Direct visualization of local interaction forces in Bi₂Sr₂CaCu₂O_8+δ vortex matter

We study the local vortex-vortex interaction force f_i of the structure frozen during a field-cooling process in an electron-irradiated Bi₂Sr₂CaCu₂O_8+d sample. We compute this magnitude from snapshots of the vortex structure obtained via magnetic decoration experiments at various fields H in the same sample. Since the observed structures correspond to the equilibrium ones frozen at T∼T_irr(H)[1], at this temperature the local modulus of f_i roughly equals the local pinning force at the decorated surface of the sample. We estimate the most probable local pinning force from the mode value of the f_i(r)distribution, f_p ^m. We found that f_p ^m grows algebraically with H and in electron-irradiated samples is 50-20% smaller than for samples with columnar defects.