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Bolmatov D, Zhernenkov M, Sharpnack L, Agra-Kooijman DM, Kumar S, Suvorov A, Pindak R, Cai YQ, Cunsolo A. Emergent Optical Phononic Modes upon Nanoscale Mesogenic Phase Transitions. NANO LETTERS 2017; 17:3870-3876. [PMID: 28548861 DOI: 10.1021/acs.nanolett.7b01324] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
The investigation of phononic collective excitations in soft matter systems at the molecular scale has always been challenging due to limitations of experimental techniques in resolving low-energy modes. Recent advances in inelastic X-ray scattering (IXS) enabled the study of such systems with unprecedented spectral contrast at meV excitation energies. In particular, it has become possible to shed light on the low-energy collective motions in materials whose morphology and phase behavior can easily be manipulated, such as mesogenic systems. The understanding of collective mode behavior with a Q-dependence is the key to implement heat management based on the control of a sample structure. The latter has great potential for a large number of energy-inspired innovations. As a first step toward this goal, we carried out high contrast IXS measurements on a liquid crystal sample, D7AOB, which exhibits solid-like dynamic features, such as the coexistence of longitudinal and transverse phononic modes. For the first time, we found that these terahertz phononic excitations persist in the crystal, smectic A, and isotropic phases. Furthermore, the intermediate smectic A phase is shown to support a van der Waals-mediated nonhydrodynamic mode with an optical-like phononic behavior. The tunability of the collective excitations at nanometer-terahertz scales via selection of the sample mesogenic phase represents a new opportunity to manipulate optomechanical properties of soft metamaterials.
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Affiliation(s)
- Dima Bolmatov
- National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States
| | - Mikhail Zhernenkov
- National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States
| | - Lewis Sharpnack
- Department of Physics, Kent State University , Kent, Ohio 44242, United States
| | | | - Satyendra Kumar
- Department of Physics, Kent State University , Kent, Ohio 44242, United States
- Division of Research and Department of Physics, University at Albany , Albany, New York 12222, United States
| | - Alexey Suvorov
- National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States
| | - Ronald Pindak
- National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States
| | - Yong Q Cai
- National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States
| | - Alessandro Cunsolo
- National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States
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Bellissima S, Neumann M, Guarini E, Bafile U, Barocchi F. Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function. Phys Rev E 2017; 95:012108. [PMID: 28208443 DOI: 10.1103/physreve.95.012108] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2016] [Indexed: 11/07/2022]
Abstract
Extending a preceding study of the velocity autocorrelation function (VAF) in a simulated Lennard-Jones fluid [Phys. Rev. E 92, 042166 (2015)PLEEE81539-375510.1103/PhysRevE.92.042166] to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion method allows for a full account and understanding of the basic dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained through the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased beyond a threshold value. While this finding agrees with the recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in both its nonpropagating and propagating aspects which are linked through dynamical transitions depending on both the thermodynamic states and the excitation wave vectors.
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Affiliation(s)
- S Bellissima
- Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy
| | - M Neumann
- Fakultät für Physik der Universität Wien, Strudlhofgasse 4, A-1090 Wien, Austria
| | - E Guarini
- Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy
| | - U Bafile
- Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy
| | - F Barocchi
- Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy
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