We call this phenomenon the Hess-Fairbank effect. Today, superfluidity is something that we can directly observe in helium isotopes and in ultra-cold atomic gases. It is conjectured to occur in extraterrestrial systems, such as neutron stars, and there is circumstantial evidence supporting its existence in other terrestrial systems, such as excitons, which are bound electron-hole pairs found in semiconductors.
Helium-4 was liquefied in , but it was only in and that scientists recognized that below the temperature of 2. In particular, the thermal conductivity of the low-temperature phase, now known as He-II, is very large, which suggests a convection mechanism, but with anomalously low viscosity.
In , Pyotr Kapitza in Moscow and John Allen and Don Misener at the University of Cambridge simultaneously performed a direct measurement of the behavior of the viscosity of the helium contained in a thin tube as a function of temperature. Both groups found a drop in He-II, which appeared discontinuously at the lambda point.
On the basis of the analogy with superconductivity, Kapitza coined the term superfluidity for this behavior. According to our modern understanding, superconductivity is nothing more than superfluidity occurring in an electrically charged system. Just as a superfluid liquid can flow forever down a narrow capillary without apparent friction, so can a current, once started in a superconducting ring — or at least for a time much longer than the age of the Universe!
In case of helium-3 atoms, the pairing is more complicated, it is called P- wave pairing. It means that internal orbital angular momentum is non-zero. You have some vector. If you think about two pairs, if you have two helium-3 atoms, you have a vector associated with angular momentum. At the same time, they have non-zero spin. So, in S, you have one spin state, so you can associate another vector to their spin. You have one vector for the angular momentum, one vector for their spin.
Now, when they are condensed, what happens is that you have a breaking of more than one symmetry of the system. A concept of a broken symmetry is a very crucial concept in the theory of superfluids and for that matter — superconductants. What it means is that if you have a system, which is described by certain Hamiltonian, has a certain of energy, it is invariant.
There are some properties of the system, which cannot be determined by the Hamiltonian. So, Hamiltonian is invariant, for example, under the direction of that spin, or the direction of this internal orbital momentum.
In the case of the condensate is invariant under the choice of the phase. We know that condensate has a phase, but the Hamiltonian will not determine what phase it has. So, the system spontaneously breaks that symmetry. In the case of the superfluids, — there are simple superfluids like helium-4 and complicated superfluids like helium-3 —more than one symmetry is broken at the same time.
In case of helium-3 not only the phase symmetry is broken, but also the direction of the spin and the direction of the internal angular momentum. It can be broken in different ways. Either you can imagine this is an angular momentum and it breaks symmetry. It means that all my atoms are relying with internal angular momentum exactly in the same way, and the spin is relying also in the same way.
This leads to an A-phase of the liquid helium. However, if I only fix the relative relations between the spin and internal angular momentum like that, only relative angle is fixed, but not each of the angles.
Then it leads to the phase B of the superfluid. The natural superfluids, such as helium-3 or helium-4, could be more or less understood with those frameworks. However, superfluidity research does not stop at just liquid helium. In particular, in the last few decades, ultracold atoms appeared as a new system to study superfluidity.
Superfluid helium's dual nature is at work again when it climbs the walls of a container. Watch this YouTube video of the effect. Any liquid will coat the sides of a dish in which it sits—thanks again to the slight attraction between atoms—but the liquid's internal friction limits how far the coating may spread.
In superfluid helium, the frictionless film slithers over the whole container, creating a sort of arena through which the superfluid can flow. If the liquid has somewhere to fall after it climbs out of the dish, it will drip from the bottom of the container until it siphons out all the superfluid pooled above it. The same principle underlies another famous demonstration in which superfluid rapidly shoots out of an open, heated glass tube packed with fine powder at the bottom.
Called the superfluid fountain , it occurs because the superfluid outside of the tube rushes in to cool down the superfluid that has been warmed by the inside of the tube. Allen, the co-discoverer of superfluidity, is said to have discovered the effect after he shined a pocket flashlight onto a glass tube of liquid helium.
Work on superfluid helium has already netted three Nobel Prizes and may yet garner more. In Penn State's Chan and Eun-Seong Kim rotated a ring full of solid helium at 26 atmospheres of pressure and found that as they cooled the helium below the critical temperature, the rotational frequency increased, just as it does with liquid helium. Half a dozen laboratories, including Beamish's, are studying the " supersolid " effect, but researchers still aren't sure which elements of the solid would condense into a single Bose—Einstein state.
The trick now is to see if the supersolid can produce the equivalent of super-leaks or other well-known super-effects. JR Minkel was a news reporter for Scientific American. Already a subscriber? Sign in. Thanks for reading Scientific American.
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