Flux dynamics of high-temperature superconductors
Flux pinning and motion in YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] crystals are studied by ac magnetic, SQUID and resistive methods. The ac response of YBCO is very anisotropic for field orientations. For H//c, in typical fields, the ac response can be linear for vortex displacement up to about 50 nm. For H//ab, the ac response is nonlinear for small vortex displacement (<0.5 nm). The large reversible vortex displacements for H//c suggest that the individual oxygen vacancies cannot be responsible for pinning. The extremely small irreversible vortex displacement for H//ab suggests that there is a very narrow-sized barrier for vortices moving across the layered structure. The linear-response ac susceptibility can be described by the formula which was used for normal-metal with a classical skin-depth. The nonlinear response can persist up to T[sub c](H) at high frequency and high amplitude, suggesting that the concept of vortex lines is relevant and pinning effect is present in high temperature regime where linear resistivity is measurable and magnetic hysteresis vanished. The presence was identified of peak-effect in YBCO crystals by an anomalous dip in temperature dependence of real part of the nonlinear ac susceptibility. The peak-effect is more pronounced for H//ab than for H//c, and disappears for field below 1 kOe. The details of the peak-effect can be understood, if the peak-effect is due to the softening of the vortex-lattice. The nonlocal effect of the tilt modulus C[sub 44] cannot be responsible. Flux creep in YBCO is studied by a SQUID method. The flux-creep characteristic is not logarithmic in time. A method of analyzing flux-creep data is given to distinguish various models. The time dependence can best fit data is a power-law M [approximately] (1 + t/[tau])[sup [minus][alpha]] or a stretched exponential M [approximately] exp([minus](t/[tau])[sup [beta]]). The relevance the flux creep to thermal activation and avalanche processes is discussed.
- Research Organization:
- Connecticut Univ., Groton, CT (United States)
- OSTI ID:
- 7267971
- Country of Publication:
- United States
- Language:
- English
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75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ALKALINE EARTH METAL COMPOUNDS
ALLOYS
BARIUM COMPOUNDS
BARIUM OXIDES
CHALCOGENIDES
COPPER COMPOUNDS
CRYSTAL LATTICES
CRYSTAL STRUCTURE
CUPRATES
HIGH-TC SUPERCONDUCTORS
INTERMETALLIC COMPOUNDS
MAGNETIC FLUX
MAGNETIC PROPERTIES
MAGNETIC SUSCEPTIBILITY
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
SUPERCONDUCTORS
TRANSITION ELEMENT COMPOUNDS
YTTRIUM COMPOUNDS
YTTRIUM OXIDES