Vortex pinning force in a superconducting niobium strip

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\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ N/m and n=1.9\ifmmode\pm\else\textpm\fi{}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.