SUPPRESSED RF DISSIPATION IN AG-107(17-IRRADIATED BI2SR2CACU2O8 SINGLE-CRYSTALS BY ENHANCED FLUX-LINE TILT MODULUS() ION)

We have studied the isothermal, magnetic field (H parallel to c) depen dent rf power P(H) dissipation (H(rf)parallel to a) in the superconduc ting state of Bi2Sr2CaCu2O8 single crystals prior to and after irradia tion with 250 MeV Ag-107(17+) ions. In the pristine state, P(H) shows an initial decrease with increase in field, reaches a minimum at H-M(T ) and increases monotonically for H>H-M(T). This behavior arises when the electromagnetic coupling between the pancake vortices in adjacent CuO layers becomes dominant on increasing the field and minimizes the distortions of the flux lines by confining the 2D vortices. In the pos t irradiated state, such an initial decrease and the minimum in P(H) i s not observed but only a much reduced rf dissipation that monotonical ly increases with held from H=0 onwards is seen. We attribute this dif ference to the strong enhancement of the tilt modulus C-44 Of the flux lines on irradiation when the pancake vortices in adjacent CuO bilaye rs are pinned along the track forming a well-stacked flux line in the field direction (parallel to c) We have also observed that the rf diss ipation disappears at a certain temperature T-sf, at which the normal core of the flux line becomes commensurate with the columnar track dia meter. (C) 1998 American Institute of Physics.