Competition between Josephson and electromagnetic interactions in Bi sub(2)Sr sub(2)CaCu sub(2)O sub(8) single crystals

We have studied the low magnetic field, high-temperature region of the H-T phase diagram of Bi sub(2)Sr sub(2)CaCu sub(2)O sub(8) (Bi2212) single crystals by measuring magnetic field (H parallel c) induced rf (H sub(rf) parallel a) dissipation in the mixed state. Normally the rf power loss P(H) versus H in a superconductor increases with the increase in the field with a minimum at H identical with 0. However, in Bi2212 the rf loss initially decreases with increasing field, shows a minimum at a certain field H sub(M) > 0 and increases continuously beyond H sub(M). We understand the unusual minimum at H sub(M) as resulting from a competition between (i) the increasing attractive electromagnetic interaction between the pancake vortices (that constitute the vortex line) in adjacent bilayers with the increasing field and (ii) the decreasing Josephson coupling between the CuO bilayers with the increase in the field. The H sub(M)(T) line defines a new boundary in the H-T phase diagram at which the tilt modulus C sub(44) of the 2D vortex stack in Bi2212 is at a maximum. It is found to obey the (1 - t super(2)) super(n) behaviour with the exponent n identical with 3/2 reminiscent of the predicted behaviour of the melting line.