5.1 Flux Pinning Mechanism
73
in a smaller value than that in (5.3). For this reason, it is not easy to estimate the
pinning force density.
Figure 5.4 shows the observed relationship between the elementary pinning force
and the contribution of one pinning center to the pinning force density for Nb at 4.2 K
and at reduced magnetic field B/μ 0 H c2 = 0.55 [1]. The straight line with a slope of
unity is the direct summation given by (5.3). This result shows that the pinning force
density obeys the relationship called the linear summation:
F p = η p N p f p
(5.4)
in the case of strong pinning. Here, η p is a constant smaller than 1 and is called the
pinning efficiency. When the pinning is weak, the pinning force density obeys the
relationship
Fig. 5.4 Relationship between the elementary pinning force (f p ) and the contribution of one pinning
center to the pinning force density (F p /N p ) for Nb at 4.2 K and at reduced magnetic field B/μ 0 H c2 =
0.55 [1]. The threshold value of the elementary pinning force will be discussed in Sect. 5.4