MAGNETIC-FLUX NOISE IN COPPER-OXIDE SUPERCONDUCTORS

We report on the magnetic flux noise in thin films of YBa2Cu3O7-x (YBC O), Tl2Ca2Ba2Cu3Ox, and TlCa2Ba2Cu3Ox and in crystals of YBCO and Bi2S r2CaCu2O8+x, measured with a Superconducting QUantum Interference Devi ce (SQUID). We ascribe the noise to the motion of flux vortices. In th e low magnetic fields in which the experiments are performed the avera ge vortex spacing always exceeds the superconducting penetration depth . The spectral density of the noise usually scales as 1/f (f is freque ncy) from 1 Hz to 1 kHz and increases with temperature to a peak which is of the same magnitude in all samples, at the transition temperatur e. Furthermore, the noise power increases with the magnitude of the ma gnetic field in which the sample is cooled, with a power-law dependenc e over several decades, whereas a supercurrent well below the critical current density applied to YBCO films suppresses the noise power by a n order of magnitude. Most of the measurements were made on YBCO films , and for this set of samples the noise decreases dramatically as the crystalline quality is improved. A model of thermally activated vortex motion is developed which explains the dependence of the noise on fre quency, temperature, magnetic field, and current. The pinning potentia l is idealized as an ensemble of symmetrical double wells, each with a different activation energy separating the two states. From the noise measurements, this model yields the distribution of pinning energies, the vortex hopping distance, the number density of mobile vortices, a nd the restoring force on a vortex at a typical pinning site. The dist ribution of pinning energies in YBa2Cu3O7-x shows a broad peak below 0 .1 eV. Over narrow temperature intervals, most samples exhibit random telegraph signals in which the flux switches between two discrete leve ls, with activation energies and hopping distances much greater than t hose deduced from the 1/f noise measurements.