Suppressed rf dissipation in ¹⁰⁷Ag¹⁷⁺ ion irradiated Bi₂Sr₂CaCu₂O₈ single crystals by enhanced flux line tilt modulus

Abstract

We have studied the isothermal, magnetic field (H c) dependent rf power P(H) dissipation (Hrf a) in the superconducting state of Bi₂Sr₂CaCu₂O₈ single crystals prior to and after irradiation with 250 MeV ¹⁰⁷Ag¹⁷⁺ ions. In the pristine state, P(H) shows an initial decrease with increase in field, reaches a minimum at HM(T) and increases monotonically for H>HM(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 post irradiated state, such an initial decrease and the minimum in P(H) is not observed but only a much reduced rf dissipation that monotonically increases with field from H = 0 onwards is seen. We attribute this difference to the strong enhancement of the tilt modulus C₄₄ of the flux lines on irradiation when the pancake vortices in adjacent CuO bilayers are pinned along the track forming a well-stacked flux line in the field direction (||c). We have also observed that the rf dissipation disappears at a certain temperature T_sf, at which the normal core of the flux line becomes commensurate with the columnar track diameter.