# Fermi statistics and Berry phase

When the positions of two fermions are exchanged adiabatically in three-dimensional space, we know that the wave function gains a factor of $-1$. Is this related to Berry's phase?

• Good question. I found this paper by Berry which connects the phase change under particle exchange to the spin of the particles. May 6, 2012 at 7:19

The answer is yes. The angular momentum of a (scalar) particle moving in the background of a Dirac magnetic monopole of an odd magnetic charge becomes half integral. (The magnetic charge must be an integer by the Dirac quantization condition.)

Under these conditions, the phase acquired by the wavefunction through a 360 degrees rotation is $-1$. By the spin statistics theorem, the particle must be quantized as a fermion, even though its dynamics is described by bosonic coordinates.

This phase can be obtained as a Berry phase. This principle was used by Witten in his seminal paper "E. Witten, Current algebra, baryons, and quark confinement, Nucl. Phys. B 223 (1983) 433-444", to demonstrate that the Skyrmions are fermions.

Witten obtained the change of sign of the Skyrmion wave function through a Berry phase computation. Here, the magnetic monopole field stems from the Wess-Zumino-Witten term in the Skyrmion Lagrangian.

A clear exposition of Witten's work with the emphasis on the analogy with the motion in the background of a Dirac monopole is given in I.J.R. Atchinson's paper : "Berry phases, magnetic monopoles and Wess Zumino terms, or how the Skyrmion got its spin"

• Nice. Do you know if there is anything similar in the case of anyons...? May 6, 2012 at 14:28
• @VijayMurthy Yes, the Berry phase (and its non-abelian generalization) play a very important role for anyons. See more here rmp.aps.org/abstract/RMP/v80/i3/p1083_1 and sciencedirect.com/science/article/pii/S0003491607001674. May 6, 2012 at 14:36
• @VijayMurthy In fact, the standard quantization of anyons is performed along similar lines. The key point is that Wess Zumino Witten terms are deform the symplectic structure, thus modify the angular momentum relations. Please see the following paper by Nair citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.148.8249, in which the relativistic anyons fractional statistics are introduced throug a magnetic monopole term in the momentum space. May 6, 2012 at 15:13
• Articles of @Heidar from arXiv arxiv.org/abs/0707.1889 and arxiv.org/abs/0711.4697.
– Dani
May 7, 2012 at 19:44