Longitudinal acoustic phonons in 3-dimensional cobalt supracrystals detected by broadband picosecond acoustics

Longitudinal acoustic phonons with few-GHz frequency were detected in three-dimensional supracrystals [1] (with face-center cubic lattice) of 7-nm cobalt nanocrystal spheres [2]. In analogy with usual atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions [3], in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (alkyl chains) which act like stiff mechanical springs holding together the nanocrystals. We induced a strain wave (i.e. a travelling phonon wavepacket) using a femtosecond pump pulse absorbed by a metal transducer and we followed its propagation inside the supracrystal monitoring the modulations in the transient pump-probe signal in reflection geometry, as in a picosecond acoustics experiment. Broadband femtosecond white-light pulses coupled to a spectrometer enable us to verify the expected dependence of the observed modulation period on the probe wavelength and the incidence angle of the laser. We could extract the speed of sound inside the supracrystal, which is 1100±100 m/s at room temperature. Very interestingly, the speed of sound is found to be strongly dependent on the sample temperature: its value increases from 760 m/s at 410K to 3140 m/s at 110K, with a change in speed/temperature slope at ≈250K. Such change can be associated to a temperature transition and is attributed to a transformation of the alkyl chain configuration from ordered to disordered.