lambda transition
/ LAM-duh tran-ZISH-un /
When water freezes to ice, it does so at a single sharp temperature, and a fixed amount of heat is dumped at that instant. Cooling liquid helium toward its superfluid state is not quite like that — there is a special temperature where it transforms, but the change sneaks up smoothly and then peaks at a knife-edge point rather than releasing a burst of heat.
The lambda transition is the temperature, around 2.17 degrees above absolute zero, at which liquid helium-4 turns into a superfluid. Its name comes from a graph: if you plot how much heat the liquid soaks up as you warm it, the curve rises, spikes to a tall sharp tip exactly at the transition, and drops — and that shape looks just like the Greek letter lambda. The transition is continuous, meaning the superfluid fraction grows gradually from zero rather than appearing all at once.
It matters as the cleanest, most precisely studied example of a continuous phase transition, a benchmark for the whole theory of how matter reorganizes itself. The common confusion is to expect it to behave like freezing, with a clear release of latent heat and a sharp boundary between two coexisting phases; instead, helium passes smoothly into its superfluid state with no latent heat, which is exactly what makes the lambda transition special.
Cool liquid helium through 2.17 kelvin and its violent boiling abruptly stops dead — above the lambda point bubbles roil all through it, but the instant it turns superfluid it conducts heat so perfectly that no bubbles can form, and the liquid falls eerily still.
At the lambda point, boiling helium suddenly goes glassy-still as it turns superfluid.
The 'lambda' is purely a description of the shape of the heat-capacity curve — it has nothing to do with wavelength or any other lambda you may meet in physics.