Scientists made two discoveries about the behavior of “supercritical matter”

Extreme physics of 'supercritical' matter may be surprisingly simple.


The matter above the critical point, the supercritical matter, was not thought of as a distinct state of matter and instead seen as a homogeneous state intermediate to liquids and gases and lacking transitions. Now, scientists at the Queen Mary University of London have made two discoveries about the behavior of ‘supercritical matter.

According to scientists, there was new physics yet to be uncovered about this matter at the supercritical state.

Scientists applied two parameters: the heat capacity and the length over which waves can propagate in the system. They made two key discoveries: 1. There is a fixed inversion point between the two where matter changes its physical properties from liquid to gas-like. 2. This inversion point is remarkably close in all systems studied, telling us that the supercritical matter is intriguingly simple and amenable to new understanding.

Kostya Trachenko, Professor of Physics at the Queen Mary University of London, said“The asserted universality of the supercritical matter opens a way to a new physically transparent picture of matter at extreme conditions. This is an exciting prospect from the point of view of fundamental physics as well as understanding and predicting supercritical properties in green environmental applications, astronomy, and other areas.”

“This journey is ongoing and is likely to see exciting developments in the future. For example, it asks whether the fixed inversion point is related to conventional higher-order phase transitions? Can it be described using the existing ideas involved in the phase transition theory, or is something new and quite different needed? As we push the boundaries of what is known, we can identify these new exciting questions and start looking for answers.”

The inapplicability of gases, liquids, and solids theories was the fundamental obstacle to comprehending supercritical matter. What physical parameters would reveal the supercritical state’s most notable characteristics remained unknown.

Scientists used two parameters to define the supercritical matter based on past knowledge of liquids at lower temperatures and pressures: 1. The first parameter is the commonly used property: the heat capacity showing how efficiently the system absorbs heat and contains essential information about the system’s degrees of freedom.

2. The second parameter is less common: this is the length over which waves can propagate in the system. This length governs the phase space available to phonons. Something interesting happens when this length reaches its smallest value possible and becomes equal to the interatomic separation.

In terms of these two parameters, the matter at extreme conditions of high pressure and temperature becomes remarkably universal.

This universality has two dimensions. First, a startling fixed inversion point that corresponds to the change between two physically distinct supercritical states—liquid-like and gas-like—can be seen in the heat capacity vs. wave propagation length plot. As it passes this inversion point, the supercritical stuff alters its fundamental physical properties. Notably, the inversion point provides a clear distinction between the two states, solving a long-standing puzzle for scientists.

Second, in all analyzed system types, the location of this inversion point is strikingly close. In stark contrast to every other known transitional point, this second universality is unique. For instance, two of these transition points—the critical point where the gas-liquid boiling line stops and the triple point where all three states of matter—liquid, gas, and solid—coexist—are distinct in various systems. On the other hand, the fact that all systems with extreme supercritical conditions have the same inversion point indicates that the supercritical matter is enticingly simple.

Journal Reference:

  1. Cillian Cockrell, Kostya Trachnko. Double universality of the transition in the supercritical state. Science Advances. 12 Aug 2022. Vol 8, Issue 32. DOI: 10.1126/sciadv.abq5183
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