Acoustic resonance in periodically sheared glass: damping due to plastic events

Using molecular dynamics simulation, we study acoustic resonance in a low-temperature model glass by applying a small periodic shear at a boundary wall. Shear wave resonance occurs as the frequency omega approaches omega= pi c(perpendicular to)/L( = 1, 2 horizontal ellipsis ). Here,c(perpendicular to)is the transverse sound speed andLis the cell width. At resonance, large-amplitude sound waves appear after many cycles even if the applied strain gamma(0)is very small. They then induce plastic events, which are heterogeneous on the mesoscopic scale and intermittent on timescales longer than the oscillation periodt(p)= 2 pi/omega. We visualize them together with the extended elastic strains around them. These plastic events serve to damp sounds. We obtain the nonlinear dampingQ(-1)= tan delta due to the plastic events near the first resonance at omega approximately equal to omega(1), which is linear in gamma(0)and independent of omega. After many resonant cycles, we observe an increase in the shear modulus (measured after switching-off the oscillation). We also observe catastrophic plastic events after a very long time (similar to 10(3)t(p)), which induce system-size elastic strains and cause a transition from resonant to off-resonant states. At resonance, stroboscopic diffusion becomes detectable.