Random telegraph signals in high-temperature superconductors

The local magnetization of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ measured with a superconducting quantum-interference device is shown to exhibit random telegraph signals (RTS’s) over narrow ranges of temperature. We believe these signals arise from the thermally activated hopping of single magnetic vortices between two pinning sites. The spectral density of the RTS is Lorentzian; from the temperature dependence of the characteristic time, we deduce pinning energies of 0.17 and 0.20 eV for samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$, respectively, at reduced temperatures of 0.99 and 0.87. The amplitudes of various RTS’s imply flux hopping distances varying from 0.2 to 30 μm. Multiple metastable states and transitions between ‘‘noisy’’ and ‘‘quiet’’ states are also observed.