Mechanism of the Lorentz-force-independent dissipation in Bi2Sr2CaCu2Oy

A novel power law was found above 65 K in the I-V characterististics of Bi₂Sr₂CaCu₂O_y single crystals with the magnetic field perpendicular to the c axis. At 80 K, the power-law exponent decreased sharply up to 1.5 T but remained nearly constant above it, suggesting an abrupt crossover to a different mechanism. An unusual field dependence of resistivity was also found, which cannot be explained by simple flux-flow theory. The above novel features of the Lorentz-force-independent dissipation can be consistently explained by a model based on two-dimensional vortex-antivortex pair breaking.