Spinc structure

In spin geometry, a spin^c structure (or complex spin structure) is a special classifying map that can exist for orientable manifolds. Such manifolds are called spin^c manifolds. C stands for the complex numbers, which are denoted $ℂ$ and appear in the definition of the underlying spin^c group. In four dimensions, a spin^c structure defines two complex plane bundles, which can be used to describe negative and positive chirality of spinors, for example in the Dirac equation of relativistic quantum field theory. Another central application is Seiberg–Witten theory, which uses them to study 4-manifolds.

Definition

Let $M$ be a $n$-dimensional orientable manifold. Its tangent bundle $TM$ is described by a classifying map $M\to \mathrm{BSO}(n)$ into the classifying space $\mathrm{BSO}(n)$ of the special orthogonal group $\mathrm{SO}(n)$. It can factor over the map ${\mathrm{BSpin}}^{\mathrm{c}}(n)\to \mathrm{BSO}(n)$ induced by the canonical projection ${\mathrm{Spin}}^{\mathrm{c}}(n)\twoheadrightarrow \mathrm{SO}(n)$ on classifying spaces. In this case, the classifying map lifts to a continuous map $M\to {\mathrm{BSpin}}^{\mathrm{c}}(n)$ into the classifying space ${\mathrm{BSpin}}^{\mathrm{c}}(n)$ of the spin^c group ${\mathrm{Spin}}^{\mathrm{c}}(n)$, which is called spin^c structure.

Let ${\mathrm{BSpin}}^{\mathrm{c}}(M)$ denote the set of spin^c structures on $M$ up to homotopy. The first unitary group $\mathrm{U}(1)$ is the second factor of the spin^c group and using its classifying space $\mathrm{BU}(1)\cong \mathrm{BSO}(2)$, which is the infinite complex projective space $ℂP^{\mathrm{\infty }}$ and a model of the Eilenberg–MacLane space $K(\mathbb{Z},2)$, there is a bijection:

${\mathrm{BSpin}}^{\mathrm{c}}(M)\cong [M,\mathrm{BU}(1)]\cong [M,ℂP^{\mathrm{\infty }}]\cong [M,K(\mathbb{Z},2)]\cong H^2(M,\mathbb{Z}).$

Due to the canonical projection ${\mathrm{BSpin}}^{\mathrm{c}}(n)\to \mathrm{U}(1)/{\mathbb{Z}}_2\cong \mathrm{U}(1)$, every spin^c structure induces a principal $\mathrm{U}(1)$-bundle or equvalently a complex line bundle.

Properties

• Every spin structure induces a canonical spin^c structure. The reverse implication doesn't hold as the complex projective plane $ℂP^2$ shows.
• Every spin^c structure induces a canonical spin^h structure. The reverse implication doesn't hold as the Wu manifold $\mathrm{SU}(3)/\mathrm{SO}(3)$ shows.
• An orientable manifold $M$ has a spin^c structure iff its third integral Stiefel–Whitney class $W_3(M)\in H^2(M,\mathbb{Z})$ vanishes, hence is the image of the second ordinary Stiefel–Whitney class $w_2(M)\in H^2(M,\mathbb{Z})$ under the canonical map $H^2(M,{\mathbb{Z}}_2)\to H^2(M,\mathbb{Z})$.
• Every orientable smooth manifold with four or less dimensions has a spin^c structure.
• Every almost complex manifold has a spin^c structure.

The following properties hold more generally for the lift on the Lie group ${\mathrm{Spin}}^k(n):=(\mathrm{Spin}(n)\times \mathrm{Spin}(k))/{\mathbb{Z}}_2$, with the particular case $k=2$ giving:

• If $M\times N$ is a spin^c manifold, then $M$ and $N$ are spin^c manifolds.
• If $M$ is a spin manifold, then $M\times N$ is a spin^c manifold iff $N$ is a spin^c manifold.
• If $M$ and $N$ are spin^c manifolds of same dimension, then their connected sum $M\mathrm{\#}N$ is a spin^c manifold.
• The following conditions are equivalent:
  • $M$ is a spin^c manifold.
  • There is a real plane bundle $E\twoheadrightarrow M$, so that $TM\oplus E$ has a spin structure or equivalently $w_2(TM\oplus E)=0$.
  • $M$ can be immersed in a spin manifold with two dimensions more.
  • $M$ can be embedded in a spin manifold with two dimensions more.

See also

• Spin^h structure

External links

• spinᶜ structure on nLab