Evidence for line vortices in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ (BSCCO) crystals were irradiated with 5.8-GeV Pb ions to produce columnar defects either along the c direction or at 45° with respect to it. Magnetization curves of the irradiated BSCCO crystals were measured with the applied field at different directions relative to the defects. The width of the magnetization curves, which reflects the pinning strength, is found to be the largest for fields along the defects. This unidirectional enhancement clearly indicates the line nature of the vortices in BSCCO.