Vortex dynamics at rf frequencies in Bi₂Sr₂CaCu₂O₈ single crystals

Abstract

We have studied the low magnetic field high temperature region of the H-T phase diagram of Bi₂Sr₂CaCu₂O₈ single crystals using the technique of non-resonant rf response at a frequency of 20 MHz. With H_rf||a, H||c, the isothermal magnetic field scans below T_C show that the frequency f(H) of the tank circuit decreases continuously with increase in H before saturating at H~H_D(T). Such a decrease in f(H) reflects increasing rf penetration into the weakly screened region between CuO bilayers. The saturation of f(H) at its lowest value for H~H_D(T) indicates complete rf penetration (and hence the disappearance of field dependence) due to the vanishing of the screening rf currents I_rf^c in those regions or equivalently when the phase coherence between adjacent superconducting layers vanishes. Therefore H_D(T) represents the decoupling of the adjacent superconducting bilayers, and hence also a 3D to 2D decoupling transition of the vortex structure. Simultaneous monitoring of the field dependent rf power dissipation P(H) shows a maximum in dP/dH at H_D(T). The observed H_D(T) line in many crystals is in excellent agreement with the (1/t-1) behavior proposed for decoupling.