Superfluidity

Of superconductivity in which electrons can flow through a metal and experience no resistance to their flow when the metal is cooled to a very low temperature. This, as was stressed, is a quantum effect and here we discuss a similar, although not directly related, phenomenon which takes place in liquid helium. The nature of the helium atom. It consists of a nucleus together with two electrons in a filled shell. Since the shell is filled their total angular momentum is zero. The nucleus also has zero angular momentum and so, therefore, does the atom as a whole it can be regarded as a particle of spin 0. Entities whose spin is an integer (0, 1, 2, . . .) multiple of h are referred to collectively as bosons (named after Bose, an Indian physicist). They are not subject to the Pauli exclusion principle unlike fermions (named after Fermi, an Italian physicist) such as electrons which have spin equal to a half-integer (1/2,3/2 , . . , ) multiple of h. This means that an assembly of helium atoms, having spin 0, can in principle all occupy the same energy state. At very low temperatures, when thermal agitation is small, a gas of helium atoms will condense into a liquid. This happens at around 4.2K and, curiously, the helium will remain as a liquid down to the lowest possible temperature-in principle, absolute zero (OK). The only way it can be solidified is to increase the pressure on it very significantly; roughly speaking to squash the atoms together. However, as it is cooled below the above liquefaction temperature a remarkable phenomenon occurs. An effective way to cool it is to pump away the helium vapour created by evaporation, which lies above the liquid helium. As it is pumped away more of the liquid evaporates and this requires energy, which is taken out of the remaining liquid, so reducing thermal agitation and, in turn, the temperature. The evaporation process is accompanied by the usual phenomenon of boiling (bubble formation) but when the temperature reaches 2.18 K the boiling suddenly stops although the evaporation process still continues. The physical interpretation is that suddenly the liquid is able to conduct heat without any resistance at all so that the heat no longer remains localized leading to the formation of bubbles. Coupled with this it is found that the liquid is able to flow down a very thin tube without experiencing any resistance to its flow. It also exhibits the remarkable behaviour of being able to climb quite quickly up the walls and out of its container as a thin film! The liquid helium has suddenly changed its nature in some way and part of it has become what is called a super fluid. As the temperature drops below 2.18 K superfluidity sets in until, around 1 K, all of the liquid becomes superfluid. This effect arises because, at a sufficiently low temperature, all of the atoms fall into the same lowest possible quantum state and they all act together. If an attempt is made to change the state of motion of one of them, for example by introducing heat at a point, the states of all of them change in a similar way. It must be stressed here, however, that this superfluidity effect is quite different in nature and origin from superconductivity. Effects such as this in solid materials (e.g. in rubidium, lithium and sodium) have also been observed recently (1995) at amazingly low temperatures of the order of 10^-7K. At such temperatures the de Broglie wavelength  of each atom is comparable to the spacing between the atoms and we have a macroscopic quantum system the whole system, as well as its components, is a quantum entity. The importance of such systems for future technology is unpredictable at this stage.