Using a SQUID-based technique, we measure the Lorentz force needed to depin a single trapped vortex in a superconducting Nb strip. The vortex is trapped by heating a portion of the strip above its critical temperature ${\mathit{T}}_{\mathit{c}}$ with a laser pulse. Between 3.0 and 5.5 K, the pinning force obeys a power law ${\mathit{f}}_{\mathit{p}}$=${\mathit{f}}_{\mathit{p}}$(0)(1-T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$, where ${\mathit{f}}_{\mathit{p}}$(0)=(4.1±0.4)×${10}^{\mathrm{-}4}$ N/m and n=1.9±0.1. We have determined at a confidence level of 99.2% that we trap single vortices, rather than multiple vortices or vortex-antivortex pairs.

• Received 24 October 1991

DOI:https://doi.org/10.1103/PhysRevLett.68.1920