The most massive particles that have been experimentally shown to exhibit wave/particle duality in the double-slit experiment are molecules, particularly complex ones such as fullerene molecules (e.g., C60, also known as buckyballs) or large organic molecules.
While the double-slit experiment is commonly associated with photons or electrons, advancements in experimental techniques have allowed scientists to extend these experiments to larger particles, including molecules. In these experiments, molecular beams are produced, and these beams are directed towards a double-slit apparatus. The resulting interference patterns on the detection screen demonstrate the wave-like behavior of the molecules.
For example, in 1999, the dual-slit interference pattern was observed using C60 fullerene molecules, which consist of 60 carbon atoms arranged in a spherical structure. This experiment provided evidence that even relatively large molecules could exhibit interference patterns, similar to what is observed with photons or electrons.
It's worth noting that as the mass and complexity of the particles increase, maintaining the coherence of the wave function becomes more challenging due to factors such as environmental interactions and decoherence. Nevertheless, experiments with increasingly larger particles continue to push the boundaries of our understanding of wave/particle duality and the limits of quantum behavior.