Anisotropic 1/f noise and motion of magnetic vortices in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Measurements of the 1/f noise have been performed on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals for currents in the a-b plane and along the c axis. In the normal state, the normalized noise spectral density ${\mathit{S}}_{\mathit{v}}$/${\mathit{V}}^2$ in the a-b plane is comparable with that along the c axis. In the superconducting transition region, a large enhancement of ${\mathit{S}}_{\mathit{v}}$/${\mathit{V}}^2$ is observed along the c axis while no such enhancement is observed in the a-b plane. Our experimental data suggest that while the 1/f noise in the normal state may be due to structural defects, a completely different noise source exists in the superconductig transition region. We discuss the 1/f noise behavior in the superconducting transition region in terms of the anisotropic nature of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and the motion of magnetic vortices.