On the nature of the transition from the spontaneously dimerized to the Neel phase in the two-dimensional J1-J2 model

TL;DR

This paper investigates the transition from a dimerized spin-liquid phase to a Neel ordered phase in a 2D frustrated Heisenberg model, revealing distinct excitation behaviors and the preservation of O(3) universality class at criticality.

Contribution

It provides a detailed analysis of the excitation spectrum near the quantum phase transition, highlighting the separation of length scales and the nature of singlet and triplet modes.

Findings

Both triplet and singlet excitations become gapless at the transition.

The singlet bound state size grows exponentially near the transition.

Critical dynamics remain in the O(3) universality class despite multiple gapless modes.

Abstract

We analyze the spectrum of the 2D S=1/2 frustrated Heisenberg model near the transition from the spontaneously dimerized spin-liquid phase into the Neel ordered phase. Two excitation branches: the triplet magnon, and the collective singlet mode, both become gapless at the transition point. However we find that the length scales associated with these modes are well separated at the quantum transition. While in the quantum disordered phase the singlet excitation has finite spectral weight and reflects the existence of spontaneous dimer order, near the transition point the size of the singlet bound state grows exponentially with the correlation length, and hence the quasiparticle residue is exponentially small. Therefore the critical dynamics remains in the O(3) universality class in spite of the four gapless modes.