Observation of mesoscopic vortex physics using micromechanical oscillators

It has long been known that magnetic fields penetrate type II superconductors in the form of quantized superconducting vortices. Most recent research in this area has, however, focused on the collective properties of large numbers of strongly interacting vortices(1,2): the study of vortex physics on the mesoscopic scale (a regime in which a small number of vortices are confined in a small. volume) has in general been hampered by the lack of suitable experimental probes. Here we use a silicon micromachined mechanical resonator to resolve the dynamics of single vortices in micrometre-sized samples of the superconductor 2H-NbSe2. Measurements at and slightly above the lower critical field, H-cl (the field at which magnetic flux first penetrates the superconductor), where only a few vortices are present, reveal a rich spectrum of sharp, irreversible vortex rearrangements. At higher fields, where tens of vortices are present, the sharp features become reversible, suggesting that we are resolving a new regime of vortex dynamics in which the detailed configuration of pinning sites, sample geometry and vortex interactions produce significant changes in the measurable vortex resonse. This behaviour can be described within the framework of interacting vortex lines in a '1 + 1'-dimensional random potential-an important (but largely untested) theoretical model for disorder-dominated systems(11,12).