Ivica Zerec

Four-well tunneling States and elastic response of clathrates.

We present resonant ultrasound elastic constant measurements of the Eu8Ga16Ge30 and Sr8Ga16Ge30 clathrates. The elastic response of the Eu clathrate provides clear evidence for the existence of a new type of four-well tunneling states, described by two nearly degenerate four level systems (FLS). The FLSs are closely linked with the fourfold split positions of Eu known from neutron diffraction density profiles. Using a realistic potential we estimate the tunneling frequencies and show that the energy gap between the two FLSs explains the observed harmonic oscillator type specific heat. In addition the quadrupolar interaction of FLSs with elastic strains explains the pronounced depression observed in elastic constant measurements. In the case of the Sr clathrate, we explain the elastic properties assuming the same type of interaction, but with Sr Einstein mode.

Kondo lattice model studied with the finite temperature Lanczos method

We investigate the Kondo Lattice Model on 2D clusters using the Finite Temperature Lanczos Method. The temperature dependence of thermodynamic and correlations functions are systematically studied for various Kondo couplings JK. The ground state value of the total local moment is presented as well. Finally, the phase diagrams of the finite clusters are constructed for periodic and open boundary conditions. For the two boundary conditions, two different regimes are found for small JK/t, depending on the distribution of non-interacting conduction electron states. If there are states within JK around the Fermi level, two energy scales, linear and quadratic in JK, exist. The former is associated with the onsite screening and the latter with the RKKY interaction. If there are no states within JK around the Fermi level, the only energy scale is that of the RKKY interaction. Our results imply that the form of the electron density of states (DOS) plays an important role in the competition between the Kondo screening and the RKKY interaction. The former is stronger if the DOS is larger around the Fermi level, while the latter is less sensitive to the form of the DOS.

Band-structure calculations for Ba6Ge25 and Ba4Na2Ge25 clathrates

Electronic band structures for Ba

Band structure calculations for Ba

_{6}

_{6}

Ge

Ge

_{25}

_{25}

and Ba

and Ba

_{4}

_{4}

Na

Na

_{2}