Thorium-229 is a unique case in nuclear physics: it presents a metastable first excited state 229m Th, just a few electronvolts above the nuclear ground state. This so-called isomer is accessible by VUV lasers, which allows transferring the amazing precision of atomic laser spectroscopy to nuclear physics. Being able to manipulate the 229 Th nuclear states at will opens up a multitude of prospects, from studies of the fundamental interactions in physics to applications as a compact and robust nuclear clock. However, direct optical excitation of the isomer or its radiative decay back to the ground state has not yet been observed, and a series of key nuclear structure parameters such as the exact energies and half-lives of the low-lying nuclear levels of 229 Th are yet unknown. Here we present the first active optical pumping into 229m Th. Our scheme employs narrow-band 29 keV synchrotron radiation to resonantly excite the second excited state, which then predominantly decays into the isomer. We determine the resonance energy with 0.07 eV accuracy, measure a half-life of 82.2 ps, an excitation linewidth of 1.70 neV, and extract the branching ratio of the second excited state into the ground and isomeric state respectively. These measurements allow us to re-evaluate gamma spectroscopy data that have been collected over 40 years.

For the Cu02 square lattice, as a parent system of high-T, superconductors, a numerical study has suggested that the effective-spin model contains a large cyclic four-spin exchange interaction J,. This paper investigates the effects of J, upon magnetic Raman scattering and upon properties of the ground state by an exact numerical method mainly for a 16-site cluster. It is found that the main Raman peak with the Bzg symmetry is shifted by 20% to the lower-energy side with a realistic magnitude for J"which is about one fourth the size of the nearest-neighbor exchange J. Accordingly a value of J extracted from analyses of experimental data becomes larger by 10% than that estimated with the use of the Heisenberg model. Furthermore the four-spin exchange enhances a shoulder due to multimagnon states at an energy of about 4J, and this result compares favorably with the experimental line shape. Detailed discussion is given on the effect of the four-spin exchange upon the ground-state properties such as the staggered magnetization, the ground-state energy, weight of various spin configurations, and the spin-spin correlation function.

Polarization analysis of the transmitted beam has been performed in a reentrant spin-glass system (Fe& "Mn")75P]6B6A13 for several concentrations x starting from the true ferromagnet to the true spin glass. The influence of some important parameters like the temperature, the sample thickness, the neutron wavelength, and the applied magnetic field, as well as the cooling conditions, has been checked. Large ferromagnetic domains of about 30 pm size are evident in the nonfrustrated alloy (x=0.07). In the weakly frustrated samples (0.22(x &0.26), smaller domains of about 4 to 6 pm are observed, which persist at low temperature. In the strongly frustrated x=0.30 sample, we observe a decrease of the domain size or of the internal magnetization at low temperature. The results in the weakly frustrated alloys support the coexistence at low temperature of a long-range longitudinal order together with frozen transverse spin components, in good agreement with the mean-field predictions. The limits of the infinite-range mean-field model are shown in the sample close to the critical point.

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