Ruixing Liang

The c axis reflectance of high-quality single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ has been measured over a wide range of temperatures and frequencies. The x=0.70 crystal has a pseudogap in the conductivity at 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which is present at room temperature. The gap deepens gradually as the temperature is lowered to 10 K with no discontinuity at ${\mathit{T}}_{\mathit{c}}$. In contrast, crystals with x=0.95 show no gap in the normal state and their superconducting state is characterized by low-lying states. These observations are consistent with phase diagrams proposed to explain NMR and neutron data.

The polar Kerr effect in the high-T_(c) superconductor YBa2Cu3O6+x was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr rotations of order approximately 1 microrad appearing near the pseudogap temperature T(*) and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature.

The magnetic field dependence of the electronic density of states at the Fermi level, $N({E}_{F},H)$, is determined in single-crystal Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.95}$ by specific heat measurements. The total specific heat is best described by including two predictions for the electronic specific heat of $d$-wave superconductivity: a ${T}^{2}$ term in zero field and an increased linear term in a magnetic field applied perpendicular to the Cu${\mathrm{O}}_{2}$ planes. The additional linear term, which implies a finite $N({E}_{F},H)$, obeys $N({E}_{F},H)\ensuremath{\propto}{(\frac{H}{{H}_{c2}})}^{\frac{1}{2}}$ as predicted by Volovik for superconductivity with lines of nodes in the gap.

The microwave surface resistance at 2.95 GHz of a very-high-quality crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ exhibits unusual nonmonotonic temperature dependence. After a sharp drop below ${\mathit{T}}_{\mathit{c}}$ by a factor of 5000 the loss rises to a peak at 35 K and then falls at lower temperature. The peak is due to a rapid decrease in the scattering of thermally activated quasiparticles below ${\mathit{T}}_{\mathit{c}}$ and this suppression of scattering suggests that the excitations responsible for the large resistivity of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ are gapped below ${\mathit{T}}_{\mathit{c}}$. A technological implication is that disorder may lower the microwave loss.