Scaling of the Berry phase close to the excited-state quantum phase transition in the Lipkin model

We study the Berry phase of the ground state and excited states in the Lipkin model. Using the Schwinger representation of spin operators, we get the expression for the Berry phase of the ground state in the thermodynamic limit and numerically calculate that of the excited states in finite sizes. We find that the Berry phase of the ground state is nonanalytic, but its derivative (with respect to the control parameter $\ensuremath{\alpha}$, see text) is finite even in the thermodynamic limit. Whereas, for the excited states, the Berry phase tends to be nonanalytic, and its derivative approaches $\ensuremath{\infty}$ in the thermodynamic limit and in the vicinity of the critical point of the excited-state quantum phase transition.