Magnetic flux noise in copper oxide superconductors

• Published in: Journal of low temperature physics Vol. 94; no. 1-2; pp. 15 - 61
• Main Authors: FERRARI, M. J, JOHNSON, M, WELLSTOOD, F. C, KINGSTON, J. J, SHAW, T. J, CLARKE, J
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer 1994
• Subjects: 360207 - Ceramics, Cermets, & Refractories- Superconducting Properties- (1992-); ALKALINE EARTH METAL COMPOUNDS; BARIUM COMPOUNDS; BARIUM OXIDES; BISMUTH COMPOUNDS; BISMUTH OXIDES; CALCIUM COMPOUNDS; CALCIUM OXIDES; CHALCOGENIDES; Condensed matter: electronic structure, electrical, magnetic, and optical properties; COPPER COMPOUNDS; COPPER OXIDES; Exact sciences and technology; Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.); HIGH-TC SUPERCONDUCTORS; MAGNETIC FLUX; MATERIALS SCIENCE; NOISE; OXIDES; OXYGEN COMPOUNDS; Physics; Properties of type I and type II superconductors; STRONTIUM COMPOUNDS; STRONTIUM OXIDES; Superconductivity; SUPERCONDUCTORS; THALLIUM COMPOUNDS; THALLIUM OXIDES; TRANSITION ELEMENT COMPOUNDS; Vortex lattices ,flux pinning, flux creep; YTTRIUM COMPOUNDS; YTTRIUM OXIDES; Magnetic flux; Hopping; Magnetization; Noise; Vortices; High-Tc superconductors; Pinning; 1/f noise; Experimental study
• ISSN: 0022-2291; 1573-7357
• DOI: 10.1007/BF00755416

The authors report on the magnetic flux noise in thin films of YBa[sub 2]Cu[sub 3]O[sub 7-x] (YBCO), Tl[sub 2]Ca[sub 2]Ba[sub 2]Cu[sub 3]O[sub x], and TlCa[sub 2]Ba[sub 2]Cu[sub 3]O[sub x] and in crystals of YBCO and Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+x], measured with a Superconducting QUantum Interference Device (SQUID). They ascribe the noise to the motion of flux vortices. In the low magnetic fields in which the experiments are performed the average vortex spacing always exceeds the superconducting penetration depth. The spectral density of the noise usually scales as 1/f (f is frequency) 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 temperature. Furthermore, the noise power increases with the magnitude of the magnetic field in which the sample is cooled, with a power-law dependence over several decades, whereas a supercurrent well below the critical current density applied to YBCO films suppresses the noise power by an order of magnitude. Most of the measurements were made on YBCO films. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, magnetic field, and current. The pinning potential 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 mobil vortices, and the restoring force on a vortex at a typical pinning site. The distribution of pinning energies in YBa[sub 2]Cu[sub 3]O[sub 7-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 levels with activation energies and hopping distances much greater than those deduced from the 1/f noise measurements.