Dissecting Superradiant Phase Transition in the Quantum Rabi Model

TL;DR

This paper investigates the microscopic mechanism behind the superradiant phase transition in the quantum Rabi model by analyzing energy evolutions of fundamental patterns, revealing how phase transition occurs at a microscopic level.

Contribution

It introduces a novel operator diagonalization approach to dissect the microscopic patterns involved in the superradiant phase transition in the quantum Rabi model.

Findings

Identification of three fundamental patterns in the QRM during phase transition

Analysis of energy evolutions reveals the roles of active, responsive, and stabilizing patterns

First microscopic explanation of how superradiant phase transition occurs in the QRM.

Abstract

The phase transition is both thermodynamically and quantum-mechanically ubiquitous in nature or laboratory and its understanding is still one of most active issues in modern physics and related disciplines. The Landau's theory provides a general framework to describe \textit{phenomenologically} the phase transition by the introduction of order parameters and the associated symmetry breakings; and is also taken as starting point to explore the critical phenomena in connection with phase transitions in renormalization group, which provides a complete theoretical description of the behavior close to the critical points. In this sense the microscopic mechanism of the phase transition remains still to be uncovered. Here we make a first attempt to explore the microscopic mechanism of the superradiant phase transition in the quantum Rabi model (QRM). We firstly perform a diagonalization in an operator space to obtain three fundamental patterns involved in the QRM and then analyze explicitly their energy evolutions with increasing coupling strengths. The characteristic behaviors found uncover the microscipic mechanism of the superradiant phase transition: one is active to drive the happening of phase transition, the second responses rapidly to the change of the active pattern and wakes up the third pattern to stablize the new phase. This kind of dissecting mechanism explains for the first time why and how happens the superradiant phase transition in the QRM and paves a way to explore the microscopic mechanism of the phase transitions happening popularly in nature.