Studies of nonlinear electrodynamics of high-temperature superconductors

Thesis/Dissertation · Thu Aug 01 04:00:00 EDT 1991

DOI:https://doi.org/10.2172/10102130· OSTI ID:10102130

Lam, Quan-Chiu H. ^[1]

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1. Univ. of California, Berkeley, CA (United States)

Nonlinear electrodynamics of high-T_c superconductors are studied both theoretically and experimentally. For powdered samples, a novel model is presented in which the metallographically observed superconducting grains in the powder are modeled as superconducting current loops of various areas with weak links. Surprising harmonic generation behavior in an arc field, H₁ cos(ωt), is predicted by the model; the power at high harmonics show sharp dips almost periodic in a superposing dc magnetic field, revealing flux quantization in the prototype loops in the model. Such oscillation of the harmonic power in dc magnetic field P_nf(H_dc), is indeed experimentally observed in powdered YBa₂Cu₃O₇. Other experimental aspects also agree with model predictions. For bulk sintered cylindrical samples, a generalized critical state model is presented. In this model, the nonlinear electrodynamics are due to flux-pinning, somewhat similar to low-temperature type-II superconductors, but with a more generalized critical current densities` dependence on magnetic field -- J_c(H)~H$$β\atop{local}$$, with β being an adjustable parameter. Experiments in ac and dc magnetic fields on a sintered cylindrical rod of YBa₂Cu₃O₇ yield unambiguous evidence of independent inter- and intragranular contributions to the complex harmonic permeability$$\tilde{μ}$$_n = μ'_n -iμ''_n. Temperature- dependence measurements reveal that, while the intragranular supercurrents disappear at T_c≥91.2 K, the intergranular supercurrents disappear at T≥86.6 K. This result is, to our knowledge, the first clear measurement of the phase-locking temperature of the 3-D matrix formed by YBa₂Cu₃O₇ grains, which are in electrical contact with one another through weak links.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 10102130
• Report Number(s): LBL--31143; ON: DE92000814
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360204
360207
BARIUM OXIDES
COMPUTER CODES
COPPER OXIDES
CRITICAL CURRENT
CYLINDERS
DATA ACQUISITION
ELECTRODYNAMICS
EXPERIMENTAL DATA
HARMONIC GENERATION
HIGH-TC SUPERCONDUCTORS
MAGNETIC FIELDS
MATHEMATICAL MODELS
SUPERCONDUCTING FILMS
YTTRIUM OXIDES