# Berry phase: Spin in a magnetic field parameter space manifold

Canonical example for Abelian Bery phase is a spin in a magnetic filed, e.g.. Usually authors calculate spin eigenstates, conclude that they don't depend on B in spherical components and so deduce that the sphere $$S^2$$ is the parameter space manifold. First question - shouldn't the parameter space actually be $$S^2 \times (0,\infty) \cong B^2/O$$, with $$O$$ being a ball center, as |B| $$\in (0,\infty)$$?

I would like to obtain more a priori understanding of the parameter space manifold identification. Working in Cartesian coordinates, for example, this $$S^2$$ identification is not transparent. My reasoning is along these lines - Fixing |B| one would naively first conclude that the $$SO(3)$$ is actually this manifold as we can orient B as we wish. However, we should ''subtract'' rotations around B and so we are lead to the coset manifold $$SO(3)/SO(2)\cong S^2$$. Letting |B| change, we finally arrive at $$B^2/O$$. Is this correct reasoning? Thing that puzzles me here is that we have spins here so $$SU(2)$$ should be more appropriate but how do we arrive to $$S^2$$ then?

The reason that the active parameter space is $$S^2$$ rather than the whole three dimensional space $$\mathbb{R}^3$$ or the ball $$B^3$$ is that the Berry curvature depends solely on the projector $$P$$ over the eigenspace, for example, an Abelian Berry curvature can be expressed as $$F = \mathrm{Tr}(P dP \wedge dP)$$ Now, spaces of projectors over one dimensional subspaces are projective spaces (which $$S^2$$ is a partiular case of): For two dimensional Hamiltonians, the projector on any one dimensional subspace can be casted in the form:

$$P(z, \bar{z}) = \frac{1}{1+\bar{z}z}\begin{bmatrix} 1 & z\\ \bar{z} & \bar{z}z \end{bmatrix}$$ with $$z = \tan \frac{\theta}{2}e^{i\phi}$$ and $$\theta$$ and $$\phi$$ are the coordinates parametrizing a unit sphere $$S^2$$. which is the one-(complex) dimensional projective space $$\mathbb{CP}^1$$.

This result generalizes as follows:

In fermionic time revesal invariant systems, the projectors are quaternionic valued and in the case of projectors on a one dimensional supspace (Abelian Berry phase), the parameter space is the one dimensional quaternionic projective space which is isomorphic to the four sphere:

$$\mathbb{HP}^1 \cong S^4$$

Please see for example: Avron, Sadun, Segert and Simon, where they obtain this parameter space as the parameter space of quadrupoles.

Another generalization is the case of the non-Abelian Wilczek-Zee phases, in this case the projectors are over higher dimensional subspaces of degenerate eigenvalues. Here, the parameter space is the Grassmann manifold:

$$\mathrm{Gr}(m,n) = U(m+n)/U(m)\times U(n)$$

where $$m$$ is the dimension of the Hamiltonian and $$n$$ is the dimension of the degenerate subspace of eigenvalues. Please see, for example, the article by Karle and Pachos.