Vortex phase diagram of YBa2Cu4O8.

We have investigated the vortex phase diagram of YBa2Cu4O8 single crystals of intermediate c-axis anisotropy by means of the magnetic torque, the DC magnetization, and the AC susceptibility. The single crystals have been grown by a self-flux method under high-pressure. The magnetization curve shows a second peak. The torque also shows the multiple peak effects. The central peak is due to the intrinsic pinning. The first torque peak at <f>θca≃90±10</f>° requires the existence of the field component parallel to the CuO2 planes. The second torque peak at <f>θca∼90±30</f>° roughly correlates with the second peak in magnetization. We summarize the vortex melting line and the lines for the various peaks in the HT diagram. We find that vortex phase diagram of YBa2Cu4O8 in H&par;c is very different from those for the YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ. [ABSTRACT FROM AUTHOR]