Finite temperature induced fermion number in the nonlinear σ model in 2+1 dimensions -: art. no. 025004

We compute the finite temperature induced fermion number for fermions coupled to a static nonlinear sigma model background in 2+1 dimensions, in the derivative expansion limit. While the zero temperature induced fermion number is well known to be topological (it is the winding number of the background), at finite temperature there is a temperature dependent correction that is nontopological-this finite T correction is sensitive to the detailed shape of the background. At low temperature we resum the derivative expansion to all orders, and we consider explicit forms of the background as a CP1 instanton or as a baby Skyrmion.