Magnetic flux noise in copper oxide superconductors

Thesis/Dissertation · Tue Jan 01 04:00:00 EST 1991

OSTI ID:7045265

Ferrari, M J

Magnetic flux noise and flux creep in thin films and single crystals of YBa[sub 2]Cu[sub 3]-O[sub 7[minus]x], Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+x], 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] are measured with a superconducting quantum interference device (SQUID). The noise power spectrum generally scales as 1/f from 1 Hz to 1 kHz, increases with temperature, and decreases in higher-quality films. It is proportional to the magnetic field B in which the sample was cooled in the range 0.1 mT < B < 3 mT. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, current, and applied magnetic field. The pinning potential is idealized as an ensemble of double wells, each with a different activation energy separating the two states. This model yields the distribution of pinning energies in the samples, the vortex hopping distance, the number density of mobile 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[minus]x] shows a broad peak below 0.1 eV. The small ambient magnetic field, and the detection of noise even in the absence of a driving force, insure that the measured pinning energies are characteristic of isolated vortices near thermal equilibrium. The observed vortex density in fields much less than 0.1 mT is too large to be explained by the ambient field suggesting a mechanism which produces trapped vortices. Vortex motion is one limitation on the sensitivity of practical SQIDs and flux transformers fabricated from copper oxide superconductors. A method involving a circulating supercurrent is introduced to improve this sensitivity. Signals produced by the motion of a single vortex are also detected, from which the hopping distances and activation energy can be extracted.

• Research Organization: California Univ., Berkeley, CA (United States)
• OSTI ID: 7045265
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360204 -- Ceramics
Cermets
&amp; Refractories-- Physical Properties
360207* -- Ceramics
Cermets
&amp; Refractories-- Superconducting Properties-- (1992-)
COPPER COMPOUNDS
CUPRATES
ELECTRONIC EQUIPMENT
EQUIPMENT
FLUXMETERS
HIGH-TC SUPERCONDUCTORS
MAGNETIC FLUX
MEASURING INSTRUMENTS
MICROWAVE EQUIPMENT
NOISE
OXYGEN COMPOUNDS
SQUID DEVICES
SUPERCONDUCTING DEVICES
SUPERCONDUCTORS
TEMPERATURE DEPENDENCE
TRANSITION ELEMENT COMPOUNDS