Competition between josephson and electromagnetic interactions in Bi₂Si₂CaCu₂O₈ single crystals

Abstract

We have studied the low magnetic field, high-temperature region of the H-T phase diagram of Bi₂Si₂CaCu₂O₈ (Bi2212) single crystals by measuring magnetic field (H || c) induced rf(H_rf || 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 = 0. However, in Bi2212 the rf loss initially decreases with increasing field, shows a minimum at a certain field H_M >0and increases continuously beyond H_M . We understand the unusual minimum at H_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_M (T) line defines a new boundary in the H-T phase diagram at which the tilt modulus C₄₄ of the 2D vortex stack in Bi2212 is at a maximum. It is found to obey the (1-t²) ⁿbehaviour with the exponent n = 3/2 reminiscent of the predicted behaviour of the melting line.