-shaped electron-like Fermi surface in both compounds. These results provide direct evidence for nesting instability giving rise to magnetic fluctuations. Our study clarifies that the electron correlation effects are changed with composition depending on the individual band.

Spin dynamics in the low-dimensional magnet TiOCl

The complex cond. at the (Verwey) metal-insulator transition in Fe3O4 was investigated at terahertz and IR frequencies. In the insulating state, both the dynamic cond. and the dielec. const. reveal a power-law frequency dependence, the characteristic feature of hopping conduction of localized charge carriers. The hopping process is limited to low frequencies only, and a cutoff frequency n1.simeq.8 meV must be introduced for a self-consistent description. On heating through the Verwey transition the low-frequency dielec. const. abruptly decreases and becomes neg. Together with the cond. spectra this indicates the formation of a narrow Drude peak with a characteristic scattering rate of about 5 meV contg. only a small fraction of the available charge carriers. The spectra can be explained assuming the transformation of the spectral wt. from the hopping process to the free-carrier cond. These results support an interpretation of Verwey transition in magnetite as an insulator-semiconductor transition with structure-induced changes in activation energy. [on SciFinder (R)]

Electronic structure of the spin- 1 2 quantum magnet TiOCl

The momentum and energy dependence of the weight distribution in the vicinity of the one-electron spectral-function singular branch lines of the 1D Hubbard model is studied for all values of the electronic density and on-site repulsion U. To achieve this goal, we use the recently introduced pseudofermion dynamical theory. Our predictions agree quantitatively for the whole momentum and energy bandwidth with the peak dispersions observed by angle-resolved photoelectron spectroscopy in the quasi-1D organic conductor TTF-TCNQ.

Heat capacity of the quantum magnet TiOCl

We have thoroughly characterized the surfaces of the organic charge-transfer salts TTF-TCNQ and

Correlation versus surface effects in photoemission of quasi-1D organic conductors

(mathrm{TMTSF}{)}_{2}{mathrm{PF}}_{6}