Superfluids Are Not Fluids

At 2.17 K, the 𝝀-point, helium undergoes a dramatic transformation: it loses all viscosity and becomes a “superfluid”, exhibiting extraordinary behaviors that defy conventional fluid dynamics. This study explores the fundamental physics underlying these phenomena. Unlike most substances, whose molecules are held together by intermolecular forces responsible for solidity and fluid viscosity, helium superfluidity arises from the absence of such attractions. Due to its simple and symmetric atomic structure, helium is largely immune to intermolecular forces except for the weak London dispersion force, which itself vanishes at sufficiently low temperatures. As a result, helium atoms in the superfluid state experience no mutual attraction and move as independent particles, leading to zero viscosity. Thus, a superfluid is not a traditional fluid but a collection of unbound atoms. The transition to the superfluid state requires breaking residual intermolecular bonds, which consumes energy and lowers the temperature, causing the transition to span a range rather than occurring at a constant temperature. Thus, the 𝝀-point marks the completion of this transition, which likely begins at temperatures above 2.6 K. This interpretation is supported by the observed curvature in specific heat near the 𝝀-point. The absence of intermolecular attraction in superfluid helium also explains why it cannot solidify merely by cooling. However, helium can be solidified under high pressure, suggesting the formation of a novel type of bond, termed a “compression bond”, resulting from the deformation of electron clouds under extreme pressure. More broadly, such bonds may also play a role in holding atoms together in metallic hydrogen and could offer insight into the bonding mechanisms in other metals.