  We study the ratchet effect in a narrow pinning-free superconductive ring
based on time-dependent Ginzburg-Landau (TDGL) equations. Voltage responses to
external dc an ac currents at various magnetic fields are studied. Due to
asymmetric barriers for flux penetration and flux exit in the ring-shaped
superconductor, the critical current above which the flux-flow state is
reached, as well as the critical current for the transition to the normal
state, are different for the two directions of applied current. These effects
cooperatively cause ratchet signal reversal at high magnetic fields, which has
not been reported to date in a pinning-free system. The ratchet signal found
here is larger than those induced by asymmetric pinning potentials. Our results
also demonstrate the feasibility of using mesoscopic superconductors to employ
superconducting diode effect in versatile superconducting devices.
