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Chiral Symmetry Breaking in Superfluid 3He-A

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Science  05 Jul 2013:
Vol. 341, Issue 6141, pp. 59-62
DOI: 10.1126/science.1236509

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Quantum Handedness

When a rotating object is placed in circulating fluid, an imbalance of pressures on either side of it causes a deflecting force called the Magnus force. The quantum analog of this effect has been predicted to appear in the low-temperature A phase of 3He, where the Cooper pairs forming the superfluid have a specific handedness. An impurity traveling through such a superfluid would experience a deflecting force in the direction determined by the chirality of the pairs. Ikegami et al. (p. 59) trapped impurities beneath the free surface of 3He, set them in motion, and demonstrated the existence of this deflecting force by measuring the differential transverse current. The sign of the deflection varied over different cooling runs, indicating that the system was choosing one or the other chirality upon entering the superfluid phase—a signature of spontaneous symmetry breaking.

Abstract

Spontaneous symmetry breaking is an important concept in many branches of physics. In helium-3 (3He), the breaking of symmetry leads to the orbital chirality in the superfluid phase known as 3He-A. Chirality is a fundamental property of 3He-A, but its direct detection has been challenging. We report direct detection of chirality by transport measurements of electrons trapped below a free surface of 3He-A. In particular, we observed the so-called intrinsic Magnus force experienced by a moving electron; the direction of the force directly reflected the chirality. We further showed that, at the superfluid transition, the system selected either right- or left-handed chirality. The observation of such selection directly demonstrates chiral symmetry breaking.

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