Magnetic measurements of the upper critical field, irreversibility line, anisotropy, and magnetic penetration depth of grain-aligned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$

We have measured the upper critical field and the irreversibility line of grain-aligned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$, with the magnetic field oriented perpendicular to the ${\mathrm{CuO}}_2$ planes. The upper critical field’s slope, ${\mathit{dH}}_{\mathit{c}2}$/dT, is -1.57 T/K, corresponding to a zero-temperature Ginzburg-Landau (GL) coherence length of 19.5 Å. The irreversibility line obeys a power-law behavior similar to that of 90-K ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Using the lower-critical-field data, we obtain the zero-temperature magnetic penetration depth ${\lambda }_{\mathit{a}\mathit{b}}$(0)=1960 Å and GL parameter ${\mathrm{\kappa }}_{\mathit{c}}$=100.