Physicists observe long-expected helium superfluid phase

Physicists have reported that they have finally observed helium 3 existing in a long-predicted type of superfluid, called the ß phase.

This is an important discovery, if it’s borne out, for reasons that partly have to do with its isotope, helium 4. Helium 4 is a fascinating substance because the helium 4 atom is a boson – a type of particle whose quantum properties and behaviour are explained by rules called Bose-Einstein statistics. Helium 3, on the other hand, is a fermion, and fermions are governed by Fermi-Dirac statistics.

Bosons and fermions have one important difference: bosons are allowed to disobey Pauli’s exclusion principle, and by doing so they can assume exotic states of matter rarely found in nature, with many unusual properties.

For example, when helium 4 is cooled below a certain temperature, it becomes a superfluid: a liquid that flows without experiencing any resistance. If you poured a superfluid into a bowl, it will be able to climb the walls of the bowl and spill out without any help. But helium 3 atoms are fermions, so they are bound to obey Pauli’s exclusion principle and can’t become a superfluid.

At least this is what physicists believed for a long time, until the early 1970s, when two independent groups of physicists found – one in theory and the other in experiments – that helium 3 could indeed enter a superfluid phase, but at a temperature 1,000-times lower than the critical temperature of helium 4. The theory group, led by Anthony Leggett at the University of Sussex, had in fact made a significant discovery.

Today, we know that the flow of superfluid helium 4 is analogous to the flow of electrons in a conventional superconductor, which also move around as if they face no resistance from the surrounding atoms. Leggett and co. found that the theory used to explain these superconductors could also be used to explain helium 3 superfluidity. This theory is called Bardeen-Cooper-Schrieffer (BCS) theory, and the materials whose superconductivity it can explain are called BCS superconductors.

Electrons are fermions and cannot ‘super-flow’. But in a BCS superconductor that has been cooled below its critical temperature, some forces in the material cause the electrons to overcome their mutual repulsion (“like charges repel”) and pair up. These electron pairs, while being made of two individual fermions, actually behave like bosons. Similarly, Leggett and co. found that helium 3 atoms could pair up to form a bosonic composite and super-flow.

Over many years, physicists used what they had learnt through these discoveries to expand our understanding of this substance. They found, among other things, that superfluid helium 3 can exist in many phases. The superfluidity would persist in each phase but with different characteristics.

Superfluid helium 3 was first thought to have two phases, called A and B. The temperature-pressure plot below clearly shows the conditions in which each phase emerges.

When physicists subjected superfluid helium 3 in its A phase to a strong magnetic field, they found another phase that they called A1, whose atom-pairs had different spin characteristics.

In 2015, a group of researchers led by Vladimir Dmitriev, at the P.L. Kapitza Institute for Physical Problems, Moscow, discovered a fourth phase, which they called the polar, or P, phase. Here, they confined helium 3 in a nematic aerogel and exposed the setup to a low magnetic field. Aerogels are ultra-light materials that are extremely porous; nematic means its molecules were arranged in parallel. The aeorogel in the Dmitriev and co. experiment was 98% porous, and whose pores “were much longer than they were wide” (source). That is, the team had found that the shape of the container in which helium 3 was confined also affected the phase of its superfluidity.

In August 2021 (preprint), the same team reported that it had observed a long-expected-to-exist fifth phase called the ß phase.

They reported that they took the setup they used to force superfluid helium 3 into the P phase, but this time exposed it to a high magnetic field. According to their paper, they found that while the superfluid earlier moved into the P phase through a single transition, as the temperature was brought down, this time it did so in two steps. First, it moved into an intermediate phase and then into the P phase. The intermediate is the ß phase.

(If this sounds simple, it wasn’t: the discoveries were each limited by the availability of specially designed instruments capable of picking up on very small-scale changes unavailable to the naked eye. Second, researchers also have had to know in advance what changes they should expect to happen in each phase, and this requires the corresponding theoretical clarity.)

I have considerably simplified helium 3’s transition from the ‘normal’ to the superfluid phase in this post. To describe it accurately, physicists use advanced mathematics and associated concepts in high-energy physics. One such concept is symmetry-breaking. When a helium 3 atom pairs up with another to form a bosonic composite, the pair must have a ‘new’ spin and orbital momentum; and their combined wavefunction will also have a ‘new’ phase. All these steps break different symmetries.

There’s a theory called Grand Unification in particle physics, in which physicists expect that at higher and higher energies, the three fundamental forces that affect subatomic particles – the strong-nuclear, the weak-nuclear and the electromagnetic – will combine into a single unified force. Physicists have found in their mathematical calculations that the symmetries that will break in this super-transition resemble those broken by helium 3 during its transition to superfluidity.

Understanding helium 3 can also be rewarding for insights into the insides of neutron stars. Neutron stars are extreme objects – surpassed in their extremeness only by black holes, which exist at the point where known theories of gravitational physics collapse into meaninglessness. A few lakh years after a neutron star is born, it is expected to have cooled sufficiently for its interiors to be composed of superfluids and superconductors.

We may never be able to directly observe these materials in their natural environment. But by studying helium 3’s various phases of superfluidity, we can get a sense of what a neutron star’s innards could be like, and whether their interactions among themselves and the neutrons on the surface could explain these objects’ still-mysterious characteristics.

Featured image: The liquid helium is in the superfluid phase. A thin invisible film creeps up the inside wall of the cup and down on the outside. A drop forms. It will fall off into the liquid helium below. This will repeat until the cup is empty – provided the liquid remains superfluid. Caption and credit: Alfred Leitner, public domain.