  We explore the occurrence of a magnetic field induced finite size effect on
the specific heat and correlation lengths of anisotropic type-II
superconductors near the zero field transition temperature Tc. Since near the
zero field transition thermal fluctuations are expected to dominate and with
increasing field strength these fluctuations become one dimensional, whereupon
the effect of fluctuations increases, it appears unavoidable to account for
thermal fluctuations. Invoking the scaling theory of critical phenomena it is
shown that the specific heat data of nearly optimally doped YBa2Cu3O7-x are
inconsistent with the traditional mean-field and lowest Landau level
predictions of a continuous superconductor to normal state transition along an
upper critical field Hc2(T). On the contrary, we observe agreement with a
magnetic field induced finite size effect, whereupon even the correlation
length longitudinal to the applied field H cannot grow beyond the limiting
magnetic length L(H). It arises because with increasing magnetic field the
density of vortex lines becomes greater, but this cannot continue indefinitely.
L(H) is then roughly set on the proximity of vortex lines by the overlapping of
their cores. Thus, the shift and the rounding of the specific heat peak in an
applied field is traced back to a magnetic field induced finite size effect in
the correlation length longitudinal to the applied field.
