Superfluid-normal fluid transition

We now recall that the classical planar rotator model may be used as a model of superfluid He4, 0 being the phase of the condensate wave function, S being related to the superfluid density ps as S = ps(hjm)2, m being the mass of a He4 atom. Thus one can have superfluid-normal fluid transition in d = 2 dimensions, despite the lack of conventional long range order This conclusion seems to be corroborated by experiments on He4 films (Bishop and Reppy, 1978). 

Liquid helium-4 has some very unusual properties since it can exist in two different liquid phases, namely, liquid helium I and liquid helium II (Fig. 1). The former is labeled the normal fluid, while the latter has been designated the superfluid since under certain conditions the fluid acts as if it had no viscosity. The phase transition between the two liquid phases is identified as the X line. Intersection of the latter with the vapor-pressure curve is known as the X point. Helium-4 has no triple point and requires a... 

The liquefied helium is subdivided into two states He I and He II with a sharp transition point of 2.18 K at 5.04 kPa, the so-called A,-point. He I behaves like a normal liquid, whereas He II exhibits interesting properties of a superfluid or quantum fluid. During expansion of liquid He I below this pressure, the previously even surface forms a sharp meniscus at the wall of the container since at the 7.-point the viscosity decreases by the factor 10 and the thermal conductivity rises by the same factor. The thermal conductivity of He II is about 200 times higher than that of copper at 20 °C. Close to the absolute zero point, the viscosity turns zero and He II becomes an inviscid superfluid. He II flows over obstacles, which lie higher than the surface of the liquid, to reach the lowest level. If two containers of different temperatures are filled with He 11 and connected to each other by a capillary or another He Il-film, He II flows from the cold container into the warmer one. 

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