Exploring molecular superfluidity in hydrogen clusters

Molecular hydrogen (H₂) has long been predicted to exhibit superfluidity-a state of zero viscosity-at extremely low temperatures. However, its existence remains under debate despite several experimental reports. In this study, we investigated the infrared transitions of methane embedded in clusters of parahydrogen molecules at 0.4 K using high-resolution helium nanodroplet spectroscopy. Our results revealed fully quantized rotational states of methane with minimal interference from surrounding H₂ molecules, enabling precise determination of the rotational constant for each hydrogen cluster. The cluster-size dependence of the determined rotational constant aligns with behavior predicted by path-integral Monte Carlo simulations, indicating that more than 60% of the hydrogen molecules in the clusters participate in quantum bosonic exchanges, a characteristic feature of superfluidity. This work provides strong experimental evidence for the existence of a superfluid phase of molecular hydrogen at 0.4 K, representing a major step forward in understanding quantum behaviors in molecular systems.