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We monitored the spectral shifts of single molecule lines under an applied ac voltage in a molecular crystal at a low temperature. When varying the voltage modulation frequency, we found pronounced resonances in the oscillating shift of the optical lines. The resonance frequencies are surprisingly low and are found anywhere in the explored region, from some tens of kHz to a few MHz. Their width and amplitude depend very steeply on temperature, with quality factors as high as a few hundred at 1.4K. Probing the resonant modes with single molecules at different locations, we find a clear spatial correlation of the modes within microcrystalline domains, extending over 10-100µm. For large amplitudes, the oscillations become anharmonic and display a range of nonlinear effects: decrease of the frequency with amplitude, hysteretic behavior depending on the scan direction and shift of the frequency whenan external strain is applied to the sample. Put together, our observations point to low-frequency acoustic modes localized at or around crystal defects. This work is closely related to earlier observations of similar resonances in Shpol'skii matrices deposited on a semiconductor. We speculate on the relation of these acoustic modes to the low-energy modes (quasi-localized modes and Boson-peak excitations) already known in disordered solids.

Single molecules detect ultra-slow oscillators in a molecular crystal excited by ac voltages