Ultimate vortex confinement studied by scanning tunneling spectroscopy.

We report a detailed scanning tunneling microscopy study of a superconductor in a strong vortex confinement regime. This is achieved in a thin nanoisland of Pb having a size d about 3 times the coherence length, and a thickness h such that h<<d<<lambda, where lambda is the London penetration depth. In this geometry the magnetic field evolution of local tunneling spectra reveals only two superconducting configurations to exist: zero and single vorticity. The normal state is reached at mu(0)H(c)=0.46 T, about 6 times the critical field of bulk Pb, with no higher order vorticity observed. Basing on our STS data, we discuss the role of local supercurrents in both configurations.