Anthony Leggett used an analogue of BCS theory to explain the second-order transition in one form of this substance; that form of it is the only known perfect Fermi fluid.
Feynman and Bijl developed the first successful method of finding the excitation spectrum of one form of this substance. A theory of order in the bosonic form of this substance was developed by Penrose and Onsager, and is known as the "off-diagonal long-range order" model. Anthony Leggett used an analogue of BCS theory to explain the second-order transition in one form of this substance; that form of it is the only known perfect Fermi fluid. One form of this substance undergoes a phase transition at 2.17 Kelvin, a temperature known as its lambda point. Both forms of this substance can become superfluidic, as demonstrated by Osheroff and Lee's experiments with its isotope 3. For 10 points, identify this substance which exists below 4.2 and 3.2 Kelvin for the 4 and 3 isotopes respectively, a condensation of the second-lightest element.
Feynman and Bijl developed the first successful method of finding the excitation spectrum of one form of this substance.
liquid helium [prompt on helium; prompt on superfluid before it is mentioned]