Flux Quantization in Thick Mesoscopic Pb Superconductors .*

We present studies of novel vortex behavior on a thick triangular-shaped mesoscopic Pb crystal using ballistic micro-Hall probe magnetometry. The Pb single crystal with dimensions of 2 microns on each edge and 0.7~$\mu $m thick was placed on a 2DEG Hall sensor with a detection area of 1~$\mu $m$^{2}$ and the magnetization was studied as a function of applied magnetic field and temperature. The critical field was found to be H$_{c}$ = 78 mT and T$_{c}$ = 7.20 K, in good agreement with values of clean, bulk Pb. Above T$_{x}$~=~6.6~K, we observe a stable Meissner state for increasing field and a remarkable quantized flux entry behavior with decreasing field. Below T$_{x}$, we observe quantized flux entry (removal) with increasing (decreasing) field. These behaviors indicate that thick mesoscopic Pb superconductors can be tuned with temperature to exhibit type I or type II superconducting behavior.