VORTEX DYNAMICS AT RF FREQUENCIES IN BI2SR2CACU2O8 SINGLE-CRYSTALS

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)paral lel to a, H parallel to c, the isothermal magnetic field scans below T -c show that the frequency f(H) of the tank circuit decreases continuo usly with increase in H before saturating at H similar to 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 l owest value for H similar to H-D(T) indicates complete rf penetration land 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 vani shes. Therefore H,(T) represents the decoupling of the adjacent superc onducting 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 obse rved H-D(T) line in many crystals is in excellent agreement with the ( l/t-1) behavior proposed for decoupling. (C) 1998 Elsevier Science B.V .