J. Giapintzakis

We have measured the resistivity tensor in one nearly twin-free crystal and one twin-free crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. The first crystal was grown with a large twin-free region. We removed the twins in the second crystal by applying a uniaxial stress. Using a modified Montgomery technique, we measured the electrical resistivities in all three crystal directions and found them to be linear in temperature above the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. Our results for ${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$ and ${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$ at room temperature are among the lowest that have been reported in the literature, indicating that our samples are of high quality. The ratio ${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$ is independent of temperature between 150 and 275 K, and its value, 2.2\ifmmode\pm\else\textpm\fi{}0.2, indicates that the Cu-O chains contribute 60% of the current when the electric field is parallel to them, with the rest of the current being contributed by the Cu-O planes. This result, at zero frequency, is in good agreement with the recent conductivity at infrared frequencies obtained by Schlesinger et al.

We have measured the magnetic field modulation of the critical current of YBCO-Au-Pb Josephson junctions fabricated on the $a$ and $b$ edges and straddling the $a\ensuremath{-}b$ corners of YBCO single crystals. In corner junctions, the critical current drops sharply at zero applied field and increases as the field is increased in either direction, in sharp contrast to the Fraunhofer diffraction patterns observed for single edge junctions. This behavior signifies a sign change of the order parameter between the $a$ and $b$ directions, strong direct evidence that the superconducting pairing state of YBCO has ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry.

Optical and Raman-scattering studies of a-, b-, and c-axis ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ between 0.025 and 5.5 eV are presented as a function of doping, and compared to the results of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ and ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$. Our doping-dependence studies show that the redistribution of spectral weight in the ${\mathrm{CuO}}_{2}$ planes of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ differs significantly from that seen in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$. We also find that the redistribution of spectral weight in the cuprates is primarily responsible for the loss of two-magnon Raman-scattering intensity with doping. Finally, we show that bound-carrier contributions comprise a significantly larger fraction of the spectral weight below 1 eV in lower-${\mathit{T}}_{\mathit{c}}$ cuprates such as the 2:1:4 compounds than in higher-${\mathit{T}}_{\mathit{c}}$ cuprates such as ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$.We suggest that the low-frequency conductivity (1 eV) in the 2:1:4 compounds is most appropriately described by a two-component picture, while that in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ is adequately described as a single component of strongly interacting carriers. In the metallic phase we find several interesting consequences of a single-component interpretation of the optical data in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$, such as a linear-in-\ensuremath{\omega} frequency-dependent scattering rate and an increase in the interaction strength with decreased carrier density. Finally, we show that the c-axis optical response in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}90 K) is characterized by a c-axis polarized Raman continuum and a Drude conductivity arising from interbilayer charge transport along the c direction. With decreased doping, the c-axis Drude response decreases dramatically, indicating a decoupling of the ${\mathrm{CuO}}_{2}$ plane bilayers in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$. By comparison, the ab-plane optical response is not strongly influenced by interbilayer decoupling, suggesting that the unusual ab-plane charge dynamics in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ persist in nearly isolated ${\mathrm{CuO}}_{2}$ plane bilayers.

We report production of nanostructured magnetic carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in $\mathrm{Ar}$ atmosphere. A combination of characterization techniques revealed that the system contains both $s{p}^{2}$ and $s{p}^{3}$ bonded carbon atoms. The material is a form of carbon containing graphite-like sheets with hyperbolic curvature, as proposed for ``schwarzite.'' The foam exhibits ferromagnetic-like behavior up to $90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets.