Decoding the glassy state: Ultrafast detection of amorphous liquid-solid transition

In 2005, Science magazine invited well-known scientists worldwide to elect the 125 greatest scientific conundrums of the 21st century. Among the five conundrums in condensed matter physics, one asks, “What is the nature of the glassy state?”

The amorphous state (commonly also called the glassy state) is a fundamental state of matter, alongside crystalline solid, liquid, and gaseous phases. It possesses two basic natures: the first is the lack of long-range order, with a randomly disordered atomic network similar to liquids but more tightly packed. This disordered nature is well understood and not in question. The second is the non-equilibrium behavior in its liquid state below the melting temperature, Tm. Unlike crystals, in which temperature changes primarily affect lattice vibrations, the amorphous liquid (also known as “supercooled liquid”) is metastable. When cooled from the equilibrium molten liquid above the Tm, it undergoes collective atomic rearrangements and structural relaxation, with relaxation times spanning well over tens of decades. The dynamic nature of the amorphous state has been debated for over 80 years.

Consequently, the central questions are, “Does amorphous have a solid state where atomic diffusion is frozen? If so, at what temperature does liquid amorphous state end and solid amorphous state begin? How can we measure it?” These questions have challenged condensed matter physicists, materials scientists, and chemists for decades.

Kauzmann was probably the first to realize the need for a fundamentally different amorphous state—the solid amorphous state. In his pioneering 1948 paper, Kauzmann defined the nature of this state as one where “atomic diffusion becomes frozen,” although he still used the word “liquid” to represent its disordered nature and called this solid amorphous state the “glassy state.” At the time, Kauzmann’s motivation was to avoid the renowned “Kauzmann paradox”: when extrapolating the entropy-temperature relationship of the amorphous liquid to low temperatures, the entropy decreases more steeply than in the crystalline solid due to the extra configurational entropy (associated with the ability for collective atomic rearrangements). Continued extrapolation suggests that if the amorphous liquid retains non-zero configurational entropy, its entropy would reach a value lower than that of the crystalline solid above absolute zero Kelvin—violating the third law of thermodynamics.