Vortex penetration in single-crystalTl2Ba2CuO6superconductors

Detailed magnetic hysteresis measurements have been performed with the applied field parallel to the ab plane in single crystals of ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CuO}}_6$. Magnetization in the descending branch of the hysteresis loop is almost zero, indicating the presence of surface barriers which determine the penetration field of vortices. Magnetization with the field tilted slightly to the ab plane shows that the two components of magnetization due to ${\mathit{H}}_{\mathrm{\parallel }}$ and ${\mathit{H}}_{\mathrm{\perp }}$ act independently. The data present clear evidence of a lock-in transition in the compounds studied.