# Excited-state quantum phase transitions in systems with two degrees of freedom: III. Interacting boson systems

## TL;DR

This paper investigates excited-state quantum phase transitions in interacting boson systems, revealing complex classical stationary points and finite-size effects, thus advancing understanding of many-body quantum dynamics.

## Contribution

It extends previous work on ESQPTs by analyzing a realistic interacting boson model with fixed angular momentum, highlighting the role of classical stationary points and phase space boundedness.

## Key findings

- ESQPTs linked to classical stationary points
- Finite-size effects due to partial separability
- Complexity from kinetic and potential energy decomposition

## Abstract

The series of articles [Ann. Phys. 345, 73 (2014) and 356, 57 (2015)] devoted to excited-state quantum phase transitions (ESQPTs) in systems with $f=2$ degrees of freedom is continued by studying the interacting boson model of nuclear collective dynamics as an example of a truly many-body system. The intrinsic Hamiltonian formalism with angular momentum fixed to $L=0$ is used to produce a generic first-order ground-state quantum phase transition with an adjustable energy barrier between the competing equilibrium configurations. The associated ESQPTs are shown to result from various classical stationary points of the model Hamiltonian, whose analysis is more complex than in previous cases because of (i) a non-trivial decomposition to kinetic and potential energy terms and (ii) the boundedness of the associated classical phase space. Finite-size effects resulting from a partial separability of both degrees of freedom are analyzed. The features studied here are inherent in a great majority of interacting boson systems.