The effect of the magnetic field on the critical behavior of Sr0:9La0:1CuO2 is explored in terms of reversible magnetization data. As the correlation length transverse to the magnetic field Hi,applied along the i-axis, cannot grow beyond the limiting magnetic length LHi, related to the average distance between vortex lines, one expects a magnetic field induced finite size effect. Invoking the scaling theory of critical phenomena we provide clear evidence for this effect. It implies that in type II superconductors there is a 3D to 1D crossover line Hpi(T). Consequently, below Tc and above Hpi(T) uperconductivity is confined to cylinders with diameter LHi(1D). Accordingly, there is no continuous phase transition in the (H,T)-plane along the Hc2-lines as predicted by the mean-field treatment.
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