Definition
Superfluid helium-4 is the phase of the isotope helium‑4 (^4He) that exhibits zero viscosity, the ability to flow without dissipative friction, and a host of other quantum mechanical phenomena when cooled below the lambda point (≈ 2.17 K at saturated vapor pressure). In this state, helium‑4 is commonly denoted as He‑II to distinguish it from the normal liquid phase (He‑I).
Overview
When liquid helium‑4 is cooled to temperatures just above absolute zero, it undergoes a second‑order phase transition at the lambda temperature (T_λ ≈ 2.172 K). Below T_λ, the liquid enters the superfluid phase, characterized by macroscopic occupation of the ground state by a Bose‑Einstein condensate of helium atoms. The superfluid exhibits remarkable properties such as frictionless flow through narrow capillaries, the ability to climb container walls (Rollin film), extremely high thermal conductivity, and quantized vortices. The phenomenon is explained by the two‑fluid model, which treats He‑II as a mixture of an inviscid superfluid component (density ρ_s) and a viscous normal component (density ρ_n) that coexist and interact.
Etymology / Origin
The word “superfluid” combines the prefix “super‑” (meaning “above” or “beyond”) with “fluid,” reflecting the material’s ability to flow without the ordinary resistance associated with conventional fluids. The term was introduced in the late 1930s following the discovery of the λ‑transition by Pyotr Kapitsa, John F. Allen, and Don Misener, who observed the anomalously low viscosity of liquid helium‑4 at millikelvin temperatures.
Characteristics
| Property | Description |
|---|---|
| Zero Viscosity | Measurements show that the superfluid component can move without shear resistance, allowing persistent currents in toroidal containers. |
| Lambda Transition | Occurs at T_λ ≈ 2.172 K (at 1 atm); specific heat exhibits a sharp peak resembling the Greek letter λ, hence the name. |
| Two‑Fluid Model | He‑II is described by two interpenetrating fluids: a superfluid fraction (ρ_s) with no entropy and a normal fraction (ρ_n) carrying entropy and viscosity. The fractions vary with temperature, with ρ_s → 0 as T → T_λ and ρ_s → ρ (total density) as T → 0 K. |
| Fountain Effect | When a heater is placed in a superfluid-filled container, the superfluid rises through a narrow channel, creating a fountain of liquid helium. |
| Second Sound | Unlike ordinary sound (first sound) which is a pressure wave, second sound is a temperature/entropy wave propagating through the coupled motion of the two fluid components. |
| Quantized Vortices | Rotational motion is supported by discrete vortex lines with circulation quantized in units of h/m_4 (Planck constant divided by the mass of a helium‑4 atom). |
| High Thermal Conductivity | He‑II conducts heat orders of magnitude more efficiently than normal liquids, often described as “thermal superfluidity.” |
| Rollin Film | A thin film of superfluid helium can creep up and over the interior surfaces of a container, eventually escaping the container under gravity. |
Related Topics
- Helium‑3 superfluidity – fermionic isotope ^3He exhibits superfluid phases at much lower temperatures, mediated by Cooper pairing.
- Bose‑Einstein condensation – the underlying quantum statistical mechanism responsible for the macroscopic ground‑state occupation in superfluid ^4He.
- Two‑fluid model – theoretical framework introduced by Lev Landau to describe the simultaneous presence of superfluid and normal components.
- Quantum vortices – topological defects in superfluids and superconductors, fundamental to the study of rotating superfluid systems.
- Lambda point – the specific temperature at which the normal-to-superfluid transition occurs in helium‑4.
- Cryogenics – the broader field concerned with low‑temperature physics, where superfluid helium‑4 is employed as a coolant and research medium.