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By using a variational Monte Carlo technique based upon Gutzwiller-projectedfermionic states, we investigate the dynamical structure factor of theantiferromagnetic $S=1/2$ Heisenberg model on the honeycomb lattice, inpresence 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-rangeantiferromagnetic order for ${J_2/J_1 \lesssim 0.23}$, plaquette valence-bondorder 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-definedmagnon mode is observed, whose dispersion is in relatively good agreement withlinear spin-wave approximation for $J_2=0$. When a nonzero second-neighborsuper-exchange is included, a roton-like mode develops around the $K$ point(i.e., the corner of the Brillouin zone). This mode softens when $J_2/J_1$ isincreased 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 atrelatively high energies. For larger values of $J_2/J_1$, valence-bond order isdetected and the spectrum of the system becomes clearly gapped, with a triplonmode at low energies. This is particularly evident for the spectrum of thedimer valence-bond phase, in which the triplon mode is rather well separatedfrom the continuum of excitations that appears at higher energies.

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