Angular dependence of c-axis magnetoresistance in Bi2Sr2CaCu2O8+δ single crystals with columnar defects

We measured the angular dependence of the c-axis magnetoresistance ρc(B) of Bi2Sr2CaCu2O8+δ irradiated with heavy ions. At temperatures near 69 K the scaling of ρc(B) with the c-axis magnetic field component Bperpendicular is broken and the in-plane field, Bparallel, affects ρc. At this temperature, at a specific field Bcr∼BΦ/2, magnetoresistance becomes independent of field orientation. This crossing point allows us to estimate the correlation length L of pancake positions along the c axis. We find L/s∼100 at B=Bcr, where s is the interlayer spacing. This provides evidence of strong enhancement of pancake alignment in the vortex liquid in crystals with columnar defects. copyright 1999 The American Physical Society