Pressure-induced lattice instabilities and superconductivity in YBa2Cu4O8 and optimally doped YBa2Cu3O7-{\delta}

Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate the pressure-induced lattice instabilities that are accompanied by T$_{\rm c}$ anomalies in YBa$_{\rm 2}$Cu$_{\rm 4}$O$_{\rm 8}$, in comparison with the optimally doped YBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm 7-\delta}$ and the non-superconducting PrBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm 6.92}$. In the first two superconducting systems there is a clear anomaly in the evolution of the lattice parameters and an increase of lattice disorder with pressure, that starts at $\approx3.7 GPa$ as well as irreversibility that induces a hysteresis. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. In complete agreement with the structural data, the micro-Raman data of the superconducting compounds show that the energy and width of the A$_{\rm g}$ phonons show anomalies at the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O$_{pl}$ bond length exhibits a strong contraction and correlate with the non-linear pressure dependence of T$_{\rm c}$. This is not the case for the non superconducting Pr sample, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.Comment: 6 pages, 7 figure

Fetal Ossification and Staining in the Chick Embryo

This 32 page thesis examines fetal ossification and staining in the chick embryo, including development, incubation, formation of the mesoderm and skeleton and methods for removal and fixation

08201 Abstracts Collection -- Design and Analysis of Randomized and Approximation Algorithms

From 11.05.08 to 16.05.08, the Dagstuhl Seminar 08201 ``Design and Analysis of Randomized and Approximation Algorithms\u27\u27 was held in the International Conference and Research Center (IBFI), Schloss Dagstuhl. During the seminar, several participants presented their current research work, and ongoing work and open problems were discussed. Abstracts of the presentations which were given during the seminar as well as abstracts of seminar results and ideas are put together in this paper. The first section describes the seminar topics and goals in general. Links to extended abstracts or full paper are provided, if available

Mass enhancement, correlations, and strong coupling superconductivity in the beta-pyrochlore KOs2O6

To assess electron correlation and electron-phonon coupling in the recently discovered beta-pyrochlores KOs2O6 and RbOs2O6, we have performed specific heat measurements in magnetic fields up to 14 T. We present data from high quality single crystalline KOs2O6, showing that KOs2O6 is a strong coupling superconductor with a coupling parameter lambda_ep \approx 1.0 to 1.6 (RbOs2O6: lambda_ep \approx 1). The estimated Sommerfeld coefficient of KOs2O6, gamma=76 to 110 mJ/(mol K^2), is twice that of RbOs2O6 [gamma=44 mJ/(mol K^2)]. Using strong-coupling corrections, we extract useful thermodynamic parameters of KOs2O6. Quantifying lambda_ep allows us to determine the mass enhancement over the calculated band electronic density of states. A significant contribution in addition to the electron-phonon term of lambda_c=1.7 to 4.3 is deduced. In an effort to understand the origin of the enhancement mechanism, we also investigate an unusual energetically low-lying phonon. There are three phonon modes per RbOs2O6, suggestive of the phonon source being the rattling motion of the alkali ion. This dynamic instability of the alkali ions causes large scattering of the charge carriers which shows up in an unusual temperature dependence of the electrical resistivity.Comment: Accepted for publication in PR

Magnetic field dependence of the oxygen isotope effect on the magnetic penetration depth in hole-doped cuprate superconductors

The magnetic field dependence of the oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth \lambda_{ab} was studied in the hole-doped high-temperature cuprate superconductors YBa_2Cu_4O_8, Y_0.8Pr_0.2Ba_2Cu_3O_7-\delta, and Y_0.7Pr_0.3Ba_2Cu_3O_7-\delta. It was found that \lambda_ab for the ^{16}O substituted samples increases stronger with increasing magnetic field than for the ^{18}O ones. The OIE on \lambda_ab decreases by more than a factor of two with increasing magnetic field from \mu_0H=0.2 T to \mu_0H=0.6 T. This effect can be explained by the isotope dependence of the in-plane charge carrier mass m^\ast_{ab}.Comment: 4 pages, two figure

Manifestations of fine features of the density of states in the transport properties of KOs2O6

We performed high-pressure transport measurements on high-quality single crystals of KOs2O6, a beta-pyrochlore superconductor. While the resistivity at high temperatures might approach saturation, there is no sign of saturation at low temperatures, down to the superconducting phase. The anomalous resistivity is accompanied by a nonmetallic behavior in the thermoelectric power (TEP) up to temperatures of at least 700 K, which also exhibits a broad hump with a maximum at 60 K. The pressure influences mostly the low-energy electronic excitations. A simple band model based on enhanced density of states in a narrow window around the Fermi energy (EF) explains the main features of this unconventional behavior in the transport coefficients and its evolution under pressure