Superfluidity ofHe4confined in microscopic bubbles in copper

We have created extremely small and independent Bose systems by confining ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in bubbles of radius of less than 110 Å in Cu foils. The size distribution and density of the bubbles were adjusted by annealing the samples and were determined by transmission electron microscopy. At low temperatures the confined helium becomes first solid, then liquid, and eventually superfluid as the mean bubble size is increased by annealing. The modification of superfluidity in this new confining geometry by size effects has been investigated by low-temperature calorimetry.