Effects of quantum instantons on the thermodynamics of the CPN-1 model

Using the 1/N expansion, we study the influence of quantum instantons on the thermodynamics of the CPN-1 model in 1+1 dimensions. We do this by calculating the pressure to next-to-leading order in 1/N, without quantum instanton contributions. The fact that the CP1 model is equivalent to the O(3) nonlinear sigma model allows for a comparison to the full pressure up to 1/N2 corrections for N=3. Assuming validity of the 1/N expansion for the CP1 model makes it possible to argue that the pressure for intermediate temperatures is dominated by the effects of quantum instantons. A similar conclusion can be drawn for general N values by using the fact that the entropy should always be positive. © 2006 The American Physical Society.