Glass-Forming Liquids and Glasses

The study of glass-forming liquids and glasses remains one of the more complicated and unexplored classes of solid-state experiments. Because they are highly amorphous, with complex non-linear dynamics at all temperatures above 2 K, they have been called the "solid state paradigm of complex systems."(1) Glass-formers are characterized by a rapid increase in viscosity (on the order of 10^-2 poise at room temperature and 10^15 poise below the glass transition temperature) with decreasing temperature, which reflects the rapidly shifting timescale of relaxation dynamics of the liquid. The slow part of liquid motion, known as a-relaxation, has been well characterized on timescales between 100 ps and longer (2, 3, 4), and stops completely below the glass transition temperature. The secondary motion, known as b-relaxation, is predicted to exist at fast timescales between 1 ps and 100 ps, and persists even in the glassy state. This timescale however has been extremely difficult to access by previous methods and its physical meaning is debated. Qualitatively, the relationship between the fast and slow motions of a liquid can be thought of as the collective motion (a-relaxation) of many particles required to release a single particle that rattles in its cage (b-relaxation) of nearest neighbors. Quantitatively, the mode coupling theory (5) makes direct predictions about the relative timescales of the two kinds of relaxation.

Current research on glass-forming liquids and glasses in the Nelson group is taking place on two fronts. The first and foremost goal is to extend the timescale of the experiment into the ps regime, where beta relaxation is measured. This is accomplished by penetrating thin metal films (about 30 molecular layers) with ultrashort pulses of light, to generate very high frequency acoustic waves. The laser system is a regenerative amplifier (Coherent RegA) and oscillator (Coherent Mira) pumped by a Coherent Innova 400 argon-ion laser, yielding 200 fs 6 mJ pulses at 150 kHz repetition rate. A single laser pulse will generate a broadband acoustic wavepacket with frequency components up to 450 GHz. The frequency dependence of wavepacket absorption and velocity (the acoustic modulus) gives a direct measurement of the distribution of timescales of liquid motion.(3) To improve signal-to-noise and to simplify data analysis tremendously, a narrow-band tunable pulseshaper (known as the Deathstar) has been recently built, allowing measurements of liquid dynamics on timescales between 1 ns and 1 ps. With this, it should be possible to make the quantitative link between fast and slow dynamics as predicted by mode coupling theory. The second goal of current research in this lab is to understand the details of acoustic propagation at such high frequencies. Not only are there fast dynamics in a glass or liquid, but there are also structural inhomogeneities which can Rayleigh-scatter phonons and complicate analysis of the motion. The separation of the static and dynamic parts of signal contribution is crucial to fully understanding glasses.