Anomalous Nanoscale Optoacoustic Phonon Mixing in Nematic Mesogens

Abstract

Recent inelastic X-ray scattering (IXS) experiments on mesogens have revealed entirely new capabilities with regards to their nanoscale phonon-assisted heat management. Mesogens such as nematic liquid crystals (LCs) are appealing systems for study because their structure and morphology can easily be tuned. We report on Q-resolved ultra-high-resolution IXS, X-ray diffraction, and THz time-domain spectroscopy experiments combined with large-scale all-atom molecular dynamics simulations on the dynamic properties of 5CB LCs. For the first time, we observe a strong mixing of phonon excitations originating from independent in-phase and out-of-phase van-der-Waals-mediated displacement patterns. The coexistence of transverse acoustic and optical modes of 5CB LCs at near room temperature is revealed through the emergent transverse phonon gap and THz light-phonon coupling taking place within the same energy range. Furthermore, our experimental observations are supported by analysis showing correlations of spontaneous fluctuations of LCs on picosecond time scales. These findings are significant for the design of a new generation of soft molecular vibration-sensitive nanoacoustic and optomechanical applications.

Rayleigh scattering and disorder-induced mixing of polarizations in amorphous solids at the nanoscale: 1-octyl-3-methylimidazolium chloride glass

“…where The high quality of the IXS data acquired permits to assign a value to L and L even at small values of q, in the interval 1-2 nm −1 . In the high-q region (q > 5 nm −1 ) an additional feature in the inelastic wings of the IXS spectrum is observed, in agreement with experimental observation in several other classes of glass [46][47][48][49][50][51][52][53][54][55][56][57][58]. Similar features are commonly related to "projection" of transverse into longitudinal dynamics.…”

“…The mixing of polarizations in a local elastically heterogeneous medium is qualitatively predicted by the elementary elasticity theory, which states that a purely polarized wave impinging on an interface between two different elastic media shall be transformed in waves with mixed polarization [45]. Features in S(q, E ) possibly attributable to acoustic excitations with mixed polarization have been observed by IXS or INS experiments [46][47][48][49][50][51][52][53][54], as well as by MD simulations [55][56][57][58] in several topologically disordered systems in the first pseudo-Brillouin zone, with a q onset typically ∼5 nm −1 . Despite this, one meets the lacking of analytical models addressing the occurrence of disorder-induced mixing of polarizations, which has, furthermore, never been related to the Rayleigh scattering in the RMT framework.…”

Molecular Picture of the Transient Nature of Lipid Rafts

“…This means that these modes do not propagate (sound velocity ∼ 0 m/s) and exist in the form of standing waves originating from the out-of-phase movements of membrane molecules. 25,38,39 MD simulations revealed optical modes above 10 meV and truncated modes close to 6−7 meV (see Figure 1b,d). The truncated excitations cannot be resolved in the longitudinal currents (Figure 1a,c) due to their close proximity to the acoustic mode.…”

“…It is noteworthy that the DHO model has previously been used to study vibrational landscapes, including but not limited to biomembranes, 21,25 simple liquids, 44,45 block copolymers, 46 and liquid crystals. 38,39 In Figure 3, there is also an indication of nondispersive "bumplike" signals at different q-values (Figure 3). These nondispersive excitations are denoted by black arrows indicating the presence of a nonpropagating mode that can exist in the form of standing waves, as calculated from eq 2 (see Figure 1) and previously measured in DMPC membranes using INS 22,43 and far-infrared spectroscopy.…”

“…57 The existence of elastic and viscous regimes in soft (noncrystalline) materials has previously been predicted on the basis of a microscopic Hamiltonian where symmetry is broken through its vibrational molecular interactions. 50 Furthermore, the viscoelastic crossover, demarcating the elastic and viscous regimes, has been experimentally observed in different soft materials including, but not limited to, biological membranes, 21,22 simple liquids, 44,45 block copolymers, 46 and liquid crystals, 38,39 alluding to its ubiquitous presence. As a result, the viscoelastic properties of soft materials primarily depend on the shear viscosity parameter, regardless of their structural complexity.…”