Instabilities of the Vortex Lattice and the Peak Effect in Single Crystal YBa2Cu4O8

We report on the measurements of the remnant magnetization, and hence critical current, in a single crystal of YBa2Cu4O8. A peak in the temperature dependence of the critical current is observed when the external magnetic field is tilted away from the a–b planes. The observed behavior is attributed to a thermally activated instability-driven vortex-lattice splitting or vortex chain formation. The nature of the peak and the possibility of a thermally-activated dimensional crossover have been discussed.