Vortex lattice melting in YBa2Cu4O8 with H parallel to ab

We report c-axis resistivity measurements in the mixed state of YBa2Cu4O8 w ith the magnetic field applied parallel to the a-, b-, and c-axes. For all orientations of the magnetic field, a "kink" is observed in the resistive t ransition, associated with the first-order melting of an anisotropic three- dimensional vortex lattice. Whereas the melting lines for H parallel to b a nd H parallel to c obey the expected relation H-m = H-0(1 - T/T-c)(n) with n = 1.5, H-m(T) for H parallel to a follows a different temperature depende nce with a lower exponent. This result is consistent with a suppression of the melting line due to a reduction in the dimensionality of YBa2Cu4O8 in t his field geometry, as observed in normal-state magnetoresistance measureme nts.