Supercritical matter, perhaps surprisingly simple

We have a fairly good understanding of how matter behaves at fairly low temperatures and pressures, but what happens to matter at high temperatures and pressures is obscure.

When the matter at high temperatures and pressures goes beyond the critical point, the difference between the liquid and gaseous states seems to disappear. This supercritical matter is thought to be hot and dense, and homogeneous. Scientists believe that there is still much new physics to be uncovered about this matter in a supercritical state.

Recently, a new study has made two key discoveries about the supercritical matter. By applying two parameters, the study found that supercritical matter exhibits certain universals and that they may be surprisingly simple. The new findings, recently published in Science Advances, are expected to help us gain a new understanding.

01 The universality of supercritical matter

The main problem in understating It is unclear which physical parameters reveal the most salient properties of the supercritical state when the traditional solid-liquid-gas demarcation line becomes blurred.

With their early understanding of liquids at lower temperatures and pressures, researchers chose two parameters to describe supercritical matter.

The first parameter is relatively common property, the heat capacity, which measures the efficiency of the system's heat absorption and contains basic information about the system's degrees of freedom.

The second parameter, which is relatively less common, is the length over which the wave can propagate in the system. This length dominates the phase space available for phonons. When this length reaches its minimum possible value and is equal to the interatomic spacing, really interesting things happen.

Scientists have found that, concerning these two parameters, matter in extreme conditions of high pressure and temperature becomes very common.

This universality consists of two aspects. First, there is a striking fixed inversion point in the plot of heat capacity versus wave propagation length for different systems.

This inversion point corresponds to the transition between two physically distinct supercritical states (liquid-like and gas-like). Upon crossing this point, the supercritical matter changes key physical properties and goes from a liquid-like state to a gas-like state. Importantly, the inversion point is the clear way to distinguish between these two states.

Second, the location of this inversion point is very close to all types of systems studied.

This is significantly different from all other known transition points. For example, two of these transition points, the three-phase point where all states of matter (liquid-gas-solid) coexist, and the critical point at the end of the gas-liquid boiling line, vary considerably from system to system.

On the other hand, this generality may also tell us that supercritical matter may be rather simple.

02 Basic understanding and practical applications

Researchers believe that, in addition to the basic understanding of states of matter and phase diagrams, the understanding of supercritical matter leads to many practical applications.

For example, hydrogen and helium are in supercritical states in gas giants such as Jupiter and Saturn, and therefore govern their physical properties. In green applications, supercritical fluids have also proven highly effective in the destruction of hazardous waste, but engineers are increasingly looking for theoretical guidance to improve the efficiency of supercritical processes.

This prevalence of supercritical matter opens the way for a new physics of matter under extreme conditions," said study corresponding author Kostya Trachenko, professor of physics at Queen Mary, University of London. This is an exciting prospect both from the point of view of fundamental physics and from the point of view of understanding and predicting supercritical properties in green environmental applications, astronomy, and other fields."

At the same time, it also opens scientists' eyes to the possibility of some exciting future developments. For example, the study also begs the question: is the fixed inversion point related to traditional higher-order phase transitions? Can it be described by using the existing ideas involved in phase transition theory, or does it require the introduction of something radically new?

As we push the boundaries of what is known, we can gradually identify these new and exciting questions and begin to find more answers.