Melting of the vortex lattice in YBa2Cu4O8 in parallel fields

The mixed state c-axis resistivity of YBa2Cu4O8 has been measured with 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 transition, associated with the first-order melting of an anisotropic three-dimensional Vortex lattice. While the melting lines for H parallel to b and 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 T dependence with a lower exponent. This result is consistent with a reduction in the dimensionality of YBa2Cu4O8 in this field geometry, as observed in normal-state magnetoresistance measurements. [S0163-1829(99)50418-9].