Dynamical properties of N\'eel and valence-bond phases in the $J_1-J_2$ model on the honeycomb lattice

TL;DR

This study uses variational Monte Carlo to analyze the dynamical structure factor of the $J_1-J_2$ Heisenberg model on the honeycomb lattice, revealing phase transitions and excitation spectra across different magnetic orders.

Contribution

It provides a detailed characterization of dynamical properties and phase transitions in the honeycomb $J_1-J_2$ model using Gutzwiller-projected fermionic states, highlighting the evolution of excitations.

Findings

Magnon mode in antiferromagnetic phase agrees with linear spin-wave theory

Roton-like mode softens and becomes gapless at the transition point

Valence-bond phases exhibit gapped spectra with well-separated triplon modes

Abstract

By using a variational Monte Carlo technique based upon Gutzwiller-projected fermionic states, we investigate the dynamical structure factor of the antiferromagnetic $S=1/2$ Heisenberg model on the honeycomb lattice, in presence of first-neighbor ($J_1$) and second-neighbor ($J_2$) couplings, for ${J_2 < 0.5 J_1}$. The ground state of the system shows long-range antiferromagnetic order for ${J_2/J_1 \lesssim 0.23}$, plaquette valence-bond order for ${0.23 \lesssim J_2/J_1 \lesssim 0.36}$, and columnar dimer order for ${J_2/J_1 \gtrsim 0.36}$. Within the antiferromagnetic state, a well-defined magnon mode is observed, whose dispersion is in relatively good agreement with linear spin-wave approximation for $J_2=0$. When a nonzero second-neighbor super-exchange is included, a roton-like mode develops around the $K$ point (i.e., the corner of the Brillouin zone). This mode softens when $J_2/J_1$ is increased and becomes gapless at the transition point, $J_2/J_1 \approx 0.23$. Here, a broad continuum of states is clearly visible in the dynamical spectrum, suggesting that nearly-deconfined spinon excitations could exist, at least at relatively high energies. For larger values of $J_2/J_1$, valence-bond order is detected and the spectrum of the system becomes clearly gapped, with a triplon mode at low energies. This is particularly evident for the spectrum of the dimer valence-bond phase, in which the triplon mode is rather well separated from the continuum of excitations that appears at higher energies.