Breaking nature's superfluid symmetry
Superfluids are an exotic state of matter in which particles flow without experiencing viscosity. Hiroki Ikegami and colleagues from the RIKEN Low Temperature Physics Laboratory in Wako have now observed another remarkable property of superfluids—the breaking of a fundamental natural symmetry in superfluid helium-3 (3He). The finding is important for many areas of physics, says Ikegami. "Spontaneous symmetry breaking is a universal and fundamental concept found in various branches of physics. It describes that nature prefers taking a less symmetric state even if underlying laws are symmetric."
Superfluidity in helium-3 occurs at a temperature of just a few thousandths of a degree above absolute zero and involves two helium-3 atoms bonding together to form Cooper pairs that are able to move without hindrance. One of the properties of a Cooper pair is its angular momentum, which describes the rotation of the pair. In the superfluid helium-3, the angular momenta of the Cooper pairs all align in the same direction: either upwards or downwards. This spontaneous choice of one direction over the other breaks one of the fundamental symmetries of superfluid helium-3 that is related to chirality—left- and right-handed mirror-image symmetry.
To detect the broken symmetry, the researchers developed a system that allowed them to observe the intrinsic Magnus force, which causes any tiny object traveling orthogonal to an orbital angular momentum to deviate sideways (Fig. 1), thus revealing the chiral 'handedness' of the fluid. The researchers injected electrons beneath a liquid helium surface to form a thin electron layer, then set the electrons in motion using electrodes placed just above the helium. The deflection of the electrons was then measured using the same electrodes.
In most measurements, the helium-3 atoms had the same orbital angular momentum alignment across the entire sample. In some cooling runs, the angular momentum of the Cooper pairs pointed upwards, in others downwards. This means that the direction is selected spontaneously despite there being no intrinsically preferred direction for the angular momentum, providing direct evidence of chiral symmetry breaking.
Sometimes the samples showed multiple domains with different orientations of orbital angular momentum, which were attributed to topological defects in the fluid. Such observations are important for further study, explains Ikegami. "One of the important consequences of symmetry breaking is the formation of topological defects such as magnetic domain walls in ferromagnets and cosmic strings in the Universe. Studying these in detail may shed light on the role of defects in the breaking of symmetries in various branches of physics."