Melting of the vortex lattice in ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ in parallel fields

The mixed state c-axis resistivity of ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ 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 $H\parallel b$ and $H\parallel c$ obey the expected relation $H_m{=H}_0(1\mathrm{}-{T/T}_c{)}^n$ with $n=1.5,\hspace{0.5em}{H}_m\mathrm{}\left(T\right)\mathrm{}$ for $H\parallel a$ follows a different T dependence with a lower exponent. This result is consistent with a reduction in the dimensionality of ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ in this field geometry, as observed in normal-state magnetoresistance measurements.