MAGNETIC-FIELD-INDUCED DIMENSIONAL CROSSOVER IN THE NORMAL-STATE OF YBA2CU4O8

YBa2Cu4O8 is unique among underdoped cuprates in that its low-T c-axis response is metallic. When a magnetic field is applied perpendicular to the CuO chains (B parallel to a), however, the c-axis resistivity r ho(c) changes dramatically and, at sufficiently high fields, rho(c)(T) begins to show localized behavior. This observation reveals the criti cal role of the CuO chains in metallizing the c axis in YBa2Cu4O8 In t he field range where rho(c)(T) is insulating, Delta rho(c)/rho(c) vari es linearly rather than quadratically with held, suggesting a new magn etoconductivity regime in a state of lower dimensionality.