Microscopic superfluidity in ⁴He clusters stirred by a rotating impurity molecule

The effective moment of inertia of a CO impurity molecule in ⁴He_N and p−(H₂)_N solvent clusters initially increases with N but then commences a nonclassical decrease at N=4 (⁴He) or N=6 (p−H₂). This suggests molecule-solvent decoupling and a transition to microscopic superfluidity. However, the quantum decoupling mechanism has not been elucidated. To understand the decoupling mechanism, a one-dimensional model is introduced in which the ⁴He atoms are confined to a ring. This model captures the physics and shows that decoupling happens primarily because of bosonic solvent-solvent repulsion. Quantum Monte Carlo and basis set calculations suggest that the system can be modeled as a stirred Tonks-Girardeau gas. This allows the N-particle time-dependent Schrödinger equation to be solved directly. Computations of the integrated particle current reveal a threshold for stirring and current generation, indicative of superfluidity.