Probing organic glasses at low temperature with variable time scale optical dephasing measurements
| Autor: | L.R. Narasimhan, Karl A. Littau, Michael D. Fayer, Y.S. Bai, A. Elschner, Dee William Pack |
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| Rok vydání: | 1990 |
| Předmět: | |
| Zdroj: | Chemical Reviews. 90:439-457 |
| ISSN: | 1520-6890 0009-2665 |
| DOI: | 10.1021/cr00101a001 |
| Popis: | Amorphous materials at low temperature have markedly different physical and thermal properties from crystals.' For example, the specific heats of crystals obey the Debye P law at low temperatures2 and the thermal conductivity also follows the same P temperature dependen~e .~ Many of these properties can be calculated because crystals have long-range spatial and orientational order.4 Twenty years ago, specific heat measurements on disordered materials revealed significant deviations from the Debye law at approximately 1 K.5-8 Measurements of thermal conductivity5y9 and dielectric responselOJ1 also deviated from crystalline behavior. These observations showed that the dynamics of ordered and disordered systems must be fundamentally different. Although ordered crystals are commonly found in nature, many other naturally occurring complex systems, such as proteins, are inherently disordered.12 Furthermore, many artificial materials, such as polymeric solids and amorphous semiconductors, are glasses. Therefore, understanding the microscopic behavior of the glassy state has been and continues to be13-15 an important problem in chemistry, physics, and materials science. Glasses are systems in which there is no translational or rotational order. More important, unlike a crystal, a glass is not in thermodynamic equilibrium. At low temperature, the equilibrium state of a substance like ethanol is crystalline. A glass is formed by rapid cooling, which traps the material in the glassy state. Thermodynamically the material should be a crystal, but kinetics prevent the system from finding the global potential minimum; Le., the kinetics at low temperature make the time scale for crystallization essentially infinitely long. Dynamics in simple crystals involve fast fluctuations about an equilibrium structure.16 In conlocal structures as well as time evolution of the nonequilibrium local structures them~e1ves.l~ Glass dynamics can occur through essentially all time scales from femtoseconds, to kiloseconds, and perhaps longer.lgZ0 Many concepts that are useful in describing dynamics in crystals cannot be extended to the amorphous state, e.g., translational symmetry, which gives rise to phonon bands and fast phonon fluctuations, providing a separation of time scales for a variety of processes.z1 In crystals at low temperatures, only the acoustic phonons are thermally excited. The phonon dispersion is well described by the Debye density of ~ t a t e s , ~ J ~ and this means that the distribution of fluctuation rates is known. A glass also has modes that are equivalent to a crystal's phonons. Even at low temperatures, however, there is a major contribution to the dynamical properties of glasses from the evolution of local structures, and it is these "extra" dynamics that make glasses fundamentally different from crystals. The anomalous heat capacities found in glasses show a term linear in temperature which is not present in crystals. This is true in such diverse substances as silicates and ceramics as well as poly(methy1 methacrylate) and Lexan polymer^.^-^>^^ In addition, heat capacities are time dependent; the heat capacity increases as time i n c r e a ~ e s . ~ ~ J ~ ~ ~ ~ ~ Thermal conductivity, which varies as P in crystals, varies as P? The phonon mean free path in a glass is substantially less than in the corresponding crystal, and a variation in the velocity of sound with temperature is seen in amorphous syst e m ~ . ~ ~ Many theoretical models have been developed to account for the various observations. The models invoke defect-induced scattering,26 localized electronic states,27 and defect and particle diffusionz8 to explain aspects of the observed behaviors. The two-level-system (TLS) model proposed independently by Anderson et al.29 and Phillips30 has been the most widely used, and it is frequently the point of departure in discussing the properties of low-temperature glasses. The TLS model postulates that, in an amorphous material, some atoms or molecules (or groups of atoms or molecules) can reside in not just one but two potential minima of the local structure potential surface (Figure 1). Each side of the double-well potential represents a distinct local structure of the glass. This is a simplified representation of what is almost certainly a complex multidimensional potential surface. At low temperatures, transitions from one side of the double well to the other represent changes in the local structure. Transitions occur by phonon-assisted tunneling. To model the complex distribution of local structures and transition rates, the TLS model states that there |
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