We study the phase diagram of the four-dimensional O(4) model with first- $\left({\beta }_1\right)$ and second- $\left({\beta }_2\right)$ neighbor couplings, especially in the ${\beta }_2<0\mathrm{}$ region, where we find a line of transitions which seems to be second order. We also compute the critical exponents on this line at the point ${\beta }_1=0\mathrm{}$ ${(F}_4$ lattice) by finite-size scaling techniques up to a lattice size of $24$, these exponents being different from the mean-field ones.

• Received 2 July 1996

DOI:https://doi.org/10.1103/PhysRevD.55.2965