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\underline{LONGITUDINAL ELASTIC CORRELATION LENGTH OF FLUX LINES }
  \underline{ALONG THE $c$-AXIS IN Bi-2212 SINGLE CRYSTAL}\\
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\underline{M. Kiuchi}, H. Yamato and T. Matsushita\\
Department of Computer Science and Electronics,\\
Kyushu Institute of Technology,\\
680--4 Kawazu, Iizuka 820, Japan\\
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The longitudinal elastic correlation length, $l_{\rm 44}$,
of flux lines in a single crystal Bi-2212 superconductor is
measured using the Campbell method under a magnetic field
parallel to the $c$-axis. It is found that $l_{\rm 44}$ is
of the order of $10 \mu$m and much longer than the distance
between the CuO$_{2}$ layers. In addition, $l_{\rm 44}$
increases with increasing magnetic field and/or temperature. 
These results agree well with the pinning correlation length
$(C_{\rm 44}/\alpha _{\rm L})^{1/2}$ estimated using the
measured critical current density, $J_{\rm c}$, where
$C_{\rm 44}=B^{\rm 2}/\mu _{\rm 0}$ is the tilt modulus of flux
line lattice, $\alpha _{\rm L}=2\pi J_{\rm c}B/a_{\rm f}$ is
the Labusch parameter and $a_{\rm f}$ is the flux line
spacing. The apparent pinning potential is estimated from
the magnetic relaxation and compared with the result of
collective flux creep in which the obtained $J_{\rm c}$ and
$l_{\rm 44}$ are used. Agreement is satisfactory. It is
concluded from these results that the flux lines are
strongly coupled over a fairly long distance along the
$c$-axis.

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