Peak effect and flux flow Page: 4 of 23
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dependent on FLL history in this non-linear region also, with factors
of two variation in the J at low E possible depending on whether the
FLL was created by decreasing H from above Hc2, increasing H from zero
or cooling the sample from above Tc in constant H. I will discuss these
history effects, which imply that the critical current depends on the
microstructure of the FLL, in more detail under the peak effect as they
provide important evidence for and against certain peak effect mecha-
nisms. There is also neutron diffraction evidence of changes in FLL
structure in the non-linear E-J regime. A broadening of the mosaic
spread of the FLL as the current is increased into this regime was ob-
served in Nb single crystals. The pinning in these samples, however,
was very low (mostly surface pinning) and the E-J curve was not history
dependent. Further neutron diffraction studies of the FLL structure
under conditions of higher pinning and E-J history dependence would be
very useful in unravelling the- true nature of this phenomenon.
It should be pointed out that the ideal flux flow behavior with
pinning referred to above has only been experimentally demonstrated in
specimens with relatively low pinning. For harder superconductors the
heat generated as a result of even small amounts of flux motion is
enough to raise the temperature of the superconductor significantly and
no linear region of flux flow is observed. Experiments using current
pulses with a low duty cycle could in principle measure the E-J curve of
a hard superconductor under approximately isothermal conditions. In
view of the importance of the flux flow concept, and its extension by
many of us to the interpretation of the volume pinning force P. as the
dynamic pinning force, such experiments should be performed.
The Dynamic Pinning Force
Somewhat after the experiments of Kim et al., Yamafuji and Irie4
pointed out that regarding the Pv, derived from Jp, as a static pinning
force was.incorrect. Rather they showed that the offset of the flux
flow E-J curve from the zero pinning ideal viscous flow curve must
arise due to the hysteretic power loss in the local velocity fluctuation
of the FLL as pins are encountered and overcome By balancing power
*Subsequently others have demonstrated that not all pins will contribute
to this h steresis but only those whose maximum force exceeds a certain
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Kramer, E.J. Peak effect and flux flow, article, January 1, 1974; Illinois. (digital.library.unt.edu/ark:/67531/metadc1017940/m1/4/: accessed February 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.