Temperature dependence of the second magnetization peak in underdoped La_2-x Sr_x CuO₄ single crystals

The characteristic magnetic fields for the second peak (SP) on the dc magnetization curves of high-temperature superconductors with random quenched disorder increase with decreasing temperature T in the low- T range. It was argued that this aspect rules out the existence of an order-disorder transition in the vortex system as the primary cause for the occurrence of the SP, and a model based on a thermally induced square-to-rhombic vortex lattice transition was recently proposed. We investigated the T variation of the field Hon for the onset of the SP in the case of underdoped La_2-x Sr_x CuO₄ single crystals (x=0.08) with the external magnetic field oriented along the c axis. It was found that in the low- T domain H_on (T) ∝1/ Tm², and an inflectionlike point in the H_on (T) dependence is still present, similar to that reported for overdoped specimens. We show that the observed behavior and the strong variation in the characteristic fields for the SP with the doping level are in agreement with a dynamic energy balance relation for the order-disorder transition in the vortex system at the SP.