Interplay between electronic correlations and coherent structural dynamics during the monoclinic insulator-to-rutile metal phase transition in VO2

We report direct observations of the structural and electronic dynamics of the photoinduced insulator-metal transition in VO(2), by means of time-resolved photoemission spectroscopy. These observations provide new insights into the processes involved in this transition. Slightly above the threshold of the photoinduced phase transition, the different response times of the electrons and the lattice reveal the electronic nature of the band gap collapse. At high excitation densities, we find that the phase transition is induced nonthermally in an ultrashort time scale. Moreover, we identify different V 3p dynamics indicating the existence of different structural pathways. These results represent a clear demonstration of the potential of time-resolved core level photoelectron spectroscopy to study ultrafast dynamics in condensed matter.

Reentrant superconductivity in Nb/Cu1-xNix bilayers

We report on the first observation of a pronounced reentrant superconductivity phenomenon in a superconductor/ferromagnet layered system. The results were obtained using a superconductor/ferromagnetic-alloy bilayer of Nb/Cu(1-x)Ni(x). The superconducting transition temperature T(c) drops sharply with increasing thickness dCuNi) of the ferromagnetic layer, until complete suppression of superconductivity is observed at d(CuNi) approximately equal to 4 nm. Increasing the Cu(1-x)Ni(x) layer thickness further, superconductivity reappears at d(CuNi) > or =13 nm. Our experiments give evidence for the pairing function oscillations associated with a realization of the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov-like state in the ferromagnetic layer.

Unconventional anisotropic superexchange in alpha'-NaV2O5

The strong line broadening observed in electron spin resonance on NaV2O5 is found to originate from an unusual type of the symmetric anisotropic exchange interaction with simultaneous spin-orbit coupling on both sites. The microscopically derived anisotropic exchange constant is almost 2 orders of magnitude larger than the one obtained from conventional estimations. Based on this result we systematically evaluate the anisotropy of the ESR linewidth in terms of the symmetric anisotropic exchange only, and we find microscopic evidence for precursor effects of the charge ordering already below 150 K.

Spin dynamics and charge order in beta-Na1/3V2O5

We present detailed electron-spin resonance investigations on single crystals of the one-dimensional vanadium-oxide bronze beta-Na(1/3)V2O5. From the angular dependence of the g value it can be concluded that the electrons are primarily located on the V1 zigzag chains. The anisotropy of the linewidth, which is determined by the intrachain symmetric anisotropic exchange interaction, favors statistic electron distribution in the metallic and blockwise charge-order in the insulating phase. The temperature dependence of the linewidth indicates the opening of a charge gap at the metal-to-insulator transition at T(MI)=132 K.

Charge order in NaV2O5 studied by EPR

We present angular dependent EPR measurements in NaV2O5 at X-band frequencies in the temperature range 4.2</=T</=670 K. A detailed analysis in terms of the antisymmetric Dzyaloshinski-Moriya and the anisotropic exchange interactions yields the following scheme of charge order: On decreasing temperature a quarter-filled ladder with strong charge disproportions, existing for T>/=100 K, is followed by zigzag charge-order fluctuations which become long range and static below T(SP) = 34 K.

[Long-term integrated radon measurement on a small scale in Vienna and communication with the population]

Long-term integrated radon measurements (alpha-track) were carried out in two housing blocks in Vienna diagonally across from each other. The arithmetic mean was 41 becquerels/cubic metre (= Bq/m3), concentrations ranging from 157 to 9 Bq/m3. The largest variation found within one block came up to a factor of 13, within one house to a factor of 8 and within one floor of one and the same house to a factor of 4. The measurements gave the opportunity to practice risk communication with the residents of the two housing blocks.