Magnetoconductivity due to quantum interference in strongly underdoped YBa2Cu3Ox - art. no. 104515

We report magnetoconductivity measurements on YBa2Cu3Ox (x=6.25 and 6.36) s ingle crystals. Our main result is that both the in-plane Delta sigma (ab) and out-of-plane Delta sigma (c) magnetoconductivities exhibit the field de pendence characteristic of "two-dimensional" quantum interference in applie d magnetic fields H parallel toc. Namely, Delta sigma (c,ab) proportional t o ln H/H-0>0, with Delta sigma (c)/sigma (c) substantially greater than Del ta sigma (ab)/sigma (ab). We interpret this result as an evidence of interl ayer incoherence in these crystals. so that the phase-coherent trajectories are mostly confined to one bilayer.