Observation of Majorana fermion states in rf-SQUIDs constructed on Pb-Bi$_2$Te$_3$ surface

Recently, much attention has been paid to search for Majorana fermions in solid-state systems. Among various proposals there is one based on radio-frequency superconducting quantum interference devices (rf-SQUIDs), in which a 4$\pi$-perioded current-phase relation is expected if Majorana fermion states exist. In this talk we report observations of truncated 4$\pi$-perioded (i.e., 2$\pi$-perioded but fully skewed) oscillatory patterns of contact resistance, on rf-SQUIDs constructed on the surface of three-dimensional topological insulator Bi$_2$Te$_3$. The results reflect the existence of Majorana fermion states in the devices.